14.1 : Properties of Alcohols

Question 1

What are the differences between alcohols and alkanes? How can this difference be reduced?

Answer 1

Alcohols :

Are less volatile
Have higher melting points
Have a greater water solubility

The differences become much smaller as the length of the carbon chain increases.

Question 2

What are the reasons for the differences in alcohols and alkanes?

Answer 2

Alkanes = non polar bonds because electronegativity of carbon and hydrogen are very similar.

Therefore, alkane molecule = non - polar.

Intermolecular forces between non-polar molecules are very weak London forces.

Alcohols have a polar O-H bond because of the difference in electronegativity of the oxygen and hydrogen atoms.

Therefore, alcohols are polar molecules.

The intermolecular forces will be very weak London forces but there will also be much stronger hydrogen bonds between the polar O-H groups.

Question 3

Describe the difference in volatility in alcohols and alkanes

Answer 3

In liquid state, intermolecular hydrogen bonds hold the alcohol molecules together.

These bonds must be broken in order to change the liquid alcohol into gas.

This require more energy than overcoming the weaker London forces in alkanes.

Therefore alcohols have a lower volatility than the alkanes with the same number of carbon atoms.

Question 4

Describe alcohols solubility in water

Answer 4

Alcohols such as methanol and ethanol are completely soluble in water, as hydrogen bonds form between the polar OH group of the alcohol and the water molecules.

Question 5

How do you increase / decrease solubility of alcohols?

Answer 5

As the hydrocarbon chain increases in size, the influence of OH group becomes relatively smaller, and so solubility decreases.

Question 6

What are primary alcohols? Give examples

Answer 6

The OH group is attached to a carbon atom that is attached to two hydrogen atoms and one alkyl group.

e.g. ethanol and methanol ( which is an exception as it is attached to no carbon atoms )

Question 7

What are secondary alcohols? Give examples

Answer 7

The OH group is attached to a carbon atom that is attached to one hydrogen atom and two alkyl groups.

e.g. propan-2-ol , propan-3-ol

Question 8

What are tertiary alcohols? Give examples

Answer 8

The OH group is attached to a carbon atoms that is attached to no hydrogen atoms and three alkyl groups.

e.g. 2-Methylpropan-2-ol , 2-methylbutan-2-ol