Nucleophilic addition
Hydride adds to acetaldehyde's carbonyl
How the Hydride adds to acetaldehyde's carbonyl works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: NaBH₄ or LiAlH₄ (H⁻ source); then H₃O⁺ workup. Conditions: EtOH (NaBH₄) or dry Et₂O/THF (LiAlH₄), 0–25 °C. Bonds formed: C–H σ; O–H (workup); bonds broken: C=O π. Stereochemistry: Planar C=O: attack from either face; new stereocentre if R groups differ. Rate and selectivity: Aldehydes > ketones > esters in reactivity.
At a glance
Step 1: Polarised carbonyl, strong nucleophile
The carbonyl carbon is electrophilic — oxygen pulls electron density away from it. Hydride (H⁻, delivered by a reducing agent) is a strong nucleophile.
Step 2: Hydride adds, π pair moves to O
Hydride's electron pair attacks the carbonyl carbon. As the new C–H bond forms, the C=O π pair moves entirely onto oxygen, since carbon can't hold five bonds.
Step 3: Alkoxide, then protonation
The result is an alkoxide, which picks up a proton on workup to give ethanol. This is the core step of every carbonyl reduction and Grignard addition.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- THF
- Tetrahydrofuran. Cyclic ether solvent; its oxygen lone pairs solvate Li⁺/Mg²⁺ so organometallics stay dissolved and reactive.
- Et2O
- Diethyl ether. Low-boiling ether solvent, classic for Grignard reagents.
- EtOH
- Ethanol. Polar protic solvent.
- LiAlH4
- Lithium aluminium hydride. Powerful hydride (H⁻) donor; reduces esters, acids, amides to alcohols/amines.
- NaBH4
- Sodium borohydride. Mild hydride donor; reduces aldehydes and ketones but not esters.
How it compares with related reactions
| Hydride adds to acetaldehyde's carbonyl | Acid-catalyzed amide hydrolysis | Acid-catalyzed hydration of an alkene | |
|---|---|---|---|
| Reagents | NaBH₄ or LiAlH₄ (H⁻ source); then H₃O⁺ workup | Amide, H₃O⁺ (HCl or H₂SO₄), H₂O | Dilute H₂SO₄ or H₃PO₄ (H₃O⁺), H₂O |
| Conditions | EtOH (NaBH₄) or dry Et₂O/THF (LiAlH₄), 0–25 °C | 6 M acid, reflux (hours) | 25–100 °C; heat and excess water favour the alcohol |
| Stereochemistry | Planar C=O: attack from either face; new stereocentre if R groups differ | No stereocentre created; α-stereocentres retained | Planar carbocation: racemic; Markovnikov regiochemistry |
| Rate / selectivity | Aldehydes > ketones > esters in reactivity | Water attack / C–N cleavage rate-limiting; far slower than ester hydrolysis | Protonation of the alkene is rate-determining |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Hydride adds to acetaldehyde's carbonyl?
NaBH₄ or LiAlH₄ (H⁻ source); then H₃O⁺ workup. Conditions: EtOH (NaBH₄) or dry Et₂O/THF (LiAlH₄), 0–25 °C.
Which bonds are formed and which are broken?
Formed: C–H σ; O–H (workup). Broken: C=O π.
What is the stereochemical outcome?
Planar C=O: attack from either face; new stereocentre if R groups differ
What controls the rate or the selectivity?
Aldehydes > ketones > esters in reactivity
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go
Related mechanisms
- Acid-catalyzed amide hydrolysis
- Acid-catalyzed hydration of an alkene
- Amide synthesis from an acid chloride and amine
- Base hydrolysis (saponification) of an ester
- Base-catalyzed transesterification
- Br₂ addition to an alkene (vicinal dihalide)
- Ester synthesis from an acid chloride and alcohol
- Fischer esterification