19.7 Nucleophilic Addition of Hydride and Grignard Reagents: Alcohol Formation

Carbonyl compounds readily undergo nucleophilic addition, making them central to building new C–H and C–C bonds. Two of the most important versions of this reaction use hydride reagents (like $NaBH_4$) and Grignard reagents ($RMgX$) to convert aldehydes and ketones into alcohols. Understanding how each nucleophile attacks, what intermediate forms, and what alcohol class you end up with is the core of this topic.

Nucleophilic Addition to Carbonyls

Mechanism of carbonyl reduction

Sodium borohydride ($NaBH_4$) delivers a nucleophilic hydride ($H^-$) to the electrophilic carbonyl carbon. The general mechanism has three key steps:

1. The $H^-$ from $NaBH_4$ attacks the electrophilic carbonyl carbon.
2. The $\pi$ bond breaks, electrons shift onto oxygen, and a tetrahedral alkoxide intermediate forms.
3. During aqueous acid workup, the alkoxide is protonated to give the final alcohol product.

A few things to keep straight about $NaBH_4$ reductions:

• Aldehydes are reduced to primary (1°) alcohols (e.g., benzaldehyde → benzyl alcohol).
• Ketones are reduced to secondary (2°) alcohols (e.g., acetophenone → 1-phenylethan-1-ol).
• $NaBH_4$ is selective: it does not reduce carboxylic acids or esters under standard conditions (e.g., benzoic acid and ethyl benzoate are left untouched).
• The reaction is second-order overall (first-order in both the carbonyl substrate and the hydride source).

Grignard reactions with carbonyls

Grignard reagents have the general formula $RMgX$ (where $X$ = Cl, Br, or I). The carbon–magnesium bond is highly polarized, so the $R$ group behaves as a strongly nucleophilic carbanion ($R^-$). Common $R$ groups include methyl, ethyl, phenyl, and vinyl.

The mechanism parallels hydride reduction, but a carbon nucleophile adds instead of $H^-$:

1. The carbanion $R^-$ attacks the electrophilic carbonyl carbon.
2. The $\pi$ bond breaks, forming a tetrahedral alkoxide intermediate (now bearing a new C–C bond and a $MgX$ counterion on oxygen).
3. Aqueous acid workup protonates the alkoxide and washes away the magnesium salts, yielding the alcohol.

The class of alcohol you get depends on the starting carbonyl:

• Formaldehyde ($HCHO$) + $RMgX$ → primary (1°) alcohol
• Other aldehydes ($RCHO$) + $R'MgX$ → secondary (2°) alcohol (e.g., propanal + $CH_3MgBr$ → 2-butanol)
• Ketones + $RMgX$ → tertiary (3°) alcohol (e.g., acetone + $CH_3CH_2MgBr$ → 2-methyl-2-butanol)

Because Grignard reagents are powerful bases and nucleophiles, they react violently with water and other protic sources. That's why Grignard reactions must be run under anhydrous conditions (dry solvents like diethyl ether or THF) before the aqueous workup step.

Hydride vs Grignard nucleophilic additions

Both reactions follow the same core pattern: nucleophilic attack on the carbonyl carbon → tetrahedral alkoxide intermediate → protonation during workup → alcohol product. The differences come down to what attacks and what product class results.

For example, starting from ethanal ($CH_3CHO$):

• $NaBH_4$ reduction gives ethanol (1° alcohol).
• Reaction with $PhMgBr$ gives 1-phenylethan-1-ol (2° alcohol).

Stereochemistry and oxidation state considerations

When a nucleophile adds to a carbonyl, the carbon goes from trigonal planar ($sp^2$) to tetrahedral ($sp^3$). If the four groups on that new tetrahedral carbon are all different, a new stereocenter is created.

With simple, unstrained ketones or aldehydes, the nucleophile can attack from either face of the planar carbonyl equally. This produces a racemic mixture (equal amounts of R and S enantiomers). Steric or electronic bias from nearby groups can sometimes favor one face, but for most problems at this level, expect a racemic product unless told otherwise.

The oxidation state of the carbonyl carbon also decreases during these reactions. An aldehyde carbon (oxidation state roughly +1) is reduced to an alcohol carbon (oxidation state roughly –1 for a 1° alcohol). The key idea: adding $H^-$ or $R^-$ to a carbonyl is a reduction of that carbon, since you're adding bonds to less electronegative atoms (H or C) and breaking a bond to the more electronegative oxygen.