functional groups
Summary
A functional group is a specific atom or group of atoms that gives an organic molecule its characteristic reactivity and properties, largely regardless of the rest of the molecule. Most testable: identify the group (carbonyl, hydroxyl, amine) to predict polarity, reactivity, and acidity or basicity.
Detail
Meaning: A functional group is a specific arrangement of atoms, often a heteroatom (O, N, S, halogen) or a pi bond, that is the main site of a molecule's reactivity. Molecules sharing a group tend to react alike regardless of the rest of the molecule. Common groups: alkene, alkyne, alcohol, ether, aldehyde, ketone, carboxylic acid, ester, amide, amine, thiol, alkyl halide.
On the MCAT: Expect to name a group from a structure, then predict behavior: polarity, hydrogen bonding, boiling point, acidity or basicity, and which reagent reacts. A carbonyl carbon is electrophilic, so nucleophiles attack it. Alcohols and carboxylic acids hydrogen-bond and boil higher than alkanes or ethers of similar mass. In spectroscopy, a broad IR peak near 3300 cm⁻¹ suggests O–H and a strong peak near 1700 cm⁻¹ suggests C=O.
Common confusion: Students mix up look-alikes. An ester (RCOOR) contains a carbonyl; an ether (ROR) does not. An aldehyde carbonyl carbon bears at least one hydrogen, while a ketone's is bonded to two carbons. An amine is basic, but an amide nitrogen is not appreciably basic because its lone pair is delocalized into the carbonyl by resonance.
Sources
- Organic Chemistry (Janice Gorzynski Smith)
- Organic Chemistry (Paula Yurkanis Bruice)
- Organic Chemistry (David Klein)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.