Class 10 Science · Chapter 4 NotesCarbon and its Compounds

Carbon has atomic number 6, with four valence electrons. To attain noble gas configuration, it could gain four electrons to form C4– or lose four to form C4+. Both are difficult: gaining four electrons would make the nucleus hold ten electrons, and losing four requires a large amount of energy. Instead, carbon shares its four valence electrons with other atoms, forming four covalent bonds. A covalent bond is formed by the sharing of an electron pair between two atoms, so both achieve a filled outermost shell. For example, two hydrogen atoms share one electron each to form H2 with a single bond. Oxygen forms a double bond in O2, nitrogen a triple bond in N2, and carbon forms four single bonds in methane (CH4). Covalent compounds have strong bonds within molecules but weak intermolecular forces, so they have low melting and boiling points and are generally poor conductors of electricity.

• Covalent bondbond formed by sharing of electron pairs.
• Single bondone shared pair; double bond: two shared pairs; triple bond: three shared pairs.
• Carbon is tetravalentit has four valence electrons.

Carbon exists in different forms in nature with different physical properties. Diamond and graphite are both made of carbon, but their structures differ. In diamond, each carbon atom is bonded to four other carbon atoms in a rigid three-dimensional structure, making diamond the hardest known substance. In graphite, each carbon atom is bonded to three other carbon atoms in the same plane, forming hexagonal layers. One bond is a double bond, satisfying carbon's valency. Graphite is smooth, slippery and a good conductor of electricity. Fullerenes are another class of allotropes; C-60 has carbon atoms arranged like a football and is named after the architect Buckminster Fuller. Diamond can be synthesised by subjecting pure carbon to very high pressure and temperature.

• Diamondhardest natural substance, rigid 3D structure.
• Graphiteslippery, good conductor, hexagonal layers.
• FullereneC-60, football-shaped molecule.

Carbon forms millions of compounds, far more than any other element. Two key properties are responsible. First, catenation: carbon atoms can bond with other carbon atoms to form long chains, branched chains or rings. Carbon atoms can be linked by single, double or triple bonds. Second, tetravalency: carbon can bond with four other atoms of carbon or other elements like hydrogen, oxygen, nitrogen, sulphur and chlorine. The carbon-carbon bond is very strong and stable, and carbon's small size allows the nucleus to hold shared electrons strongly. These features give rise to a vast number of stable compounds. Compounds containing only carbon and hydrogen are called hydrocarbons. Saturated hydrocarbons have only single bonds (alkanes), while unsaturated hydrocarbons have double bonds (alkenes) or triple bonds (alkynes).

• Catenationability of carbon to form bonds with other carbon atoms.
• Tetravalencycarbon has a valency of four.
• Hydrocarbonscompounds of carbon and hydrogen only.

Carbon chains can be straight, branched or cyclic. For example, butane (C4H10) can have two different arrangements of carbon atoms: a straight chain (n-butane) and a branched chain (isobutane). Both have the same molecular formula but different structures; such compounds are called structural isomers. Cyclohexane (C6H12) is a ring compound. Benzene (C6H6) is another cyclic compound with alternating single and double bonds. Saturated hydrocarbons are called alkanes; unsaturated hydrocarbons with one or more double bonds are alkenes, and those with triple bonds are alkynes. The general formula for alkanes is CnH2n+2, for alkenes CnH2n, and for alkynes CnH2n–2.

• Structural isomerssame molecular formula, different structures.
• AlkanesCnH2n+2; Alkenes: CnH2n; Alkynes: CnH2n–2.
• Cyclic compoundscarbon atoms arranged in a ring.

In a hydrocarbon chain, one or more hydrogen atoms can be replaced by other elements such as halogens, oxygen, nitrogen or sulphur. These replacing elements are called heteroatoms, and the group they form is called a functional group. Common functional groups include alcohol (–OH), aldehyde (–CHO), ketone (–CO–), carboxylic acid (–COOH) and halo groups (–Cl, –Br). A homologous series is a series of compounds with the same functional group, in which successive members differ by a –CH2– unit. For example, methanol (CH3OH), ethanol (C2H5OH), propanol (C3H7OH) and butanol (C4H9OH) form a homologous series. As molecular mass increases, melting and boiling points increase, but chemical properties remain similar because they are determined by the functional group.

• Functional groupatom or group that gives characteristic properties.
• Homologous seriessame functional group, successive members differ by –CH2–.
• Physical properties change gradually; chemical properties are similar.

The names of carbon compounds are based on the carbon chain and the functional group. First, identify the number of carbon atoms in the chain: one carbon is methane, two is ethane, three is propane, and so on. If a functional group is present, it is indicated by a prefix or suffix. For example, an alcohol group is shown by the suffix –ol, so a three-carbon alcohol is propanol. A ketone group is shown by –one, so a three-carbon ketone is propanone. If the carbon chain is unsaturated, the ending ‘ane’ is replaced by ‘ene’ for a double bond or ‘yne’ for a triple bond. For example, a three-carbon chain with a double bond is propene, and with a triple bond is propyne. If the suffix begins with a vowel, the final ‘e’ of the chain name is removed before adding the suffix.

• Prefix or suffix indicates the functional group.
• Alcohol–ol; Aldehyde: –al; Ketone: –one; Carboxylic acid: –oic acid.
• Unsaturated‘ene’ for double bond, ‘yne’ for triple bond.

Carbon compounds undergo several important reactions. Combustion: carbon and its compounds burn in oxygen to release heat and light, forming carbon dioxide and water. Saturated hydrocarbons generally burn with a clean blue flame, while unsaturated hydrocarbons burn with a yellow sooty flame. Oxidation: alcohols can be oxidised to carboxylic acids using oxidising agents like alkaline potassium permanganate or acidified potassium dichromate. Addition reaction: unsaturated hydrocarbons add hydrogen in the presence of catalysts such as palladium or nickel to form saturated hydrocarbons; this is used in hydrogenation of vegetable oils. Substitution reaction: saturated hydrocarbons react with chlorine in the presence of sunlight, where chlorine replaces hydrogen atoms one by one.

• CombustionC + O2 → CO2 + heat and light.
• Oxidationalcohol → carboxylic acid using oxidising agents.
• Additionunsaturated + H2 → saturated (catalyst: Ni/Pd).
• SubstitutionCH4 + Cl2 → CH3Cl + HCl (sunlight).

Ethanol (CH3CH2OH) is a liquid at room temperature, commonly called alcohol. It is the active ingredient in alcoholic drinks, a good solvent used in medicines, and soluble in water in all proportions. Ethanol reacts with sodium to give sodium ethoxide and hydrogen gas. Heating ethanol with excess concentrated sulphuric acid at 443 K dehydrates it to ethene. Ethanoic acid (CH3COOH) is a carboxylic acid; a 5-8% solution in water is vinegar, used as a preservative. It is a weak acid. It reacts with ethanol in the presence of an acid catalyst to form an ester (esterification), with a base like sodium hydroxide to form sodium acetate and water, and with carbonates and hydrogencarbonates to give salt, carbon dioxide and water.

• EthanolCH3CH2OH; reacts with Na to give H2 gas.
• Ethanoic acidCH3COOH; weak acid, vinegar is 5-8% solution.
• Esterificationacid + alcohol → ester (sweet-smelling).
• Ethanoic acid + NaHCO3 → sodium acetate + H2O + CO2.

Soaps are sodium or potassium salts of long-chain carboxylic acids. A soap molecule has two ends: a hydrophilic (water-loving) ionic end and a hydrophobic (water-repelling) hydrocarbon tail. When soap is added to water, the molecules arrange themselves in clusters called micelles, with the hydrocarbon tails pointing inwards and ionic ends outwards. The oily dirt gets trapped in the centre of the micelle, forming an emulsion that is rinsed away with water. Soap does not work well in hard water because calcium and magnesium ions present in hard water react with soap to form an insoluble scum. Detergents are sodium salts of sulphonic acids or ammonium salts with long hydrocarbon chains; their charged ends do not form insoluble precipitates with calcium and magnesium ions, so they work effectively even in hard water.

• Soapsodium/potassium salt of long-chain carboxylic acid.
• Micellecluster with hydrophobic tails inside, hydrophilic ends outside.
• Hard water + soap → scum (insoluble precipitate).
• Detergents work in hard water because they do not form scum.