Hydrocarbons MCQs for NEET — Chemistry Questions with Answers

The NCERT states, 'However, the carbon atom of the branch that attaches to the root alkane is numbered 1 as exemplified below.'

As per NCERT Rule 5, 'If the two substituents are found in equivalent positions, the lower number is given to the one coming first in the alphabetical listing.'

The NCERT text clearly defines: 'Unsaturated hydrocarbons are those, which contain at least one carbon-carbon double or triple bond.'

Heptane is a straight-chain saturated hydrocarbon with only single C-C bonds, falling under the definition of an alkane. Propene is an alkene, Ethyne is an alkyne, and Benzene is an aromatic hydrocarbon.

The NCERT summary states: 'Alkanes show conformational isomerism due to free rotation along the C–C sigma bonds. Out of staggered and the eclipsed conformations of ethane, staggered conformation is more stable as hydrogen atoms are farthest apart.'

The NCERT text explains: 'Alkenes exhibit geometrical (cis-trans) isomerism due to restricted rotation around the carbon–carbon double bond.' For geometrical isomerism, each carbon of the double bond must be attached to two different groups. In but-2-ene ($CH_3-CH=CH-CH_3$), both carbons of the double bond are attached to a hydrogen and a methyl group, allowing for cis and trans isomers. Pent-1-ene ($CH_2=CH-CH_2-CH_2-CH_3$) has one carbon of the double bond attached to two hydrogen atoms, preventing geometrical isomerism.

The NCERT text states: 'Functional group isomerism: Two or more compounds having the same molecular formula but different functional groups are called functional isomers and this phenomenon is termed as functional group isomerism. For example, the molecular formula C3H6O represents an aldehyde and a ketone.' Propanal is an aldehyde and propanone is a ketone, both having the formula $C_3H_6O$ but different functional groups.

The NCERT text defines isomerism: 'The phenomenon of existence of two or more compounds possessing the same molecular formula but different properties is known as isomerism. Such compounds are called as isomers.' This difference in properties arises due to differences in their structures.

The NCERT defines carbon types: 'Tertiary carbon is attached to three carbon atoms and neo or quaternary carbon is attached to four carbon atoms.' In 2,2-dimethylpropane ($C(CH_3)_4$), the central carbon atom is bonded to four other carbon atoms (making it a quaternary or 4° carbon), and each of the four methyl groups has a carbon atom bonded to only one other carbon atom (making them primary or 1° carbons).

The problem in the NCERT text (Problem 9.1) asks to 'Write structures of different chain isomers of alkanes corresponding to the molecular formula C6H14. Also write their IUPAC names.' The solution lists five isomers for $C_6H_{14}$: n-Hexane, 2-Methylpentane, 3-Methylpentane, 2,3-Dimethylbutane, and 2,2-Dimethylbutane. These are all chain isomers.