Alcohol Structures and Isomers

An oxygen atom in a formula does not automatically make a compound an alcohol. Find how the oxygen is bonded, then build structures by counting every atom and checking each atom’s bonds.

Recognise the alcohol group

The hydroxyl group

The alcohols studied here contain one hydroxyl group, -O-H, attached to a saturated carbon atom: that carbon has only single bonds. This is often written -OH.

Oxygen has two bonds, one to carbon and one to hydrogen. An OH within the -C(= O)OH group of a carboxylic acid belongs to a different functional group; it does not make that acid an alcohol.

A group is not an ion

The -OH group is covalently bonded within an alcohol molecule. It is not a separate hydroxide ion, OH⁻. Dissolving ethanol in water does not release hydroxide ions as an alkali does, so ethanol is not an alkali.

Count the hydrogen on oxygen too

For open-chain, saturated alcohols with one OH group, the grouped general formula is CₙH₂ₙ₊₁OH. The hydrogen in OH is additional to the 2n + 1 hydrogens written before it. The corresponding molecular formula is CₙH₂ₙ₊₂O.

For ethanol, n = 2: C₂H₅OH contains two C, six H and one O, so its molecular formula is C₂H₆O. A molecular formula gives totals; it does not prove that an OH group is present or show where it is attached.

Draw and compare C1–C4 structures

The unbranched series

Build the unbranched alcohols

Each molecule contains one oxygen bonded to a carbon and a hydrogen. Methanol has one carbon, ethanol two, and propan-1-ol three in an unbranched chain.

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View figure data

Atoms in each displayed structure

Alcohol	Molecular formula	Carbon atoms	Position of -OH
methanol	CH4O	1	on the carbon
ethanol	C2H6O	2	on an end carbon
propanol	C3H8O	3	on an end carbon

Atoms in each displayed structure

Alcohol	Molecular formula	Carbon atoms	Position of -OH
methanol	CH4O	1	on the carbon
ethanol	C2H6O	2	on an end carbon
propanol	C3H8O	3	on an end carbon

The names methanol to butanol identify the unbranched series. A position number distinguishes structures when the OH group can be attached at different places. The table gives the structures with OH on an end carbon.

Change branching or OH position

Keep four carbons while changing the skeleton

Butan-1-ol has a continuous four-carbon chain. The branched structure has a central CH attached to two CH3 groups and a CH2OH group. Both contain four carbons, ten hydrogens and one oxygen.

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Compare carbon skeletons without changing atom totals

Alcohol	Molecular formula	Condensed structural formula	Carbon skeleton
butan-1-ol	C4H10O	CH3CH2CH2CH2OH	Unbranched
2-methylpropan-1-ol	C4H10O	(CH3)2CHCH2OH	Branched

Compare carbon skeletons without changing atom totals

Alcohol	Molecular formula	Condensed structural formula	Carbon skeleton
butan-1-ol	C4H10O	CH3CH2CH2CH2OH	Unbranched
2-methylpropan-1-ol	C4H10O	(CH3)2CHCH2OH	Branched

The OH group need not be on an end carbon. These condensed structural formulae show other valid alcohols; the supplied names help you distinguish them:

Parentheses collect attached groups rather than extending the main chain. In CH₃-CH(OH)-CH₃, OH attaches to the middle carbon; in (CH₃)₃C-OH, three CH3 groups attach to the central carbon.

Changing OH position or branching can give structural isomers: the molecular formula remains the same, but atom connections differ. Rotating a drawing or reading the same chain from the other end does not create a new structure.

Build a drawing in four steps

1. Draw the carbon skeleton, including each branch.
2. Attach O–H to the chosen carbon with a single C–O bond.
3. Add hydrogens until each carbon has four bonds. Oxygen already has two.
4. Count every atom, including H on oxygen, and compare with the molecular formula.

Making ethanol

Ethanol Production compares the two production routes.

Fermentation

Find the glucose-to-ethanol equation and conditions.

Hydration

Find the ethene-and-steam equation and conditions.

Ethanol as a fuel

Combustion requires counting the oxygen already in ethanol.

Oxidising ethanol

Oxidation changes ethanol into ethanoic acid. Use that specific example rather than assuming every alcohol structure forms an acid in the same way.

Check the bonds and atom totals

• The OH hydrogen counts towards the total; C₄H₉OH contains ten H, not nine.
• A branch carbon still counts towards n.
• An OH group uses one of its attached carbon’s four bonds.
• The formula CₙH₂ₙ₊₂O alone does not identify an alcohol: check for a C–O–H connection.

Make the structure visible

A molecular formula cannot replace a requested structural drawing. Show the bonds needed to locate OH and any branches. If asked for a displayed formula, show every atom and bond, including O–H.

Draw and check two structures

Explain oxygen exclusion

The fermentation example is beside the production explanation.

Compare production routes

The factory-choice example is with the route comparison.

Count oxygen in combustion

The combustion example is with the fuel equation.

Interpret an oxidation observation

The oxidation example is with the reagent and product.

Try without prompts

Explain a failed fermentation

Try the fermentation-temperature question.

Practise and check

Use the Organic Chemistry topic check to practise and check your understanding.