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Alcohols

An alcohol is an organic molecule with a hydroxyl group, $$\ce{-OH}$$.

The general formula for the homologous series of simple alcohols is: $$$\text{C}_\text{n}\text{H}_{2\text{n}+1}\text{OH}$$$

Unlike alkanes and alkenes, alcohols are usually liquid at room temperature and pressure.

Alcohols exhibit isomerism. However, not all isomers contain a hydroxyl group. In other words, not all isomers are alcohols.

Isomers of <span style=$$\ce{C3H8O}$$. Propan-1-ol (left) and propan-2-ol (middle) differ in the placement of the $$\ce{-OH}$$ group, and methoxyethane (right), is not an alcohol." src="https://www.toktol.com//Content/images/transparent.gif" onload="conditionalLoadImage(this, 'https://toktolweb.blob.core.windows.net/courseimages/', 'Chem11_Alcohols_01_AlcoholIsomers.png', false, {}, 'https://toktolwebcdn.blob.core.windows.net/quizimages/')" name="Chem11_Alcohols_01_AlcoholIsomers.png" sub="Isomers of $$\ce{C3H8O}$$. Propan-1-ol (left) and propan-2-ol (middle) differ in the placement of the $$\ce{-OH}$$ group, and methoxyethane (right), is not an alcohol." />
Isomers of $$\ce{C3H8O}$$. Propan-1-ol (left) and propan-2-ol (middle) differ in the placement of the $$\ce{-OH}$$ group, and methoxyethane (right), is not an alcohol.

Alcohols are highly flammable. Like alkanes and alkenes, alcohols can undergo combustion to give carbon dioxide and water. This process is extremely exothermic. As a result, alcohols can be used as fuels.

In Brazil, alcohol is available commercially as a fuel for vehicles.
In Brazil, alcohol is available commercially as a fuel for vehicles.

The first four alcohols in the homologous series are shown in the table below.

Name Diagram Formula
Methanol
$$\ce{CH3OH}$$
Ethanol
$$\ce{C2H5OH}$$
Propanol
$$\ce{C3H7OH}$$
Butanol
$$\ce{C4H9OH}$$

There are two main methods for producing ethanol.

Fermentation converts glucose to alcohol and carbon dioxide. The reaction is controlled by an enzyme (zymase) provided naturally by yeast (a microorganism).

$$$\text{glucose}\ce{->[\text{zymase}]}\text{ethanol}+\text{carbon dioxide}$$$

The yeast dies when the concentration of alcohol reaches approximately 15%. Higher concentrations of fermented ethanol can only be achieved using distillation.

Alcohol can also be made industrially by the addition of steam to ethene. This reaction requires heat and an acid catalyst to break the double bond.

$$$\ce{CH2=CH2 + H2O -> CH3-CH2OH}$$$

A comparison of the methods is given in the table.

Fermentation Steam addition
Made in batches Made continuously
Expensive Cheap (after initial costs)
Slow Fast
Uses plant sugars Uses ethene
Used to make alcoholic beverages. Not used to make beverages. Used to make industrial solvents.