MCAT Organic Chemistry Review

Analyzing Organic Reactions

Answers to Concept Checks

·        4.1

1.    An acid–base reaction will proceed when the acid and base react to form conjugate products that are weaker than the reactants.

2.    Amphoteric species can act as either an acid or a base. Water, bicarbonate, and dihydrogen phosphate are common amphoteric species in biological systems.

3.    pKa = –log Ka, where Ka is the equilibrium constant for the dissociation of an acid. pKa indicates acid strength: a stronger acid has a lower (or even negative) pKa.

4.    Alcohols, aldehydes and ketones, carboxylic acids, and most carboxylic acid derivatives act as acids. Amines and amides act as bases.

·        4.2

1.    Nucleophilicity and electrophilicity are based on relative rates of reactions and are therefore kinetic properties. Acidity and basicity are measured by the position of equilibrium in a protonation or deprotonation reaction and are therefore thermodynamic properties.

2.    The four main determinants of nucleophilicity are charge (more negative = better nucleophile), electronegativity (more electronegative = worse nucleophile), steric hindrance (larger = worse nucleophile), and the solvent (protic solvents can protonate or hydrogen bond with the nucleophile, decreasing its reactivity).

3.    A substitution reaction will proceed when the nucleophile is a stronger base (more reactive) than the leaving group.

4.    Greater positive charge increases electrophilicity, and better leaving groups increase electrophilicity by making the reaction more likely to proceed.

5.    Good leaving groups can stabilize the extra electrons that result from heterolysis. Weak bases (the conjugate bases of strong acids) are good leaving groups. Resonance stabilization and inductive effects from electron-withdrawing groups also improve leaving group ability.

·        4.3

1.    Good oxidizing agents have a high affinity for electrons or have high oxidation states. Examples include O2, O3, Cl2, permanganate (MnO4-), chromate (CrO42-), dichromate (CrO72-), and pyridinium chlorochromate. These compounds often contain a metal and a large number of oxygen atoms.

2.    Good reducing agents have low electronegativities and ionization energies or contain a hydride ion (H). Examples include sodium, magnesium, aluminum, zinc, sodium hydride (NaH), calcium dihydride (CaH2), lithium aluminum hydride (LiAlH4), and sodium borohydride (NaBH4). These compounds often contain a metal and a large number of hydrides.

3.    Carbon dioxide, carboxylic acid, ketone, alcohol, methane

·        4.4

1.    The two reactive centers are the carbonyl carbon, which is electrophilic, and the α-hydrogens, which are acidic.

2.    SN1 reactions are most likely to occur on tertiary carbons where a carbocation can be most easily stabilized.

3.    SN2 reactions are most like to occur on methyl or primary carbons because these reactions are easily inhibited by steric hindrance.

·        4.5

1.    1. Know your nomenclature; 2. Identify the functional groups; 3. Identify the other reagents; 4. Identify the most reactive functional group(s); 5. Identify the first step of the reaction; 6. Consider stereoselectivity

2.    If there are no reagents other than the reactants, then the properties of the functional groups on the reactants themselves (acid–base; nucleophile–electrophile) will determine the outcome.