Nucleophilic Addition Mechanism

Every reaction of aldehydes and ketones with nucleophiles in this chapter follows the same mechanistic template. Once you know this template, the rest of the chapter is just learning which specific nucleophiles attack and what additional steps follow. Sections 6.5 through 6.10 all apply the template with different nucleophiles. Knowing the template is 80% of the work.

The Universal Three-Step Template

Step 1: Nucleophile attacks the carbonyl carbon. The nucleophile’s lone pair (or bond electrons for anionic reagents like hydride) flows to the electrophilic C. The pi bond of the C=O breaks, and the electrons flow onto the oxygen, making it an alkoxide (negatively charged oxygen).

Arrows: one from Nu’s lone pair → C, one from the C=O pi bond → O.

Result: a tetrahedral alkoxide intermediate - the carbon is now sp³ with four bonds (two original R groups, the new bond to Nu, and the O⁻).

Step 2: Protonation. The alkoxide oxygen picks up a proton from wherever one is available (solvent, acid catalyst, conjugate acid of the nucleophile).

Arrow: from alkoxide’s lone pair → H.

Result: a neutral tetrahedral alcohol with Nu attached to what was the carbonyl carbon.

Step 3 (reaction-specific): further transformations. Some additions stop here (hydride, Grignard). Others go further (hemiacetal → acetal, hemiaminal → imine, cyanohydrin stays as an alcohol-nitrile).

Watch the addition play out below. The Bürgi-Dunitz trajectory (107° from the C=O axis) is the canonical approach angle. Switch between nucleophiles to see how hydride, Grignard, alkoxide, water, and amines each take the same template to a different product.

Under Base Catalysis

When the nucleophile is already a good anion (hydroxide, alkoxide, hydride from NaBH₄, carbanion from Grignard), no acid is needed. The neutral nucleophile attacks, oxygen picks up a proton from solvent during aqueous workup, and you are done.

Base catalysis essentially means: use an already-activated anionic nucleophile. The mechanism: attack → alkoxide → protonation on workup.

Under Acid Catalysis

When the nucleophile is weak and neutral (water, alcohol, amine), acid catalysis speeds up the reaction dramatically. The mechanism shifts to:

Step 1a: Protonate the carbonyl oxygen. The oxygen’s lone pair accepts an H⁺, forming a protonated carbonyl (C=OH⁺). This gives the carbon even more δ⁺ character (essentially an oxocarbenium ion), making it a stronger electrophile.

Step 1b: Neutral nucleophile attacks the activated carbonyl. Weak nucleophiles can now attack because the electrophile is much stronger. The pi bond breaks, oxygen becomes neutral (it already accepted a proton, so now after pi collapse it is a neutral O-H).

Step 2: Deprotonation of the Nu. The newly-added nucleophile carries a proton that needs to leave. A water molecule or other base removes it.

Acid catalysis is essential for water addition to carbonyls, hemiacetal/acetal formation, imine formation, and most equilibrium-driven carbonyl reactions.

Why Addition Is Favorable

The net reaction converts a C=O pi bond (~180 kJ/mol) into a new C-Nu sigma bond (~350 kJ/mol) and a C-OH single bond (~360 kJ/mol after protonation). We trade one pi bond (~180 kJ/mol) for two sigma bonds (~700 kJ/mol combined). That is a large exothermic shift.

The reaction is reversible if the nucleophile is a good leaving group and conditions permit reversal. Water addition to aldehydes, for example, is reversible and generally lies toward the starting materials for most ketones (see Section 6.5).

Kinetics and Substrate Preference

Aldehydes react faster than ketones because:

1. Aldehydes are more electrophilic (one alkyl donor vs. two).
2. Aldehydes are less hindered sterically (one H + one R vs. two R).

Rate order: formaldehyde > aldehyde > ketone.

Within ketones, smaller ketones react faster than larger ones (less steric hindrance). Acetone is faster than di-isopropyl ketone.

Stereochemistry of Addition

If the nucleophile attacks a flat carbonyl (sp² carbon), it approaches from either face. For a prochiral ketone (two different alkyl groups plus two different faces), this produces a racemic mixture - equal amounts of each stereoisomer.

To get a single stereoisomer, you need either:

1. A pre-existing chiral environment in the substrate (diastereotopic faces).
2. A chiral nucleophile or a chiral catalyst (asymmetric synthesis, usually beyond MCAT scope).

For achiral substrates with achiral nucleophiles, expect racemic product mixtures. This is why Grignard additions to ketones usually yield racemic tertiary alcohols.

The Tetrahedral Intermediate and Its Fate

The tetrahedral alkoxide intermediate is the key branch point:

• If it protonates cleanly → you get the alcohol product (most Grignard, hydride additions).
• If a leaving group is attached to it (like an OR or Cl) → it can collapse to kick out the leaving group and re-form a new carbonyl. This is the pattern for carboxylic acid derivatives (Ch 9) - aldehydes and ketones do not have this option because they have no good leaving group.
• If water is lost → gives an imine (when amine is nucleophile) or an enol (when another oxygen is in the mix). This is the pattern for imine formation and acetal formation.

Keeping track of whether the tetrahedral intermediate keeps all its attachments, loses water, or ejects a leaving group is the key to reading any carbonyl mechanism.