Difficulty: Intermediate | Prerequisites: Part 1 of these notes (alcohol classification, acid-base chemistry), carbonyl basics

Big Picture

Oxidation and reduction are the two most important transformations alcohols undergo, and they are reversible partners of each other. This section covers how to reduce carbonyls to alcohols (using hydride reagents) and how to oxidise alcohols back to carbonyls (using chromium-based reagents). Knowing when to use NaBH4 vs. LiAlH4 vs. Jones reagent vs. PCC is one of the most heavily tested skills in this course. The reagent choice controls the product, and the mechanism explains why.

TL;DR

Carbonyls are reduced to alcohols by NaBH4 (mild, protic solvent, no workup) or LiAlH4 (stronger, aprotic solvent, requires acid workup). Alcohols are oxidised back to carbonyls by chromium reagents: Jones reagent oxidises 1° alcohols all the way to carboxylic acids and 2° alcohols to ketones, while PCC and PDC stop at the aldehyde for 1° alcohols. Tertiary alcohols cannot be oxidised (no H on the carbon to eliminate).

Key Terms

OIL RIG

Oxidation Is Loss, Reduction Is Gain. In organic chemistry, this refers specifically to bonds to hydrogen: oxidation loses H (or gains bonds to O), reduction gains H.

[O] and [R]

Shorthand notation. [O] = oxidation, [R] = reduction.

Nucleophilic hydride

An H⁻ equivalent delivered by a reducing agent (NaBH4 or LiAlH4). Think of it as a hydrogen atom carrying a pair of electrons, ready to attack an electrophilic carbonyl carbon.

NaBH4 (sodium borohydride)

A mild hydride reducing agent. Used in polar protic solvents (MeOH, EtOH). No separate workup step required. In simple terms, the gentler option that tolerates protic solvents.

LiAlH4 (lithium aluminium hydride, LAH)

A stronger, more reactive hydride reducing agent. Must be used in polar aprotic solvents (THF, Et2O). Requires an acidic aqueous workup step (H2O/HCl). Think of it as the heavy-duty version that reacts with protic solvents, so you must keep water away until the workup.

NaH (sodium hydride)

A base only, not a reducing agent. The hydride in NaH is too ionic (least stable) to act as a nucleophile. It simply deprotonates. Do not confuse with NaBH4 or LiAlH4.

Workup step

The acid-water quench (e.g. H2O, HCl) added after a LiAlH4 reduction. Its purpose is to neutralise charged species (alkoxides) so the product alcohol can be isolated. Think of it as converting the metal alkoxide salt into a free alcohol you can actually extract.

Jones reagent (chromic acid)

An aqueous acidic chromium(VI) oxidant. Made from CrO3 + H2O + H2SO4, or Na2Cr2O7 + H2O + H2SO4, or K2Cr2O7 + H2O + H2SO4. Oxidises 2° alcohols to ketones and 1° alcohols all the way to carboxylic acids.

PCC (pyridinium chlorochromate)

A milder chromium-based oxidant used in DCM (dichloromethane). Oxidises 1° alcohols to aldehydes and stops there. In simple terms, the gentle chromium reagent that does not over-oxidise.

PDC (pyridinium dichromate)

Functionally similar to PCC. Also used in DCM, also stops at the aldehyde for 1° alcohols.

Chromate ester

The intermediate formed when the alcohol reacts with the chromium reagent. An E2-type concerted elimination from this intermediate produces the carbonyl. This is why tertiary alcohols do not oxidise: there is no hydrogen on the carbon to eliminate.

Core Content

Hydride Reduction of Carbonyls

The carbonyl carbon is electrophilic because of resonance: the C=O bond has a resonance structure with a positive carbon and negative oxygen. This makes the carbon susceptible to nucleophilic attack by hydride.

Reduction step: The nucleophilic hydride (H⁻) attacks the electrophilic carbonyl carbon, breaking the pi bond and forming a C-H bond. The oxygen picks up a negative charge (alkoxide).

Workup step (for LiAlH4 only): An electrophilic proton from H2O/HCl protonates the alkoxide to give the free alcohol. Do not write H⁻ as the proton source; use H3O⁺ instead.

NaBH4 vs. LiAlH4: Choosing the Right Reagent

NaBH4 (sodium borohydride):

• Polar protic solvent (MeOH, EtOH)

• No workup step needed (solvent protonates the alkoxide)

• Milder reducing agent

• Acts as nucleophile only (not basic enough to deprotonate most functional groups)

• Stability: covalent B-H interaction, most stable of the anionic hydrides

LiAlH4 (LAH):

• Polar aprotic solvent (THF, Et2O), because it reacts violently with protic solvents

• Workup step necessary (H2O, HCl)

• Stronger [R] agent (also basic)

• Acts as both nucleophile and base (base character happens first)

• Stability: middle ground, ionic enough to remove electron density from hydrogen

• Avoids bad ABC by using aprotic solvents

NaH (sodium hydride):

• Base only, never a reducing agent

• Least stable (fully ionic interaction)

• Deprotonates alcohols to alkoxides, does not reduce carbonyls

Summary: Carbonyl Reduction Products by Substrate

• Aldehyde + NaBH4/MeOH → primary alcohol (1°)

• Ketone + LiAlH4/THF, then H2O/HCl → secondary alcohol (2°)

• Tertiary alcohol + LiAlH4 → no reaction (no carbonyl to reduce; you cannot reduce a fully substituted carbon)

• Formaldehyde (H2CO) + NaBH4/MeOH → methanol

Mechanism of NaBH4 Reduction

• Hydride attacks carbonyl carbon (irreversible step).

• The resulting alkoxide is protonated by MeOH (the protic solvent).

• MeOH replaces the delivered hydride on boron, forming MeO-BH3, then (MeO)2BH2, and so on.

• Each NaBH4 molecule can deliver up to 4 hydrides (reducing 4 equivalents of carbonyl).

• Final boron by-product: B(OMe)4⁻

Mechanism of LiAlH4 Reduction

• Hydride from Al-H attacks the carbonyl carbon in THF.

• The resulting alkoxide coordinates to aluminium as a Lewis acid-base pair.

• This repeats 4 times total (one Al delivers 4 hydrides).

• After all reductions, the acidic workup (H2O, HCl) protonates the alkoxide intermediates.

• By-product: Al(OH)4⁻

Deuterium Labelling

Deuterium (D) versions of these reagents (NaBD4, LiAlD4 or LAD) place D exactly where H⁻ would go (on the former carbonyl carbon). The workup proton (H or D from solvent) goes on the oxygen. Knowing the mechanism lets you predict exactly where each isotope ends up, which is a common exam question.

Chromium Oxidation of Alcohols

Reduction is reversible: the reverse of reducing a carbonyl to an alcohol is oxidising an alcohol back to a carbonyl.

Jones oxidation (chromic acid, aqueous acidic conditions):

• 2° alcohol → ketone

• 1° alcohol → carboxylic acid (passes through the aldehyde, but does not stop there)

• 3° alcohol → no reaction (no C-H on the carbinol carbon to eliminate)

Three equivalent recipes for chromic acid: CrO3/H2O/H2SO4, Na2Cr2O7/H2O/H2SO4, K2Cr2O7/H2O/H2SO4. The important reagent in all cases is H3O⁺ in the presence of Cr(VI).

Mechanism of Jones oxidation:

• The alcohol attacks the chromium reagent, displacing water, to form a chromate ester.

• An E2-type concerted elimination removes the C-H and the chromium leaving group simultaneously, producing the carbonyl.

• The key difference from a standard E2: the base here is a weak base (water), not a strong one.

Why 1° alcohols go all the way to carboxylic acids under Jones conditions:

• The aldehyde intermediate forms a hydrate (gem-diol) in the aqueous acidic environment.

• The hydrate looks like an alcohol to the chromium reagent, so it gets oxidised again.

• Result: the aldehyde cannot be isolated; the final product is a carboxylic acid.

Aldehyde Synthesis from Primary Alcohols: PCC and PDC

PCC (pyridinium chlorochromate): Cl-CrO3⁻ with a pyridinium counterion. Used in DCM.

PDC (pyridinium dichromate): (CrO3)2O2²⁻ with two pyridinium counterions. Also used in DCM.

Both stop the oxidation of 1° alcohols at the aldehyde stage. They do not over-oxidise to carboxylic acids.

Why they stop at the aldehyde: The reaction is run in DCM, not aqueous acid. Without water and acid present, the aldehyde cannot form a hydrate, so there is nothing that resembles an alcohol for the reagent to oxidise further.

PCC and PDC are milder alternatives to the Jones reagent. They also oxidise 2° alcohols to ketones, same as Jones.

Selectivity Summary

• PCC or PDC in DCM + a molecule with both a 1° and a 2° alcohol: the 1° gives an aldehyde, the 2° gives a ketone.

• Jones reagent + the same molecule: the 1° goes all the way to a carboxylic acid, the 2° gives a ketone.

• Tertiary -OH is never touched by any of these chromium reagents.

Formulas / Reagent Summary

Real-World Applications

Chromium oxidation of alcohols is one of the foundational transformations in synthetic organic chemistry and pharmaceutical manufacturing. PCC and Jones reagent are classic methods for producing aldehyde and ketone intermediates in drug synthesis. Sodium borohydride reduction is widely used in industrial settings because of its mildness, including in the production of flavour and fragrance chemicals.

Common Misconceptions

• Students frequently confuse NaBH4 with NaH. NaH is a base only. NaBH4 is a nucleophilic reducing agent. They do very different things.

• Students often forget that LiAlH4 requires an aprotic solvent and a separate workup step. Writing "LiAlH4, MeOH" is wrong and would result in a violent reaction.

• A very common mistake is thinking PCC and Jones give the same product from a 1° alcohol. They do not. Jones gives the carboxylic acid; PCC gives the aldehyde.

• Students sometimes think tertiary alcohols can be oxidised. They cannot, because there is no hydrogen on the carbinol carbon to eliminate in the chromate ester intermediate.

Why It Matters / Exam Flags

⚠️ The NaBH4 vs. LiAlH4 reagent choice (solvent, workup, strength) is tested constantly.

⚠️ The deuterium labelling question (where does D end up?) tests whether you understand the mechanism, not just the reagent.

⚠️ Jones reagent vs. PCC/PDC for 1° alcohols is a classic selectivity question. Know that the aqueous environment is what causes over-oxidation.

⚠️ The chromate ester mechanism and the E2-type elimination step explain why 3° alcohols are inert. Expect a "why" question about this.

Quick Self-Test

1. True or false: NaBH4 requires an acidic aqueous workup.

1. Fill in the blank: LiAlH4 must be used in a polar _________ solvent.

1. True or false: PCC oxidises a primary alcohol to a carboxylic acid.

1. Fill in the blank: Tertiary alcohols cannot be oxidised because there is no _________ on the carbinol carbon.

1. True or false: NaH can reduce a ketone to a secondary alcohol.

Answers: 1. False (no workup needed; the protic solvent handles protonation). 2. Aprotic. 3. False (PCC stops at the aldehyde). 4. Hydrogen (C-H bond). 5. False (NaH is a base only).

Practice Q&A

Q: You have a primary alcohol and need to make the corresponding aldehyde. Which reagent and solvent do you choose, and why not Jones reagent?

A: PCC (or PDC) in DCM. Jones reagent would over-oxidise the primary alcohol past the aldehyde to the carboxylic acid, because the aqueous acidic conditions allow the aldehyde to form a hydrate that gets oxidised again.

Q: Draw or describe the product of treating a ketone with NaBD4 in EtOH. Where does the deuterium end up?

A: The deuterium ends up on the former carbonyl carbon (the carbon that was C=O). The -OH proton comes from the EtOH solvent (H, not D). The mechanism delivers D⁻ to the electrophilic carbon, and the protic solvent protonates the resulting alkoxide.

Q: Why does LiAlH4 require a workup step but NaBH4 does not?

A: LiAlH4 is used in aprotic solvent (THF), so there is no proton source to quench the alkoxide intermediate during the reaction. The acid workup (H2O/HCl) protonates the aluminium alkoxide complex. NaBH4 is used in protic solvent (MeOH), which protonates the alkoxide in situ.

Q: A student treats a tertiary alcohol with CrO3, H2O, H2SO4. What happens?

A: No reaction. The oxidation mechanism requires elimination of a C-H bond from the chromate ester intermediate, and a tertiary alcohol has no hydrogen on its carbinol carbon.

Q: Explain the difference in stability and function among NaH, NaBH4, and LiAlH4.

A: NaH has the least stable hydride (fully ionic) and acts as a base only. NaBH4 has the most stable hydride (covalent B-H bonds) and acts as a nucleophile only (mild reducer). LiAlH4 is intermediate in stability, acts as both nucleophile and base, and is the stronger reducing agent.

Connections to Other Topics

• Hydride reduction mechanisms reappear in Chapter 20 with carboxylic acid derivatives (esters, amides).

• Chromium oxidation and the concept of oxidation state connect to the broader redox framework used throughout organic and biological chemistry.

• Deuterium labelling is a general mechanistic probe used in physical organic chemistry and biochemistry.

Related Terms / Search Tags

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