Chapter 2, Problem 50DSP

Some Biochemical Reactions of Alkanes

Alkanes occur naturally in places other than petroleum deposits—in insects, for example. The waxy alkanes dispersed in its cuticle help protect an insect from dehydration. Some insects use volatile alkanes to defend themselves or communicate with others of the same species. Alkanes even serve as starting materials that the insect converts to other biologically important substances.

The major biosynthetic pathway leading to alkanes is by enzyme-catalysed decarboxylation (loss of ${\text{CO}}_2$) of fatty acids, compounds of the type ${\text{CH}}_3{\left({\text{CH}}_2\right)}_n{\text{CO}}_2\text{H}$ in which n is an even number and the chain has $14$ or more carbons.

${\text{CH}}_3{\left({\text{CH}}_2\right)}_n{\text{CO}}_2\text{H}\to {\text{CH}}_3{\left({\text{CH}}_2\right)}_{n-1}{\text{CH}}_3{\text{+ CO}}_2$

Biochemical conversion of alkanes to other substances normally begins with oxidation.

In addition to alkanes, the oxidation of drugs and other substances occurs mainly in the liver and is catalyzed by the enzyme cytochrome $\text{P-450}$. Molecular oxygen and nicotinamide adenine dinucleotide (NAD) are also required.

Oxidation by microorganisms has been extensively studied and is often selective for certain kinds of $\text{C-H}$ bonds. The fungus Pseudomonas oleovorans, for example, oxidizes the ${\text{CH}}_3$ groups at the end of the carbon chain of $4$-methyloctane faster than the ${\text{CH}}_3$ branch and faster than the ${\text{CH}}_2$ and CH units within the chain.

Tridecane $\left[{\text{CH}}_3{\left({\text{CH}}_2\right)}_{11}{\text{CH}}_3\right]$ is a major component of the repellent that the stink bug Piezodorus guildinii releases from its scent glands when attacked. What fatty acid gives tridecane on decarboxylation?

${\text{CH}}_3{\left({\text{CH}}_2\right)}_{10}{\text{CO}}_2\text{H}$

${\text{CH}}_3{\left({\text{CH}}_2\right)}_{11}{\text{CO}}_2\text{H}$

${\text{CH}}_3{\left({\text{CH}}_2\right)}_{12}{\text{CO}}_2\text{H}$

${\text{CH}}_3{\left({\text{CH}}_2\right)}_{13}{\text{CO}}_2\text{H}$