CBSE Class 10 Chemistry Carbon and Its Compounds Notes Set 10

Carbon and Organic Compounds

Organic chemistry is the chemistry of hydrocarbons and their derivatives. Carbon is tetravalent and forms strong covalent bonds with itself (catenation) and with H, O, N, S and the halogens, giving chains, branched chains and rings. Organic compounds are generally covalent, have low melting and boiling points, are insoluble in water but soluble in organic solvents, burn, and often show isomerism.
Urea, made by Wohler from ammonium cyanate, was the first organic compound made in a laboratory; it disproved the vital force theory proposed by Berzelius.

Classification and Homologous Series

1. Open chain (acyclic or aliphatic) compounds: straight or branched. Closed chain compounds: alicyclic and aromatic.
2. A homologous series is a family with the same functional group and similar chemical properties, in which successive members differ by a CH₂ group (14 u). Physical properties change gradually.
3. General formulae: alkanes \( C_nH_{2n+2} \), alkenes \( C_nH_{2n} \), alkynes \( C_nH_{2n-2} \).

IUPAC Nomenclature

1. Select the longest chain that contains the principal functional group or multiple bond; its length gives the root (meth, eth, prop, but, pent, hex).
2. Number the chain so that the functional group, then the multiple bond, then the substituents receive the lowest numbers.
3. Name branches as prefixes in alphabetical order, with di, tri for repeats; halogens are prefixes (bromo, chloro).
4. Suffixes: alcohol –ol, aldehyde –al, ketone –one, carboxylic acid –oic acid, ester –oate. Example: \( \ce{CH3-CH(COOH)-CH2-CH3} \) is 2-methylbutanoic acid.

Isomerism

Isomers have the same molecular formula but different structures.
1. Chain isomerism: different carbon skeletons, for example pentane and 2-methylbutane. Hexane, C₆H₁₄, has five chain isomers.
2. Position isomerism: the same skeleton but the functional group or multiple bond at a different place, for example propan-1-ol and propan-2-ol.
3. Functional isomerism: different functional groups, for example ethanoic acid and methyl methanoate, or an alcohol and an ether.

Alkanes, Alkenes and Alkynes

1. Alkanes have only single bonds (bond length 1.54 Å); they are fairly unreactive and give substitution reactions.
2. Alkenes have one C=C (1.34 Å) and alkynes one C≡C (1.20 Å); they are unsaturated and give addition reactions.
3. Bond energy order: C≡C > C=C > C–C, while bond length runs the other way.
4. Test for unsaturation: alkenes and alkynes decolourise bromine water and alkaline potassium permanganate, but alkanes do not. Only terminal alkynes give a white precipitate with ammoniacal silver nitrate.
5. Ethyne: made by dehydrohalogenation of a dihaloalkane with alcoholic KOH or from calcium carbide and water; it trimerises to benzene in a red hot iron tube; its oxygen flame (above 2500 °C) is used for welding. Ethyne and nitrogen both have 14 electrons.
6. Unsaturated fatty acids contain at least one C=C bond.