Alcohols Flashcards
1
Q
How do you name alcohols?
A
- The suffix -ol is added to the stem name of the longest carbon chain
- The position of the alcohol functional group in the chain is indicated using a number
2
Q
How are alcohols and alkanes different?
A
- Although same number of carbon atoms e.g. methane and methanol
1. Alcohols are less volatile
2. Alcohols have higher melting points
4. Greater water solubility than the corresponding alkanes - The differences become much smaller as the length of the carbon chain increases
3
Q
Why are alkanes and alcohols different?
A
- Alkanes have non-polar bonds because the electronegatvitiy of hydrogen and carbon are very similar and so alkanes molecules are therfore non-polar
- The intermolecular forces between non-polar molecule are very weak London forces
- Alcohols have a polar O-H bond because of the difference in electronegatvcity of the oxygen and hydrogen atoms. Alcohols are therefore polar
- The intermolecular forces will be very weak London forces but there will also be much stronger hydrogen bonds between the polar O-H bonds
4
Q
Explain the difference in volatility and boiling points
A
- In the liquid state, intermolecular hydrogen bonds hold the alcohol molecules together
- These bonds must be broken in order to change the liquid alcohol into a gas
- This requires more energy than overcoming the weaker London forces in alkanes, so alcohols have a lower volatility than the alkanes with the same number of carbon atoms
5
Q
Explain the difference in solubility in water
A
- A compound that can form hydrogen bonds with water is far more water-soluble than a compound that cannot
- Alkanes are non-polar molecules and cannot form hydrogen bonds with water
- 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
- As the hydrocarbon chain increases in size, the influence of the -OH group becomes relatively smaller, and the solubility of longer-chain alcohols becomes more like that of hydrocarbons, solubility decreases
6
Q
What does the classification of alcohols depend on?
A
- Alcohols can be classified as primary, secondary or tertiary
- This classification depends on the number of hydrogen atoms and the alkyl groups attached to the carbon atoms that contains the alcohol functional group
7
Q
Describe a primary alcohol
A
- In a primary alcohol the -OH group is attached to a carbon atom that is attached to two hydrogen atoms and one alkyl group
- Methanol with three hydrogen atoms and no carbon atoms attached, is an expectation that is still classified as a primary alcohol
- Methanol and ethanol, the two simplest alcohols in the alcohols homologous series are both primary alcohols
8
Q
What is a secondary alcohol?
A
- In a secondary alcohol the -OH group is attached to a carbon atom that is attached to one hydrogen atoms and two alkyl groups
- -Propan-2-ol and pentan-3-ol are both examples of secondary alcohols
9
Q
What is a tertiary alcohol?
A
- In a tertiary alcohol the -OH group is attached to a carbon atoms that is attached to no hydrogen atoms and three alkyl groups
- 2-Methylpropan-2-ol and 2-methylbutan-2-ol re both examples of secondary alcohols
10
Q
Describe the combustion of alcohols
A
- Alcohols burn completely in a plentiful supply of oxygen to produce carbon dioxide and water
- The reaction is exothermic
- As the number of carbon atoms in the alcohol chain increases the quantity of heat released per mole also increases
11
Q
What happens in the oxidation of alcohols?
A
- Primary and secondary alcohols can be oxidised by an oxidising agent
- The usual oxidising mixture is a solution of potassium dichromate(VI) k2Cr2O7, acidified with dilute sulphuric acid H2SO4
- If the alcohol is oxidised the orange solution containing dichromate (VI) ions is reduced to a green solution containing chromium (II) ions
12
Q
Describe the oxidation of primary alcohols
A
- Primary alcohols can be oxidised to either aldehydes or carboxylic acids
- The product of the oxidation depends on the reaction conditions used because aldehydes themselves also oxidised to carboxylic acids
13
Q
How do you prepare aldehydes?
A
- On gentle heating of a primary alcohols with acidified potassium dichromate, an aldehyde is formed
- To ensure that the aldehyde is prepared rather than the carboxylic acid, the aldehyde is distilled out of the reaction mixture as it forms
- This prevents any further reaction with the oxidising agent
- The dichromate (VI) ions change colour from orange to green
14
Q
How do you prepare carboxylic acids?
A
- If a primary alcohol is heated strongly under reflux, with an excess of acidified potassium dichromate (VI), a carboxylic acid is fromed
- Use of an excess of the acidified potassium dichromate (VI) ensures that all of the alcohol is oxidised
- Heating under reflux ensures that any aldehyde formed initially in the reaction undergoes oxidation to the carboxylic acid
15
Q
In summary what is the difference between preparing for a carboxylic acid against an aldehyde?
A
- When preparing an aldehyde, use distillation to remove the aldehyde form the reaction mixture
- When preparing the carboxylic acid, heat the acid under reflux
