General Introduction to Organic Compounds

Most of the foodstuffs that we consume every day such as sugar, fats,

starch, vinegar, etc are basically organic compounds. Even though the

organic compounds have been known to man since prehistoric times,

their study practically began from the eighteenth century! The term

“organic compound” was coined by Berzelius in 1807. Let’s explore

more about these compounds

Introduction to Organic Compounds

Earlier people thought that compounds which are obtained from plants

and animals are organic compounds and compounds which are

obtained from minerals, non-living sources are termed as inorganic

compounds. However, the modern definition of organic compounds is

a bit different to this.

An organic compound is defined as any compound whose molecules

contain carbon and hydrogen ( also known as ” hydrocarbons” ) or

compound that is the derivative of it. The branch of science which

deals with the scientific study of structure, properties and reactions of

hydrocarbons and their derivatives is known as organic chemistry.

Browse more Topics under Organic Chemistry

● Classification of Organic Compounds

● Isomerism

● Nomenclature of Organic Compounds

● Purification of Organic Compounds

● Qualitative Analysis of Organic Compounds

● Quantitative Analysis of Organic Compounds

● Structural Representations of Organic Compounds

● Types of Organic Reactions

● Fundamental Concepts of Organic Reaction Mechanism

Characteristics of Organic Compounds

The general characteristics of Organic Compounds include:

● Can be isolated as well as prepared in laboratory

● Comprise almost 90% of all known compounds.

● Mostly built up of only three elements- carbon, hydrogen and

oxygen. Other elements like halogen, nitrogen as well as

phosphorous are also present but to a lesser extent.

● Possess complex structures and high molecular weights

● Their properties are decided by certain active atom or group of

atoms known as the functional group.

● They are mostly insoluble in water but soluble in organic

solvents.

● They are combustible in nature

● Chemical reactions involving organic compounds proceed at

slower rates.

Characteristics due to Presence of Covalent Bonds

A covalent bond is a chemical bond that involves the sharing of

electron pairs between atoms that in turn results in a balance of

attractive and repulsive forces between the atoms. The presence of a

covalent bond renders certain characteristics to the organic

compounds. These include:

● Low melting points and boiling points in comparison to the

inorganic compounds.

● Organic acids and bases are less stronger and thus they have a

limited dissociation in an aqueous medium.

● They exhibit the phenomenon of isomerism in which a single

molecular formula represents several organic compounds

differing in physical and chemical properties.

● They are volatile in nature.

General Characteristics of Members of Homologous Series

A Homologous Series is a group of organic chemical compounds,

usually listed in the order of increasing size, that have a similar

structure (and hence, also similar properties) and whose structures

differ only by the number of CH2– CH2 units in the main carbon

chain. They possess the following general characteristics:

● A general formula describes the members of the homologous

series

● Successive members differ from each other by CH2CH2

● Physical properties change regularly with increasing number of

carbon atoms.

● Members have similar chemical properties because they have

same functional group.

● Members of the homologous series can be prepared using the

same method.

Importance of Organic Compounds

● Organic compounds are important because all living organisms

contain carbon.

● While carbohydrates, proteins and fats, the basic structures of

life, are organic compounds

● They are the basic components of many of the cycles that drive

the earth. For example, the carbon cycle that includes the

exchange of carbon between plants and animals in

photosynthesis and cellular respiration.

● Organic compounds combine with metals to form

organometallic compounds. These compounds are important

industrially. They are used as catalysts, promoters, analysers as

well as stabilizers.

Learn more about Types of Organic Reactions here.

Solved Questions For You

Q 1. Which of these is not a property of organic compounds?

a. They possess complex structures and high molecular weights

b. They are combustible

c. They have high melting as well as boiling points

d. They have low melting as well as boiling points

Ans: The correct answer is c. They have high melting as well as

boiling points.

Classification of Organic Compounds

Organic compounds constitute about 90% of all compounds. That’s

quite a lot, isn’t it? In order to study such a vast number of

compounds, it is necessary to classify them into categories. Let us

know more about the Classification of Organic Compounds as well as

the general categories into which organic compounds are divided.

Classification of Organic Compounds

Depending upon the arrangement of carbon atoms in their structure,

organic compounds are broadly categorized into

● Acyclic or Open Chain compounds

● Cyclic or Closed Chain compounds

The following diagram will give you a clear idea about the

classification of organic compounds:

Browse more Topics under Organic Chemistry

● General Introduction to Organic Compounds

● Isomerism

● Nomenclature of Organic Compounds

● Purification of Organic Compounds

● Qualitative Analysis of Organic Compounds

● Quantitative Analysis of Organic Compounds

● Structural Representations of Organic Compounds

● Types of Organic Reactions

● Fundamental Concepts of Organic Reaction Mechanism

Acyclic or Open Chain Compounds

The carbon atoms are present in the form of an open chain.This chain

may either be a straight chain or a branched chain. These were initially

known as Aliphatic compounds because the compounds of this class

were derived from either animal or vegetable fats

● Straight Chain Compounds: The carbon skeleton is in the form

of a straight chain. Examples:

n-Propane CH3-CH2-CH3

Propene CH2=CH-CH3

● Branched Chain Compounds: The carbon skeleton is in the

form of a branched chain. Examples: Isobutylene

Cyclic or Closed Chain Compounds

They are marked by the presence of one or more closed chains or ring

of atoms in their structure. Depending on whether there is a presence

of any other atom apart from carbon in the constitution of the ring,

they are further classified as:

● Homocyclic or Carbocyclic Compounds

● Heterocyclic Compounds

Homocyclic or Carbocyclic Compounds

The rings in these compounds are entirely made up of carbon atoms.

No other atom is present in the ring skeleton.These can be further

divided into two sub-classes:

● Alicyclic Compounds

● Aromatic Compounds

Alicyclic Compounds

Their name is attributed to their resemblance to Aliphatic compounds

in their properties. The examples of this category include

cyclopropane, cyclobutane, cyclopentane, cyclohexane, etc.

Aromatic Compounds

These are cyclic unsaturated compounds. They derive their name from

the Greek word Aroma which means “fragrant smell” since most of

these compounds bear a pleasant smell. These are further classified

into two types:

● Benzenoid Aromatic Compounds: They are characterized by

the presence of one or more fused or isolated benzene rings as

well as their derivatives in their structure. Depending upon the

number of benzene rings that are fused together in their

structure, they can be further classified as Monocyclic,

Bicyclic, Tricyclic.

● Non-Benzenoid aromatic Compounds: They are characterized

by the presence of a single benzene ring to which other groups

are attached.

Aniline

Bicyclic and Tricyclic Compounds

These are characterized by the presence of two or more rings in their

structure.Examples include Naphthalene, Phenanthrene as well as

Anthracene.

Naphthalene

Phenanthrene

1.

Anthracene

Non-Benzenoid Aromatic Compounds

Aromatic compounds that contain other highly unsaturated rings in

place of the benzene ring are called non-benzenoid aromatic

compounds. Examples include

Azulene

Tropolone

Heterocyclic Compounds

When one or more heteroatoms such as oxygen, nitrogen, sulphur,

boron, silicon etc, are present in the ring such compounds are known

as heterocyclic compounds.

● Alicyclic heterocyclic compounds: Aliphatic heterocyclic

compounds that contain one or more heteroatoms in their rings

are called alicyclic heterocyclic compounds.

● Aromatic heterocyclic compounds Aromatic heterocyclic

compounds that contain one or more heteroatoms in their ring

skeleton are called aromatic heterocyclic compounds.

Solved Questions For You

Que: Which of these is not an aromatic compound?

a.

b.

c.

d.

Ans: The correct answer is option b. It is the structure of cyclohexane

which is an alicyclic compound.

Isomerism

You have probably come across the words ‘isomers and isomerism’ in

your previous classes. But, do you know all about them? Isn’t it funny

that with the same chemical formula, two compounds exhibit different

properties? Are they twins or something? Well, no! In this chapter, let

us study about isomerism in greater detail. We will look at its types

and examples as well.

What is Isomerism?

It is a phenomenon where two or more compounds have the same

chemical formula but possesses different structural formulas, that is,

different properties. This is mainly because of different structural or

spatial arrangements. Isomers are the compounds exhibiting

isomerism.

Types of Isomerism

Basically, there are two types. They are:

● Structural Isomerism

● Stereoisomerism

However, these are again of many subtypes as shown in the figure:

Structural Isomerism

Isomers are structural isomers when they have the same molecular

formula but different structures, as in how they are linked to each

other. Structural isomerism is further of the following types. Let’s

learn about these types one-by-one.

Learn the different Characteristics of Organic Compounds here.

1) Chain Isomerism

Isomers are chain isomers when two or more compounds have the

same molecular formula but differ in the branching of carbon atoms.

For example, we can represent C5H12 as three compounds:

CH3CH2CH2CH2CH3– pentane

2) Position Isomerism

Isomers are position isomers when the two or more compounds differ

in the position of the functional group or substituent atoms. For

example, we can represent C3H7OH in two arrangements:

CH3CH2CH2OH -Propan-1-ol

3) Functional Isomerism

Isomers are functional isomers when the two or more compounds have

an identical molecular formula but differ in the functional group

present. These isomers are functional isomers. For example, we can

represent C3H6O as a ketone and as an aldehyde.

4) Metamerism

This is exhibited by compounds due to the presence of different alkyl

chains on either side of the functional group. For example, we can

represent C4H10O as ethoxyethane (C2H5OC2H5) and

methoxypropane (CH3OC3H7).

Read more about Nomenclature of Organic Compounds here in detail.

Stereo Isomerism

Stereoisomerism is a phenomenon in which compounds have the same

molecular formula but differ in the relative positioning or orientation

of atoms in space. Stereoisomers are the compounds exhibiting

stereoisomerism. We can further classify stereoisomerism into:

● Geometric Isomerism: it is shown by molecules in which their

spatial positions are locked to each other due to the presence of

a ring structure or a double bond.

● Optical Isomerism: Two or more compounds that have the

same molecular arrangement but differ in the optical activity

are optical isomers.

Learn more about different types of Organic Reaction here.

Solved Examples for You

Q1: What is a chiral carbon?

Ans: If all the four valencies of carbon are satisfied by four different

atoms or four different groups of atoms, then carbon is known as

chiral carbon.

Q2: What is tautomerism?

Ans: It is the type of isomerism in which two functional isomers exist

together in equilibrium. The two forms existing in equilibrium are

called as tautomers.

Nomenclature of Organic Compounds

How did you get your name? Easy. Maybe your parents or your

relatives decided. But, who gave names to the organic compounds?

Why are they called what they are called? Have you ever wondered?

In this chapter, we will look at the concept of nomenclature of organic

compounds. We will see how these compounds get their names. Let’s

begin.

Nomenclature of Organic Compounds

In earlier days, people knew organic compounds by their common

names. For example, methane was ‘marsh gas’. This is because we

found it in marshy places. With the evolution of so many organic

compounds and continuous addition of new compounds, dealing with

trivial names became a difficulty.

Therefore, scientists introduced a proper method in order to name the

organic compounds. This uniform system for naming the compounds

is the IUPAC system, which is the International Union of Pure and

Applied Chemistry.

Features of the Trivial System

The name of an organic compound, when in a non-systematic manner

or vernacular name is what is known as a trivial system. There are no

particular set of rules for the trivial name of the compound. In this

system, names are usually simple like acetic acid, toluene, and phenol

etc. For example, tartaric acid is a carbolic acid that we usually find in

tamarind. But in IUPAC, it is 2,3-dihydroxy-1,4-Butanedioic acid.

Browse more Topics under Organic Chemistry

● General Introduction to Organic Compounds

● Classification of Organic Compounds

● Isomerism

● Purification of Organic Compounds

● Qualitative Analysis of Organic Compounds

● Quantitative Analysis of Organic Compounds

● Structural Representations of Organic Compounds

● Types of Organic Reactions

● Fundamental Concepts of Organic Reaction Mechanism

Drawbacks of this System

● Many trivial names are present for a single compound. For

example, Phenol has different names like hydroxybenzene,

carbolic acid, and phenol.

● This system is limited to few compounds in each group. For

example, the first two members of the carbolic acid family

have trivial names, formic acid, and acetic acid respectively but

carbolic acid with more atoms does not have any trivial names.

● There are no particular guidelines for naming complex

compounds.

Chemical Nomenclature: IUPAC Rules

According to the IUPAC system, the nomenclature of organic

compounds consists of the following parts:

1) Steps Involved

● Longest Chain Rule: In this step, all we have to do is identify

the parent hydrocarbon and give the name to it. The parent

chain of the compound is usually the longest chain of carbon

atoms. This chain could be straight or of a different shape.

● Lowest number of Locants: We start the numbering of the

carbon atoms in the longest chain from the end that gives the

lowest number to the carbon atoms carrying the substituents.

● Multiple Presence of the same substituent: Prefixes such as di,

tri, etc. are added to the substituents that are present twice,

thrice respectively in the parent chain.

● Naming the various substituents: If more than one substituent is

present, then we need to arrange the substituents in an

alphabetical order of their names.

● Naming different substituents at equivalent positions: If we

find the presence of two different substituents on the same

position from the two ends, what do we do? In such cases, the

substituent first in the alphabetical order gets the lowest

number.

● The Naming of Complex Substituents: We name the complex

substituent when the substituent on the parent chain has a

branched structure (i.e complex structure). We name these

substituents as a substituted alkyl group. It is also important to

note that the carbon atom of this substituent gets the number 1.

We write the name of these type of substituents in brackets.

The final name will be in format : Locant + Prefix + Root + Locant +

Suffix. Now, we will look at some more details of nomenclature of

organic compounds.

2) Word root

It indicates the number of carbon atoms in the longest selected carbon

chain. For example, C1 is ‘Meth’ and C5 is ‘Pent’.

3) Suffix

A suffix is generally a functional group in the molecule which follows

the word root. We can divide it into:

● Primary suffix: We write it immediately after the word root.

For example, in alkanes the suffix is ane.

● Secondary suffix: We write it after the primary suffix. For

instance, if a compound has alkane and alcohol group attached

to it, the naming will be alkanol, -ol being the suffix for

alcohol.

4) Prefix

We add the prefix to the word root while naming the compound. It

indicates the presence of substituent groups or side chains in the

organic molecule. It reveals the cyclic and acyclic nature of the

compound.

● Primary prefix: Indicates whether the molecule is cyclic or not.

For example, for cyclic compounds the prefix used is cyclo.

● Secondary prefix: Indicates the presence of substituent groups

or any side chain. For example –CH3is known as Methyl and

–Br is Bromo.

Types of Chemical Nomenclature

1) Compositional Nomenclature of Organic Compounds

This term denotes the named constructions based on the composition

of species or substances being named, against the systems that involve

structural composition or information. One among them is the

generalised stoichiometric name. Substances or the elements are

named with multiple prefixes in order to give the overall

stoichiometry of an element or a compound.

When there are more components, then we divide them into 2 classes

namely, electropositive and electronegative components. These names

will sound like salt names and this does not imply the chemical nature

and behaviour of those species. Examples: Sodium Chloride – NaCl,

Trioxygen – O3, Phosphorous trichloride – PCl3

2) Substitutive Nomenclature of Organic Compounds

It is based on the approach where parent hydride is changed by

replacement of hydrogen atoms with other atoms or a group of atoms.

It is a system where we name the organic compounds using functional

groups as the suffix or prefix to the name of the parent compound. We

use this system in naming compounds derived from hydrides of

specific group elements in the periodic table.

Similar to that of carbon, these elements may form rings and chains

that will have many derivatives. Rules come in handy in naming the

parent or main compounds and their substituents. Hydrides belonging

to group 13-17 of the periodic table get the suffix – ane. For example

– Borane, Phosphane, and oxidane etc. Examples: 1, 1-difluoro

trisilane (SiH3.SiH2.Si.HF2), Trichlorophosphine(PCl3)

Solved Example for You

Q: Write a note on the additive nomenclature of organic compounds.

Ans: We use this method for the coordination compounds even though

it has wide applications. An example for its application is

pentaamminechlorocobalt (III) chloride – [CoCl(NH3)5]Cl2.

Chloride will have the prefix ‘chloro’ while ligand will have

‘chlorido’.

For example: PCl3 – trichloridophosphorus, [CoCl3 (NH3)3] –

tri-ammine-trichloridocobalt.

Purification of Organic Compounds

Almost everything that we see these days is impure, isn’t it? The water

we drink and the food we eat also need to go through levels of

purification processes. Similar is the case with organic compounds.

There are several methods of purification of organic compounds. Why

are these important and how do we do it? Let us learn all that in this

chapter!

Types of Purification

A large number of methods are available for the purification of

substances. The choice of method, however, depends upon the nature

of substance (whether solid or liquid). It also depends on the type of

impurities present in it. We commonly use these methods for

purification of substances:

● Simple crystallisation

● Fractional crystallisation

● Sublimation

● Simple distillation

● Fractional distillation

● Distillation under reduced pressure

● Steam distillation

● Azeotropic distillation

● Chromatography

Browse more Topics under Organic Chemistry

● General Introduction to Organic Compounds

● Classification of Organic Compounds

● Isomerism

● Nomenclature of Organic Compounds

● Qualitative Analysis of Organic Compounds

● Quantitative Analysis of Organic Compounds

● Structural Representations of Organic Compounds

● Types of Organic Reactions

● Fundamental Concepts of Organic Reaction Mechanism

Let us now study about these methods in brief for better

understanding.

Simple Crystallisation

This is the most common method that we use to purify organic solids.

For crystallisation, a suitable solvent is one

● which dissolves more of the substance at a higher temperature

than at room temperature

● in which impurities are either insoluble or dissolve to an extent

that they remain in solution (in the mother liquor) upon

crystallisation

● which is not highly inflammable and

● which does not react chemically with the compound to be

crystallized. The most commonly-used solvents for

crystallisation are water, alcohol, ether, chloroform, carbon-

tetrachloride, acetone, benzene, petroleum ether etc.

Fractional Crystallisation

It is the process of separation of different components of a mixture by

repeated crystallisations. In the first step, we dissolve the mixture in a

solvent in which the two components have different solubilities. When

we cool a hot saturated solution of this mixture, the less soluble

component crystallises out first while the more soluble substance

remains in solution.

The mother liquor left after crystallisation of the less soluble

component is again concentrated and then we allow it to cool. Hence,

we obtain the crystals of the more soluble component.

Sublimation

Certain organic solids on heating directly change from solid to vapour

state without passing through a liquid state. These substances are

sublimable. This process is sublimation.

We use this process for the separation of sublimable volatile

compounds from non-sublimable impurities. We use this for the

purposes of purification of camphor, naphthalene, anthracene, benzoic

acid, Iodine and salicylic acid etc containing non-volatile impurities.

Simple Distillation

Distillation is the joint process of vapourisation and condensation. We

use this method for the purification of liquids which boil without

decomposition and contain non-volatile impurities. We can also use

this method for separating liquids having sufficient difference in their

boiling points.

Fractional Distillation

We can use this process to separate a mixture of two or more miscible

liquids which have boiling points close to each other. We carry out

this process by using fractionating columns. The fractionating column

is a special type of long glass tube that has obstructions to the passage

of the vapour upwards and that of liquid downwards. This method can

separate a mixture of acetone (b. p. 330 K) and methyl alcohol (b. p.

338 K) or a mixture of benzene and toluene.

Distillation under Reduced Pressure

We use this method for the purification of high boiling liquids and

liquids which decompose at or below their boiling points. Practical

examples include the crude oil industry, sugarcane industry etc.

Steam Distillation

This method is applicable for the separation and purification of those

organic compounds (solids or liquids) which:

● are insoluble in water

● are volatile in steam

● possess a high vapour pressure (10-15 mm Hg) at 373 K and

● contain non-volatile impurities.

Azeotropic Distillation

An azeotropic mixture is a mixture having a constant boiling point.

The most familiar example is a mixture of ethanol and water in the

ratio of 95.87: 4.13 (a ratio present in rectified spirit). It boils at

78.13oC. We can’t separate the constituents of an azeotropic mixture

by fractional distillation. Hence, we have to use a special type of

distillation (azeotropic distillation) for separating the constituents of

an azeotropic mixture.

In this method, we use the third compound in distillation. The process

uses the fact that dehydrating agents like diethyl ether etc. depress the

partial pressure of one of the original components. As a result, the

boiling point of that component raises sufficiently and thus, the other

component will distil over.

Chromatography

This is a modern method that we can use for the separation of

mixtures into its components, purification of compounds and also test

the purity of compounds. The name chromatography comes from the

Greek word ‘chroma’ meaning colour and ‘graphy’ for writing

because the method was first used for the separation of coloured

substances found in plants. This method was described by Tswett in

1906.

Principle of Chromatography

The technique of chromatography uses the difference in the rates at

which the components of a mixture move through a porous medium

(stationary phase) under the influence of some solvent or gas (moving

phase).

Thus, this technique consists of two phases- one is a stationary phase

of the large surface area while the second is a moving phase which is

allowed to move slowly over the stationary phase. The stationary

phase is either a solid or a liquid while the moving phase may be a

liquid or a gas. There are also some other methods of purification like

differential extraction and other chemical methods.

Solved Examples for You

Question: Give two practical applications of simple crystallisation.

Answer: Practical applications of simple crystallisation include:

● Sugar having an impurity of common salt can be crystallized

from hot ethanol since sugar dissolves in hot ethanol but

common salt does not.

● A mixture of benzoic acid and naphthalene can be separated

from hot water in which benzoic acid dissolves but naphthalene

does not.

Qualitative Analysis of Organic Compounds

How do you think scientists can recognise the various organic

compounds? Is it easy? Well, ‘easy’ is a relative term! However, the

qualitative analysis that helps them know what compound is what. Just

like your teachers conduct tests to know which subject you are bad at,

scientists also have a number of tests to know which is which

compound. In this chapter, we will study the Qualitative analysis of

organic compounds. However, before we start, let us go through all

the basic tests first.

Qualitative Analysis of Elements

The most commonly occurring elements in organic compounds are

carbon, hydrogen, oxygen, nitrogen, sulphur and halogen elements.

There is no direct method for the detection of oxygen. For detecting

nitrogen, sulphur and halogens, we can use the sodium fusion test

(Lassaigne’s test).

Sodium Fusion Test

This test is used for the qualitative analysis of elements nitrogen,

sulphur and halogen in Organic compounds. In order to detect them, it

is necessary to convert them into ionisable inorganic substances.

Browse more Topics under Organic Chemistry

● General Introduction to Organic Compounds

● Classification of Organic Compounds

● Isomerism

● Nomenclature of Organic Compounds

● Purification of Organic Compounds

● Quantitative Analysis of Organic Compounds

● Structural Representations of Organic Compounds

● Types of Organic Reactions

● Fundamental Concepts of Organic Reaction Mechanism

1) Test for Nitrogen

We can detect cyanide ion and hence, nitrogen ion in the sample by

the Prussian blue test. The filtered alkaline solution resulting from the

action of water upon the sodium fusion is treated iron (II) sulphate and

thus, forms sodium hexacyanoferrate (II).

Upon boiling the alkaline iron (II) salt solution, some iron (III) ions

are insensibly produced by the action of air. Now, we add dilute

sulphuric acid to dissolve the iron (II) and (III) hydroxides. The

hexacyanoferrate (II) reacts with the iron (III) salt, producing iron (III)

hexacyanoferrate (II), Prussian blue. A Prussian blue precipitate or

colouration indicates that nitrogen is present.

FeSO4 + 6NaCN → Na4[Fe(CN)6] + Na2SO4

3Na4[Fe(CN)6] + 2Fe2(SO4)3 → Fe4[Fe(CN)6]3 +

6Na2SO4

2) Test for Halogens (Nitrogen and Sulphur Absent)

We acidify a portion of the fusion solution with dilute nitric acid. We,

then add an excess of silver nitrate solution. A precipitate indicates the

presence of a halogen. We decant the mother liquor and treat the

precipitate with dilute aqueous ammonia solution. If the precipitate is

white and readily soluble in ammonia solution, chlorine is present. In

case, it is pale yellow and difficulty soluble, bromine is present. If it is

yellow and insoluble, then iodine and bromine may be confirmed by

some more tests.

3) Test for Halogens (Nitrogen and/or Sulphur Present)

Cyanide and sulphide ions both interfere with this test for halide by

forming silver cyanide and silver sulphide precipitates. If nitrogen or

sulphur is present, we must remove the interfering ions. To remove

cyanide and sulphide ions, we have to acidify the fusion solution with

dilute nitric acid. Then, we have to evaporate it to half of the original

volume to expel hydrogen cyanide and/or hydrogen sulphide which

may be present.

Qualitative Analysis of Functional Groups

1) Alcoholic –OH group

We can detect the alcoholic group by the following tests:

● Sodium Metal Test: We conduct this test on the basis of the

appearance of effervescence due to the liberation of hydrogen

gas in reactions of sodium with alcohol.

2R – OH + 2Na → 2RONa + H2

● Acetyl Chloride Test: Acetyl chloride reacts vigorously with

primary and secondary alcohols with the evolution of hydrogen

chloride. The hydrogen chloride gives white fumes of

ammonium chloride with ammonium hydroxide.

● Ceric Ammonium Test: To the sample, we add a few drops of

ceric ammonium nitrate and shake well. The appearance of

pink or red colour indicates the presence of an alcoholic group.

2ROH + (NH4)2Ce(NO3)6 → (ROH)2Ce(NO3)4 +

2NH4NO3

2) Carbonyls (Aldehydes and Ketones)

● 2,4-dinitrophenyl hydrazine test: We add a small amount (2

drops or 0.05 – 0.1g) of the substance to 3 ml of

2,4-dinitrophenyl hydrazine reagent and shake well. A

crystalline precipitate indicates the presence of a carbonyl

compound. Occasionally the precipitate is oily at first but this

becomes crystalline upon standing.

3) Aldehydes

● Schiff’s Test: We dissolve the given compound in alcohol and

then add 1-2ml of Schiff’s reagent. The appearance of pink, red

or magenta colour confirms the presence of aldehyde group.

● Tollen’s Test (Silver Mirror Test): We add 3-4 drops of the

liquid to the Tollen’s reagent. We heat the container. A shining

mirror precipitate confirms the presence of the aldehyde.

2Ag(NH3)2+ + RCHO + 3OH– → RCOO– + 2Ag¯ +

4NH3 + 2H2O

4) Carboxyl Group

We can identify Carboxylic acid by the following tests:

● Sodium Bicarbonate test: We add Sodium bicarbonate

(NaHCO3) to the 1 ml of the sample. A pinch of effervescence

indicates the presence of a carboxylic group.

RCO2H + NaHCO3 → RCOONa + CO2 + H2O

● Ester Test: We warm a small amount of the acid with two parts

of absolute ethanol and one pare of concentrated sulphuric acid.

We cool the solution and pour it continuously into aqueous

Na2CO3 solution. A sweet, fruity smell of an ester confirms the

presence of ester.

5) Amino Group

The most important basic nitrogen compounds are the primary,

secondary and tertiary amines and they dissolve in mineral acids and

change red litmus to blue.

● Chemical classification of the amine function: The

classification of primary, secondary or tertiary amines is done

by means of the reaction with nitrous acid.

● Nitrous Acid Test: We add 2g of the substance to 5 ml of 2 M

HCl acid. Then, we cool it and add 2 ml of ice-cold 10%

aqueous NaNO2 solution slowly by means of a dropper. If we

obtain a clear solution, with a continuous evolution of nitrogen

gas, the substance is a primary amine.

RNH2 + HONO → ROH + H2O + N2

Solved Example for You

Q: Fehling’s test is used for the detection of what?

Ans: Fehling’s reagent is a blue coloured basic solution of

bistartratocuprate(II) complex. Fehling’s test is used to detect the

presence of an aldehyde group. It differentiates between aldehyde and

ketone group present in the same organic compound.

Quantitative Analysis

We might have learnt about the qualitative analysis of various organic

compounds. However, without knowing the exact quantity of each of

the compounds, we won’t be able to use them in the best possible way.

So, how do we do that? Quantitative analysis will help us! This form

of analysis also involves certain tests! Yeah, you can never escape

tests in chemistry! Let us start with the basics of what quantitative

analysis is. 

What is Quantitative Analysis?

Quantitative analysis is an analysis method that we can use to

determine the quantity of the elements or molecules produced during

the reaction. Organic compounds consist of carbon and hydrogen. It

also comprises of elements like oxygen, nitrogen, phosphorus, sulphur

and halogens. We will now explain the various methods for the

measurement of percentage composition of elements in an organic

compound.

Quantitative Analysis of Halogens

Carius Method

Here, we heat a mixture of organic compound and fuming nitric acid

in the presence of silver nitrate contained in a tube (hard glass). This

tube is known as the Carius tube in a furnace. Carbon and hydrogen

present in the organic compound oxidise to carbon dioxide and water

respectively.

The halogens present in the organic compound reacts with the silver

nitrate to form the corresponding silver halide (AgX). After this, we

filter, wash, dry and weigh them.

Calculations

Let us decide the mass of the given organic compound as mg. Suppose

the total mass of the compound, AgX formed = m1 g. We know that 1

mol of AgX has 1 mol of X. So, in m1 g of AgX,

Mass of halogen = (atomic mass of X × m1 g)(molecular mass of

AgX)

Percentage of halogen = (atomic mass of X × m1 × 100)(molecular

mass of AgX × m)

Quantitative Analysis of Sulphur

In this segment, we continue to a process of heating a fixed mass of an

organic compound containing sulphur in a Carius tube. This tube has

sodium peroxide or fuming nitric acid. Sulphur present in the organic

compound oxidises into sulphuric acid. It, then, precipitates as barium

sulphate by the addition of barium chloride solution in water.

We then wash, filter, dry and weigh the precipitate. The mass of

barium sulphate that we use in calculating the percentage of sulphur in

the given organic compound.

Calculations

Let us consider the mass of organic compound = mg. Let the mass of

the compound, barium sulphate formed = m1 g. We know that 32 g

sulphur is present in 1 mol of BaSO4. Therefore, 233 g BaSO4

contains 32 g sulphur

⇒ M1 g of BaSO4 contains 32 × m1/233 g of sulphur

∴ Percentage of sulphur = 32 × m1 × 100/233 × m

Solved Example for You

Q: What is Aluise’s method?

Ans: It is a method that we use to calculate the weight of oxygen in an

organic compound. In this method, we heat the organic compound

containing oxygen with graphite. CO gets formed. We convert this CO

quantitatively into CO2 on reaction with I2O5. Thus, we get the weight

of the oxygen in the compound.

Structural Representations of Organic Compounds

All elements around us display certain properties. This is largely

dependent on their structure and chemical properties. In order to study

and understand the various concepts of such elements in a holistic

manner, it is pertinent to take a view of the structural representation of

such elements and compounds. This will help us in determining the

arrangement of atoms within an element. It will also enhance our

understanding of elements and compounds. Let us understand the

structural representation of different organic compounds.

Understanding the Structural Representation of Organic Compounds

We can represent organic compounds in various ways when it comes

to their structure. There are different types of structures of organic

compounds. Depending on the convenience of making the structure of

an organic compound, one can choose the structure that he wants to

use to depict the compound.

The Lewis structure is considered to be one of the most popular ways

to depict the structure of an organic compound. However, as the size

of the compound increases, it becomes difficult to portray it with the

use of Lewis structure. This is why many other structures have been

introduced, which can be used to portray the structure of an organic

compound. Let us take a look at some of these structures.

Browse more Topics under Organic Chemistry

● General Introduction to Organic Compounds

● Classification of Organic Compounds

● Isomerism

● Nomenclature of Organic Compounds

● Purification of Organic Compounds

● Qualitative Analysis of Organic Compounds

● Quantitative Analysis of Organic Compounds

● Types of Organic Reactions

● Fundamental Concepts of Organic Reaction Mechanism

The Complete Structural Formulae

The Lewis structures can be simplified easily by representing the two

– electron covalent bond by a single dash. Such a type of structural

formula focuses on the electrons involved in the bond formation.

A single dash represents a single bond, a double dash represents a

double bond and a triple dash represents a triple bond respectively.

Lone pairs of electrons on heteroatoms such as oxygen, halogens and

more, may or may not be shown.

Condensed Structural Formulae

We can further abbreviate complete structural formulae by omitting

some or all the dashes that represent covalent bonds. Identical

repetitive units are put in parenthesis and subscripts are used to

indicate their repetition. For example, we can further condense

CH3CH2CH2CH2CH2CH3 to CH3(CH2)4CH3.

The role of parentheses is very important in condensed structural

formulae. We can further condense these structural formulae by

enclosing the repetitive structural unit within a bracket and placing an

integer as a subscript that indicates the number times the structural

unit gets repeated. Further examples of this phenomenon are:

● CH3CH2CH2CH2CH2CH2CH3 changes to CH3 (CH2)5CH3

● CH3CH2CH2CH2CH2CH2CH2COOH changes to

CH3(CH2)6COOH

Bond Line Structural Representation

In a Bond line structural representation, carbon and hydrogen atoms

are not shown and the lines representing carbon-carbon bonds are

drawn in a zig-zag fashion. Only heteroatoms are written in bond line

representation. The terminals in the bond – line structures denote

methyl (CH3) groups unless indicated otherwise by a functional group,

while the line junctions denote carbon atoms bonded to an appropriate

number of hydrogen molecules required to satisfy the valency of the

carbon atoms.

Cyclic compounds are usually represented by bond line formulae as

well. It is a simple, short and convenient method of representing

organic molecule. Each carbon on the line end or intersection is

attached to the required number of hydrogen atoms. Thus, the terminal

denotes CH3 group and an unsubstituted intersection denotes a CH2

group.

Polygon Formulae

There are many organic compounds, in which carbon atoms are not

joined in a chain but they are actually joined in a ring. These cyclic

compounds are usually represented by polygon without showing

carbon and hydrogen atoms.

The corners of a polygon represent a carbon atom and the sides of a

polygon denote a carbon-carbon bond. Similarly, if an atom or a group

of atoms other than hydrogen is attached to carbon, then that atom or a

group of atoms manifest in this structure.

A Solved Question for You

Q: Discuss the concept of wedge and dash representation and

sawhorse projection.

Ans: Wedge and dash projection is a means of representing a molecule

(drawing). In this three types of lines come in use to represent the

three-dimensional structure:

● solid lines to represent bonds which are in the plane of the

paper;

● dashed lines to represent bonds that extend away from the

viewer;

● wedge-shaped lines to represent bonds oriented facing the

viewer.

Sawhorse projection a way of representing an organic compound from

a rather oblique angle to study its conformations. Therefore, it is a

very efficient way of studying conformations of a compound along

with its optical character. This is because it is easily convertible into

Fischer projection and Newman projections. In this representation, we

observe two carbon atoms bonded to each other along with the groups

attached to them from an edge view unlike that in Newman projection

in which we observe it from the front view.

Types of Organic Reactions

Reactions happen all around us. Do you think there are types of

organic reactions? Yes. When someone tries to study organic

chemistry, the study of the types of organic reactions becomes an

inherent part of such study module. Let us take a look at the types of

organic reactions that scientists have been able to decipher until now. 

Types of Organic Reactions

There are five main types of organic reactions that can take place.

They are as follows:

● Substitution reactions

● Elimination reactions

● Addition reactions

● Radical reactions

● Oxidation-Reduction Reactions

Let us study each of these reactions in detail, to understand more

about them.

(Source: wikihow)

1) Substitution Reactions

In a substitution reaction, generally, one atom or a group of atoms

take place of another atom or a group of atoms which leads to the

formation of an altogether new substance. We can take an example of

C – Cl bond, in which the carbon atom usually has a partial positive

charge due to the presence of highly electronegative chlorine atoms.

In a nucleophilic substitution reaction, it is important that the

nucleophile must have a pair of electrons and it also should have a

high affinity for the electropositive species in comparison to the

substituent which was originally present in the element. In order for

the substitution reaction to occur, there are certain conditions that have

to be present such as maintaining low temperatures same as room

temperature.

Also, a strong base such as NaOH has to be in dilute form because

suppose if the base is of higher concentration, there are chances of

dehydrohalogenation taking place in the reaction. And, the solution

needs to be in an aqueous state such as water for the reaction to take

place. The types of Substitution reactions include nucleophilic

substitution reactions and electrophilic substitution reactions.

Browse more Topics under Organic Chemistry

● General Introduction to Organic Compounds

● Classification of Organic Compounds

● Isomerism

● Nomenclature of Organic Compounds

● Purification of Organic Compounds

● Qualitative Analysis of Organic Compounds

● Quantitative Analysis of Organic Compounds

● Structural Representations of Organic Compounds

● Fundamental Concepts of Organic Reaction Mechanism

2) Elimination Reactions

There are certain reactions which involve the elimination and removal

of the adjacent atoms. After these multiple bonds are simultaneously

formed and there is a release of small molecules as products as a

result. One of the examples of a typical elimination reaction is the

conversion of ethyl chloride to ethylene.

CH3CH2Cl → CH2= CH2 + HCl

In the above reaction, the eliminated molecule is HCl, which can form

out of the combination of H+ from the carbon atom which is on the

left side and Cl– from the carbon atom which is on the right side.

3) Addition Reactions

An addition reaction is simply just the opposite of an elimination

reaction. In an addition reaction, the components or molecules of A

and B are added to the carbon-carbon multiple bonds and this is called

an addition reaction. In the reaction given below when HCl is added to

ethylene, it will give us ethylene chloride.

HCl + CH2 = CH2 → CH3CH2Cl

4) Radical Reactions

Most of the organic reactions involve radicals and their movement.

Addition of a halogen to a typically saturated hydrocarbon involves

free radical mechanism. There are usually three stages involved in a

radical reaction which are, initiation, propagation, and termination.

Initially when the weak bond is broken initiation of the reaction takes

place with the formation of free radicals. After that when the halogen

is added to the hydrocarbon a radical is produced and finally, it gives

alkyl halide.

A Solved Question for You

Q: In the context of an organic reaction, explain why such reactions

take place.

Ans: In case of all organic reactions we can study that during a

reaction, the solution will reach an equilibrium that favours the more

stable side. In most cases, these will be reaction products as written

from left to right, however, sometimes we must predict which side is

favoured as in the case of acid-base reactions.

This can be demonstrated through DGº = – RT. In Keq = DH – TDS. If

Keq > 1, in this case, the energy is released to the surroundings which

are an exergonic reaction, the negative value of DGº, reaction

favoured. If Keq, < 1, energy is absorbed from the surroundings which

demonstrate an endergonic reaction, positive value of DGº, reaction

not favoured.

Energy changes in a reaction can be illustrated by Energy Diagrams in

which the highest energy point in a reaction step is called the

transition state and the energy required to go from reactant to

transition state is the activation energy. If a reaction occurs in more

than one step, it must involve species that are neither the reactant nor

the final product. Each step has its own free energy of activation.

Fundamental Concepts of Organic Reaction Mechanism

As we know the branch of chemistry that deals with the study of

hydrocarbons and their derivatives is known as Organic Chemistry.

But are you guys aware regarding the concepts of organic reaction

mechanism? In this chapter, let us now understand the fundamental

concepts of organic reaction mechanism in detail. 

Introduction to Organic Chemistry

The Shapes of Carbon Compounds

Both ‘s’ and ‘p’ orbitals are involved in hybridization, inorganic or

carbon compounds which further leads to three types of hybridization

which are sp3 (in alkanes) – Tetrahedral in shape sp2 (in alkenes) –

Linear molecule – Planar structure sp (in alkynes).

Functional Group

The functional groups are atom or group of atoms which are further

joined in a specific manner and determines the chemical properties of

the organic compound. Few of the examples are the hydroxyl group

(—OH), carboxylic acid group (—COOH) and aldehyde group

(—CHO) etc.

Homologous Series

A homologous series refers to as a family of organic compounds that

have similar chemical properties, the same functional group, and the

successive members differ from each other in the molecular formula

by —CH2 units. By the same general molecular formula, the members

of a homologous series can be represented.

Nomenclature of Organic Compounds

Common name: Organic compounds were named after the sources of

origin, before the IUPAC system of nomenclature. For example, urea

was so named because it was obtained from the urine of mammals.

Formic acid was so named since it was extracted from red ants called

Formica.

The Fundamental Concepts in Organic Reaction Mechanism

Fission of a Covalent Bond

A covalent bond can undergo Fission in two ways:

● Homolytic Fission: Also referred to as Homolysis, Homolytic

fission refers to the process wherein each of the atoms acquires

one of the bonding electrons.

● Heterolytic Fission: Also referred to as Heterolysis, Heterolytic

Fission refers to the process wherein when the bond is broken,

one of the atoms acquires both of the bonding electrons.

In case B is more electronegative than A, further to acquires both the

bonding electrons and becomes negatively charged. The products of

heterolytic fission are ions.

Organic Reaction Mechanism: Inductive Effect

The frequently observed electron displacement effects in the substrate

molecules are as following: it is a permanent effect which comes into

existence when an electron is withdrawing or an electron donating

group is attached to a chain of singly bonded carbon atoms.

The displacement of sigma-electrons along a saturated carbon chain

due to the presence of an electron withdrawing group or electron

repelling group at one end of the chain resulting in the development of

partial positive or partial negative charges in the decreasing order of

magnitude is called an inductive effect or I effect.

Organic Reaction Mechanism: Electromeric Effect

Electromeric effect or E effect refers to the complete transfer of the

shared pair of pie electrons of multiple bonds to one of the shared

atoms in the presence of an attacking reagent.

Resonance Effect (Mesomeric Effect)

Resonance refers to the phenomenon in which a molecule is

represented by several electronic structures which do not differ much

in their energy contents and are obtained by the oscillation of pie

electrons. Such structures are called canonical forms and the molecule

is said to be a resonance hybrid of these canonical forms.

The permanent effect involving the transfer of electron relayed

through pie electrons of multiple bonds in a chain of carbon atoms in a

molecule is called the mesomeric effect. The mesomeric effect is a

permanent effect and comes into existence in the following two cases:

● when electron withdrawing or electron pumping group is in

conjugation with a pie bond.

● when an atom or group having at least one lone pair of electron

is in conjugation with a pie bond.

Solved Examples for You

Question 1. Which of the following represents the correct IUPAC

name for the compounds concerned?

a. 2, 2-Dimethylpentane or 2-Dimethylpentane

b. 2, 4, 7-Trimethyloctane or 2, 5, 7- Trimethyloctane

c. 2-Chloro-4-methylpentane or 4-Chloro-2-methylpentane

d. But-3-yn- l-ol or But-4-ol-yne.

Answer:

a. 2, 2-Demethylpentane

b. 2, 4, 7-Trimethyloctane.

c. Alphabetical order of substituents: 2- Chloro-4-methylpentane

d. But-3-yn-l-ol. Lower locant for the principal functional group,

i.e., alcohol.

Question 2. Draw formulas for the first five members of each

homologous series beginning with the following compounds,

a. H—COOH

b. CH3COCH3

c. H—CH=CH2

Answer:

(a) CH3—COOH

CH3CH2—COOH CH3CH2CH2—COOH

CH3CH2CH2CH2—COOH

(b) CH3COCH3

CH3COCH2CH3

CH3COCH2CH2CH3

CH3COCH2CH2CH2CH3

CH3CO(CH3)4CH3

(c) H—CH=CH2

CH3CH=CH2

CH3CH2CH=CH2

CH3CH2CH2CH=CH2

CH3CH2CH2CH2CH=CH2

Question 3. Which of the two: O2NCH2CH2O– or CH3CH2O– is

more stable and why?

Answer: O2N——<——- CH2——<——- CH2 —<——- O– is more

stable than CH3——<——-CH2——<——-O- because NO2 group

has -I-effect and hence it tends to disperse the -ve charge on the

O-atom. In contrast, CH3CH2 has +I-effect. It, therefore, tends to

intensify the -ve charge and hence destabilizes it.

Question 4. Explain why (CH3)3C+ is more stable than CH3C+H2.

Answer: (CH3)3C+ has nine alpha hydrogens and has nine

hyperconjugation structures while CH3C+H2 has three alpha

hydrogens and has three hyperconjugation structures,

therefore(CH3)3C+ is more stable than CH3C+H2.