Branched Alkanes

Alkanes

Lesson Outcomes:

• At the end of this topic, students should be

able to:-
– Identify alkane and haloalkane/ alkyl halide.
– Explain structure and physical properties of

alkane

– Name and draw alkane by using IUPAC

nomenclature

– Write the equation for the preparation of

alkane.

– Write the equation for the reactions of alkane.
– Describe the sources and uses of alkane

Hydrocarbons

Aromatic

Aliphatic

Hydrocarbons
• simplest organic compounds
• contain only carbon and hydrogen

Introduction to Hydrocarbons

Hydrocarbons

Aromatic

Aliphatic

Alkanes

Alkynes

Alkenes

Class of Hydrocarbons

Hydrocarbons

Aliphatic

Alkanes

•

Alkanes are

hydrocarbons in
which all of the
bonds are single
bonds.

C

C

H

H

H

H

H

H

Hydrocarbons

Aliphatic

Alkenes

•

Alkenes are

hydrocarbons that
contain a carbon-
carbon double
bond.

C

C

H

H

H

H

Hydrocarbons

Aliphatic

Alkynes

•

Alkynes are

hydrocarbons that
contain a carbon-
carbon triple bond.

HC

CH

Hydrocarbons

Aromatic

•

The most

common aromatic
hydrocarbons are
those that contain a
benzene ring.

H

H

H

H

H

H

Alkanes

• Alkanes are saturated hydrocarbons
• Contain only carbon and hydrogen in their

molecules.

• General formula of CnH2n+1
• All carbons in alkanes are sp3 hybridized

and form only single bond.

Naming of Branched Alkanes –

IUPAC Nomencalture

RULE 1
Identify the longest carbon chain (not necessary

straight) and write parent name of the compound.

CH3-CH-CH2-CH3

CH3

Butane

CH3-CH-CH2-CH2-CH3

CH2-CH3

Hexane

RULE 2
Circle the substituent group atom, number the

longest carbon chain making sure the substituent
is at lower number.

CH3-CH-CH2-CH2-CH3

CH2-CH3

CH3-CH-CH2-CH3

CH3

1

2

3

4

1

2

3

4

5

6

methyl

methyl

IUPAC name:
2-methylbutane

IUPAC name:
3-methylhexane

RULE 3
Use prefix in writing the

name of a compound
if there is more than
one substituent.

No. of
substituent

Prefix
used

2

di

3

tri

4

tetra

5

penta

6

hexa

C

C

H3C

CH3

H

H

CH3 CH3

1

2

3

4

2,3-dimethylbutane

Notes: numbers are
separated by comma

RULE 4
Arrange substituents according to alphabetical order
Example: bromo> cyclopropyl>ethyl > isopropyl >

methyl.

However, prefixes such as di-, tri-, tetra-, sec-, tert-,

are not considered except the prefix “iso”

RULE 5
When a compound has two possible longest carbon

chains, select the chain/parent with more
substituents.

CH3-CH-CH3

CH3-CH2-CH-CH2-CH2-CH2-CH3

CH3-CH-CH3

CH3-CH2-CH-CH2-CH2-CH2-CH3

1

1

2

2

3

4

4

3

5

5

6

6

7

7

3-ethyl-2-methylheptane

Cycloalkanes

• Cyclic alkanes are called cycloalkanes
• General formula CnH2n
• Name as the parent alkane but add cyclo-

cyclopropane
cyclobutane
cyclopentane

ethylcyclopentane

Naming of Cycloalkanes

CH2CH3

 Name any alkyl groups on the ring in the usual
way.

 List substituents in alphabetical order and count
in the direction that gives the lowest number.

3-ethyl-1,1-dimethylcyclohexane

CH2CH3

H3C

CH3

Questions

1. Write structural formulae of C5H12. Label

class of C and H atoms. Ie 1º, 2º, 3º.

2. Name the following compounds

according to IUPAC nomenclature

Physical properties of alkanes

a)

Physical state

•

At room temperature (25°C) and atmosphere pressure (1 atm),
methane, ethane, propane and butane are gases, the C5-C17 are
liquids and C18 and above are solid.

b) Boiling Point
•

Generally, boiling point of alkanes increase with higher number of C
or relative molecular mass.

•

Example:

•

This is because as the size of alkanes (relative molecular mass)
increases, the strength of Van der Waals forces between molecules
increases.

Butane,
C4H10 >

Ethane,
C2H6>

Methane,
CH4

0°C

-89°C

-162°C

Cont… Physical properties of

alkanes

• Isomers of alkanes however have different

boiling point due to branching.

• From the above table, boiling point of 2,2-

dimethylpropane < 2-methybutane < pentane

Pentane

2-methylbutane

2,2-dimethylpropane

37°C

28.5°C

9°C

Cont… Physical properties of

alkanes

c) Solubility

– Alkanes are not soluble in H2O because they

are non-polar whereas water is a polar
solvent. Besides they cannot form hydrogen
bond with water.

– However, it is soluble in non-polar solvents

such as ether and tetrachloromethane
because it is non-polar.

+ H2

C

C

H

C

C

H

H

H

H

H

H

H

H

H

Hydrogenation of alkenes

Pt / Pd / Ni

alkene

hydrogen

alkane

Example:

C2H4 + H2 C2H6

Ni

ethene

ethane

Preparation of alkanes

Hydrolysis of Grignard reagent
(alkylmagnesium halide)

CH3CH2CH2MgCl + H2O

CH3CH2CH3 + Mg(OH)Cl
H+

RMgX + H2O  RH + Mg(OH)X

H+

alkylmagnesium

halide
alkane

Example:

Propylmagnesium

chloride

Propane

Chemical Properties of Alkanes

• Alkanes are LESS reactive compared to

alkenes, alkynes , arenes and others.

• This is because carbon and hydrogen atoms in

alkanes have similar electronegativity values
making the C-H bond not polar. So, alkanes are
not attacked by nucleophile and electrophile.

• Besides, alkanes do not have unpaired /

unshared electrons to be reactive towards acids
or electrophiles.

Cont…..Chemical Properties of

Alkanes

a)

Combustion

•

Complete combustion forms carbon dioxide
and water whereas incomplete combustion
forms carbon monoxide or carbon and water.

•

The reaction is EXOTHERMIC

C4H10

2
13 O2
4CO2
5H2O

C4H10

2
9 O2
4CO

5H2O

C4H10

2
5 O2
4C

5H2O

Complete combustion of butane

Incomplete combustion of butane

Incomplete combustion of butane

b) Halogenation

•React with chlorine/ bromine/ iodine gas in the presence of
ultraviolet light or high temperature to form haloalkane or
alkyl halide.

•The mechanism involved is free-radical substitution which
C-H bond is broken and new C-X bond is formed.

•Further substitution will result in a mixture of products.

CH4 + Cl2
CH3Cl + HCl

CH3Cl + Cl2
CH2Cl2 + HCl

CH2Cl2 + Cl2
CHCl3 + HCl

CHCl3 + Cl2
CCl4 + HCl

uv

uv

uv

uv

• The reactivity of the halogens decreases in the

following order :

F2 > Cl2 > Br2 > I2

• Bromine reacts with alkanes in the same way as

chlorine but at a lower rate.

• Fluorine is very reactive and difficult to control.
• Iodine is generally unreactive.
• Chlorination and bromination are exothermic.
• Energy input in the form of heat or light is

necessary to initiate the halogenation.

Mechanism of free radical

substitution

• Free radical substitution is a chain (series)

reaction that involve in the three step
reactions:

i) initiation

ii) propagation

iii) termination

i) Initiation

The chain is initiated (started) by UV light breaking

a chlorine molecule into free radicals. This
process is called photochemical homolytic
fission/cleaveage.

Cl2 2Cl●

Heat / light

• Unlike methane and ethane, alkanes with more than two

carbon atoms can form more than one
monohalogenation product based on different
classification of carbon atom that given different IUPAC
name.

Natural Source of Hydrocarbon-
Crude Oil

• The major source of alkanes is crude oil or

petroleum.

• Petroleum is a liquid mixture of thousands of

compounds, most of them hydrocarbons, which
are formed from the decomposition of ancient
plants and animals.

• The petroleum industry is concerned with the

separation of the thousands of hydrocarbon in
crude oil to produce useable products.

• The different components in crude oil are

separated by fractional distillation.

Fractional Distillation of Crude
oil

• Fractional distillation is a separation process

based on the different boiling points of the
compounds.

• The crude oil is separated into fractions. Each

fraction consists of a mixture of hydrocarbon
which boils over a limited range of temperature.

• The table below lists the different fractions

obtained from crude oil.

Fraction

Molecular size

Boiling point

range / °C

Uses

Petroleum

gas

C1 to C4
<20

Fuel for domestic

gas cookers and for

heating

Light

petroleum

C5 to C6
20 to 60

Organic solvent

Light
naphta

C6 to C7
60 to 100

Organic solvent

Petrol

C5 to C12
40 to 205

Fuel for motor

vehicles

Kerosene

C12 to C18
175 to 325

Fuel for jet engines

Gas oil/
Diesel

C18 to C25
275 to 400

Fuel for diesel

engines

Lubricating

oil

C20 to C34
>400

Lubricants

Bitumen

> C34
Solid

residue

For road surfacing

and rooling

• Besides being used as fuels, some of these fractions

are used to manufacture thousands of other
compounds such as plastics, detergents, paints,
polymers, synthetic rubber and medicines.

• The petroleum fractions are easier to vaporise and

are therefore more useful fuels.

• In the petrochemical industry, the heavier fractions

are broken down into lighter fractions by a process
called cracking and reforming.

• Cracking

–converts high molecular weight
hydrocarbons to more useful, low molecular
weight ones
i) thermal cracking (by heat)
ii) catalytic cracking (with the aid of catalyst)

• Reforming

–increases branching of hydrocarbon chains
–branched hydrocarbons have better burning
characteristics for automobile engines

Petroleum refining

Cracking

• Cracking is a process where large hydrocarbon

molecules (from crude oil) are broken down into
smaller and more volatile molecules.

• There are two types of cracking, thermal cracking

and catalytic cracking.

• Thermal cracking makes use of high temperature

and high pressure to bring about the cracking
process.

• Catalytic cracking, as the name implies, makes

use of suitable catalysts for the process, which
can be carried out at lower temperature and
pressure. The catalysts used are alumina
(aluminium oxide), silica (silicon dioxide) or
zeolites.

• Catalytic cracking produces more branched

chain alkanes than thermal cracking.