Topic 14 Alcohols

Question 1

Compare the physical properties of alcohols with alkanes.

Answer 1

BOILING POINTS

• alkanes have non-polar bonds because the electronegativity of hydrogen and carbon are very similar therefore alkane molecules are non-polar intermolecular forces between non-polar molecules are very weak London forces
• alcohols have a polar O-H bond so are therefore polar intermolecular forces will be formed as well as London forces. Alcohols can also form much stronger hydrogen bonds between polar hydroxyl groups
• therefore more energy is required to break apart the intermolecular forces between alcohols than alkanes resulting in a higher boiling point

VOLATILITY
in liquid state alcohols have hydrogen bonding holding the molecules together, this is much harder to overcome than the weak London forces in alkanes so therefore require more energy to overcome meaning alcohols have a lower volatility.

SOLUBILITY IN WATER
alcohol compounds can form hydrogen bonds with water so is far more soluble than alkanes. As the carbon chain length increases the effect of the OH group decreases meaning water solubility decreases.

Question 2

How can ethylene glycol be used as antifreeze?

Answer 2

Water freezes at 0C but ethylene glycol freezes at -13C but when the two are mixed the freezing point can be as low as -40C meaning that the ice on a windshield quickly defrosts

Question 3

How do we classify alcohols?

Answer 3

Alcohols can be split into three categories: primary, secondary and tertiary.
This is dependent on how many carbons and alkyl groups the carbon, that is bonded to the hydroxyl group, is bonded to.

E.g. this secondary alcohol propan-2-ol

    H  H  H
     |   |   |
H-C-C-C-H
     |    |   |
    H OH H

This is secondary because the carbon bonded to the hydroxyl group is also bonded to two other carbons.

Question 4

What happens in the combustion of alcohols?

Answer 4

Alcohols burn completely in a plentiful supply of oxygen to produce CO2 and water.
E.g Ethanol
C2H5OH + 3O2 -> 2CO2 +3H2O

The reaction is exothermic releasing a large quantity of energy in the form of heat. As the length of the carbon chain increases so does the quantity of heat

Question 5

What happens when a primary alcohol is oxidised?

Answer 5

PREPARATION OF ALDEHYDES
-on gentle heating of primary alcohols with acidified potassium dichromate an aldehyde is formed.
-we use a distillation set up the aldehyde is distilled out as the reaction takes place, this is because it has a lower b.p than the alcohol so evaporates upon formation where it quickly enters a condensing tube and is collected
-the oxidising agent is acidified potassium dichromate which is reduced, turning from an orange to a green solution containing chromium (III) ions
E.g. Butan-1-ol is oxidised to form butanal and water

PREPARATION OF CARBOXYLIC ACIDS
-upon the primary alcohol is heated strongly under reflux with an excess of acidified potassium dichromate a carboxylic acid is formed
-use of all the excess potassium dichromate ensures that all the alcohol is oxidised
-heating under reflux ensures that any aldehyde formed initially in the reaction also undergoes oxidation to the carboxylic acid as it is condensed and falls back into the mixture
E.g. Butan-1-ol is oxidised to form butanoic acid and water

Question 6

What happens when a secondary alcohol is oxidised?

Answer 6

PREPARATION OF KETONES
-secondary alcohols are oxidised to form ketones, it is not possible to oxidised them further with acidified dichromate ions
-to ensure completion the reaction undergoes reflux with the oxidising mixture, once again the dichromate ions change from orange to green
E.g.
Propan-2-ol is oxidised to form propanone and water

Question 7

what happens when a tertiary alcohol is oxidised?

Answer 7

Tertiary alcohols cannot be oxidised and the acidified dichromate ions remains orange when added to a tertiary alcohol

Question 8

What happens when an alcohol is dehydrated?

Answer 8

-water is removed fro the starting material (alcohols)
-an alcohol is heated in reflux with an acid catalyst ( usually concentrated H2SO4 or H3PO4)
-the product of the reaction is an alkene and water
- this is I an example of an elimination reaction as 1 molecule is broken in two
-secondary alcohols can form two different alkenes depending on which hydrogens are used to make water
E.g. pentan-2-ol
H H H H H
| | | | |
H-C-C-C-C-C-H
| | | | |
H H H O H
^ H ^

Either of the marked hydrogens could be used meaning EZ stereoisomers can be formed. Either Z or E pent-2-ene

Question 9

What happens in a substitution reaction of an alcohol to form a haloalkane?

Answer 9

-to make a haloalkane you must first make a hydrogen halide. To do this you heat a mixture of sodium halide and sulfuric acid
E.g. sodium bromide to form hydrogen bromide
- NaBr + H2SO4 -> NaHSO4+ HBr
- the HBr formed reacts with the alcohol to produce the haloalkane
E.g.
propan-2-ol + hydrogen bromide -> 2-bromopropane + water
-however both of these reactions take place at once
-NaBr +H2SO4 + C3H7OH -> NaHSO4 +C3H7Br +H2O