Chapter 14 Flashcards
What functional group do alcohols contain?
-OH
Compared to alkanes of the same length, alcohols:
Are less volatile
Have higher melting points
Are more soluble in water
What happens between alcohols and other hydrocarbons when the length of the carbon chain increases?
The differences between them become less significant
What is the polarity in alcohols?
Alcohols have a polar O-H bond because of the difference in electronegativity of the oxygen and hydrogen atoms
Therefore alcohol molecules are polar
Why are alcohols less volatile than alkanes?
They contain strong hydrogen bonds between molecules which require more energy to overcome
Why are alcohols more soluble in water than alkanes?
Alcohols are polar molecules therefore hydrogen bonds can be formed between the polar -OH group of the alcohol and the water molecules
Why does the solubility of alcohols decrease as the chain length increases?
Because the influence of the -OH group becomes smaller
How are alcohols classified?
Into primary, secondary and tertiary
What are primary alcohols?
In a primary alcohol, the -OH group is attached to a carbon atom that is attached to two hydrogen atoms and one alkyl group
What are secondary alcohols?
In a secondary alcohol the -OH group is attached to a carbon atom that is attached to one hydrogen atom and two alkyl groups.
What are tertiary alcohols?
In a tertiary alcohol, the -OH group Is attached to a carbon atom that is attached to no hydrogen atoms and three alkyl groups.
Describe the combustion of alcohols
Alcohols burn completely in a plentiful supply of oxygen to produce carbon dioxide and water. The reaction is exothermic and releases a large quantity of energy. As chain length increases, the quantity of heat released per mole also increases
e.g.
C2H5OH(l) + 3O2(g) –> 2CO2(g) + 3H2O(l)
Describe the oxidation of alcohols
Primary and secondary alcohols can be oxidised by an oxidising agent, usually a solution of acidified potassium dichromate. Tertiary alcohols do not oxidise with an oxidising agent.
When an alcohol is oxidised, a colour change from orange to green occurs as the dichromate ions become chromium ions.
The product depends on the reaction method used:
Primary alcohols form aldehydes when distilled and carboxylic acids when heated under reflux.
Secondary alcohols form ketone when heated under reflux.
What is the colour change when alcohols are oxidised?
Orange to green
What are the products of the oxidation of primary alcohols?
Carboxylic acids
Aldehydes
Why are there multiple products when primary alcohol are oxidised?
The product of the oxidation depends on the reaction conditions used because aldehydes are themselves oxidised to carboxylic acids
Describe the process of oxidising a primary alcohol to form an aldehyde
The primary alcohol is gently heated with acidified potassium dichromate to form an aldehyde.
The method used is distillation to ensure any aldehyde formed is removed before it oxidises further into a carboxylic acid.
Describe the process of oxidising a primary alcohol to form a carboxylic acid
The primary alcohol is heated strongly under reflux, with an excess of acidified potassium dichromate.
Excess ensures that the alcohol is fully oxidised
Reflux ensures that any aldehyde initially formed also undergoes oxidation to the carboxylic acid
Describe the process of oxidising a secondary alcohol to form ketones
The secondary alcohol is heated under reflux with the oxidising mixture. The dichromate ions once again change colour from orange to green.
Do tertiary alcohols oxidise with potassium dichromate?
No they do not, they can only be oxidised through combustion
How is water removed from alcohols and what is the reaction called?
The reaction is dehydration which is when a water molecule is removed from an alcohol.
The alcohol is heated under reflux in the presence of an acid catalyst such as concentrated phosphoric acid.
The product is an alkene.
How are haloalkanes formed from alcohols?
Alcohols take part in substitution reactions with hydrogen halides to form haloalkanes.
The alcohol is heated under reflux with sulfuric acid and a sodium halide.
