Organic Chemistry

A hydrocarbon contains carbon and hydrogen only. Natural gas is mainly methane; crude oil is a mixture of hydrocarbons. Both are non-renewable because they form far more slowly than they are consumed. Burning a fuel releases energy, while the same hydrocarbon feedstock may also be valuable for manufacturing chemicals and plastics.

As hydrocarbon molecules get larger, melting and boiling points generally rise overall, viscosity increases, and volatility decreases. Smaller fractions are useful fuels; larger fractions may be cracked into more sought-after smaller molecules. The amount supplied by fractional distillation need not match the demand for each fraction. Larger molecules generally have stronger intermolecular attractions, so more energy is needed to separate them and the liquid resists flow more. These are trends within comparable members, not a claim that every individual melting point rises smoothly.

Bioethanol can be made from sugars in a crop such as sugarcane and is renewable if the crop is regrown. Growing plants take in carbon dioxide, partly offsetting carbon dioxide released when the ethanol burns. Fossil fuels instead add carbon stored underground to the active carbon cycle. Biofuel is not automatically emission-free: cultivation, processing, transport and land-use change also affect its impact.

A homologous series has a common general formula, similar chemical properties and a gradual change in physical properties as molecule size and mass increase. Successive members differ by CH₂. Alkanes are saturated hydrocarbons: all carbon-carbon bonds are single. For an unbranched or branched acyclic alkane, the general formula is C_nH_2n+2.

To draw a displayed formula, draw the carbon-carbon connections first, then add individual C-H bonds until each carbon has four bonds. A condensed CH₃ group stands for three separate hydrogens bonded to that carbon. Count atoms after drawing; moving the page or bending a chain does not create a new compound.

Alkanes are generally unreactive, but burn and undergo substitution with chlorine in ultraviolet light. Complete combustion: CH₄(g) + 2O₂(g) -> CO₂(g) + 2H₂O(l), showing cooled products. Limited oxygen can produce carbon monoxide and/or soot. Substitution: CH₄(g) + Cl₂(g) -> CH₃Cl(g) + HCl(g), under UV. One H is replaced by Cl; further substitution is possible.

Alkenes are unsaturated hydrocarbons containing a carbon-carbon double bond. For acyclic molecules with one C=C bond, their general formula is C_nH_2n. Ethene is CH₂=CH₂; propene is CH₂=CH-CH₃. Each carbon still has four bonds in total.

Aqueous bromine distinguishes an alkene from an alkane under the usual test conditions without UV. The alkene decolourises orange/brown bromine water because bromine adds across C=C. Ethene forms CH₂Br-CH₂Br. The alkane does not show this addition reaction. The colour change is an observation; unsaturation is the inference.

Cracking breaks larger hydrocarbons into smaller molecules using strong heating, often with a catalyst. Products can include smaller alkanes, alkenes and hydrogen. Examples of possible balanced changes are C₁₀H₂₂ -> C₈H₁₈ + C₂H₄, and C₂H₆ -> C₂H₄ + H₂. Actual cracking produces a mixture; use atom conservation to deduce a missing product.

Polyunsaturated food molecules contain more than one C=C bond. Hydrogenating unsaturated vegetable oils with hydrogen and a nickel catalyst reduces the number of double bonds, making the product more solid and useful in margarine manufacture. This is addition, not polymerisation.

A polymer is a large molecule built from many smaller monomer molecules. Different monomers give different repeating units and properties. In addition polymerisation, the C=C bonds open to link monomers into a chain; no small molecule is eliminated. Poly(ethene) forms from ethene and is used in plastic bags and clingfilm.

Ethene gives the repeat unit [-CH₂-CH₂-]_n. Propene, CH₂=CH-CH₃, gives [-CH₂-CH(CH₃)-]_n. The CH₃ side group stays attached; it does not become a third carbon in the two-carbon backbone repeat.

Many plastics are non-biodegradable: natural biological processes do not break them down readily. Persistent waste can harm wildlife through ingestion or entanglement, accumulate in habitats and fragment into small pieces. Burning plastics releases carbon dioxide and, depending on the plastic and conditions, other pollutants. Disposal shifts a problem unless its consequences are considered.