Chemistry » Organic Molecules » Organic Molecular Structures

The Alcohols

The Alcohols

An alcohol is any organic compound where there is a hydroxyl functional group \((-\text{OH})\) bound to a carbon atom. The general formula for a simple alcohol is \(\color{red}{\textbf{C}_{\textbf{n}}\textbf{H}_{\textbf{2n+1}}\textbf{OH}}\).

The simplest and most commonly used alcohols are methanol and ethanol (see figure below).

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The (a) structural, (b) condensed structural and (c) molecular formula representations of methanol.

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The (a) structural, (b) condensed structural and (c) molecular formula representations of ethanol. (d) An atomic model of ethanol.

There are three possible types of carbon atoms – primary, secondary and tertiary. A primary carbon is attached to only one other carbon atom. A secondary carbon is attached to two other carbon atoms, while a tertiary carbon atom is attached to three other carbon atoms.

Note:

Make sure you understand the difference between primary, secondary and tertiary carbons:

  • Primary

    A primary carbon is a carbon atom bonded to one other carbon atom.

  • Secondary

    A secondary carbon is a carbon atom bonded to two other carbon atoms.

  • Tertiary

    A tertiary carbon is a carbon atom bonded to three other carbon atoms.

For example:

\(\text{CH}_{3}\text{CH}_{2}\text{CH}_{3}\), carbon 1 and carbon 3 are primary carbon atoms because they are only bonded to one other carbon atom. Carbon 2 is a secondary carbon atom because it is bonded to two other carbon atoms.

\(\text{C}(\text{CH}_{3})_{3}\text{X}\), The central carbon atom in this compound is a tertiary carbon atom because it is bonded to three other carbon atoms.

A primary alcohol has the hydroxyl (\(-\text{OH}\)) group bonded to a primary carbon atoms. Similarly, a secondary alcohol has the hydroxyl bonded to a secondary carbon atom and a tertiary alcohol has the hydroxyl bonded to a tertiary carbon atom.

There can be a functional group attached to these different types of carbon atom. When a hydroxyl \((-\text{OH})\) functional group is attached to a primary carbon atom it is called a primary alcohol. For a secondary alcohol the hydroxyl is bonded to a secondary carbon atom. When the hydroxyl is bonded to a tertiary carbon atom it is a tertiary alcohol. Examples are given below.

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A (a) primary (butan-1-ol), (b) secondary (butan-2-ol) and (c) tertiary (2-methylpropan-2-ol) alcohol.

The alcohols have a number of different uses:

  • methylated spirits is ethanol with methanol added

    Fact:

    Methanol is toxic. If ingested it forms formic acid or formate salts, which damage the central nervous system and can cause blindness, coma or death.

  • all alcohols are toxic, but in low concentrations ethanol can be used in alcoholic drinks

  • ethanol is the only alcohol used in alcoholic drinks

  • ethanol is used as an industrial solvent

  • methanol and ethanol can both be used as a fuel and they burn more cleanly than petrol or diesel

  • ethanol is used as a solvent in medical drugs, perfumes and plant essences

  • ethanol is an antiseptic

  • ethanol can be used as a preservative for fixing and preserving specimens as well as for preserving food.

Extra Notes: Alcohols or Alkanols

Alcohols or alkanols are the compounds containing one or more hydroxyl groups (-OH). The alcohols contain the -OH group attached to alkyl group

Alkanols are classified as mono-, di- and trihydric alcohols according to the number of -OH groups’ contained in their molecules. Some examples of mono-, di- and trihydric alcohols are as follows:

Primary (1°), Secondary (2°) and Tertiary (3°) Alkanols

Monohydric alcohols are classified as primary, secondary or tertiary alcohols depending upon whether the hydrogen group is attached to a primary, secondary or tertiary carbon atom, For example:

Further Reading:

Alcohols | Organic Chemistry

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