Chemistry · Hydrocarbons · NEET
The molecular formula only counts how many atoms of each kind are present. It does NOT tell you HOW those atoms are joined. C5H12 always has 5 carbons and 12 hydrogens, but you can join the 5 carbons in a straight line, or with one branch, or with two branches. Each joining pattern is a different real compound with different boiling points. Same atom count, different structure = structural isomers.
Chain isomerism: the carbon skeleton changes (straight chain vs branched). Position isomerism: the skeleton stays the same, but a double bond, substituent, or group sits at a different carbon. Functional isomerism: the same formula gives a DIFFERENT functional group (like an alcohol vs an ether, or an aldehyde vs a ketone). Trick to remember: chain = shape of carbon, position = place of group, functional = kind of group.
Yes. All three are C5H12 (5 carbons, 12 hydrogens) but the carbon skeleton is different: n-pentane is a straight chain, isopentane (2-methylbutane) has one branch, neopentane (2,2-dimethylpropane) has two branches. Because only the carbon skeleton changes, they are CHAIN isomers. This matters for NEET because more branching lowers the boiling point (n-pentane > isopentane > neopentane).
No. In structural (constitutional) isomerism the atoms are actually JOINED in a different order - the bonds themselves are different. In stereoisomerism (like cis-trans or optical isomers) the bonds are joined in the SAME order, but the atoms point differently in space. So structural = different connectivity; stereo = same connectivity, different 3D arrangement. NEET loves to test this line.
Metamerism is a special sub-type of structural isomerism. It happens when different alkyl groups sit on either side of the SAME functional group. Example: CH3-O-CH2CH2CH3 (methyl propyl ether) and CH3CH2-O-CH2CH3 (diethyl ether) are both C4H10O but have different alkyl groups around the oxygen. NEET has directly asked which pair are metamers, so remember: same functional group, chopped into different alkyl halves.
Draw the longest chain first, then keep shortening the chain and moving carbons as branches. C5H12 gives exactly 3: (1) straight 5-carbon chain (n-pentane), (2) 4-carbon chain with one CH3 branch (2-methylbutane), (3) 3-carbon chain with two CH3 branches on the middle carbon (2,2-dimethylpropane). You cannot make a fourth without repeating one of these. Always name each to make sure you did not draw the same one twice.
A compound with a molecular formula of C6H14 has two tertiary carbons. Its IUPAC name is:
Statement I: The boiling point of three isomeric pentanes follows the order n-pentane > isopentane > neopentane. Statement II: When branching increases, the molecule attains a shape of sphere. This results in smaller surface area for contact, due to which the intermolecular forces between the spherical molecules are weak, thereby lowering the boiling point. Choose the most appropriate answer.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is when two or more compounds have the same molecular formula but their atoms are joined in a different order, so they are different substances with different properties. The three main types for NEET are chain, position and functional isomerism.
Chain isomerism (different carbon skeleton), position isomerism (same skeleton but the group or double bond is at a different carbon), and functional isomerism (same formula but a different functional group, such as an alcohol vs an ether). Metamerism is a minor extra type sometimes asked.
Chain: n-pentane and 2-methylbutane (both C5H12). Position: propan-1-ol and propan-2-ol (both C3H8O, the OH is on a different carbon). Functional: propanal (an aldehyde) and propanone/acetone (a ketone), both C3H6O.
It underpins many hydrocarbon questions: counting isomers, naming branched alkanes, and explaining why branched isomers boil at lower temperatures. NEET repeatedly asks you to identify isomer type or count isomers, so mastering it earns easy marks and prevents confusion with stereoisomerism.
No. Cis-trans (geometrical) isomerism is a STEREOisomerism, because the atoms are joined in the same order but arranged differently in 3D space. Structural isomers differ in the actual order in which atoms are bonded.
