Carbon and its Compounds

Covalent bond

A bond formed by two atoms sharing a pair of electrons so that both reach a noble gas configuration. Bonds inside the molecule are strong but forces between molecules are weak, so covalent compounds have low melting points and conduct electricity poorly.

Why does carbon form covalent bonds rather than gaining or losing electrons?

Carbon has four valence electrons. Gaining four would leave six protons holding ten electrons, which is difficult; losing four would need a large amount of energy. Sharing its electrons lets carbon reach a noble gas configuration without either problem.

Allotropes of carbon

Forms of carbon that differ in how the atoms are bonded. In diamond each carbon bonds to four others in a rigid 3-D structure, making it the hardest substance; in graphite each bonds to three in hexagonal layers, so it is slippery and conducts electricity. Fullerenes such as C-60 are a third class.

Catenation

Carbon's ability to bond with other carbon atoms to form long chains, branched chains and rings. Silicon forms chains of only seven or eight atoms, and those compounds are very reactive, whereas carbon-carbon bonds are strong and stable.

Tetravalency

Carbon's valency of four, which lets it bond with four other atoms of carbon or of other elements. Its small size lets the nucleus hold the shared electron pairs strongly, which makes these bonds exceptionally stable.

Unsaturated hydrocarbon

A compound of carbon and hydrogen with at least one double or triple bond between carbon atoms: an alkene or an alkyne. Unsaturated compounds are more reactive than saturated alkanes, whose carbon atoms are joined only by single bonds.

Structural isomers

Compounds with the same molecular formula but different structures. Butane, C4H10, can have its four carbon atoms in a straight chain or in a branched chain, giving two different compounds.

Functional group

An atom or group of atoms, such as -OH, -Cl or -Br, that replaces hydrogen on a carbon chain and gives the compound its characteristic properties, whatever the length or shape of the chain.

Homologous series

A series of carbon compounds with the same functional group in which successive members differ by a -CH2- unit. Their chemical properties stay similar, while melting point, boiling point and solubility change gradually as molecular mass increases.

General formula of alkenes

CnH2n, where n = 2, 3, 4 and so on. Ethene, C2H4, is the first member, followed by C3H6, C4H8 and C5H10.

How is the functional group shown in the name of a carbon compound?

By a prefix or suffix on the name of the carbon chain: chloro- or bromo- for haloalkanes, -ol for alcohols, -al for aldehydes, -one for ketones, -oic acid for carboxylic acids, -ene and -yne for double and triple bonds. If the suffix starts with a vowel the chain name drops its final 'e', as in propanone.

How does the flame show whether a hydrocarbon is saturated or unsaturated, and what does a blackened cooking vessel mean?

Saturated hydrocarbons generally burn with a clean flame, while unsaturated ones give a yellow flame with black smoke. Limited air also causes incomplete combustion and soot, so a vessel blackening on a stove means the air holes are blocked and fuel is being wasted.

Oxidising agent

A substance that adds oxygen to another. Alkaline potassium permanganate and acidified potassium dichromate are oxidising agents because they convert alcohols into carboxylic acids.

What happens when ethanol is warmed with alkaline potassium permanganate or acidified potassium dichromate?

Ethanol, CH3CH2OH, is oxidised to ethanoic acid, CH3COOH, because the reagent adds oxygen to it. Added to warm ethanol, the colour of alkaline potassium permanganate disappears at first and only persists once it is in excess.

Addition reaction

A reaction in which an unsaturated hydrocarbon adds hydrogen across its multiple bond in the presence of a catalyst such as nickel or palladium, becoming saturated. It is used to hydrogenate vegetable oils with a nickel catalyst.

What is a substitution reaction, and what happens when methane reacts with chlorine in sunlight?

A reaction in which one atom or group takes the place of another. Saturated hydrocarbons are fairly unreactive, but in sunlight chlorine replaces their hydrogen atoms one by one: CH4 + Cl2 --sunlight--> CH3Cl + HCl.

What is esterification, and what is its equation?

An acid reacting with an alcohol to form an ester. Ethanoic acid warmed with absolute ethanol and a little concentrated sulphuric acid as catalyst gives a sweet-smelling ester: CH3COOH + CH3CH2OH --acid--> CH3COOC2H5 + H2O. Esters are used in perfumes and flavourings.

What is saponification, and what is its equation?

Treating an ester with sodium hydroxide, which converts it back into the alcohol and the sodium salt of the acid: CH3COOC2H5 + NaOH --> C2H5OH + CH3COONa. The name reflects its use in making soap.

What happens when ethanoic acid reacts with sodium carbonate, and what is the equation?

It forms sodium acetate, water and carbon dioxide gas, which can be tested with lime water: 2CH3COOH + Na2CO3 --> 2CH3COONa + H2O + CO2. Sodium hydrogencarbonate reacts the same way: CH3COOH + NaHCO3 --> CH3COONa + H2O + CO2.

Micelle

A cluster of soap molecules in water with their hydrophobic hydrocarbon tails pointing inwards and their ionic ends on the surface. Ion-ion repulsion keeps micelles apart so they stay in solution as a colloid, and they are large enough to scatter light, which makes soap solution cloudy.

How does soap remove oily dirt that water alone cannot?

Oil does not dissolve in water. Each soap molecule has an ionic end that interacts with water and a carbon chain that interacts with oil, so the molecules surround the oily dirt with the chains inside, forming micelles and an emulsion. The dirt is held in the water and rinsed away.

Detergent

A cleansing agent that is generally the sodium salt of a sulphonic acid, or an ammonium salt with chloride or bromide ions, with a long hydrocarbon chain. Detergents are used in shampoos and products for washing clothes.

Why are detergents better cleansing agents than soaps in hard water?

Soap reacts with the calcium and magnesium salts that make water hard, forming an insoluble scum, so more soap is needed and little foam forms. The charged ends of detergent molecules do not form insoluble precipitates with calcium and magnesium ions, so detergents stay effective.

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