Chapter 14: Alcohols Flashcards
- Where do alcohols inherit their physical and chemical properties?
- How are alcohols named?
- Alcohols contain the –OH functional group, known as the hydroxyl group. The hydroxyl group is responsible for both the physical and chemical properties of the alcohols.
- 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.
What is the IUPAC name of this alcohol?
Compare the physical properties of alcohols with alkanes of the same number of carbon atoms. Explain these differences.
- Alcohol is less volatile, has higher melting points, and greater water solubility than the corresponding alkanes. The differences become much smaller as the length of the carbon chain increases.
- These differences can be explained by considering the polarity of the bonds in both the alkanes and alcohols.
- The alkanes have non-polar bond because the electronegativity of hydrogen and carbon are very similar.
- The alkane molecules are therefore non-polar.
- The 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.
- Alcohol molecules 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 groups.
The difference in the boiling points of the alkanes and alcohols can be seen in Figure 3.
Give an explanation for this relationship.
In the liquid state, intrmoleculas hydrogen bonds hold the alcohol molecules together. These bond 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 bonds.
Explain why alcohols are soluble in water but alkanes are not.
- Alkanes are non-polar molecules and cannot form hydrogen bonds with water.
- Alcohols are completely soluble in water, as hydrogen bonds form between the polar –OH group of the alcohol and the water molecules.
- However, 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.
- What are primary alcohols?
- What are secondary alcohols?
- What are tertiary alcohols?
- In a primary alcohol the –OH group is attached to one 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 exception that is still classified as a primary alcohol).
- In a secondary alcohol the –OH group is attached to a carbon atom that is attached to one hydrogen atom and two alkyl groups.
- 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.
Describe the general method for oxidising alcohols.
Primary alcohols can be oxidised to either aldehydes or carboxylic acids.
Describe how aldehydes can be prepared from primary alcohols.
- On gentle heating of primary alcohols with acidified potassium dichromate (a strong oxidising agent), 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 orange dichromate(VI) ions are reduced to a green solution containing chromium(III) ions.
Give the full equation, showing the displayed formula, for the oxidation of butan-1-ol with acidified potassium dicrhomate(VI) to form butanal.
Primary alcohols can be oxidised to either aldehydes or carboxylic acids.
Describe how carboxylic acids can be prepared from primary alcohols.
- If a primary alcohol is heated strongly under reflux, with an excess of acidified potassium dicrhomate(VI), a carboxylic acid is form.
- 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 also undergoes oxidation to the carboxylic acid.
Give the complete oxidation of butan-1-ol to form butanoic acid.
What is the order of oxidation for primary alcohols?
primary alcohols → aldehyde → carboxylic acid
Describe the oxidation of secondary alcohols.
- Secondary alcohols are oxidies to ketones. It is not possible to further oxidise ketones using acidified dichromate(VI) ions.
- To ensure the reaction goes to completion, the secondary alcohol is heated under reflux with the oxidising mixture.
- The dichromate(VI) ions once again change colour from orange to green.
Give the full equation, including displayed formula, for the oxidation of propan-2-ol to propanone.
Give the equation, including the displayed formula, for the dehydration of the alcohol cyclohexanol.
Describe the substitution reactions of alcohols.
- Alcohols react with hydrogen halides to form haloalkanes. When preparing a haloalkane, the alcohol is heated under reflux with sulfuric acid and a sodium halide.
- As a result, hydrogen bromide is formed in situ (in place).
