Explain why a much better yield of primary amine is obtained from the reaction of an alkyl halide with azide ion (-N3), followed by catalytic hydrogenation. (Hint: An alkyl azide is not nucleophilic.)

Ch. 9 - Substitution and Elimination Reactions of Alkyl Halides

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Bruice 8th Edition

Ch. 9 - Substitution and Elimination Reactions of Alkyl Halides

Problem 17b

Chapter 10, Problem 17b

Explain why a much better yield of primary amine is obtained from the reaction of an alkyl halide with azide ion (^-N₃), followed by catalytic hydrogenation. (Hint: An alkyl azide is not nucleophilic.)

Verified step by step guidance

Step 1: Begin by understanding the reaction sequence. The alkyl halide (CH₃CH₂CH₂Br) reacts with azide ion (N₃⁻) in a nucleophilic substitution reaction to form an alkyl azide (CH₃CH₂CH₂N=N=N). This is an SN2 reaction where the azide ion replaces the bromine atom.

Step 2: Recognize the role of the alkyl azide intermediate. The alkyl azide is not nucleophilic, meaning it does not readily react further with other nucleophiles. This prevents side reactions that could reduce the yield of the desired primary amine.

Step 3: Understand the catalytic hydrogenation step. The alkyl azide undergoes reduction in the presence of hydrogen gas (H₂) and a catalyst such as palladium on carbon (Pd/C). This process breaks the nitrogen-nitrogen bonds in the azide group, converting it into a primary amine (CH₃CH₂CH₂NH₂) and releasing nitrogen gas (N₂).

Step 4: Compare this method to other approaches for synthesizing primary amines. Direct nucleophilic substitution of an alkyl halide with ammonia often leads to over-alkylation, producing secondary and tertiary amines instead of the desired primary amine. The use of azide avoids this issue because the alkyl azide intermediate does not undergo further nucleophilic reactions.

Step 5: Conclude that the reaction sequence involving azide ion followed by catalytic hydrogenation is highly selective for producing primary amines. The lack of side reactions and over-alkylation ensures a much better yield of the desired product.

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Key Concepts

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

Nucleophilicity

Nucleophilicity refers to the ability of a species to donate an electron pair to form a chemical bond. In organic reactions, nucleophiles attack electrophiles, leading to bond formation. Azide ions (N3-) are strong nucleophiles, but when they form alkyl azides, the resulting compound is not nucleophilic due to the resonance stabilization of the azide group, which makes it less reactive.

Alkyl Azides

Alkyl azides are organic compounds containing an alkyl group attached to an azide functional group (-N3). They are typically formed through nucleophilic substitution reactions involving alkyl halides and azide ions. Alkyl azides are stable and can undergo catalytic hydrogenation to yield primary amines, making them useful intermediates in organic synthesis.

Catalytic Hydrogenation

Catalytic hydrogenation is a chemical reaction that involves the addition of hydrogen (H2) to a compound in the presence of a catalyst, often palladium (Pd) or platinum (Pt). In the context of alkyl azides, hydrogenation converts the azide group into an amine, resulting in a higher yield of primary amines. This process is efficient because it selectively reduces the azide without affecting other functional groups.

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Related Practice

Textbook Question

What is the product of the reaction of bromoethane with each of the following nucleophiles?

c. (CH₃)₃N

d. CH₃CH₂S^-

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Textbook Question

Rank the following alkyl halides from most reactive to least reactive in an S_N1 reaction:

2-bromo-2-methylpentane, 2-chloro-2-methylpentane, 3-chloropentane, and 2-iodo-2-methylpentane.

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Textbook Question

Draw the stereoisomers that are formed from the following S_N1 reactions:

a. 3-bromo-3-methylpentane and methanol

b. 3-chloro-3-methylhexane and methanol

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Textbook Question

What is the product of the reaction of bromoethane with each of the following nucleophiles?

a. CH₃CH₂CH₂O⁻

b. CH₃C≡C⁻

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Textbook Question

Which of the following reactions take place more rapidly when the concentration of the nucleophile is increased?

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Textbook Question

Explain why the reaction of an alkyl halide with ammonia gives a low yield of primary amine.

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