Alcohols
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ALCOHOLS
What are alcohols?
Alcohols are compounds in which one or more hydrogen atoms in an alkane have been replaced by an -OH group. For the purposes of UK A level, we will only look at compounds containing one -OH group.
Some examples of an alcohol would be
The different kinds of alcohols
Alcohols fall into different classes depending on how the -OH group is positioned on the chain of carbon atoms. There are some chemical differences between the various types.
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Primary alcohols
In a primary (1°) alcohol, the carbon which carries the -OH group is only attached to one alkyl group.
Some examples of primary alcohols include:
Notice that it doesn’t matter how complicated the attached alkyl group is. In each case there is only one linkage to an alkyl group from the CH2 group holding the -OH group.
There is an exception to this. Methanol, CH3OH, is counted as a primary alcohol even though there are no alkyl groups attached to the carbon with the -OH group on it.
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Secondary alcohols
In a secondary (2°) alcohol, the carbon with the -OH group attached is joined directly to two alkyl groups, which may be the same or different.
Examples:
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Tertiary alcohols
In a tertiary (3°) alcohol, the carbon atom holding the -OH group is attached directly to three alkyl groups, which may be any combination of same or different.
Examples:
Physical properties of alcohols
Boiling Points
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The boiling point of an alcohol is always much higher than that of the alkane with the same number of carbon atoms.
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The boiling points of the alcohols increase as the number of carbon atoms increases.
The effect of van der Waals forces on the boiling points of the alcohols:
Hydrogen bonding isn’t the only intermolecular force in alcohols. There are also van der Waals dispersion forces and dipole-dipole interactions. The hydrogen bonding and the dipole-dipole interactions will be much the same for all the alcohols, but the dispersion forces will increase as the alcohols get bigger. These attractions get stronger as the molecules get longer and have more electrons. That increases the sizes of the temporary dipoles that are set up. This is why the boiling points increase as the number of carbon atoms in the chains increases. It takes more energy to overcome the dispersion forces, and so the boiling points rise.
The effect of van der Waals forces on the comparison between alkanes and alcohols:
Even if there wasn’t any hydrogen bonding or dipole-dipole interactions, the boiling point of the alcohol would be higher than the corresponding alkane with the same number of carbon atoms.
Compare ethane and ethanol:
Ethanol is a longer molecule, and the oxygen brings with it an extra 8 electrons. Both of these will increase the size of the van der Waals dispersion forces and so the boiling point.
If you were doing a really fair comparison to show the effect of the hydrogen bonding on boiling point it would be better to compare ethanol with propane rather than ethane. The length would then be much the same, and the number of electrons is exactly the same.
Solubility of alcohols in water
The small alcohols are completely soluble in water. Whatever proportions you mix them in, you will get a single solution.
However, solubility falls as the length of the hydrocarbon chain in the alcohol increases. Once you get to four carbons and beyond, the fall in solubility is noticeable, and you may well end up with two layers in your test tube.