Organic Chemistry: Hydrocarbons, Alcohols and Polymers
Crude Oil and Fractional Distillation
Crude oil is a finite resource and a vital feedstock for the petrochemical industry. It is a mixture of hundreds of different hydrocarbons — molecules made up of only hydrogen and carbon atoms. The most common hydrocarbons in crude oil are alkanes.
Crude oil is separated into useful fractions by fractional distillation. The oil is heated so that most of it vaporises. The vapours rise up a tall fractionating column that is hot at the bottom and cool at the top. Different fractions condense at different heights depending on the boiling points of their molecules. Shorter-chain molecules have weaker forces between them and lower boiling points, so they condense near the top; longer-chain molecules with stronger forces between them condense near the bottom. The main fractions include refinery gases, petrol (gasoline), kerosene, diesel, fuel oil and bitumen.
Shorter-chain hydrocarbons have lower boiling points, lower viscosity and are more flammable. They are collected near the top of the fractionating column. Longer-chain hydrocarbons condense near the bottom, are more viscous and less easily ignited.
Alkanes
Alkanes are saturated hydrocarbons — they have only single bonds between carbon atoms. Their general formula is \(C_nH_{2n+2}\). The first four members are methane (\(CH_4\)), ethane (\(C_2H_6\)), propane (\(C_3H_8\)) and butane (\(C_4H_{10}\)). A common exam error is writing the formula as C\(_n\)+H\(_{2n+2}\) with a plus sign — it is \(C_nH_{2n+2}\) with the subscripts joined directly.
Alkanes burn in oxygen. Complete combustion (with excess oxygen) produces carbon dioxide and water:
$$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$$
Incomplete combustion (limited oxygen) produces carbon monoxide and/or carbon (soot) alongside water. Carbon monoxide is a toxic, colourless, odourless gas that binds to haemoglobin and prevents oxygen transport.
Cracking
There is greater demand for shorter-chain hydrocarbons (for fuels and as chemical feedstocks) than for longer-chain ones. Cracking converts surplus long-chain alkanes into shorter, more useful molecules. Two methods are used: catalytic cracking (passing hydrocarbon vapour over a hot catalyst at moderate temperatures) and steam cracking (mixing with steam at very high temperatures). Both methods always produce a mixture of shorter alkanes and alkenes. For example:
$$C_{10}H_{22} \rightarrow C_8H_{18} + C_2H_4$$
The alkene produced (here ethene, \(C_2H_4\)) is a key raw material for making polymers. You can distinguish alkenes from alkanes using bromine water: alkenes decolourise orange bromine water to colourless (the bromine adds across the double bond), whereas alkanes produce no colour change because they are saturated.
Alkenes have the general formula \(C_nH_{2n}\) and contain at least one \(C{=}C\) double bond. Because the double bond is a site of high electron density, alkenes undergo addition reactions: molecules like hydrogen, water or bromine add across the double bond, breaking it and forming a single-bonded product. This is why alkenes are far more reactive than alkanes and are used as starting materials in chemical synthesis.
Alcohols
Alcohols are organic compounds that contain a hydroxyl functional group (–OH). The first four are methanol (\(CH_3OH\)), ethanol (\(C_2H_5OH\)), propanol (\(C_3H_7OH\)) and butanol (\(C_4H_9OH\)). Ethanol is the alcohol in alcoholic drinks and is also used as a fuel and solvent.
Ethanol can be produced in two ways. Fermentation uses yeast to convert glucose into ethanol and carbon dioxide under anaerobic (no oxygen) conditions at around 25–35 °C — above 40 °C the yeast enzymes denature; below ~15 °C the reaction is too slow. Hydration of ethene (a reversible reaction requiring steam and a catalyst) is a faster industrial method used when ethene is available from cracking. Fermentation uses a renewable starting material (plant sugars) whereas hydration uses a finite resource (crude oil-derived ethene).
Fermentation: glucose → ethanol + carbon dioxide, using yeast under anaerobic conditions at approximately 25–35 °C. Too hot and the yeast enzymes denature; too cold and the rate is unacceptably slow. The ethanol produced can be used as a biofuel or purified for drinks.
Carboxylic Acids
Carboxylic acids contain the –COOH functional group. The simplest examples are methanoic acid (\(HCOOH\)) and ethanoic acid (\(CH_3COOH\), the acid in vinegar). Carboxylic acids dissolve in water to form weakly acidic solutions. They react with carbonates to produce a salt, water, and carbon dioxide — for example, ethanoic acid with sodium carbonate produces sodium ethanoate, water and carbon dioxide. They also react with alcohols (in the presence of an acid catalyst) to form esters and water.
Addition polymers are made when many small alkene monomer molecules join together by opening their \(C{=}C\) double bonds. No other product is formed — all atoms end up in the polymer chain. The reaction has 100% atom economy. Common addition polymers include poly(ethene) made from ethene, and poly(propene) made from propene. The repeat unit of the polymer has the same number of carbon and hydrogen atoms as the monomer, but no double bond.
Addition polymers are widely used in packaging, containers, pipes and clothing fibres. However, they are generally non-biodegradable and can persist in the environment for a very long time, creating significant waste management challenges.
Condensation polymers are formed when two different types of monomer join together and a small molecule — usually water — is eliminated at each bond. Unlike addition polymerisation, the atom economy is less than 100% because water is lost. Examples include nylon (a polyamide, formed from diamines and dicarboxylic acids) and polyester (formed from diols and dicarboxylic acids). These materials are used in textiles, bottles and ropes. Because the monomers have two functional groups each (one at each end of the molecule), very long chains form.
Addition polymerisation: alkene monomers join with no other products (100% atom economy). Condensation polymerisation: monomers with two functional groups join, releasing water each time (atom economy below 100%). Examples: poly(ethene) is addition; nylon and polyester are condensation polymers.