Question

The gas-phase decomposition of acetaldehyde can be represented by the overall chemical equations.

${\mathrm{CH}}_3\mathrm{CHO}\to {\mathrm{CH}}_4+\mathrm{CO}$

It is thought to occur through a sequence of reactions.

localid="1663685249297" $$\begin{gathered} {\mathrm{CH}}_3\mathrm{CHO}\to {\mathrm{CH}}_3+\mathrm{CHO}\left(Initiation\right) \\ {\mathrm{CH}}_3+{\mathrm{CH}}_3\mathrm{CHO}\to {\mathrm{CH}}_4+{\mathrm{CH}}_2\mathrm{CHO}\left(Initiation\right) \\ {\mathrm{CH}}_2\mathrm{CHO}\to \mathrm{CO}+\mathrm{CH}\left(\mathrm{Pr}opogation\right) \\ {\mathrm{CH}}_3+{\mathrm{CH}}_3\to {\mathrm{CH}}_3{\mathrm{CH}}_3\left(Ter\min ation\right) \end{gathered}$$

Show that this reaction mechanism corresponds to a chain reaction, and identify the initiation, propagation, and termination steps.

Short answer

$$\begin{gathered} {\mathrm{CH}}_3\mathrm{CHO}\to {\mathrm{CH}}_3+\mathrm{CHO}\left(Initiation\right) \\ {\mathrm{CH}}_3+{\mathrm{CH}}_3\mathrm{CHO}\to {\mathrm{CH}}_4+{\mathrm{CH}}_2\mathrm{CHO}\left(Initiation\right) \\ {\mathrm{CH}}_2\mathrm{CHO}\to \mathrm{CO}+\mathrm{CH}\left(\mathrm{Pr}opogation\right) \\ {\mathrm{CH}}_3+{\mathrm{CH}}_3\to {\mathrm{CH}}_3{\mathrm{CH}}_3\left(Ter\min ation\right) \end{gathered}$$

Step-by-step solution

Chain reaction

A chain reaction usually proceeds through three steps:

Chain initiation: The process in which reactive intermediates are generated.

Chain propagation: The step in which products are formed from the reactive intermediates

Chain termination: The step in which two reactive intermediates combine to give the corresponding product.

Overall equation for the decomposition of acetaldehyde

The overall equation for the decomposition of acetaldehyde are given below

${\mathrm{CH}}_3\mathrm{CHO}\to {\mathrm{CH}}_4+\mathrm{CO}$

The suggested mechanism is as follows

$$\begin{gathered} 1.{\mathrm{CH}}_3\mathrm{CHO}\stackrel{{\mathrm{k}}_1}{\to }{\mathrm{CH}}_3+\mathrm{CHO} \\ 2.{\mathrm{CH}}_3+{\mathrm{CH}}_3\mathrm{CHO}\stackrel{{\mathrm{k}}_2}{\to }{\mathrm{CH}}_4+{\mathrm{CH}}_2\mathrm{CHO} \\ 3.{\mathrm{CH}}_2\mathrm{CHO}\stackrel{{\mathrm{k}}_3}{\to }\mathrm{CO}+{\mathrm{CH}}_3 \\ 4.{\mathrm{CH}}_3+\mathrm{C}{\mathrm{H}}_3\stackrel{{\mathrm{k}}_4}{\to }{\mathrm{CH}}_3\mathrm{CH} \end{gathered}$$

Reactive intermediates

A reactive intermediate is a chemical species which doenot appear in the overall balanced chemical reaction, but are generated in several steps of the reaction, the reactive intermediates in the reaction $C{H}_3,CHO,C{H}_{2}O$ they are formed in several steps of the reaction,but do not appear in balance equation.

Chain propagation step

In the first step ${\mathrm{CH}}_3\mathrm{and}\mathrm{CHO}$are generated so it is called as chain initiation step.

Chain elongation step

In the second and third steps, products such as ${\mathrm{CH}}_4,\mathrm{CO}$along with the intermediates ${\mathrm{CH}}_3,{\mathrm{CH}}_2\mathrm{O}$are generated, and hence these steps are called chain propagation steps.

Chain termination step

In the last step known as the chain termination step reactive intermediate combine to give a product.

 Chain Reaction

From the above points, it is concluded that the decomposition of acetaldehyde follows a chain reaction mechanism.

$$\begin{gathered} 1.{\mathrm{CH}}_3\mathrm{CHO}\stackrel{{\mathrm{k}}_1}{\to }{\mathrm{CH}}_3+\mathrm{CHO}\left(\mathrm{Initiation}\right) \\ 2.{\mathrm{CH}}_3+{\mathrm{CH}}_3\mathrm{CHO}\stackrel{{\mathrm{k}}_2}{\to }{\mathrm{CH}}_4+{\mathrm{CH}}_2\mathrm{CHO}\left(\mathrm{initiation}\right) \\ 3.{\mathrm{CH}}_2\mathrm{CHO}\stackrel{{\mathrm{k}}_3}{\to }\mathrm{CO}+{\mathrm{CH}}_3\left(\mathrm{Propogation}\right) \\ 4.{\mathrm{CH}}_3+\mathrm{C}{\mathrm{H}}_3\stackrel{{\mathrm{k}}_4}{\to }{\mathrm{CH}}_3{\mathrm{CH}}_3\left(\mathrm{Termination}\right) \end{gathered}$$