Carbon and its Compounds — Notes

Carbon (2, 4) cannot easily form $\ce{C^4-}$ (six protons cannot hold ten electrons) or $\ce{C^4+}$ (too much energy needed). It shares electrons: a shared pair is a covalent bond. One, two and three shared pairs make single, double and triple bonds: $\ce{H2}$ (single), $\ce{O2}$ (double), $\ce{N2}$ (triple); methane has four C–H single bonds; carbon dioxide has two C=O double bonds.

Covalent compounds have low melting and boiling points (weak forces between molecules) and are generally poor conductors of electricity (no ions).

Allotropes of carbon: diamond — each carbon bonded to four others in a rigid 3-D network; hardest natural substance; non-conductor. Graphite — each carbon bonded to three others in hexagonal layers; layers slide; good conductor. Fullerenes — e.g. $\ce{C60}$, shaped like a football.

• Catenation: carbon links with other carbon atoms to form long chains, branched chains and rings. (Silicon forms chains of only about 7–8 atoms, and these are reactive.)
• Tetravalency: a valency of 4 lets carbon bond with four other atoms — carbon, hydrogen, oxygen, nitrogen, sulphur, chlorine and others.
• Carbon atoms are small, so the bonds are strong and the compounds stable. These properties give carbon millions of compounds.

Hydrocarbons contain only carbon and hydrogen.

• Saturated (alkanes, single bonds only): $C_nH_{2n+2}$ — methane, ethane, propane, butane, pentane, hexane.
• Unsaturated: alkenes (one C=C), $C_nH_{2n}$ — ethene, propene; alkynes (one C≡C), $C_nH_{2n-2}$ — ethyne.
• Carbon skeletons can be straight, branched or rings (cyclohexane $\ce{C6H12}$; benzene $\ce{C6H6}$ with alternating double bonds).
• Structural isomers: same molecular formula, different structures, e.g. butane and isobutane ($\ce{C4H10}$).

Functional groups decide the properties: halo (–Cl, –Br), alcohol (–OH), aldehyde (–CHO), ketone (>C=O), carboxylic acid (–COOH).

Homologous series: same functional group; successive members differ by $\ce{CH2}$ (14 u); same general formula; similar chemical properties; gradation in physical properties (melting and boiling points rise with molecular mass).

Naming (up to six carbons): root for the number of carbons (meth, eth, prop, but, pent, hex) — count the carbon of –CHO or –COOH — then a prefix or suffix:

• halogen: chloro-, bromo- (chloropropane);
• alcohol: -ol (propanol); aldehyde: -al (propanal); ketone: -one (propanone);
• carboxylic acid: -oic acid (propanoic acid); double bond: -ene; triple bond: -yne. Drop the final "e" of the root before a suffix that starts with a vowel.

• Combustion: carbon compounds burn in oxygen to give carbon dioxide and water with heat and light. Saturated hydrocarbons usually give a clean blue flame; unsaturated ones a yellow, sooty flame. Limited air gives incomplete combustion (soot, carbon monoxide).
• Oxidation: alcohols are oxidised to carboxylic acids by alkaline potassium permanganate or acidified potassium dichromate (oxidising agents).
• Addition: unsaturated compounds add hydrogen in the presence of a nickel or palladium catalyst (hydrogenation), e.g. vegetable oils → vanaspati.
• Substitution: in sunlight, chlorine replaces hydrogen atoms of alkanes one by one.

Ethanol ($\ce{C2H5OH}$): colourless liquid, soluble in water; used in medicines (tincture of iodine, cough syrups), sanitisers, as a solvent and as fuel blended with petrol. Methanol is highly poisonous; denatured alcohol contains methanol and a blue dye.

• With sodium: sodium ethoxide + hydrogen.
• With hot concentrated sulphuric acid at 443 K: dehydration to ethene.

Ethanoic acid ($\ce{CH3COOH}$): weak acid; 5–8 % solution is vinegar; pure acid freezes at 290 K (glacial acetic acid).

• Esterification: acid + alcohol (acid catalyst) → sweet-smelling ester + water. Esters are used in perfumes and flavours.
• Saponification: ester + sodium hydroxide → alcohol + sodium salt of the acid.
• With bases → salt + water; with carbonates and hydrogencarbonates → salt + water + carbon dioxide.

Soaps are sodium or potassium salts of long-chain carboxylic acids. Each molecule has an ionic, hydrophilic head and a long hydrocarbon, hydrophobic tail.

Cleansing action: in water, soap molecules form micelles — tails point inwards into the oily dirt, ionic heads point outwards into the water. The micelles form an emulsion and are rinsed away with the dirt.

Hard water contains calcium and magnesium salts, which react with soap to form insoluble scum; much soap is wasted. Detergents (sodium salts of sulphonic acids, or ammonium salts with chloride or bromide ions) do not form insoluble precipitates with these ions, so they lather in hard water. They are used in shampoos and washing powders.