Organic Chemistry 2 · Reaction Mechanism

Oxidation, Reduction, and Alcohol Protecting Groups

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In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Study tools

In 30 seconds

In organic chemistry, oxidation increases carbon-oxygen bonding (or decreases carbon-hydrogen bonding) while reduction does the reverse. Aldehydes and ketones are reduced to alcohols with (mild and selective) or (strong, and it also reduces esters, acids, amides, and epoxides). Primary alcohols oxidize to aldehydes with , Swern, or Dess-Martin conditions and all the way to carboxylic acids with strong chromium oxidants; secondary alcohols give ketones; and tertiary alcohols do not oxidize. Alcohol protecting groups such as silyl ethers temporarily mask the acidic O-H so strong bases and organometallic reagents can react elsewhere, and are removed later with fluoride or mild acid.

Why this matters

Oxidation-state control is central to metabolism and drug metabolism. In the liver, cytochrome P450 enzymes catalyze hydroxylation, a biological oxidation that turns lipophilic drugs into more polar, more readily excreted metabolites; ethanol itself is oxidized through acetaldehyde to acetate. The same selectivity logic a medicinal chemist uses to choose PCC versus chromic acid is what drug-metabolism scientists use to predict where a candidate molecule will be oxidized. Silyl protecting groups are a workhorse of pharmaceutical process chemistry, letting chemists assemble complex drug scaffolds while keeping reactive alcohol groups out of the way.

The college version

1. Oxidation states and the oxidation/reduction ledger

For organic reactions, set aside half-cell potentials. Judge oxidation level by counting bonds to oxygen versus bonds to hydrogen on a given carbon. Adding a C-O bond or losing a C-H bond is oxidation; the reverse is reduction. This gives a clean ladder for single-carbon units: alkane below alcohol below aldehyde/ketone below carboxylic acid below carbon dioxide.

2. Aldehyde and ketone reduction

Aldehyde and ketone reduction converts a carbonyl into an alcohol. Both NaBH4 and LiAlH4 deliver hydride (H^-) to a carbonyl carbon, converting the C=O into an alcohol. NaBH4 is mild and works in protic solvents such as methanol or ethanol; it reduces aldehydes and ketones but leaves esters, carboxylic acids, and amides untouched, which makes it selective. LiAlH4 is far more reactive: it also reduces esters, carboxylic acids, amides, and even epoxides, and it must be used in anhydrous ether solvents (THF or diethyl ether) because it reacts violently with water and alcohols.

3. Primary and secondary alcohol oxidation (and knowing when it stops)

Primary alcohol oxidation and secondary alcohol oxidation follow different endpoints:

Primary alcohol to aldehyde with PCC, Swern, or Dess-Martin conditions (anhydrous, they stop at the aldehyde).
Primary alcohol to carboxylic acid with strong chromium oxidants such as Jones reagent (chromic acid), which overoxidize through the aldehyde hydrate.
Secondary alcohol to ketone with any of the above; there is no risk because a ketone has no oxidizable hydrogen on the carbonyl carbon.
Tertiary alcohol gives no reaction, because there is no carbon-hydrogen bond on the carbinol carbon to remove.

How it works

Choose a target transformation and list every functional group present.
Pick a reductant or oxidant whose functional-group tolerance matches those groups.
If a strong base or nucleophile is required, protect acidic O-H groups as silyl ethers first.
Run the key transformation.
Deprotect under fluoride or mild acid to restore the alcohol.
Confirm the product by spectroscopy before proceeding.

Common confusions

Do not confuse	With	Difference
NaBH4	LiAlH4	NaBH4 is mild and alcohol-tolerant; LiAlH4 is strong and water-intolerant
Aldehyde product	Carboxylic acid product	Mild, anhydrous oxidants give the aldehyde; aqueous Cr(VI) gives the acid
PCC	Jones/chromic acid	PCC stops at the aldehyde; Jones overoxidizes to the acid
Secondary alcohol	Tertiary alcohol	Secondary oxidizes to a ketone; tertiary does not oxidize
Protecting group	Leaving group	A protecting group is removed later to restore the original group; a leaving group departs permanently
Deprotection	Deprotonation	Deprotection removes a protecting group; deprotonation removes a proton

Memory aids

"OIL RIG for carbon: Oxidation Is Loss (of H, or gain of O); Reduction Is Gain (of H, or loss of O)." For reagent choice, remember "PCC Pauses, Chromic acid Continues" - mild oxidants pause at the aldehyde, chromium continues to the acid.

Quick review

Topic Recap

increases C-O bonding and reduction increases C-H bonding. Aldehydes and ketones are reduced with NaBH4 or LiAlH4, the latter being far stronger. Alcohol oxidation products are set by the oxidant: mild, anhydrous reagents (PCC, Swern, Dess-Martin) give aldehydes from primary alcohols and ketones from secondary alcohols, while aqueous chromium(VI) overoxidizes primary alcohols to carboxylic acids; tertiary alcohols are inert. Silyl protecting groups mask acidic O-H groups during base-sensitive steps and are removed with fluoride or acid.

Knowledge Check

Which reagent would you choose to convert a primary alcohol to an aldehyde without overoxidation?
Why does LiAlH4, but not NaBH4, reduce esters and carboxylic acids?
What product forms when a tertiary alcohol is treated with PCC?
Why must LiAlH4 reactions be run in anhydrous ether solvents?
How does a TMS protect an alcohol against a Grignard reagent?

Answers and Rationales

PCC, Swern, or Dess-Martin conditions; they are anhydrous, so the aldehyde cannot hydrate and overoxidize to the acid.
LiAlH4 is a much stronger, more reactive hydride source than NaBH4, so it can attack the less electrophilic ester/acid carbonyl, whereas NaBH4 is too mild for those substrates.
No reaction; tertiary alcohols lack a C-H bond on the carbinol carbon to be removed in the oxidation step.
LiAlH4 reacts violently with water and protic solvents, releasing hydrogen gas and destroying the reagent, so strictly dry ether solvents are required.
The silyl ether replaces the acidic O-H proton, so the Grignard cannot be protonated (quenched) by the alcohol and can instead react at the intended carbonyl.

Eli explains

The same idea, in plain words

Explain it like I’m 10

Oxidation and reduction are a bookkeeping ledger for carbon. Each time a carbon gains a bond to oxygen, it gives up some electron density and its oxidation state goes up; each time it gains a bond to hydrogen, it gains electron density and its oxidation state goes down. A carbon bonded only to hydrogens (methane, CH₄) is the most reduced; a carbon bonded to two or three oxygens (a carboxylic acid or carbon dioxide) is the most oxidized.

Think of it like a staircase. Each step up the staircase adds an oxygen or removes a hydrogen: alkane to alcohol to aldehyde/ketone to carboxylic acid. Reagents are the movers that push you up (oxidants) or down (reductants) the staircase. The skill in synthesis is picking a mover that stops on exactly the floor you want, such as a mild oxidant that lands on the aldehyde floor without overshooting to the acid floor.

Where it stops being exact: the staircase is a useful ranking, but a formal oxidation number for carbon in every molecule can be fiddly, and the same reagent can behave differently depending on the solvent, temperature, and what other functional groups are nearby. The model guides predictions; the detailed outcome always depends on the real conditions.

Simple Example

Ethanol (CH₃CH₂OH) is a primary alcohol. Treating it with PCC in dichloromethane removes two hydrogens and gives acetaldehyde (CH₃CHO), an aldehyde. Treating the same ethanol with strong aqueous chromic acid instead gives acetic acid (CH₃COOH), because the aldehyde formed in situ hydrates and oxidizes further. Same starting material, two different movers, two different floors.

Worked example

The overoxidation pathway shows why mild, anhydrous oxidants are needed to make aldehydes:

In aqueous chromium(VI) media, the primary alcohol coordinates to chromium and loses a hydrogen, forming the aldehyde.
Water is present, so the aldehyde establishes an equilibrium with its gem-diol hydrate.
Only the hydrate has the two C-O bonds that allow a second oxidation; the hydrate's C-H is removed and the carboxylic acid forms.
Because the equilibrium continuously feeds hydrate back into the pool, the reaction runs to the acid. PCC, Swern, and Dess-Martin conditions exclude water, so step 2 never occurs and the aldehyde accumulates as the product.

For protecting-group chemistry, the key events are: the alcohol O-H is deprotonated by a base such as imidazole; the alkoxide attacks the electrophilic silicon of a chlorosilane such as TMSCl; chloride leaves; and a stable silyl ether (R-O-SiR'₃) forms. The O-H proton is now gone, so strong bases (organolithiums, Grignards) cannot quench themselves against it. Deprotection reverses this with fluoride (TBAF), which attacks silicon because of silicon's strong Si-F bond, liberating the free alcohol.

Key takeaways

High yield: NaBH4 reduces aldehydes and ketones but NOT esters or acids; LiAlH4 reduces all of them.
High yield: PCC, Swern, and Dess-Martin stop primary alcohols at the aldehyde because they are anhydrous.
High yield: Aqueous chromium(VI) oxidizes primary alcohols all the way to carboxylic acids via the hydrate.
High yield: Secondary alcohols always give ketones; tertiary alcohols do not oxidize.
Silyl ethers protect alcohols against strong bases and organometallic reagents.
Deprotection of silyl ethers uses fluoride (TBAF) or mild acid.
LiAlH4 requires strictly anhydrous ether solvents.
A reaction-selection strategy matches the reagent's functional-group tolerance to every group present in the substrate.
Reactive-reagent safety: LiAlH4 and chromium(VI) are hazardous; handle only under approved institutional procedures.

Keep learning

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Study tools & related lessonsYou’ll learn to · Key vocabulary · Related

You’ll learn to

Define oxidation and reduction for organic molecules in terms of bonds to oxygen and hydrogen, and track oxidation-state trends across a series of carbon compounds.
Compare NaBH4 and LiAlH4 as carbonyl reductants, including which functional groups each can reduce and why their reaction conditions differ.
Predict the oxidation product of primary, secondary, and tertiary alcohols using PCC, Swern, Dess-Martin, or chromium-based oxidants, and explain overoxidation.
Describe why and how alcohols are protected as silyl ethers (TMS), when protection is needed, and how the protecting group is removed.

Key vocabulary

Organic oxidation: Increasing C-O bonding or decreasing C-H bonding
Organic reduction: Increasing C-H bonding or decreasing C-O bonding
Oxidation state: A bookkeeping number for electron ownership on carbon
NaBH4: Mild hydride reductant usable in alcohol solvents
LiAlH4: Strong hydride reductant, anhydrous only
PCC: Pyridinium chlorochromate, a mild oxidant
Swern oxidation: DMSO plus oxalyl chloride plus base at low temperature
Dess-Martin periodinane: Mild, neutral hypervalent-iodine oxidant
Chromium-based oxidants: Jones/chromic acid reagents
Overoxidation: Oxidation proceeding past the desired product
Protecting group: A temporary mask on a reactive functional group
Silyl ether: An R-O-SiR'₃ derivative of an alcohol
TMS protection: Attaching a trimethylsilyl group to oxygen
Deprotection: Removing the protecting group

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