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 in air (making them good fuels): 2C 2 H 6 (g) + 7O 2 (g)  4CO 2 (g) + 6H 2 O(l)  H = –2855 kJ 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 and even death.

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

Free Radical Substitution reaction For a reaction between an alkane and bromine to occur, C-H and Br-Br bonds must break. 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. or 1-bromohexane.

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. 1,1 dibromohexane Free Radical Substitution reaction

R E P E A T I N G S T E P S Mechanism of chlorination of methane CHAIN REACTION “hydrogen abstraction” “dissociation”

4. Termination Steps These steps stop the chain reaction “recombinations” Mechanism of chlorination of methane