Removing an alcohol group (a hydroxyl, or -OH) from an organic molecule is one of the most common transformations in chemistry. The approach you choose depends on what you want in its place: a double bond, a plain C-H bond, or a different functional group entirely. Here are the major methods, how they work, and when to use each one.
Acid-Catalyzed Dehydration: Replacing -OH With a Double Bond
The most straightforward way to remove an alcohol group is dehydration: you eliminate the -OH along with a hydrogen atom from the neighboring carbon, forming a carbon-carbon double bond (an alkene) and water. This is typically done by heating the alcohol with a concentrated strong acid such as sulfuric acid or phosphoric acid.
Not all alcohols dehydrate equally well. Tertiary alcohols, where the carbon bearing the -OH is attached to three other carbon groups, react the most easily because they form the most stable intermediate (a carbocation). Secondary alcohols require somewhat harsher conditions, and primary alcohols are the most resistant. Methyl alcohol cannot undergo this reaction at all because no stable carbocation can form. The general order of reactivity is tertiary > secondary > primary.
Under standard lab conditions, concentrated acid and heat (often with distillation to drive off the product) are needed. Under hydrothermal conditions, where water itself acts as both solvent and catalyst at temperatures around 200 to 350 °C under pressure, even secondary alcohols dehydrate rapidly without added acid. Primary alcohols remain sluggish even under those extreme conditions because they can only proceed through a slower, more demanding elimination pathway.
The main limitation of dehydration is selectivity. With certain substrates you can get mixtures of alkene products, including rearranged ones. Heating cyclohexanol with dilute hydrochloric acid at 225 °C, for example, produces cyclohexene but also gives about 14% methylcyclopentenes from skeletal rearrangement.
Mild Dehydration With Specialty Reagents
When your molecule contains acid-sensitive functional groups, strong acid and high temperatures are off the table. Two reagents handle this situation well.
The Burgess reagent selectively converts secondary alcohols into sulfamate ester intermediates in refluxing THF (about 66 °C). The actual elimination to an alkene can require a higher-boiling solvent like toluene (about 111 °C). Because the reagent targets secondary alcohols preferentially, it offers a degree of chemoselectivity you don’t get with sulfuric acid.
Martin sulfurane is a mild, neutral dehydrating agent that tolerates a wide variety of functional groups, including esters, ethers, carbonyls, and carbamates. It converts beta-hydroxy ketones cleanly into alpha,beta-unsaturated carbonyl compounds (enones), making it especially useful in complex natural product synthesis. It has been used in total syntheses of platencin, tubulysins, and acutiphycin, among others. One quirk: some secondary alcohols undergo oxidation to ketones instead of elimination, so the outcome depends on the substrate’s structure.
Complete Removal: Replacing -OH With a Hydrogen
Sometimes you don’t want a double bond at all. You want to replace the -OH with a hydrogen, converting the alcohol carbon into a simple C-H bond. This is called deoxygenation, and it requires a different strategy because you’re breaking a C-O bond without introducing unsaturation.
The Barton-McCombie Deoxygenation
The classic method is the Barton-McCombie reaction. You first convert the alcohol into a thiocarbonyl derivative (such as a xanthate or thiocarbamate), then treat it with a radical chain reagent that strips away the thiocarbonyl group and delivers a hydrogen atom. The traditional radical source is tributyltin hydride, which works well but is highly toxic and difficult to remove from products.
Photocatalytic Deoxygenation
A newer, more practical approach avoids tin reagents altogether. The alcohol is first converted to a benzoate ester, then treated with a photocatalyst and formate salts. Light activates the photocatalyst, which generates a powerful reducing radical from the formate. A mild acid (formic acid buffered with zinc formate) promotes the fragmentation step, and the net result is clean replacement of the -OH with a hydrogen atom. This system works well for benzylic alcohols and uses inexpensive, low-toxicity reagents. It runs under air-tolerant conditions, which makes it considerably more practical than the tin-based version.
Two-Step Sulfonate Reduction
Another reliable approach converts the alcohol to a sulfonate ester (such as a tosylate), turning the -OH into a good leaving group. A strong metal hydride reducing agent then displaces the sulfonate and delivers a hydrogen. This two-step sequence works well for primary and secondary alcohols but can be limited by steric crowding at tertiary centers, where making the sulfonate ester is difficult.
Catalytic Hydrodeoxygenation
In industrial and materials chemistry contexts, alcohols are deoxygenated using hydrogen gas over heterogeneous metal catalysts at elevated temperatures. This approach, called hydrodeoxygenation (HDO), is especially important for converting biomass-derived molecules (like the sugar alcohols glycerol, xylitol, and sorbitol) into useful chemicals and fuels.
Effective HDO catalysts typically pair a late transition metal that activates hydrogen (such as rhodium, iridium, or platinum) with an oxygen-loving metal like molybdenum on a silica support. The combination matters: rhodium-molybdenum, iridium-molybdenum, and platinum-molybdenum catalysts all achieved around 80% yields of deoxygenated product in laboratory tests on secondary and tertiary alcohols. Palladium-molybdenum catalysts showed a striking selectivity, efficiently deoxygenating tertiary alcohols while leaving primary and secondary alcohols untouched. This selectivity opens the door to removing one -OH group from a molecule that has several, without affecting the others. Nickel-molybdenum catalysts performed poorly across the board, yielding only about 5% product, because nickel failed to form the stable alloy with molybdenum that the other metals achieved.
For biomass polyols with multiple -OH groups, bifunctional catalysts combining noble metals (platinum, ruthenium, iridium) with promoters like rhenium or tungsten on tailored supports can selectively cleave specific C-O bonds while leaving others intact. The challenge is avoiding unwanted C-C bond breaking or ring formation, which scrambles the molecular skeleton.
Converting -OH Into a Leaving Group
Many alcohol removal strategies share a common first step: converting the -OH into something that’s easier to displace. Hydroxyl groups themselves are poor leaving groups, so chemists routinely activate them before the key bond-breaking step. Options include converting the alcohol to a halide (chloride or bromide), a sulfonate ester (tosylate or mesylate), a thiocarbonyl derivative (for radical deoxygenation), or a benzoate ester (for photocatalytic methods).
The Mitsunobu reaction takes a different approach, using a redox-driven coupling to replace the -OH directly with a nucleophile from an acid partner. It works under mild conditions and inverts the stereochemistry at the alcohol carbon, which can be an advantage or a disadvantage depending on your goals. For sterically crowded alcohols, stronger acid partners like 4-nitrobenzoic acid help push the reaction forward.
Choosing the Right Method
- Want an alkene? Use acid-catalyzed dehydration for simple substrates, or the Burgess reagent or Martin sulfurane for sensitive molecules.
- Want a C-H bond (complete removal)? Use the Barton-McCombie deoxygenation, the photocatalytic benzoate ester method, or the tosylate/hydride two-step sequence.
- Working with a tertiary alcohol? Dehydration is easiest. For selective deoxygenation in the presence of other -OH groups, palladium-molybdenum catalysts offer unique selectivity.
- Need to avoid strong acid or heat? Martin sulfurane, the Burgess reagent, or photocatalytic deoxygenation all operate under mild, neutral conditions.
- Processing bulk biomass? Catalytic hydrodeoxygenation with hydrogen gas and bimetallic catalysts is the standard industrial approach.