Alcohols and Carboxylic Acids for the ESAT

Alcohols as a Homologous Series

Alcohols are a homologous series of organic compounds characterised by the presence of the hydroxyl functional group, $-OH$. The first four members of the series demonstrate a clear pattern in their molecular structure:

• Methanol: $CH_3OH$
• Ethanol: $C_2H_5OH$
• Propanol: $C_3H_7OH$
• Butanol: $C_4H_9OH$

Based on these members, we can derive the general formula for alcohols as $C_nH_{2n+1}OH$, where $n$ is the number of carbon atoms.

Worked Example: Determining Alcohol Formulae

1. What is the formula of an alcohol with six carbon atoms? Applying the general formula where $n = 6$: $C_6H_{(2 \times 6) + 1}OH = C_6H_{13}OH$.

2. What is the formula of an alcohol with a total of 20 hydrogen atoms? In the general formula $C_nH_{2n+1}OH$, there are $2n + 1$ hydrogens in the alkyl chain and 1 hydrogen in the hydroxyl group. Total hydrogens = $2n + 2 = 20$. Solving for $n$: $2n = 18$, so $n = 9$. The formula is $C_9H_{19}OH$.

Nomenclature of Alcohols

Naming alcohols follows the standard IUPAC rules with specific additions:

1. The suffix for the name is -ol.
2. The prefix indicates the number of carbon atoms, followed by -an- to show the chain is saturated (e.g., methan-, ethan-).
3. For chains of 3 or more carbons, a number is inserted between the prefix and suffix to indicate the position of the $-OH$ group (e.g., propan-1-ol).

Note that methanol ($CH_3OH$) and ethanol ($C_2H_5OH$) do not require a number because the hydroxyl group can only be in one possible position.

Worked Example: Naming Alcohols

• $CH_3CH(OH)CH_3$: This has a three-carbon chain with the $-OH$ on the second carbon. It is propan-2-ol.
• $CH_3CH_2CH_2CH(OH)CH_2CH_3$: This has a six-carbon chain with the $-OH$ on the third carbon. It is hexan-3-ol.
• $CH_3CH_2CH_2CH_2OH$: This is a four-carbon chain with the $-OH$ on the first carbon. It is butan-1-ol.

For a straight-chain alcohol with five carbon atoms, three structural isomers exist depending on the $-OH$ position:

1. pentan-1-ol: $CH_3CH_2CH_2CH_2CH_2OH$
2. pentan-2-ol: $CH_3CH_2CH_2CH(OH)CH_3$
3. pentan-3-ol: $CH_3CH_2CH(OH)CH_2CH_3$

Reaction of Alcohols with Sodium

Alcohols react with sodium metal in a manner similar to water, as both contain an $-OH$ bond. However, while the reaction of sodium with water is fast and vigorous, the reaction with alcohols is significantly slower and more moderate.

Sodium reacts with water to produce sodium hydroxide and hydrogen gas: $2Na + 2H_2O \rightarrow 2NaOH + H_2$

Sodium reacts with ethanol to produce sodium ethoxide and hydrogen gas: $2Na + 2C_2H_5OH \rightarrow 2C_2H_5ONa + H_2$

Sodium ethoxide is a strong alkali in aqueous solution. This reaction can be used as a chemical test for the presence of the hydroxyl group or as a safer way to dispose of small amounts of sodium metal.

Worked Example: Sodium and Methanol

The balanced equation for the reaction of sodium with methanol is: $2Na + 2CH_3OH \rightarrow 2CH_3ONa + H_2$ The organic product is sodium methoxide.

Carboxylic Acids

Carboxylic acids form a homologous series containing the carboxyl functional group, $-COOH$. Their general formula is $C_nH_{2n+1}COOH$.

Nomenclature of Carboxylic Acids

1. The suffix is -oic acid.
2. The prefix indicates the total number of carbon atoms, including the carbon in the carboxyl group.
3. No position number is needed because the $-COOH$ group always occurs at the end of the chain (position 1).

For example, $CH_3COOH$ has two carbons in total and is called ethanoic acid. If $n = 4$ in the general formula, the molecule is $C_4H_9COOH$. This contains 5 carbon atoms in total, making it pentanoic acid. The first member of the series occurs when $n = 0$, giving $HCOOH$ (methanoic acid).

Chemical Properties of Carboxylic Acids

Carboxylic acids are weak acids. In aqueous solution, they only partially dissociate into ions: $RCOOH(aq) \rightleftharpoons RCOO^{-}(aq) + H^{+}(aq)$

They undergo typical acid reactions:

• With metals: forms a salt and hydrogen. For example: $2CH_3COOH + Mg \rightarrow (CH_3COO)_2Mg + H_2$ (Magnesium ethanoate).
• With bases: forms a salt and water. For example: $C_2H_5COOH + NaOH \rightarrow C_2H_5COONa + H_2O$ (Sodium propanoate).
• With carbonates: forms a salt, water, and carbon dioxide. For example: $2CH_3COOH + K_2CO_3 \rightarrow 2CH_3COOK + CO_2 + H_2O$ (Potassium ethanoate).

Note that the salts end in -oate, and the anion is typically written first in the chemical formula (e.g., $(CH_3COO)_2Mg$).

Formation of Esters

Carboxylic acids react with alcohols in the presence of an acid catalyst (typically concentrated sulfuric acid) to produce esters and water. This is a reversible reaction known as esterification.

In the naming of an ester, the first part comes from the alcohol (the alkyl group) and the second part comes from the carboxylic acid (the carboxylate salt name).

For example, the reaction between ethanoic acid and methanol:

Worked Example: Predicting Esters

• Ethanoic acid + ethanol: Produces ethyl ethanoate. Formula: $CH_3COOC_2H_5$.
• Methanoic acid + ethanol: Produces ethyl methanoate. Formula: $HCOOC_2H_5$.
• Propanoic acid + methanol: Produces methyl propanoate. Formula: $C_2H_5COOCH_3$.