REAGENT RECOGNITION EXAM 3

LiAlH₄ (LAH)

Used for:
Strong reduction of carbonyl compounds.

Converts:

• Aldehyde → 1° alcohol

• Ketone → 2° alcohol

• Ester → 1° alcohol

• Carboxylic acid → 1° alcohol

Important:

• Strong reducing agent

• Goes all the way to alcohol

• Used in dry ether (Et₂O), then H₂O workup

NaBH₄

Used for:
Reducing aldehydes and ketones.

Converts:

• Aldehyde → 1° alcohol

• Ketone → 2° alcohol

Does NOT reduce:

• Esters

• Carboxylic acids

PCC

Used for:
Oxidizing alcohols without overoxidation.

Converts:

• 1° alcohol → Aldehyde

• 2° alcohol → Ketone

Important:
Stops at the aldehyde.

Chromic Acid (H₂CrO₄)

Used for:
Strong oxidation of alcohols and aldehydes.

Converts:

• 1° alcohol → Carboxylic acid

• 2° alcohol → Ketone

• Aldehyde → Carboxylic acid

Hot KMnO₄

Used for:
Strong oxidation.

Converts:

• 1° alcohol → Carboxylic acid

• 2° alcohol → Ketone

• Aldehyde → Carboxylic acid

Swern Oxidation

Used for:
Mild oxidation of alcohols.

Converts:

• 1° alcohol → Aldehyde

• 2° alcohol → Ketone

Important:
Stops before carboxylic acid.

DIBAL-H

Used for:
Selective reduction to aldehydes.

Converts:

• Ester → Aldehyde

• Nitrile → Aldehyde

Important:

• Stops at aldehyde

• Usually used at −78°C

LiAlH(O-t-Bu)₃

Used for:
Selective reduction of acid chlorides.

Converts:

• Acid chloride → Aldehyde

Important:
Mild enough to stop at the aldehyde.

Grignard Reagent (RMgX)

Used for:
Making new carbon-carbon (C–C) bonds.

Converts:

• Formaldehyde → 1° alcohol

• Aldehyde → 2° alcohol

• Ketone → 3° alcohol

• Nitrile → Ketone (after H₃O⁺ workup)

Important:

• Strong nucleophile/base

• Cannot be used with acidic hydrogens (H₂O, ROH, RCOOH, etc.)

Organolithium (RLi)

Used for:
Making new carbon-carbon (C–C) bonds.

Converts:
Same reactions as Grignard reagents.

Important:
Even more reactive than Grignard reagents.

HCN

Used for:
Forming cyanohydrins.

Converts:

• Aldehyde/Ketone → Cyanohydrin

Important:
Adds OH and CN to the carbonyl carbon.

ROH + H⁺

Used for:
Forming hemiacetals.

Converts:

• Aldehyde/Ketone → Hemiacetal

Important:
Uses 1 equivalent of alcohol.

Excess ROH + H⁺

Used for:
Forming acetals.

Converts:

• Hemiacetal → Acetal

Important:
Produces 2 OR groups on the same carbon.

HOCH₂CH₂OH + H⁺ (Ethylene glycol)

Used for:
Making a cyclic acetal.

Converts:

• Aldehyde/Ketone → Cyclic acetal

Important:
Used as a protecting group for carbonyls.

H₃O⁺ / H₂O

Used for:
Acid workup or removing protecting groups.

Converts:

• Alkoxide → Alcohol

• Cyclic acetal → Aldehyde/Ketone

Tollens' Reagent (Ag(NH₃)₂⁺)

Used for:
Testing for aldehydes.

Converts:

• Aldehyde → Carboxylic acid

Important:
Positive test = silver mirror.
Ketones = no reaction.

Ag₂O, OH⁻

Used for:
Oxidizing aldehydes.

Converts:

• Aldehyde → Carboxylic acid

X₂ + OH⁻ (Cl₂, Br₂, or I₂)

sed for:
Base-promoted α-halogenation.

Converts:

• α-H → α-X

Important:

• Forms an enolate first.

• With a methyl ketone, gives the haloform reaction (carboxylate + CHX₃).

X₂ + H⁺

Used for:
Acid-catalyzed α-halogenation.

Converts:

• α-H → α-X

Important:
Forms an enol first.
Usually only one halogen is added.

H⁺

Used for:
Acid catalysis.

Converts:

• Carbonyl → Protonated carbonyl (oxonium ion)

• Carbonyl ⇌ Enol

Important:
Starts:

• Acid-catalyzed halogenation

• Hemiacetal/acetal formation

• Cyclic acetal formation

Heat (Δ)

Used for:
Dehydration.

Converts:

• β-Hydroxy aldehyde/ketone → α,β-unsaturated aldehyde/ketone

Important:
Removes H₂O and forms a conjugated product.

How do I choose the correct reagent?

• Reduce everything to an alcohol? → LAH

• Reduce only an aldehyde/ketone? → NaBH₄

• Oxidize to an aldehyde? → PCC or Swern

• Oxidize to a carboxylic acid? → Chromic acid or KMnO₄

• Make an aldehyde from an ester/nitrile? → DIBAL-H

• Make an aldehyde from an acid chloride? → LiAlH(O-t-Bu)₃

• Make a new C–C bond? → Grignard (RMgX) or RLi

• Make a cyanohydrin? → HCN

• Make a hemiacetal? → ROH + H⁺

• Make an acetal? → Excess ROH + H⁺

• Protect a carbonyl? → Ethylene glycol + H⁺

• Remove a protecting group? → H₃O⁺/H₂O

• Test for an aldehyde? → Tollens' reagent

• Halogenate under basic conditions? → X₂ + OH⁻

• Halogenate under acidic conditions? → X₂ + H⁺

• Dehydrate an aldol product? → Heat (Δ)