Reactions of Alcohols

Alcohols are versatile functional groups that participate in a range of important reactions: they can be oxidized to aldehydes, ketones, or carboxylic acids depending on their structure and the reagent used; their otherwise-poor hydroxyl leaving group can be upgraded for substitution reactions; and they can protect reactive carbonyl groups as acetals and ketals. This article walks through each of these transformations, closing with a full worked example of a protecting-group strategy in action.

Key Takeaways

• Primary alcohols: PCC oxidizes them only to an aldehyde; stronger reagents (chromic acid, Jones oxidation) push them all the way to a carboxylic acid.

• Secondary alcohols oxidize to ketones (e.g., with Na₂Cr₂O₇) and can't go any further.

• Tertiary alcohols don't oxidize under normal conditions — no hydrogen on the carbinol carbon for the oxidant to remove.

• The hydroxyl group is a poor leaving group; converting an alcohol to a mesylate or tosylate creates a resonance-stabilized leaving group suitable for SN1/SN2 substitution.

• Acetals (from aldehydes) and ketals (from ketones) form when a carbonyl reacts with two equivalents of alcohol or one diol under acid catalysis — a reversible way to protect a reactive carbonyl.

• Acetals/ketals are stable to base and to reducing/nucleophilic reagents; treatment with aqueous acid deprotects them back to the original carbonyl.

• Protecting groups enable selective reactions: protecting a ketone as a ketal lets LiAlH₄ reduce a co-existing ester without also reducing the ketone.

Oxidizing Primary Alcohols: Aldehydes vs. Carboxylic Acids

Primary alcohols (RCH₂OH) can be oxidized to two different products depending on how strong the oxidizing conditions are.

Treated with pyridinium chlorochromate (PCC), a primary alcohol is oxidized only as far as an aldehyde (RCHO). PCC is a mild, anhydrous oxidant that selectively stops at the aldehyde stage rather than continuing to a carboxylic acid.

Under stronger, aqueous oxidizing conditions — chromic acid (H₂CrO₄) or Jones oxidation (CrO₃, H₂SO₄ in acetone) — the same primary alcohol is oxidized all the way to a carboxylic acid (RCOOH). This gives chemists control: the choice of reagent alone determines whether a primary alcohol stops at the aldehyde or continues to the carboxylic acid.

Oxidizing Secondary and Tertiary Alcohols

Secondary alcohols (R₂CHOH) are oxidized to ketones (R₂C=O), commonly using dichromate salts (Na₂Cr₂O₇ in acidic solution) as the oxidizing agent. Unlike primary alcohols, secondary alcohols can't be pushed any further — once oxidized to a ketone, there's no additional hydrogen on that carbon left to remove, so the reaction stops there regardless of how strong the oxidant is.

Tertiary alcohols (R₃COH) don't oxidize at all under normal conditions. The carbon bearing the hydroxyl group has no hydrogen atom attached to it, and oxidation requires removing a hydrogen from that carbon — without one available, there's nothing for the oxidizing agent to act on.

Turning a Poor Leaving Group into a Good One: Mesylates and Tosylates

In its native form, the hydroxyl group is a poor leaving group — it doesn't readily depart during nucleophilic substitution reactions like SN1 or SN2, which limits how useful alcohols are as substitution substrates on their own.

To get around this, alcohols can be converted into:

• Mesylates (-OSO₂CH₃) — methanesulfonate esters

• Tosylates (-OSO₂C₆H₄CH₃) — para-toluenesulfonate esters

Both are far better leaving groups than a plain hydroxyl group. When either one departs, the resulting sulfonate anion delocalizes its negative charge by resonance across multiple oxygen atoms, making it a highly stable, weak conjugate base — and a stable conjugate base means an easy-to-displace leaving group. Converting an alcohol to a mesylate or tosylate makes it far more reactive toward nucleophiles in substitution reactions.

Protecting Carbonyls with Acetals and Ketals

Alcohols also play a key role as protecting groups for carbonyl chemistry. Aldehydes and ketones are highly reactive at their carbonyl carbon, which makes them vulnerable to unwanted side reactions whenever a chemist needs to run a separate reaction elsewhere in the same molecule.

To temporarily shield a carbonyl, it's reacted with two equivalents of an alcohol or one diol (a molecule with two hydroxyl groups), under acid catalysis. This forms:

• An acetal, from an aldehyde

• A ketal, from a ketone

Acetals and ketals are stable under basic and neutral conditions, effectively hiding the reactive carbonyl from reagents that would otherwise attack it.

When the carbonyl is needed again, the protecting group is removed by treating the compound with aqueous acid, which hydrolyzes the acetal or ketal back into the original aldehyde or ketone.

Worked Example: Selective Reduction Using a Protecting Group

Protecting groups matter most when a molecule has two functional groups that would otherwise both react with the same reagent. Consider a molecule that contains both a ketone and an ester, where the goal is to reduce only the ester.

Step 1 — Protect the ketone. Because ketones are readily reduced, the ketone is first protected before any reduction takes place. Reacting the ketone with a diol in the presence of an acid catalyst (H⁺) converts it into a ketal, which is stable under the reductive conditions coming next.

Step 2 — Reduce the ester. With the ketone safely protected, the compound is treated with lithium aluminum hydride (LiAlH₄), which reduces the ester group to a primary alcohol. The ketal is untouched throughout this step, since it's no longer an electrophilic carbonyl carbon for LiAlH₄ to attack.

Step 3 — Deprotect the ketal. Once the reduction is complete, the ketal is hydrolyzed back to the original ketone by treatment with aqueous acid (H₂O, H⁺), reversing the protection step from Step 1.

The final product contains both a ketone (regenerated) and a primary alcohol (from the reduced ester) — a selective reduction of just the ester. Without the protecting-group step, treating the original molecule directly with LiAlH₄ would have reduced both the ketone and the ester.

Common MCAT Mistakes

• Assuming a strong oxidant always over-oxidizes a primary alcohol. The product is reagent-dependent, not just alcohol-dependent — PCC deliberately stops at the aldehyde, while chromic acid/Jones oxidation push all the way to the carboxylic acid.

• Forgetting why tertiary alcohols resist oxidation. It's not that they're chemically inert — the carbinol carbon simply has no hydrogen for the oxidant to remove, so there's no mechanistic pathway to a carbonyl.

• Treating mesylates/tosylates as changing the alcohol's reactivity toward oxidation. They're leaving-group upgrades for substitution chemistry (SN1/SN2), not oxidation reagents — don't confuse the two reaction categories.

• Losing track of protecting-group order of operations. A ketal must be installed before the incompatible reagent (like LiAlH₄) is added, and removed only afterward with aqueous acid — reversing that order defeats the point of protecting the group at all.

MCAT-Style Concept Check

Question: A chemist treats a primary alcohol with PCC. What is the expected product, and why does the reaction stop there?

• A) A carboxylic acid, because PCC is a strong aqueous oxidant

• B) An aldehyde, because PCC is a mild, anhydrous oxidant that doesn't push the oxidation further

• C) A ketone, because PCC removes a hydrogen from the adjacent carbon

• D) No reaction, because primary alcohols lack a hydrogen on the carbinol carbon

Answer: B

Explanation: PCC (pyridinium chlorochromate) is a mild, anhydrous oxidizing agent that oxidizes a primary alcohol only as far as the aldehyde stage. Reaching the carboxylic acid requires a stronger, aqueous oxidant such as chromic acid or Jones oxidation (CrO₃, H₂SO₄ in acetone). Option A describes the aqueous-oxidant outcome, not PCC's; option C misapplies the ketone-forming pathway, which belongs to secondary alcohols; option D is incorrect since primary alcohols do have a carbinol hydrogen, which is exactly what makes oxidation possible in the first place.

FAQ

Why does PCC stop at the aldehyde while chromic acid oxidizes all the way to a carboxylic acid?

PCC is a mild, anhydrous oxidant, so it lacks the conditions needed to push the reaction past the aldehyde stage. Chromic acid and Jones oxidation are stronger, aqueous oxidizing conditions that continue oxidizing the aldehyde intermediate all the way to a carboxylic acid.

Why can't secondary or tertiary alcohols be oxidized to carboxylic acids?

Secondary alcohols oxidize only to ketones because, once that carbon has been converted to a carbonyl, there's no remaining hydrogen on it to remove for further oxidation. Tertiary alcohols can't oxidize at all, since the carbinol carbon never had a hydrogen to begin with.

Why convert an alcohol to a mesylate or tosylate before a substitution reaction?

The hydroxyl group is a poor leaving group and resists departing during SN1/SN2 reactions. Converting it to a mesylate or tosylate creates a resonance-stabilized sulfonate leaving group that departs far more readily, making the carbon a viable substitution substrate.

What's the difference between an acetal and a ketal, and why are they useful?

An acetal forms from an aldehyde and a ketal forms from a ketone, both by reaction with two equivalents of alcohol (or one diol) under acid catalysis. Both are stable under basic/neutral conditions, letting a chemist temporarily "hide" a reactive carbonyl from other reagents, then regenerate it later with aqueous acid.