ammonia
Summary
Ammonia (NH₃) is a small neutral molecule with a lone pair on nitrogen, so it acts as a weak base and as a nucleophile. Its most testable organic use is reacting with electrophiles such as alkyl halides to make amines, a reaction that often overalkylates.
Detail
Meaning: Ammonia, NH₃, is trigonal pyramidal, with an sp³ nitrogen carrying one lone pair. The lone pair lets it accept a proton to form ammonium, NH₄⁺ (pKa of NH₄⁺ about 9.25), or attack an electrophilic carbon as a nucleophile. It reacts with alkyl halides by SN2 to give amines, with acyl chlorides or anhydrides to give amides, and with aldehydes and ketones to give imines. It hydrogen bonds, so it dissolves readily in water.
Equation: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, with Kb = [NH₄⁺][OH⁻]/[NH₃]. Concentrations are in mol/L and Kb is dimensionless (about 1.8 × 10⁻⁵ at 25 °C).
On the MCAT: Expect amine synthesis passages: excess NH₃ with an alkyl halide, the Gabriel synthesis as a route to clean primary amines, reductive amination, and the Strecker synthesis of α-amino acids (aldehyde + NH₃ + HCN, then hydrolysis). Questions may ask you to predict products or to compare basicity and boiling point with amines.
Common confusion: Students assume the reaction stops at the primary amine. The product is usually as nucleophilic as ammonia, or more so, so it keeps reacting and gives mixtures unless NH₃ is in large excess. Also, NH₄⁺ has no free lone pair and is not a nucleophile; below about pH 9 most of it is present as NH₄⁺.
Sources
- Organic Chemistry
- Organic Chemistry: Structure and Function
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.