Ch. 19 - Amines

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 19 - Amines

Problem 15

Chapter 19, Problem 15

Propose a mechanism to show the individual alkylations that form this quaternary ammonium salt.

Verified step by step guidance

Step 1: Identify the starting material, which is a secondary amine (CH3CH2NH2). The nitrogen atom has a lone pair of electrons, making it nucleophilic and capable of attacking electrophiles like CH3Br.

Step 2: In the first alkylation step, the lone pair on the nitrogen attacks the methyl group of CH3Br, displacing the bromide ion (Br⁻) and forming a tertiary amine (CH3CH2N(CH3)H). This is an SN2 reaction.

Step 3: In the second alkylation step, the tertiary amine (CH3CH2N(CH3)H) undergoes another SN2 reaction with CH3Br. The lone pair on the nitrogen attacks another CH3 group, displacing Br⁻ and forming a quaternary ammonium salt (CH3CH2N(CH3)2⁺Br⁻).

Step 4: The reaction continues with excess CH3Br, allowing the tertiary amine to undergo further alkylation. The nitrogen attacks a third CH3 group, forming the final quaternary ammonium salt (CH3CH2N(CH3)3⁺Br⁻).

Step 5: The reaction is facilitated by NaHCO3, which neutralizes any acidic byproducts formed during the alkylation steps, ensuring the reaction proceeds efficiently to completion.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Alkylation Mechanism

Alkylation is a chemical reaction where an alkyl group is transferred to a nucleophile, often involving a substrate with a lone pair of electrons. In the context of quaternary ammonium salt formation, the mechanism typically involves nucleophilic attack by the nitrogen atom on an alkyl halide, such as CH3Br, leading to the formation of a new carbon-nitrogen bond.

Quaternary Ammonium Salts

Quaternary ammonium salts are positively charged ions formed when a nitrogen atom is bonded to four alkyl or aryl groups. This structure results in a stable, tetrahedral geometry around the nitrogen. The presence of a quaternary ammonium salt is significant in organic synthesis and can influence the solubility and reactivity of compounds.

Role of NaHCO3

Sodium bicarbonate (NaHCO3) acts as a mild base in the alkylation reaction, helping to deprotonate the nitrogen atom in the amine, enhancing its nucleophilicity. This increased nucleophilicity allows for more effective attack on the alkyl halide, facilitating the formation of the quaternary ammonium salt through multiple alkylation steps.

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Related Practice

Textbook Question

(a) Propose a mechanism for the reaction of 2-bromopyridine with sodium amide to give 2-aminopyridine.

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Textbook Question

We have considered nucleophilic aromatic substitution of pyridine at the 2-position and 3-position but not at the 4-position. Complete the three possible cases by showing the mechanism for the reaction of methoxide ion with 4-chloropyridine. Show how the intermediate is stabilized by delocalization of the charge onto the nitrogen atom.

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Textbook Question

(b) When 3-bromopyridine is used in this reaction, stronger reaction conditions are required and a mixture of 3-aminopyridine and 4-aminopyridine results. Propose a mechanism to explain this curious result.

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Textbook Question

Give the products expected from the following reactions.

(a) acetyl chloride + ethylamine

(b)

(c)

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Textbook Question

Show how you would use direct alkylation to synthesize the following compounds.

(a) benzyltrimethylammonium iodide

(b) pentan-1-amine

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Textbook Question

What would happen in the synthesis of sulfanilamide if the amino group were not protected as an amide in the chlorosulfonation step?

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