Structural formulae – carboxylic acids and esters Notes

Carboxylic Acids – Structure, Formulae and Properties

The Carboxyl Group (–COOH)

1. Carboxylic acids are organic compounds that contain the carboxyl functional group, written as –COOH.
2. The carboxyl group is made of:
   1. A carbonyl group: C=O
   2. A hydroxyl group: –OH
   3. joined to the same carbon atom.
3. So the functional group looks like: $$–C(=O)OH–C(=O)OH$$
4. General formula (simple carboxylic acids / alkanoic acids): $$C_nH_{2n+1}COOH$$ (or often written simply as R–COOH, where R is an alkyl group.)

Drawing Structural and Displayed Formulae

Example

Ethanoic Acid (CH₃COOH)

• Molecular formula: CH₃COOH or C₂H₄O₂
• Condensed structural formula: CH₃COOH or CH₃–COOH

Physical Properties of Carboxylic Acids

Because of –COOH, carboxylic acids:

1. Form hydrogen bonds with each other → relatively high boiling points
2. Form hydrogen bonds with water → small acids (like methanoic and ethanoic acid) are soluble in water
3. Are weak acids, partially ionising in water: $$\mathrm{CH}_3 \mathrm{COOH}_{(\mathrm{aq})} \rightleftharpoons \mathrm{CH}_3 \mathrm{COO}_{(\mathrm{aq})}^{-}+\mathrm{H}_{(\mathrm{aq})}^{+}$$​

Typical Reactions of Carboxylic Acids

1. With reactive metals (e.g. Mg, Zn)
   → Salt + hydrogen gas $$2 \mathrm{CH}_3 \mathrm{COOH}+\mathrm{Mg} \rightarrow\left(\mathrm{CH}_3 \mathrm{COO}\right)_2 \mathrm{Mg}+\mathrm{H}_2$$
2. With bases (alkalis)
   → Salt + water $$\mathrm{CH}_3 \mathrm{COOH}+\mathrm{NaOH} \rightarrow \mathrm{CH}_3 \mathrm{COONa}+\mathrm{H}_2 \mathrm{O}$$ The salts are called carboxylates (e.g. sodium ethanoate).
3. With alcohols
   → Esters + water (this is esterification, see below).

Uses of Carboxylic Acids

1. Ethanoic acid (acetic acid):
   1. Main acid in vinegar, used as a food preservative.
2. Citric acid:
   1. Used in cleaning products and as a food additive (sour taste).
3. Used as starting materials in making polymers, pharmaceuticals and dyes.

Esters – Structure, Formulae and Their Relationship to Carboxylic Acids

Ester Functional Group and General Structure

1. Esters are derivatives of carboxylic acids.
2. They contain the ester functional group: $$–COO–$$
3. General structural formula for a simple ester: $$R–COO–R′$$
   1. $R–COO$ part (acyl group) comes from the carboxylic acid.
   2. $R'$ (alkyl group) comes from the alcohol.

Drawing Structural and Displayed Formulae for Esters

Example

Ethyl Ethanoate

Formed from:

• Ethanoic acid (CH₃COOH)
• Ethanol (CH₃CH₂OH)

Structural formula: $$\mathrm{CH}_3 \mathrm{COOCH}_2 \mathrm{CH}_3$$

Or written more clearly as: $$\mathrm{CH}_3-\mathrm{C}(=\mathrm{O})-\mathrm{O}-\mathrm{CH}_2 \mathrm{CH}_3$$

Relationship Between Carboxylic Acids, Alcohols and Esters

Esters are formed in an esterification (condensation) reaction:

Carboxylic acid + Alcohol ⇌ Ester + Water

General form: $$\mathrm{R}-\mathrm{COOH}+\mathrm{R}^{\prime}-\mathrm{OH} \rightleftharpoons \mathrm{R}-\mathrm{COO}-\mathrm{R}^{\prime}+\mathrm{H}_2 \mathrm{O}$$

Naming Carboxylic Acids and Esters

1. Carboxylic acids
   1. Take the alkane name and replace “-e” with “-oic acid”.
      1. Methane → methanoic acid
      2. Ethane → ethanoic acid
      3. Propane → propanoic acid
2. Esters
   1. First part: alkyl group from the alcohol (methyl, ethyl, propyl, etc.).
   2. Second part: acid part from the carboxylic acid, with ending “–oate”.

Properties and Uses of Esters – Linking Structure to Function

Physical Properties of Esters

Because esters have the –COO– group but no –OH group:

1. They are less polar than carboxylic acids.
2. They cannot hydrogen bond to each other as strongly as carboxylic acids.
3. They usually have:
   1. Lower boiling points than carboxylic acids of similar molar mass
   2. Higher volatility (they evaporate easily)
4. Their solubility in water is lower than that of small carboxylic acids, especially as the carbon chain gets longer.

Smell and Taste – Fragrances and Flavourings

1. Many short-chain esters are:
   1. Volatile (easily evaporate)
   2. Have pleasant, fruity smells
2. This makes them ideal for:
   1. Perfumes and body sprays
   2. Food flavourings (to mimic fruit flavours)

Esters as Solvents

1. Because esters are:
   1. Moderately polar, but also
   2. Have non-polar hydrocarbon chains,
2. they are good at dissolving many organic substances such as:
   1. Paint and ink components
   2. Nail polish resins
   3. Adhesive ingredients

Hydrolysis – Breaking Esters Back Down

The reverse of esterification is hydrolysis:

Ester + water → carboxylic acid + alcohol

1. Acid-catalysed hydrolysis: reversible.
2. Base-catalysed hydrolysis (saponification): produces a carboxylate salt and is effectively irreversible.

This reaction is important in:

1. Digestion of fats and oils (which are esters of glycerol and fatty acids).
2. Soap making (saponification of triglycerides).