Ch. 4 - Analyzing Organic Reactions

5% of MCAT Organic content

What is a Lewis acid? Lewis base?

Lewis acid: Electron acceptor with vacant orbitals or positively polarized atoms.

Lewis base: Electron donor with a lone pair of electrons, often anions.

Define Brønsted–Lowry acids and bases.

• Acids: Proton donors.

• Bases: Proton acceptors.

What is an amphoteric molecule? Give an example.

molecule that can act as either an acid or base.

ex. water

What is the acid dissociation constant (Ka) and its significance?

• Measures acidity; corresponds to the dissociation of HA into H⁺ and A⁻.

• Lower pKa = stronger acid.

Which functional groups are commonly acidic?

• Alcohols, aldehydes, ketones, carboxylic acids, and derivatives.

• α-Hydrogens near carbonyls are also acidic.

Which functional groups are commonly basic?

Amines and amides.

What is a nucleophile?

• "Nucleus-loving" molecule with lone pairs or π bonds.

• Increased electron density; often negatively charged.

Factors affecting nucleophilicity:

• Charge: More negative → stronger nucleophile.

• Electronegativity: Lower electronegativity → stronger nucleophile.

• Steric hindrance: Less bulky → stronger nucleophile.

• Solvent: Protic solvents hinder nucleophilicity.

What is an electrophile?

• "Electron-loving" molecule with a positive charge or polarization.

• ex. Alcohols, aldehydes, ketones, carboxylic acids.

What makes a good leaving group?

• Can stabilize additional charge via resonance or induction.

• Weak bases (conjugates of strong acids) are good leaving groups.

What is an SN1 reaction?

• Two-step reaction:

  1. Leaving group leaves → forms a carbocation.

  2. Nucleophile attacks carbocation → racemic mixture.

• Prefers more substituted carbons for carbocation stability.

• Rate = k[R–L].

What is an SN2 reaction?

• One-step reaction: Nucleophile attacks as the leaving group leaves.

• Requires a backside attack → inversion of stereochemistry.

• Prefers less substituted carbons (avoids steric hindrance).

• Rate = k[Nu:][R–L].

What is oxidation, and what assists it?

• Increase in oxidation state.

• Assisted by oxidizing agents that accept electrons.

  • ex: CrO₃, PCC, Na₂Cr₂O₇.

What is reduction, and what assists it?

• Decrease in oxidation state.

• Assisted by reducing agents that donate electrons.

  • ex: LiAlH₄.

What functional groups are most oxidized?

Carboxylic acids > aldehydes/ketones > alcohols.

What is chemoselectivity?

Reactions tend to target the most oxidized functional group.

How can functional groups be protected?

• Diols protect aldehydes/ketones.

• Alcohols can be converted to tert-butyl ethers.

Steps to solving reaction problems:

1. Know the nomenclature.

2. Identify the functional groups.

3. Identify the reagents.

4. Determine the most reactive functional group.

5. Identify the first step of the reaction.

6. Consider stereoselectivity.