JEE Chemistry Organic Chemistry Some Basic Principles and Techniques Notes Set 03

Classification of Organic Compounds

Organic compounds are first split by the arrangement of the carbon skeleton.
Acyclic (open chain) compounds have a straight or branched chain, for example propane and but-1-ene. Cyclic (closed chain) compounds contain a ring. Cyclic compounds are of two kinds: homocyclic or carbocyclic, in which the ring has carbon atoms only, and heterocyclic, in which at least one ring atom is not carbon (N, O or S).

Carbocyclic compounds are further divided into alicyclic (cyclohexane, cyclohexene) and aromatic compounds (benzene and its derivatives). Heterocyclic examples are pyridine (N), furan (O) and thiophene (S).

Homologous Series

A homologous series is a family of compounds with the same functional group and the same general formula, in which each member differs from the next by a CH₂ unit. Members show a gradual change in physical properties (boiling point, density) and similar chemical properties.
Common general formulas: alkane C_nH_2n+2; alkene and cycloalkane C_nH_2n; alkyne C_nH_2n−2; alkanol C_nH_2n+2O; aldehyde and ketone C_nH_2nO; carboxylic acid and ester C_nH_2nO₂.

Hybridisation of Carbon

A simple way to find the hybridisation: count the sigma bonds plus lone pairs. Two gives sp, three gives sp² and four gives sp³. Every double bond has one sigma and one pi bond, and every triple bond has one sigma and two pi bonds. In an acyclic molecule the number of sigma bonds is one less than the number of atoms.

Primary, Secondary, Tertiary and Quaternary Carbons

A carbon atom is called primary (1°), secondary (2°), tertiary (3°) or quaternary (4°) if it is bonded to one, two, three or four other carbon atoms. A hydrogen atom takes the same label as the carbon it is attached to. In toluene, for example, the three hydrogens of CH₃ are 1° and the five ring hydrogens are 2°.
Alcohols and alkyl halides are classified by the carbon that carries OH or X, but amines are classified by the number of alkyl groups on nitrogen. So (CH₃)₃C–OH is a tertiary alcohol and (CH₃)₃C–NH₂ is a primary amine.

Common Alkyl and Other Groups

Common Names to Remember

Worked Examples

Illustration 1. State the hybridisation of each carbon in CH₃–CH=CH–C≡CH.
Answer:
From the left: CH₃ is sp³, the two double-bond carbons are sp², and the two triple-bond carbons are sp.
Order: sp³, sp², sp², sp, sp.

Illustration 2. Find the number of sigma and pi bonds in CH₂=CH–C≡N.
Answer:
Atoms: 3 C + 3 H + 1 N = 7, so sigma bonds = 7 − 1 = 6.
Pi bonds: one in C=C and two in C≡N = 3.
Answer: 6 sigma and 3 pi bonds.

Illustration 3. How many 1°, 2° and 3° hydrogen atoms does 2-methylbutane have?
Answer:
Structure: CH₃–CH(CH₃)–CH₂–CH₃.
Three CH₃ groups give 9 hydrogens of 1°. The CH₂ gives 2 hydrogens of 2°. The CH gives 1 hydrogen of 3°.
Answer: 9, 2 and 1.