Q10P - Organic Chemistry (Mcmurry)

Question

How would you prepare the following carboxylic acids?

(a)${\mathbf{\left(}{\mathbf{CH}}_{\mathbf{3}}\mathbf{\right)}}_{\mathbf{3}}\mathbf{ }{\mathbf{CCO}}_{\mathbf{2}}\mathbf{H}\mathbf{ }\mathbf{from}\mathbf{ }{\mathbf{\left(}{\mathbf{CH}}_{\mathbf{3}}\mathbf{\right)}}_{\mathbf{3}}\mathbf{ }\mathbf{CCI}$

(b) ${\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{CO}}_{\mathbf{2}}\mathbf{H}\mathbf{ }\mathbf{from}\mathbf{ }{\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{Br}$ 

Step-by-Step Solution

Answer

(a)

The below synthesis route is used to prepare the target carboxylic acid from tertiary alkyl chloride.

$$\begin{gathered} {\left({\mathrm{CH}}_3\right)}_3\mathrm{CCI} \underset{3. {\mathrm{H}}_3{\mathrm{O}}^{+} }{\overset{1. \mathrm{Mg}, \mathrm{ether} \\ 2.{\mathrm{CO}}_2, \mathrm{ether}}{\to }} {\left({\mathrm{CH}}_3\right)}_3{\mathrm{CCO}}_2\mathrm{H} \end{gathered}$$ 

(b)

The following methods are used to prepare the target carboxylic acid from primary alkyl bromide.

 

Method 1:

$$\begin{gathered} {\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{Br} \underset{3. {\mathrm{H}}_3{\mathrm{O}}^{+} }{\overset{1. \mathrm{Mg}, \mathrm{ether} \\ 2.{\mathrm{CO}}_2, \mathrm{ether}}{\to }} {\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CO}}_2\mathrm{H} \end{gathered}$$

Method 2:

${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{Br} \underset{2. {\mathrm{H}}_2{\mathrm{O}}^{+} }{\overset{1. \mathrm{NaCN} }{\to }} {\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CO}}_2\mathrm{H}$

Step1: Preparation of the target carboxylic acid from tertiary alkyl chloride. (a)

In the first step, the given starting material is t-butyl chloride, ${\left({\mathrm{CH}}_3\right)}_3 \mathrm{CCI}$ which is reacted with magnesium metal in the presence of ether solvent to produce t-butylmagnesium chloride, ${\left({\mathrm{CH}}_3\right)}_3 \mathrm{CMgCI}$ which is known as a Grignard reagent. 

In the second step, the Grignard reagent is reacted with carbon dioxide gas via a nucleophilic carbonyl addition reaction, followed by the protonation by adding dilute mineral acid as the third step, to give ${\left({\mathrm{CH}}_3\right)}_3 {\mathrm{CCO}}_2\mathrm{H}$ as the target carboxylic acid.

$$\begin{gathered} {\left({\mathrm{CH}}_3\right)}_3\mathrm{CCI} \stackrel{\mathrm{Mg}, \mathrm{ether}}{\to } {\left({\mathrm{CH}}_3\right)}_3\mathrm{CMgCI} \stackrel{{\mathrm{CO}}_2, \mathrm{ether}}{\to } {\left({\mathrm{CH}}_3\right)}_3{\mathrm{CCO}}_2\mathrm{MgCI} \\ \downarrow {\mathrm{H}}_3{\mathrm{O}}^{+} \\ {\left({\mathrm{CH}}_3\right)}_3{\mathrm{CCO}}_2\mathrm{H} \end{gathered}$$ 

Step 2: Preparation of the target carboxylic acid from primary alkyl bromide. (b)

Method 1:

In the first step, the starting material is given as ${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{Br}$ (n-propylbromide)is treated with magnesium metal in the presence of ether solvent to produce n-propylmagnesium bromide, ${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{MgBr}$ .

In the second step, the Grignard reagent generated in the first step is further reacted with carbon dioxide gas to the magnesium salt of the carboxylic acid. In the final step, the carboxylate anion is acidified with dilute mineral acid such as hydrochloric acid to yield ${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CO}}_2\mathrm{H}$.

$$\begin{gathered} {\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{Br} \stackrel{\mathrm{Mg}, \mathrm{ether}}{\to } {\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{MgBr} \stackrel{{\mathrm{CO}}_2, \mathrm{ether}}{\to } {\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CO}}_2\mathrm{MgBr} \\ \downarrow {\mathrm{H}}_3{\mathrm{O}}^{+} \\ {\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CO}}_2\mathrm{H} \end{gathered}$$

Method 2:

${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{Br}$ is a type of the primary alkyl halide that undertakes a second order nucleophilic substitution reaction with $\mathrm{NaCN}$ (acts as a nucleophile in this reaction).

This results in the formation of the corresponding nitriles. The hydrolysis of the newly formed nitriles in the presence of aqueous acid to produce the target carboxylic acid as shown below.

 

${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{Br} \stackrel{\mathrm{NaCN} }{\to } {\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{CN} \stackrel{{\mathrm{H}}_3{\mathrm{O}}^{+} }{\to }{\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CO}}_2\mathrm{H}$