Alcohols, Carboxylic Acids, and Acid Derivatives (5D) - MCAT Chemical and Physical Foundations of Biological Systems

$\text{R-COOR'}$. Esters comprise a carbonyl group attached to an alkoxy group, where R and R' are alkyl chains.

Carboxylic acid. Carboxylic acids have lower pKa values due to resonance stabilization of their conjugate bases compared to alcohols.

$\text{R-OH}$. Alcohols feature a hydroxyl group bonded to a carbon atom in an alkyl chain, denoted by R representing the alkyl substituent.

Polar $\text{O-H}$ bond (H-bond donor and acceptor). The electronegativity difference in the O-H bond creates polarity, enabling alcohols to donate and accept hydrogen bonds with water molecules.

Aldehyde. Mild oxidants selectively convert primary alcohols to aldehydes by removing two hydrogens without further oxidation.

Carboxylic acid. Strong oxidants fully oxidize primary alcohols to carboxylic acids by adding an oxygen and removing hydrogens.

Ketone. Secondary alcohols lose two hydrogens during oxidation, forming ketones due to the carbon's attachment to two alkyl groups.

No oxidation (unless C-C bond cleavage occurs). Tertiary alcohols lack the necessary hydrogen on the carbinol carbon for typical oxidation, preventing reaction unless bonds break.

$\text{R-COOH}$. Carboxylic acids contain a carbonyl and hydroxyl group on the same carbon, represented with R as the alkyl chain.

Carboxylate ($\text{R-COO}^-$). Deprotonation of the carboxylic acid's hydroxyl group yields the carboxylate anion, stabilized by resonance.

Ammonia or an amine ($\text{NH}_3$ or $\text{RNH}_2$). Amines or ammonia act as nucleophiles, attacking the carbonyl and displacing chloride to form the amide bond.

Ester. Alcohols serve as nucleophiles in acyl substitution, displacing the leaving group to yield esters.

Amide. Amines function as nucleophiles, attacking the carbonyl and expelling the leaving group to produce amides.

Negative charge delocalized over two oxygens. Resonance in the carboxylate ion distributes the negative charge across two oxygen atoms, enhancing stability and acidity.

$pK_a \approx 4$ to $5$. The pKa reflects the strength of carboxylic acids, influenced by resonance stabilization of the conjugate base.

$pK_a \approx 16$ to $18$. Alcohols have higher pKa values because their conjugate bases lack resonance stabilization, making them weaker acids.

$\text{R-CO-O-CO-R}$. Acid anhydrides consist of two acyl groups linked by an oxygen atom, often represented symmetrically for simplicity.

$\text{R-CONH}_2$ (or $\text{R-CONR'}_2$). Amides have a carbonyl group bonded to a nitrogen atom, which may be substituted with hydrogens or alkyl groups.

$\text{R-COSR'}$. Thioesters replace the oxygen in esters with sulfur, linking the carbonyl to a thioalkyl group.

Acid chloride. Acid chlorides have the best leaving group, chloride, making them highly susceptible to nucleophilic attack.

Amide. Amides possess a poor leaving group due to nitrogen's resonance donation, reducing reactivity in substitution reactions.

Acid chloride $>$ anhydride $>$ ester $\approx$ thioester $>$ amide. Reactivity decreases with poorer leaving groups and increased resonance stabilization from the heteroatom.

$\text{Cl}^-$. Chloride is a weaker base and better leaving group than alkoxide in acyl substitution reactions.

Ester (Fischer esterification). Acid catalysis protonates the carbonyl, enabling nucleophilic attack by alcohol and water elimination to form the ester.