13.1_Alkanes_3

13.1 13.1 ALKANESALKANES

CH EM I S T RY U N I TCH EM I S T RY U N I TS K 0 2 7S K 0 2 7

OBJECTIVES:OBJECTIVES:

CHAPTER 13 : HYDROCARBONSCHAPTER 13 : HYDROCARBONS

13.1 ALKANES13.1 ALKANES

1. State the natural sources of alkanes

2. Describe the combustion of alkanes in

-excess oxygen

-limited oxygen

S K 0 2 7 C H EM I S T RYS K 0 2 7 C H EM I S T RY

-limited oxygen

3. Explain the unreactivity of alkanes

4. Explain the halogenation reaction of alkanes

5. Explain the free radical subsitution mechanism

for methane, ethane and propane.

6. Explain the monosubstitution of alkane containing

equivalent type of hydrogen atoms as in neopentane

• Natural gas

• crude oil (petroleum)

Nature sources of Alkanes



Natural gas contains primarily methane (70%) and ethane (10%), with some propane (15%).

Crude oil is a mixture of liquid alkanes and other hydrocarbons


Reaction of alkanes with oxygen

• Complete combustion (in excess of oxygen) of

alkanes to give carbon dioxide gas, water and heat.

CH4 + 2O2 → CO2 + 2H2O + heat



4 2 2 2

C2H6 + 7/2 O2 → 2CO2 + 3H2O + heat

• In limited supply of oxygen, combustion of alkanes

produces carbon monoxide and water.

CH4 + O2 → CO + 2H2O + heat

C2H6 + 5/2O2 → 2CO + 3H2O + heat


Alkanes are nonreactive (inert) towards most reagents such

as acids, alkalis, oxidising agents or reducing agents because:

• are saturated hydrocarbons with no functional group; do

not undergo addition reactions.

Chemical Properties of Alkanes



not undergo addition reactions.

• are non-polar, electronegativity values of C & H almost

similar.

•have strong C — C and C — H covalent bonds

• have no unpaired electrons.

However under right conditions, alkanes may undergo

substitution rxn e.g.halogenation.

Halogenation of Alkanes

Alkanes react with halogens to produce haloalkanes in the presence of light or when heated.

R–H + X2

R–X + HX hv

or ∆



or ∆

The halogenation is a free-radical substitution reaction.

The mechanism involves initiation, propagation and

termination steps.

In the example; If CH3Cl is allowed to react with more

chlorine, further chlorination produces mixture of

CH2Cl2,CHCl3 & CCl4.

Example:

CH4+ Cl

2CH

3Cl + HCl

hv

Mechanism : Chlorination of methane

CH4 + Cl2 CH3Cl + HClhv

The equation of reaction is;

Mechanism



Step 1 : Chain initiation step:

In the presence of ultra-violet light / heat, the covalent

bond in the chlorine molecule undergoes homolyticfission to produce chlorine free radicals, Cl•.

Cl Cl

heatCl Cl+


Step 2 : Chain propagation step:

Cl+H3C H CH3 + HCl




These involve reaction between a free radical species

(Cl•) and a molecule (CH4) to produce a new free radical

species (CH3•).

CH3• propagates a chain reaction when it reacts with

another chlorine molecule to form CH3Cl and Cl•

Cl Cl ClCH3 + CH3Cl +


Step 3 : Chain termination step:

Cl2Cl + Cl




The chain is terminated when two free radicals

combine to form molecules.

CH3 + Cl CH3Cl

CH3 + CH3CH3CH3


Note: in excess of chlorine, the propagation steps proceed with the reaction between chlorine free radical with

chloromethane to produce dichloromethane.

The reaction may continue to produce trichloromethane

and finally tetrachloromethane.

CH3Cl + Cl• → CH2Cl• + HCl



CH3Cl + Cl• → CH2Cl• + HCl

CH2Cl• + Cl2 → CH2Cl2 + Cl•

CH2Cl2 + Cl• → CHCl2• + HCl

CHCl2• + Cl2 → CHCl3 + •Cl

CHCl3 + Cl• → CCl3• + HCl

CCl3• + Cl2 → CCl4 + •Cl

2nd propagation

3rd propagation

4th propagation

CH3Cl + HClCH

4+ Cl2 →

hv

methane

Product of monochlorination of alkanes



CH3CH

2Cl + HCl

methane

CH3CH

3+ Cl2 →

hv

ethane


CH3CH

2CH2Cl +

CH3CHClCH3 + HClCH

3CH

3CH3 + Cl2 →

hv

propane

Product of monochlorination of alkanes



C

CH3

H3C

CH3

CH2Cl + HClC

CH3

H3C

CH3

CH3+ Cl2 →

hv

neopentane


• Bromine is less reactive toward alkanes than chlorine.

• Therefore bromine is more selective.• The % product yield is based on the stability of the free radical, 3o > 2o > 1o.

CH CHCH

Bromination of alkanes



+C

CH3

H3C CH3

Br

C

CH3

H3C CH2Br

H

>99 %

(major)trace

(minor)

C

CH3

H3C CH3

H

+ Br2

hν

127oC

C

CH3

H3C CH3

H

+ Cl2

hν

25oC

C

CH3

H3C CH2Cl

H

C

CH3

H3C CH3

Cl

+

37 % 63 %

The difference in selectivity between chlorination and bromination of alkanes need to be kept in mind when one wishes to prepare an alkyl halide from an alkane:

Halogenation of alkanes



1. Because chlorination of an alkane yields every possible monochloride, it is used only when all the hydrogen in an alkane are equivalent.

2. Bromination is normally used only to prepare tertiary alkyl bromides from alkanes.


Exercises:

When a few drops of liquid bromine are added to

neopentane and the mixture is exposed to sunlight,

a reaction occurs.



a) State two observations that prove that a reaction

has indeed occurred.

b) Write the chemical equation for the reaction

between bromine and neopentane.

c) Write the mechanism for the reaction.


Write the equation and mechanism for mono

substitution reaction; chlorination of propane to

yield 2-chloropropane.

Exercises:



yield 2-chloropropane.


Date post:	26-May-2017
Category:	Documents
Upload:	syamira-mddahin
View:	213 times
Download:	0 times

13.1_Alkanes_3

Documents