C11: Organic Chemistry

Natural gas, mainly methane, and crude oil are non-renewable energy resources. Crude oil is a hydrocarbon mixture separated by fractional distillation into fractions that compete as fuels and chemical feedstocks.

Bioethanol from sugarcane is renewable. Carbon dioxide released during burning can be partly offset by carbon dioxide absorbed during plant growth, but a complete sustainability discussion also considers land, processing, transport, and competing uses.

A homologous series shares a general formula and chemical behaviour while physical properties change gradually with molecular size. Alkanes are saturated CnH2n+2 hydrocarbons; draw and name methane to propane. They undergo combustion and chlorine substitution.

Increasing molecular size generally raises melting point, boiling point, and viscosity within a homologous series. The required unbranched alkane structures are methane $\ce{CH4}$, ethane $\ce{CH3-CH3}$, and propane $\ce{CH3-CH2-CH3}$. Alkanes are generally unreactive apart from combustion and substitution by chlorine. In ultraviolet light, a chlorine atom replaces a hydrogen atom, forming a chloroalkane and hydrogen chloride.

Alkenes are unsaturated $C_nH_{2n}$ hydrocarbons. The required unbranched structures are ethene $\ce{CH2=CH2}$ and propene $\ce{CH2=CH-CH3}$. Cracking makes smaller alkenes and hydrogen to match demand for smaller refinery molecules. Aqueous bromine is orange or brown before addition and becomes colourless with an alkene; it remains orange or brown with an alkane under the same test conditions. In addition reactions, the carbon-carbon double bond opens so bromine or hydrogen atoms join the two carbon atoms. Alkenes also burn and form addition polymers. Hydrogenation converts unsaturated vegetable oil into solid margarine; polyunsaturated means several carbon-carbon double bonds are present.

Alcohols form a homologous series containing the $\ce{-OH}$ group. The required unbranched structures are methanol $\ce{CH3OH}$, ethanol $\ce{CH3CH2OH}$, and propanol $\ce{CH3CH2CH2OH}$. In complete combustion they form carbon dioxide and water, and they can be oxidised to carboxylic acids. During fermentation, yeast enzymes convert glucose into ethanol and carbon dioxide under warm conditions without oxygen.

Carboxylic acids form a homologous series containing the $\ce{-CO2H}$ group. Ethanol becomes ethanoic acid through oxidation by atmospheric oxygen or acidified potassium manganate(VII).

A polymer is a large molecule built from repeating monomer units. Ethene forms poly(ethene) by addition polymerisation for uses such as bags and clingfilm. Deduce a repeat unit from the monomer by opening the double bond, and reverse the process to identify the monomer.

Non-biodegradable plastic accumulates when discarded. Physical recycling can remelt sorted poly(ethene) into pellets, while chemical recycling can crack plastic into fuel. Evaluate collection, sorting, contamination, energy, cost, jobs, resource use, emissions, and pollution rather than declaring one method universally best.