Functional Groups

Recognizing the Core Groups

MCAT organic questions start with fast identification: alcohols ($-OH$), ethers ($R-O-R$), aldehydes (terminal $C=O$ with H), ketones (internal $C=O$), carboxylic acids ($-COOH$), esters ($-COOR$), amides ($-CONR_2$), amines, thiols, and phosphates. In IUPAC naming, the highest-priority group becomes the suffix (carboxylic acid > ester > amide > aldehyde > ketone > alcohol > amine), with lower-priority groups named as substituents.

Properties from Structure

Hydrogen-bond donors (alcohols, acids, amides) boil far higher than similar-mass ethers or alkanes; carboxylic acids dimerize and boil highest. Acidity tracks conjugate-base stability: carboxylic acids ($pK_a \approx 4-5$) beat phenols ($\approx 10$) beat alcohols ($\approx 16$). Amines are the common organic bases.

Oxidation Levels and Unsaturation

Oxidation climbs the ladder alkane to alcohol to aldehyde/ketone to carboxylic acid; reduction descends it. Primary alcohols oxidize to aldehydes (PCC) or acids (strong oxidants); secondary alcohols give ketones; tertiary alcohols resist oxidation. Degrees of unsaturation come from $\text{DoU} = \frac{2C + 2 + N - H - X}{2}$

Key Takeaways

• Suffix priority: carboxylic acid outranks every other group.
• Hydrogen bonding explains boiling point and solubility rankings.
• Acidity order: carboxylic acid > phenol > water/alcohol > alkyne.
• Count degrees of unsaturation before proposing any structure.