Reactions of Alkanes

IB

Combustion of alkanes

Alkanes are unreactive as a family because of the strong C–C and C–H bonds as well as them being nonpolar compounds. At room temperature alkanes do not react with acids, bases, or strong oxidizing agents. Alkanes do undergo

combustion

fuels): in air (making them good 2C 2 H 6 (

g

) + 7O 2 (

g

)  kJ 4CO 2 (

g

) + 6H 2 O(

l

) 

H

= –2855 Complete combustion produced carbon dioxide and water while incomplete may produces a combination of carbon monoxide, carbon and water in addition to carbon dioxide. Carbon dioxide contributes to global warming while carbon monoxide is toxic; hemoglobin binds to carbon monoxide in preference to oxygen causing suffocation

Products of combustion

Complete combustion produces: carbon dioxide water vapour while incomplete may produces a combination of : carbon monoxide carbon water vapour carbon dioxide. Carbon dioxide contributes to global warming.

Carbon monoxide is toxic; hemoglobin binds to carbon monoxide in preference to oxygen causing suffocation and even death.

Substitution Reaction

• In the presence of light alkanes undergo

substitution reaction

with halogens. RH + Br 2  RBr + HBr • In a substitution reaction, one atom of a molecule is removed and re placed or substituted by another atom or group of atoms. • Mechanism of subtitution reaction involves free radicals.

Free Radical Substitution reaction

2 2 2 2 3  Br 2 2 2 2 1-bromohexane 2 2 For a reaction between an alkane and bromine to occur, C-H and Br-Br bonds must break.  HBr The C-H bond is stronger than Br-Br bond (check bond enthalpy in databooklet). Therefore, the reaction proceeds by first the breakage of Br-Br bond, which is brought about by UV light. Br-Br bond can be broken in one of two ways. Br 2 2Br

or

.

Br 2 Br :   Br 

Free Radical Substitution reaction

When the bond is broken, either •

the bond pair can be equally shared between the two atoms producing two bromine atoms (called free radicals), or

•

The bond pair goes with one atom producing a positive and a negatively charged ions of bromine.

The first type of bond breakage producing free radicals is referred to as a

homolytic fission

and the second

heterolytic fission

. •

Homolytic fission because the bond pairs are equally distributed, or particles that are the same in every way is produced.

•

homolytic fission of the halogen takes place.

In the next step, the free radical removes a hydrogen atom from the alkane forming hydrogen bromine and a free radical of the alkane.

CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 -H + Br•



CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 • + HBr

Free Radical Substitution reaction

• The free radical goes on to react with a molecule of chlorine and regenerate another chlorine free radical.

CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 • + Br 2



CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 Br + Br•

And so on. Because this reaction, once initiated, can keep itself going is referred to as a chain reaction. The reaction can conducted with any halogen and the mechanism would be the same. Not only that, more than one hydrogen can be substituted.

2 2 2 2 3  2Br 2 2 2 1,1 dibromohexane 2 2 2  2HBr

S T E P S R E P E A T I N G

Mechanism of chlorination of methane

CHAIN REACTION 1. Initiation

..

: Cl ..

..

Cl ..

:

light “dissociation”

2 ..

: Cl ..

.

a free radical 2. Chain Propagation (first step)

CH 3 H + ..

: Cl ..

.

“hydrogen abstraction” 3. Chain Propagation (second step)

H ..

Cl ..

: + .

CH 3

methyl radical

.

CH 3 + ..

: Cl ..

..

Cl ..

: CH 3 ..

Cl ..

: + ..

: Cl ..

.

feeds back into step two

Mechanism of chlorination of methane

4. Termination Steps

2 ..

: Cl ..

.

CH3 .

+

. CH 3 ..

: Cl ..

.

+

. CH 3 “recombinations” ..

..

: Cl ..

Cl ..

: CH 3 CH 3 ..

: Cl ..

CH3

These steps stop the chain reaction