Generally, bond strength decreases down the periodic table, and acidity therefore increases. Also,
the more electronegative an atom, the higher the acidity. When these two trends oppose each
other, low bond strength takes precedence.
For the common functional groups on the MCAT, the α-hydrogens of carbonyl compounds deserve
special note. α-hydrogens are connected to the α-carbon, which is a carbon adjacent to the
carbonyl. Because the enol form of carbonyl-containing carbanions is stabilized by resonance, these
are acidic hydrogens that are easily lost. We will go into greater depth about enolate chemistry in
Chapters 7 through 9 of MCAT Organic Chemistry Review.
COMMON FUNCTIONAL GROUPS
We can also apply these acid and base rules directly to the functional groups that appear on the
MCAT. Functional groups that act as acids include alcohols, aldehydes and ketones (at the α-
carbon), carboxylic acids, and most carboxylic acid derivatives. These compounds are therefore
easier to target with basic (or nucleophilic) reactants because they readily accept a lone pair.
Amines and amides are the main functional groups that act as bases—keep an eye out for these
compounds in the formation of peptide bonds. The nitrogen atom of an amine can form coordinate
covalent bonds by donating a lone pair to a Lewis acid.
MCAT Concept Check 4.1:
Before you move on, assess your understanding of the material with these questions.When will an acid–base reaction proceed, based on the strength of the reactants and
products?What does it mean for a molecule to be amphoteric? What biologically relevant molecules
are also characteristically amphoteric?