Ch. 21 - Amino Acids, Peptides, and Proteins

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook

Bruice 8th Edition

Ch. 21 - Amino Acids, Peptides, and Proteins

Problem 20

Chapter 22, Problem 20

Why is excess ammonia used in the preceding reaction?

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Ammonia (NH₃) is a nucleophile, meaning it donates a pair of electrons to form a bond with an electrophile. In the reaction, ammonia is used to attack the electrophilic center, typically a carbon atom in a substrate like an alkyl halide or acyl compound.

Excess ammonia is used to drive the reaction toward the formation of the desired product. This is based on Le Chatelier's principle, which states that increasing the concentration of a reactant (in this case, ammonia) will shift the equilibrium toward the product side.

Using excess ammonia minimizes the likelihood of side reactions. For example, in reactions involving alkyl halides, excess ammonia helps prevent the formation of secondary or tertiary amines by limiting the availability of the alkyl halide for further substitution reactions.

Excess ammonia also ensures that the reaction proceeds to completion by maintaining a high concentration of the nucleophile, which increases the reaction rate and the yield of the desired product.

Finally, excess ammonia can act as a solvent in some cases, providing a medium for the reaction to occur and ensuring that all reactants are well-dissolved and able to interact effectively.

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Key Concepts

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

Le Chatelier's Principle

Le Chatelier's Principle states that if a system at equilibrium is disturbed, the system will adjust to counteract the disturbance and restore a new equilibrium. In the context of reactions involving ammonia, using excess ammonia can shift the equilibrium position towards the products, enhancing the yield of the desired product.

Ammonia as a Nucleophile

Ammonia (NH3) acts as a nucleophile due to the presence of a lone pair of electrons on the nitrogen atom. This property allows ammonia to attack electrophilic centers in organic molecules, facilitating various reactions, such as nucleophilic substitutions or additions, which are often key in organic synthesis.

Reaction Yield and Selectivity

In organic chemistry, the yield of a reaction refers to the amount of product formed relative to the theoretical maximum. Using excess reagents, like ammonia, can improve both the yield and selectivity of a reaction by ensuring that the limiting reactant is fully consumed, thus driving the reaction to completion and minimizing by-products.

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Related Practice

Textbook Question

Explain the order of elution (with a buffer of pH 4) of the following pairs of amino acids through a column packed with Dowex 50:

a. aspartate before serine

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Textbook Question

A mixture of seven amino acids (glycine, glutamate, leucine, lysine, alanine, isoleucine, and aspartate) is separated by chromatography. Explain why only six spots show up when the chromatographic plate is coated with ninhydrin and heated.

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Textbook Question

What alkyl halide is used in the acetamidomalonic ester synthesis to prepare

a. lysine?

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Textbook Question

What amino acid is formed using the N-phthalimidomalonic ester synthesis when the following alkyl halides are used in the third step?

b. CH₃SCH₂CH₂Br

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