Alkenes, Alcohols and Carboxylic Acids
Alkenes
Alkenes are a homologous series of hydrocarbons that contain a carbon–carbon double bond (\(\text{C}=\text{C}\)). This double bond makes them unsaturated — they have fewer hydrogen atoms than the equivalent alkane with the same number of carbon atoms.
The general formula for alkenes is \(\text{C}_n\text{H}_{2n}\). The first three members are: ethene (\(\text{C}_2\text{H}_4\)), propene (\(\text{C}_3\text{H}_6\)), and butene (\(\text{C}_4\text{H}_8\)). Alkenes are more reactive than alkanes because of the \(\text{C}=\text{C}\) double bond.
Test for alkenes: Add bromine water (orange) to the substance. If an alkene is present, the bromine water is decolourised (turns colourless) because bromine adds across the double bond in an addition reaction. Alkanes do not decolourise bromine water because they have no double bonds.
Alkenes undergo addition reactions because the double bond can open up and bond with other atoms:
- With hydrogen (hydrogenation): Produces an alkane. \(\text{C}_2\text{H}_4 + \text{H}_2 \rightarrow \text{C}_2\text{H}_6\) (using a nickel catalyst at 150 °C).
- With water/steam (hydration): Produces an alcohol. \(\text{C}_2\text{H}_4 + \text{H}_2\text{O} \rightarrow \text{C}_2\text{H}_5\text{OH}\).
- With bromine: Produces a dibromo compound. \(\text{C}_2\text{H}_4 + \text{Br}_2 \rightarrow \text{C}_2\text{H}_4\text{Br}_2\).
Alcohols
Alcohols contain the functional group \(-\text{OH}\) (hydroxyl group). The general formula is \(\text{C}_n\text{H}_{2n+1}\text{OH}\). The first three members are methanol (\(\text{CH}_3\text{OH}\)), ethanol (\(\text{C}_2\text{H}_5\text{OH}\)), and propanol (\(\text{C}_3\text{H}_7\text{OH}\)).
Combustion: Alcohols burn in air to produce carbon dioxide and water. \(\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}\). This makes them useful as fuels (e.g. ethanol as a biofuel).
With sodium: Alcohols react with sodium metal to produce hydrogen gas and a sodium alkoxide. \(2\text{C}_2\text{H}_5\text{OH} + 2\text{Na} \rightarrow 2\text{C}_2\text{H}_5\text{ONa} + \text{H}_2\). This reaction is less vigorous than sodium with water.
Oxidation: Alcohols can be oxidised to form carboxylic acids. This happens when ethanol is left exposed to air (bacterial oxidation turns wine into vinegar), or when an acidified potassium dichromate solution is heated with the alcohol — the solution changes from orange to green.
Ethanol is produced in two ways: (1) Fermentation of sugars using yeast at 25–30 °C in anaerobic conditions (\(\text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2\)), and (2) Hydration of ethene with steam using a phosphoric acid catalyst at 300 °C and 60–70 atm.
Carboxylic Acids
Carboxylic acids contain the functional group \(-\text{COOH}\) (carboxyl group). The first three members are methanoic acid (\(\text{HCOOH}\)), ethanoic acid (\(\text{CH}_3\text{COOH}\)), and propanoic acid (\(\text{C}_2\text{H}_5\text{COOH}\)).
Carboxylic acids are weak acids — they do not fully dissociate (ionise) in water. This means a solution of ethanoic acid has a higher pH (is less acidic) than a solution of hydrochloric acid at the same concentration.
Carboxylic acids react like other acids:
- With metal carbonates: Produce a salt, water, and carbon dioxide. \(2\text{CH}_3\text{COOH} + \text{Na}_2\text{CO}_3 \rightarrow 2\text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2\)
- With alcohols: Produce an ester and water in a condensation reaction (requires an acid catalyst). \(\text{CH}_3\text{COOH} + \text{C}_2\text{H}_5\text{OH} \rightarrow \text{CH}_3\text{COOC}_2\text{H}_5 + \text{H}_2\text{O}\)
Esters often have fruity smells and are used in flavourings and perfumes.
The three homologous series are linked: alkenes can be hydrated to make alcohols, and alcohols can be oxidised to make carboxylic acids. Understanding these links helps you see how organic chemistry fits together.