Carbon and its Compounds

The atomic number of carbon is 6. The electron arrangement is 2, 4. It needs four more electrons to complete an octet.

Gaining four would make C⁴⁻, but six protons cannot hold ten electrons. Losing four would make C⁴⁺, and that costs a huge amount of energy. So carbon shares electrons and forms covalent bonds. In methane, carbon shares one pair with each of four hydrogens.

The bond inside the molecule is strong. The pull between molecules is weak. Melting and boiling points stay low, and these compounds usually do not conduct electricity. No ions are formed.

Diamond and graphite are both only carbon. The difference is how the atoms join. In diamond every carbon is joined to four other carbons. The frame is rigid and spreads in three directions. Diamond is the hardest natural substance.

In graphite every carbon is joined to three other carbons in the same plane. Hexagonal sheets sit one above another. One bond behaves like a double bond, so the valency is satisfied. The layers slide, so graphite feels slippery. Because of free electrons it conducts electricity.

In C60 sixty carbons sit in the shape of a football. This is a fullerene. The name comes from the dome of the architect Buckminster Fuller. Small diamonds are also made by putting pure carbon under very high pressure and temperature.

Carbon joins its own atoms into chains. This is catenation. A chain may be straight, branched or a ring. Silicon also makes short chains, but they stay very reactive. The carbon-carbon bond is strong and the atom is small, so the compounds last.

Compounds with only single bonds are saturated, such as ethane C2H6. A double or triple bond between carbon atoms makes the compound unsaturated. Ethene C2H4 has a double bond, ethyne C2H2 a triple bond. Unsaturated compounds are more reactive.

Compounds of only carbon and hydrogen are hydrocarbons. The saturated ones are alkanes, general formula CnH2n+2. Alkenes with one double bond are CnH2n. Alkynes with one triple bond are CnH2n−2. C4H10 has two structures. Compounds with the same formula and different structures are structural isomers.

If another atom or group stands in place of hydrogen, it is a functional group. That group decides the chemical behaviour, whether the chain is long or short.

The name starts from the carbon count: 1 meth, 2 eth, 3 prop, 4 but, 5 pent. A three-carbon chain is propane. If the suffix starts with a vowel, drop the final e. Propane − e + one = propanone.

A double bond replaces ane with ene, a triple bond with yne. Three carbons and a double bond = propene. Three carbons and a triple bond = propyne.

Carbon and most of its compounds give heat and light on burning. In plenty of air a saturated hydrocarbon gives a clean blue flame. An unsaturated compound gives a yellow flame and black smoke.

If air is short, even a saturated compound burns incompletely and gives soot. That is why a gas or kerosene stove has air holes. A blackened pan means the holes are blocked and fuel is being wasted.

Coal and petroleum contain some nitrogen and sulphur. Their oxides dirty the air. Coal sometimes only glows red and gives heat, because a flame forms when gases burn. The yellow of a candle flame comes from half-burnt carbon particles.

A substance that adds oxygen to another is an oxidising agent. Alkaline KMnO4 and acidified potassium dichromate turn alcohols into acids. Ethanol becoming ethanoic acid is oxidation, because oxygen is added to the molecule.

Unsaturated hydrocarbons add hydrogen in the presence of nickel or palladium and become saturated. The long chains in vegetable oils are usually unsaturated. Hydrogen is added on a nickel catalyst to turn them into solid fat. Oils with unsaturated chains are the better choice for cooking. A catalyst changes the speed and is not used up itself.

Saturated hydrocarbons stay sluggish with most reagents. In sunlight chlorine quickly takes the place of hydrogen. CH₄ + Cl₂ → CH₃Cl + HCl This is substitution. One atom takes the place of another.

The melting point of ethanol is 156 K and the boiling point is 351 K. This is the alcohol of everyday speech. It is a solvent in medicines, such as tincture of iodine. It dissolves in water in every proportion.

A little dilute ethanol causes drunkenness. Even a small amount of pure ethanol (absolute alcohol) can be lethal. Long use damages health. Methanol is more dangerous still. The liver turns it into methanal, which can harm the optic nerve and cause blindness. Industrial alcohol is made denatured by adding methanol and a blue dye, so that nobody drinks it.

At 443 K, excess concentrated sulphuric acid removes water from ethanol and makes ethene. The concentrated acid is a dehydrating agent here. C₂H₅OH → C₂H₄ + H₂O

The common name of ethanoic acid is acetic acid. A 5–8% solution in water is vinegar, a preservative for pickles. The melting point of the pure acid is 290 K. In a cold winter it freezes, so it is called glacial acetic acid.

Carboxylic acids are weaker than mineral acids. HCl is almost fully ionised, ethanoic acid is not. With a base, a salt and water form: NaOH + CH₃COOH → CH₃COONa + H₂O

Soap is a sodium or potassium salt of a long-chain carboxylic acid. The ionic end is friendly with water, that is hydrophilic. The carbon tail is friendly with oil, that is hydrophobic.

Inside water the tails hide in the middle of the drop and the ionic heads stay outside. That cluster is a micelle. Heads of the same charge do not come close, so the micelle stays suspended like a colloid and the dirt rinses away. Micelles scatter light, so a soap solution looks cloudy.