Chemistry 11Unit 10 – Organic Chemistry

Part II – Alkanes, cycloalkanes and

derivatives

1. Introduction to hydrocarbons

As we have seen in Chapter 8, carbon can form

maximum four bonds with neighboring atoms. In

addition, carbon demonstrates a very unique

characteristic of catenation; carbon atoms can

connect together in arbitrary length.

When a compound is formed by only carbon and

hydrogen, it is called a hydrocarbon. There are a

huge number of types of hydrocarbons, both

aliphatic and aromatic, and they can be found

mainly in crude oil.

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Some of the hydrocarbons extracted from petroleum. 3

Hydrocarbon compounds can be divided into two

classes:

(1) Aromatic: compounds that have flat and ring

structures with resonance bonds. We will discuss it

later in the chapter.

(2) Aliphatic: compounds that are either open-

chain or cyclic, saturated or unsaturated, but do

not contain the aromatic ring. (In other words, non-

aromatic.)

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Depending on the structural features, aliphatic

hydrocarbons can be divided into several families.5

Before we proceed to studying different classes of

hydrocarbons in detail, we need to learn how to

write the structures of hydrocarbons correctly.

There are 3 ways that we can represent the structure

of a hydrocarbon. Namely:

(1) Full structure

(2) Condensed structure

(3) Carbon skeleton

Of course, we can also describe the compound by

means of its molecular formula, but it is not unique.

Many compounds may have the same formula.

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For example: Propane molecule C3H8

Full structure:

Condensed structure:

Carbon skeleton:

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2. Alkanes

(a) Structures

Alkanes form the simplest class of organic

compounds in which all members contain only

single-bonded carbon and hydrogen atoms.

Each carbon atom in an alkane

molecule is bonded to four

neighboring atoms (either

carbon or hydrogen), and is in

tetrahedral geometry. (Recall

the Lewis structures and VSEPR)

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Example: Methane (CH4)

All alkane molecules fulfill the general molecular

formula 𝐂𝒏𝐇𝟐𝒏+𝟐, where 𝑛 is the number of carbon

atoms. Molecules that possess the same general

molecular formula form a homologous series.

Examples:

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C2H6 C4H10 C6H14

C3H8 C5H12 C7H16

In addition to the linear structure, alkanes can also

exist in branched configurations. The structures

having the same molecular formula but different

arrangements are called structural isomers or

constitutional isomers.

For instance, C5H12 has 3 possible structural isomers;

namely

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The number of possible isomers increases dramatically

with the number of carbon atoms. 11

0

1E+09

2E+09

3E+09

4E+09

5E+09

0 10 20 30

Nu

mb

er

of

iso

me

rs

Number of carbon atoms

Number of structural isomers of alkanes

Interestingly, if you re-plot the data in logarithmic

scale, the following graph will be obtained:12

1

10

100

1000

10000

100000

1000000

0 5 10 15 20

log

(Nu

mb

er

of

iso

me

rs)

Number of carbon atoms

Number of structural isomers of alkanes

(b) Nomenclature

(i) For straight-chained alkanes, the naming consists of

two steps:

Step 1: Count the number of carbon atoms and

determine the appropriate prefix:

Step 2: Add the ending –ane to the prefix.

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n prefix n prefix

1 Meth- 6 Hex-

2 Eth- 7 Hept-

3 Prop- 8 Oct-

4 But- 9 Non-

5 Pent- 10 Dec-

Example: Name the following compound.

Example: Draw the structure of a straight chain

nonane.

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(ii) For branched alkanes, the procedures of naming

are little bit more complicated.

Let’s consider the following alkane:

Step 1: Identify the longest straight carbon chain in

the molecule. It is called the parent chain.

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Step 2: Identify all the side chains

attached to the parent chain. Each

side chain, called substituent, is

named by combining the prefix,

which indicates the number of

carbon atoms involved, with the

ending –yl.

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The longest

consecutive

chain contains

9 carbon

atoms

Step 3: Number the parent chain starting either from

the left or from the right and label the locations of

substituents. Choose the scheme that gives the

lowest set of numbers.

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Substituents on carbons

3, 4, 5, 5, 6, 6

It is preferred!

Substituents on carbons

4, 4, 5, 5, 6, 7

It is not preferred!

Step 4: Following the alphabetically order, name the

substituent by the carbon to which it is attached,

followed by a dash, and then the name of the alkyl

group.

e.g. 3-methyl in

Step 5: When there is more than one such a

substituent on the parent chain, name the substituent

by the carbons, in ascending order, followed by a

dash, and then the name of the alkyl group with the

numeric prefix.

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Note that the numeric prefix is different from the

prefix used to designate the carbon chain.

Example: 2,3-dimethyl in

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Number Prefix Number Prefix

1 Mono- 3 Tri-

2 Di- 4 Tetra-

On carbon

#2On carbon

#3

The compound should therefore be named as

follows:

4,5,5-triethyl-3,6,6-trimethylnonane

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Practice: Name the following alkanes.21

(c) Properties

Since C and H have similar values of electronegativity,

C-H and C-C bonds are essentially non-polar, making

alkane molecules non-polar overall.

Due to the lacking of bond dipoles, alkane molecules

are held together by only weak London force. This

results in relatively low melting and boiling points

compared to other organic compounds.

Alkanes are soluble to non-polar or slightly polar

solvents such as benzene, chloroform and ether.

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The following diagram shows the melting and boiling

points of straight chain alkanes.

The higher the number of carbons, the higher the

molecular mass, and the higher the m.p. and b.p.

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Branched alkanes have lower melting and boiling

points than their linear counterparts.

The key: symmetry and packing of molecules

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(d) Cycloalkanes

When C atoms are connected in a ring form, the

resulting alkane is called cycloalkane.

Cycloalkanes are named in a similar way as the linear

alkanes except that the prefix cyclo- is added to the

name of the parent chain.

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Cyclopropane

Cyclobutane

Cyclopentane

Cyclohexane

Because of the ring connectivity, cycloalkanes have

two hydrogen less than the linear counterparts and

share the general formula 𝐂𝐧𝐇𝟐𝐧.

Despite the ring structure, cycloalkanes are not

planar at all, even for the smallest member

cyclopropane. It is in contrast to the aromatic

compounds such as benzene which is fully planar.

(We will discuss it later in this Unit).

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Substituted cycloalkanes are named in the same

way as branched alkanes except for the followings:

(1) No number is needed to indicate the position

of the side chain if there is only one substituent.

(2) If there are two or more substituents, arrange

them in alphabetical order, and the carbon on

which the first substituent is attached is labeled

“1”. The remaining carbons are numbered either

clockwise or counter-clockwise in such a way a

smallest set of numbers is obtained.

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Cycloalkanes can exist as substituents, and to

denote them as side chains, we add the suffix –yl to

the name to the parent cycloalkanes.

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Methylcyclohexane 1-ethyl-3-methylcyclohexane

Substituent Name Substituent Name

C3H5- Cyclopropyl C5H9- Cyclopentyl

C4H7- Cyclobutyl C6H11- Cyclohexyl

(e) Haloalkanes

The hydrogen atoms on an alkane molecule can be

replaced by halogen atoms (i.e., F, Cl, Br or I) to yield

the derivative called haloalkanes or alkyl halides.

To name alkyl halides, we simply consider halogen

groups as substituents and label them with

appropriate positions and prefixes.

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Group Name Group Name

F- Fluoro- Br- Bromo-

Cl- Chloro- I- Iodo-

Practice: Name the following compounds.30

2-fluorobutane 2,4-dichloropentane

2-bromo-3-ethyl-3-fluoropentane

Some properties of alkyl halides

(1) C-X bonds are mostly polar, yet they tend to be

insoluble in water.

(2) Alkyl halides have a various level of reactivity

depending on the type of halogen. Alkyl halides

with fluorine are unreactive (e.g. Teflon) while those

with iodine are quite reactive (e.g. CH3I as

pesticide).

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