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CHAPTER 11 :
INTRODUCTION TO
ORGANIC CHEMISTRY
CHAPTER 11 : INTRODUCTION TO ORGANIC CHEMISTRY
11.1 Introduction
11.2 Empirical molecular and structural formulas
11.3 Functional groups and homologous series
11.4 Classification of carbon atoms in organic molecules
11.5 Isomerism
11.6 Reactions in organic compound
Organic and Inorganic Compound
Organic compound Inorganic compound
were defined as
compounds that
could be obtained
from living
organisms
were those that
came from
nonliving sources
Some examples of carbon compounds in our daily lives :-
CH4
methane (a component of natural gas)
OCOCH3
COOH
CH3 CHCOOH
NH2
Methyl salicylic acid (aspirin-a drug)
alanine (amino acid-a protein component)
NCH3
CO2CH3
OCO
cocaine (a pain killer)
11.2 EMPIRICAL, MOLECULAR AND
STRUCTURAL FORMULAE
Empirical formula is the simplest formula that
shows the relative numbers of the different kinds
of atoms in a molecule.
Molecular formula is a formula that states the
actual number of each kind of atom found in the
molecule. Example : C2H4 , C2H4O2
Quantitative example :-
A sample of hydrocarbon contains 85.7 % carbon and
14.3 % hydrogen by mass. Its molar mass is 56.
Determine the empirical formula and molecular formula
of the compound.
Solution :-
Element C H
Mass (g) 85.7 14.3
Moles (n) 85.7 12.0
14.3 1.0
Smallest ratioSmallest ratio 7.1427.142= 1
14.437.142= 2
= 7.142 = 14.3
Empirical formula = CH2
n (empirical molar mass)= molar mass
n ( 12 x 1 + 1 x 2 ) = 56 n = 4Molecular formula = C4H8
Practice Exercise:
1) A complete combustion of 10.0 g of compound X,
CxHyOz forms 12.0 g H2O and 22.0 g CO2. Its
molar mass is 60.
a) Determine the percentage composition of
C, H and O.
b) Determine the molecular formula of
compound X.
2) A complete combustion of 0.6 g of hydrocarbon,
CxH12 forms 1.98 g CO2. Determine the percentage
of C and H in the hydrocarbon and write the
molecular formula.
Structural formula shows the order in which
atoms are bonded together
Representation of structural formula :-
a) Condensed Structure
b) Expanded Structure
c) Skeletal Structure
d) 3-Dimensional formula
e) Ficher Projection
a) Condensed Structure
In condensed formulae all the hydrogen atoms
that are attached to a particular carbon are
usually written immediately after that carbon
Example :
C4H9Cl CH3CHCH2CH3 or CH3CH(Cl)CH2CH3
Condensed structure
Cl
b) Expanded Structure
Expanded structures indicate the way in which
the atoms are attached to each other and are not
representations of the actual shapes of the
molecules.
Example :
C4H9ClC C C C
H H
H H HCl
H H H H
Expanded structure
c) Skeletal Structure
This structure shows only the carbon skeleton
The hydrogen atoms that are assumed to be present, are not written.
Other atoms such as O, Cl, N and etc. are shown
Example :
CH3CH(Cl)CH2CH3
Cl
=1.
H2C CH2
H2C CH2
2.
=
CH2=CHCH2OH 3. =OH
Practice Exercise :
Rewrite each of the following structures using skeletal formula :-
O
CH3CH2CH2C CH3
(CH3)2CHCH2CH2CH(CH3)CH2CH3
CH2= CHCH2CH2CH = CHCH3
O
CH3 CH2 CH ( CH3 ) CH2 C OH
1.
2.
3.
4.
d) 3 - Dimensional formula (wedge - dashed wedge - wedge)
Describes how the atoms of a molecule are arranged in space
Example :
C
Br
H
H
H(Bromoethane)
C
Br
HH
HC
H
BrH
HC
H
HBr
H
Indication :-
bonds that lie in the plane of the page bonds that lie behind the plane bonds that project out of the plane of the paper
OR OR
e) Fischer Projection
Vertical lines represent bonds that project behind the plane of paper
Horizontal lines represent bonds that project out of the plane of paper
The intersection of vertical and horizontal lines represent a carbon atom, that is stereocentre
Example :
2 – butanol , CH3CH(OH)CH2CH3
CH3
HO
CH2CH3
H
CH3
H
CH2CH3
OHOR
11.3 FUNCTIONAL GROUPS AND
HOMOLOGOUS SERIES
A functional group is an atom or group of
atoms in an organic molecule which characterized
the molecule and enables the molecule to react
in specific ways (determines its chemical properties)
Some important functional groups in organic compounds :-
Homologous Homologous SeriesSeries
Functional Functional GroupGroup
General General FormulaFormula
IUPAC IUPAC nomenclaturenomenclature
Prefix- -suffixPrefix- -suffix
ExampleExample
alkanealkane nonenone CCnnHH2n+22n+2 -ane-ane CHCH44
methanemethane
alkenealkene C = C C = C (double (double
bond)bond)
CCnnHH2n2n -ene-ene CHCH22=CH=CH22
etheneethene
alkynesalkynes C C C C (triple (triple
bond)bond)
CCnnHH2n-22n-2 -yne-yne CH CH CH CH
ethyneethyne
Homologous Homologous SeriesSeries
Functional Functional GroupGroup
General General FormulaFormula
IUPAC IUPAC nomenclaturenomenclature
Prefix- -suffixPrefix- -suffix
ExampleExample
arenearene CCnnHH2n-62n-6 -benzene-benzene
alcoholalcohol ––OH OH (hydroxyl)(hydroxyl)
CCnnHH2n+12n+1OHOH alkanolalkanol CHCH33CHCH22OH OH
ethanol ethanol
etherether ––OR OR
(alkoxy)(alkoxy) CCnnHH2n+22n+2OO alkoxyalkanealkoxyalkane CHCH33OCHOCH33
methoxymethanemethoxymethane
haloalkanehaloalkane ––X X (halogen)(halogen)
CCnnHH2n+12n+1XX haloalkanehaloalkane CHCH33CHCH22ClCl
chloroethanechloroethane
aromatic ring
CH3
methylbenzene
Homologous Homologous SeriesSeries
Functional Functional GroupGroup
General General FormulaFormula
IUPAC IUPAC nomenclaturenomenclature
Prefix- -suffixPrefix- -suffix
ExampleExample
aldehydealdehyde CCnnHH2n2nOO alkanalalkanal CHCH33C=OC=O
ketoneketone CCnnHH2n2nOO alkanonealkanone CHCH33C=O C=O
carboxylic carboxylic acidacid
CnHCnH2n2nOO22 alkanoic acidalkanoic acid CHCH33C=OC=O
C
O
H
carbonylH
ethanal
C
O
carbonylCH3
propanone
C OH
O
carboxylOH
ethanoic acid
Homologous Homologous SeriesSeries
Functional Functional GroupGroup
General General FormulaFormula
IUPAC IUPAC nomenclaturenomenclature
Prefix- -suffixPrefix- -suffix
ExampleExample
acyl acyl
chloridechloride CCnnHH2n+12n+1
COClCOCl
alkanoyl alkanoyl
chloridechloride CHCH33C=OC=O
esterester CCnnHH2n2nOO22 alkyl alkyl
alkanoatealkanoate CHCH33COOCHCOOCH33
amideamide CCnnHH2n+12n+1
CONHCONH22 -amide-amide CHCH33CONHCONH22
amineamine -NH-NH22 CCnnHH2n+12n+1
NHNH22 -amine-amine CHCH33NHNH22
C
O
Cl
acylCl
ethanoyl chloride
C
O
O C
ester
C
O
NH2
ethyl ethanoate
amideethanamide
amino methanamine
11.4 CLASSIFICATION OF CARBON AND HYDROGEN ATOMS IN ORGANIC
MOLECULES
Carbon atom classified primary (1o)
secondary (2o)
tertiary (3o)
quarternary (4o)
depending on the number of carbon atoms bonded to it
A primary carbon – directly bonded to one other
carbon atom
(has 1 adjacent carbon atom)
C
H
H
CH3
H
Example :
1o carbon
1o H
A secondary carbon – directly bonded to two other
carbon atoms
(has 2 adjacent carbon atoms)
C
H
CH3
H CH3
Example :
2o carbon
2o H
A tertiary carbon – directly bonded to three other
carbon atoms
(has 3 adjacent carbon atoms)
C
CH3
CH3
H CH3
Example :
3o carbon
3o H
A quarternary carbon – directly bonded to four other
carbon atoms
(has 4 adjacent carbon atoms)
CCH3
CH3
CH3
CH3
Example :
4o carbon
Similarly, a hydrogen atom is also classified as
primary, secondary or tertiary depending on the
type of carbon to which it is bonded.
1° hydrogen atom bonded to a 1° C atom
2° hydrogen atom bonded to a 2° C atom
3° hydrogen atom bonded to a 3° C atom
Classification of haloalkanes (alkyl halides)
Alkyl halides are classified based on the carbon atom
to which the halogen is directly attached.
1° alkyl halide – the halogen atom is bonded to a primary carbon atom
2° alkyl halide – the halogen atom is bonded to a secondary carbon atom
3° alkyl halide – the halogen atom is bonded to a tertiary carbon atom
H C
H
H
C
H
H
Cl
H C
H
H
C
H
C
H
H
HCl
H C
H
H
C C
H
H
HCl
CH3
1° alkyl chloride
1° C
2° alkyl chloride
2° C
3° alkyl chloride
3° C
Classification of alcohols
Alcohols are classified based on the carbon atom
to which the hydroxyl group is directly attached.
1° alcohol – the hydroxyl group is attached to a 1° carbon atom
2° alcohol – the hydroxyl group is attached to a 2° carbon atom
3° alcohol – the hydroxyl group is attached to a 3° carbon atom
H C
H
H
C
H
H
OH
H C
H
H
C
H
C
H
H
HOH
H C
H
H
C C
H
H
HOH
CH3
1° alcohol
1° C
2° C
2° alcohol
3° alcohol
3° C
Classification of amines
Amines are classified based on the number of alkyl
groups or carbon atoms that are directly attached
to the nitrogen atom
1° amine – N is bonded to one alkyl group
2° amine – N is bonded to two alkyl groups
3° amine – N is bonded to three alkyl groups
H3C N
H
H
H3C N H
CH3
H3C N
CH3
CH3
N bonded to one alkyl group
A primary (1°) amine
N bonded to two alkyl group
A secondary (2°) amine
N bonded to three alkyl group
A tertiary (3°) amine
ISOMERISM
Structural/Constitutional Isomerism Stereoisomerism
Isomerism
Chain isomerism
Positional isomerism
Functional group
isomerism
Geometric isomerism
Optical isomerism
Isomerism is the existence of different compounds
with the same molecular formula but different
structural formulae
Isomers – different compounds that have same
molecular formula
Two types of isomerism
structural isomerism
stereoisomerism
different order of attachment of atoms
different spatial arrangement of atoms in molecules
Structural isomerism
Chain/skeletal isomerism
Structural isomers are different compounds with
the same molecular formula but differ in the order
of attachment of atoms
Positional isomerism
Functional group
isomerism
The isomers differ in the carbon skeleton (different carbon chain)
a) Chain/skeletal isomerism
They possess the same functional group and belong to the same homologous series
Example :
C5H12 :
CH3CH2CH2CH2CH3
CH3CHCH2CH3
CH3
CH3-C-CH3
CH3
CH3
b) Positional isomerism
These isomers have a substituent group in different positions in the same carbon skeleton
Example :
C3H7Cl i) CH3CH2CH2Cl
1-chloropropane
CH3CHCH3
Cl
2-chloropropane
C4H8 ii) CH2=CHCH2CH3 CH3CH=CHCH3
1-butene 2-butene
1,2-dimethylbenzene
iii) C8H10 CH3
CH3
CH3
CH3
1,3-dimethylbenzene
CH3
CH3
1,4-dimethylbenzene
c) Functional group isomerism
These isomers have different functional groups and belong to different homologous series with the same general formula
Different classes of compounds that exhibit functional group isomerism :-
General formulaGeneral formula Classes of compoundsClasses of compounds
CnH2n+2O alcohol and ether
CnH2nO aldehyde and ketone
CnH2n alkene and cycloalkane
CnH2nO2 carboxylic acid and ester
Example :
i) C2H6O CH3CH2OH
ethanol
CH3OCH3
dimethyl ether
ii) C3H6O CH3CH2C-H
Opropanal
CH3C-CH3
Opropanone
iii) C3H6O2 CH3CH2C-OH
O
propanoic acid
CH3C-O-CH3
O
methyl ethanoate
Stereoisomerism
Geometric Isomerism Optical Isomerism
a) Geometric isomerism
occurs only in two classes of compounds :
Alkenes & cyclic compound
(because of rigidity in molecules)
Geometric isomers (also called cis-trans isomers)
are stereoisomers that differ by groups being
on the same side (cis-isomer) or opposite
sides (trans-isomer) of a site of rigidity in a molecule
The requirements for geometric isomerism :
i) restricted rotation about a C=C,double bond, in alkenes or a C-C single bond in cyclic compounds
ii) each carbon atom of a site of restricted rotation has two different groups attached to it
Examples :
H3C CH3
H H
C C=
i)
cis-2-butene
H3C
C= C
CH3
H
H
trans-2-butene
ii) H3C CH2CH3
C= C
H CH3
trans-3-methyl-2-pentene
H3C CH3
C = CH CH2CH3
cis-3-methyl-2-pentene
iii)HH
CH3 CH3
cis-1,2-dimethylcyclohexane
H
HCH3
CH3
trans-1,2-dimethylcyclohexane
Cl
Cl
H
H
iv)
Cl
Cl
HH
cis-1,3-dichlorocyclopentane trans-1,3-dichlorocyclopentane
If one of the doubly bonded carbons has 2 identical
groups, geometric isomerism is not possible.
Examples :
CH3CH2i)
C = C
H
HH3C
2-methyl-2-butene
Hii)
C= C
CH3
CH3Cl
1-chloro-2-methylpropene
cis-trans isomers have similar chemical properties
but different physical properties
They differ in melting and boiling points and
solubility due to different polarity of the molecules
cis-isomers polar molecules
trans-isomers non-polar
• Melting point: trans- isomer > cis-isomer• Boiling point: cis-isomer > trans- isomer
• Stability: trans-isomer > cis-isomer
b) Optical isomerism
If a beam of light is passed through a piece of
polarizer prism, the emergent light vibrates in a
single plane, hence it is called a plane-polarized
light
Optically active compounds have the ability to
rotate plane-polarized light
The angle of rotation can be measured with an
instrument called polarimeter
Schematic representation of a polarimeter containing an optically active sample :
clockwise rotation – plus sign (+) / dextrorotarory
anticlockwise rotation – minus sign (-) / levorotorary
The requirements for optical isomerism :-
i) molecule contains a chiral carbon or chiral centre
(carbon atom with 4 different groups attached to it)
ii) molecule is not superimposable with its mirror image
A representation of a chiral molecule with
3-dimensional formula :-
P
CQ
RS
* PQRS *designates chiral centre
Enantiomers are a pair of mirror-image molecules
that are not superimposable (must have one or more
chiral carbons)
Examples :
i) 2-butanol, CH3CHCH2CH3
OH
C*
CH2CH3
H3C
OHH
C
CH2CH3
CH3HOH
enantiomers
:-
ii) 2-hydroxypropanoic acid, CH3CHCOOH
OH
:-
COOH COOH
OH HO H H
CH3 CH3
enantiomers
A racemic mixture or racemate is an equimolar
mixture of enantiomers which is optically inactive
because the two components rotate plane-polarized
light equally (same degree of rotation but in opposite
direction– so they can cancel each other’s rotation)
A pair of enantiomers have identical chemical and
physical properties but differ in the direction of
rotation of plane-polarized light
A compound with n chiral centers can have a
maximum of 2n stereoisomers
If a molecule contains two or more chiral centers,
diastereomers may exist
Diastereomers are stereoisomers that are not
mirror images of each other
All physical properties of diastereomers are usually
different from one another
Example :
The 4 stereoisomers of 2-amino-3-hydroxybutanoic acid CH3CH-CHCOOH are shown below using
OH NH2
Fischer projection formula:-
COOH COOH
CH3 CH3
H
H
H
H
NH2 H2N
OH HO
enantiomers
COOH
NH2
H H
HO
CH3
COOH
H H OH
H2N
enantiomersCH3
A B
C D
Four pairs of diastereomers are identified :
A and C ; A and D ; B and C ;
B and D
Meso compound is a stereoisomer that has more
than one chiral centres and that is superimposable
on its mirror image because of the presence of an
internal plane of symmetry, hence it is optically
inactive (does not cause a rotation of plane-polarized
light)
Example : Tartaric acid , HOOCCH(OH)CH(OH)COOH
COOH
OH
OH
H
H
COOH
COOH
COOH
HO
HO
H
H
COOH
COOH
H
H
OH
OH
plane of symmetry
plane of symmetry
rotate 180o
P Q
identical
At first glance, P and Q are assumed to be enantiomers
But if compound Q is rotated 180o in the plane of the paper, it is actually identical to compound P, therefore P and Q are superimposable mirror images
P and Q are the same compound
It is a meso compound
COOH
OH
OH
H
H
COOH
COOH
COOH
R S
COOH
COOH
H HO
H OH *not a plane of symmetry
*not a plane of symmetry
rotate 180o
different
H OH
H HO
R and S are related as mirror images and are not superimposable even if rotated 180o
Thus R and S constitute an enantiomeric pair
There are 2 pairs of diastereomers :
P and R & P and S
Further examples of meso compounds:
CH3
CH3
Cl
Cl
H
H
CHO
CHO
HO
HO
H
HH OH plane of symmetry
11.6 REACTIONS OF ORGANIC COMPOUNDS
11.6.1 Types of Covalent Bond Cleavage/Fission
All chemical reactions involved bond breaking and bond making
Two types of covalent bond cleavage :-
Homolytic cleavage Heterolytic cleavage
a) Homolytic Cleavage
Occurs in a non-polar bond involving two atoms of
similar electronegativity
A single bond breaks symmetrically into two equal
parts, leaving each atom with one unpaired electron
Free radicals are formed in homolytic cleavage
X X X + X ≡ 2X•• • • •
free radicals
b) Heterolytic Cleavage
Occurs in a polar bond involving unequal sharing of electron pair between two atoms of different electronegativities
A single bond breaks unsymmetrically and both the bonding electrons are transferred to the more electronegative atom
Cation and anion are formed in heterolytic cleavage
A B••
A ••- + B+ A is more
electronegative
A+ + B••- B is more
electronegative
cationanion
anioncation
Carbocations and free radicals are intermediates in organic reactions.
They are unstable and highly reactive
11.6.2 Reaction Intermediates
a) Carbocation
Also called carbonium ion
A very reactive species with a positive charge on a carbon atom
Carbocation is formed in heterolytic cleavage
Example :
(CH3)3C — Cl
(CH3)3C+
carbocation
+ Cl-
anion
Chlorine is more electronegative than carbon and the C—Cl bond is polar
The C—Cl bond breaks heterolitically and both the bonding electrons are transferred to chlorine atom to form anion and carbocation
b) Free Radical
A very reactive species with an unpaired electron
Formed in homolytic cleavage
Cl – Cl
Example :
uvCl •
free radicals
+ Cl •
C C C • + C•
H3C H CH3 • + H •
11.6.3 Relative Stabilities of Carbocations and Free Radicals
Carbocation and free radical primary
secondary
tertiary
depending on the number of carbon atoms directly bonded to the :-
• positively charged carbon atom (for carbocation)
• carbon atom with unpaired electron (for free radical)
The stability of carbocation increases with the number of alkyl groups present
The alkyl groups are electron-releasing relative to hydrogen, thus help to stabilize the positive charge on the carbocation
Carbocation Stability:
H C H < R C H < R C H < R C R
H H R R
+ + + +methyl cation
primary(1°)
secondary(2°)
tertiary(3°)
Increasing stability
As the number of alkyl groups attached to the positively charged carbon atom increases, the stability of carbocation increases
Likewise, the stability of free radical increases as more alkyl groups are attached to the carbon atom with unpaired electron
Free Radical Stability :
H C H < R C H < R C H < R C R
H H R R
methyl radical
primary(1°)
secondary(2°)
tertiary(3°)
Increasing stability
11.6.4 Reagents and Sites of Organic Reactions
a) Electrophile (E+)
Means ‘electron loving’
An electron-deficient species and electron-pair acceptor that attacks a part of a molecule where the electron density is high
An electrophile can be either neutral or positively charged
Examples of electrophiles :-
1. cations such as H+, H3O+, NO2+, Br+ etc.2. carbocations.3. Lewis acids such as AlCl3, BF3 etc.4. oxidizing agents such as Cl2, Br2 and etc
Examples of electrophilic sites in organic molecules :-
• molecules with low electron density around a polar bond such as :-
+ - + - + -C = O C – X C – OH
carbonyl haloalkanes hydroxyl compound
b) Nucleophile (Nu-)
Means ‘nucleus loving’
An electron-rich species and electron-pair donor that attacks a part of a molecule where the electron density is low
A nucleophile can be either neutral or negatively charged
Examples of nucleophiles :-
1. anions such as OH-, RO-, Cl-, Cn- etc.2. carbanions. (species with –ve charge on C atoms)3. Lewis bases which can donate lone pair electrons such as NH3, H2O etc.
Examples of nucleophilic sites in organic molecules :-
molecules with high electron density around the carbon-carbon multiple bond such as :-
-C=C- (alkenes) , -CC-(alkynes),
(benzene ring) and etc.
11.6.5 Types of Organic Reactions
The four main types of organic reactions are:
• Addition
• Substitution
• Elimination
• Rearrangement
1.Addition reaction
a) Electrophilic addition b) Nucleophilic addition
A reaction in which atoms or groups added to a multiple bond
a) Electrophilic Addition
Initiated by an electrophile, which attacks a nucleophilic site of a molecule
Typical reaction of unsaturated compounds such as alkenes and alkynes
Example :
CH3CH=CH2 + Br2 CH3CHBrCH2Brroomtemperature
electrophile
b) Nucleophilic Addition
Initiated by a nucleophile, which attacks an electrophilic site of a molecule
Typical reaction of carbonyl compounds
Example :
CH3 C CH 3 + HCN
O
CH3 C CH3
OH
CN
+
H+ CN-
2. Substitution Reaction
A reaction in which an atom or group in a molecule is replaced by
another atom or group
a) Free-radical Substitution
b) Electrophilic Substitution
c) Nucleophilic Substitution
a) Free-radical Substitution
Substitution which involves free radicals as intermediate species
Example :
CH3CH3 + Cl2 CH3CH2Cl + HCl uv light
b) Electrophilic Substitution
Typical reaction of aromatic compounds
The aromatic nucleus has high electron density, thus it is nucleophilic and is tend to electrophilic attack
Example :
+ Br2
Fe
catalystBr + HBr
electrophile
Br Br
c) Nucleophilic Substitution
Typical reaction of saturated organic compounds bearing polar bond as functional group, such as haloalkane and alchohol
Example :
CH3CH2Br + OH-(aq) CH3CH2OH + Br-(aq)
nucleophile
3. Elimination Reaction
A reaction in which atoms or groups are removed from adjacent carbon atoms of a molecule to form a multiple bond (double or triple bond)
Elimination reaction results in the formation of unsaturated molecules
CH3CH2OH CH2= CH2 + H2O Conc. H2SO4
Example :
4. Rearrangement Reaction
A reaction in which atoms or groups in a molecule change position
Occurs when a single reactant reorganizes the bonds and atoms
Example :
H C C
H OH
R
tautomerisation
H C C R
H
H O
enol keto (more stable)
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