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Chapter 22: Problem 569

Reaction of acid chlorides with sodium azide, \(\mathrm{NaN}_{3}\), yields acyl azides, RCON \(_{3}\). When heated, these undergo the Curtius rearrangement to amines, \(\mathrm{RNH}_{2}\), or, in a non-hydroxylie solvent, to isocyanates, RNCO. Using the structure for the azide, suggest a mechanism for the rearrangement.

Short Answer

Expert verified
The mechanism for the Curtius rearrangement involves the following steps: 1. Acid chlorides (R-COCl) react with sodium azide (NaN3) through nucleophilic acyl substitution to form acyl azides (RCON3). 2. Upon heating, acyl azides undergo Curtius rearrangement through cyclic intermediate rearrangement, resulting in the formation of isocyanates (RNCO). 3. If water is present, isocyanates can further react with it to form amines (RNH2) and carbon dioxide (CO2) via carbamic acid intermediate.

Step by step solution

01

Structure of azide

The azide ion, N3-, has the structure: \[\hspace{1cm} \overset{-}{N} = \overset{+}{N} - \overset{}{N} \hspace{1cm} (1)\] To react with the acid chlorides, the azide will act as a nucleophile in the reaction.
02

Formation of acyl azides

Acid chlorides (R-COCl) react with sodium azide to form acyl azides (RCON3). The reaction occurs through nucleophilic acyl substitution, as shown below: 1. The negative end of azide ion \((1)\) acts as a nucleophile, attacking the carbonyl carbon of the acid chloride, forming a tetrahedral intermediate. 2. The chloride ion, being a good leaving group, leaves and acyl azide is formed. This reaction is favored since the azide ion is a strong nucleophile while the chloride ion is a good leaving group. The reaction is as follows: \[ RCOCl + \mathrm{NaN}_{3} \rightarrow RCON_{3} + \mathrm{NaCl} \]
03

Curtius rearrangement

When acyl azides are heated, they undergo Curtius rearrangement. The reaction proceeds through the following steps: 1. The nitrogen-nitrogen double bond in the acyl azide (RCON3) attacks the carbonyl carbon, forming a cyclic intermediate. 2. The cyclic intermediate rearranges by breaking the N-N single bond and migrating the R group to the central nitrogen atom. 3. The resulting isocyanate (RNCO) is formed. In a non-hydroxylic solvent, the rearrangement ends at this step. The reaction is as follows: \[ \mathrm{RCON}_{3} \xrightarrow[\mathrm{Heating}]{} \mathrm{RNCO}+ \mathrm{N}_{2} \]
04

Formation of amines (Optional step)

If water is present, isocyanates can react with water to form amines (RNH2) and carbon dioxide (CO2). The reaction proceeds through the following steps: 1. Water molecule acts as a nucleophile and attacks the carbon of the isocyanate, forming a carbamic acid intermediate. 2. The carbamic acid intermediate breaks down, losing a carbon dioxide molecule, and forming an amine (RNH2). The reaction is as follows: \[ \mathrm{RNCO} + \mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{RNH}_{2} + \mathrm{CO}_{2} \]
05

Summary of Curtius rearrangement mechanism

1. Acid chlorides react with sodium azide to form acyl azides through nucleophilic acyl substitution. 2. Acyl azides undergo Curtius rearrangement upon heating, forming isocyanates through migration of the R group to the central nitrogen atom. 3. If water is present, isocyanates can react with it to form amines and carbon dioxide through carbamic acid intermediate.

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