Chapter 4 · Question 6
What are structural isomers? Using the molecular formula , draw the two possible structural isomers. How do chains, branches, and rings expand the diversity of carbon compounds?
Q6
What are structural isomers? Using the molecular formula , draw the two possible structural isomers. How do chains, branches, and rings expand the diversity of carbon compounds?
Answer Revealed
Direct Answer:
Structural isomers are compounds with the same molecular formula but different structures (different arrangement of atoms). For , the two isomers are: (i) n-butane — a straight chain of four carbon atoms (), and (ii) isobutane — a branched chain with three carbons in a row and one methyl () branch on the central carbon. Carbon chains can be straight (n-butane), branched (isobutane), or cyclic (cyclohexane , benzene ). These three modes — chains, branches, and rings — combined with single, double, and triple bonds and the possibility of various functional groups — generate the enormous diversity of organic compounds.
Simple Explanation
Isomers are like Lego structures built from the same bricks but arranged differently. Butane () has two ways to arrange its four carbon atoms: in a straight line (n-butane) or shaped like a Y (isobutane). Both have exactly and atoms, but their atoms are connected in a different order. Adding branches, closing chains into rings (like cyclohexane's hexagon), and combining these with different bond types and functional groups gives carbon the ability to build millions of different compounds.
Exam-Ready Structure
Structural isomerism is a direct consequence of carbon's catenation and tetravalency: 1. Definition: Structural isomers are compounds that have the identical molecular formula but different structural arrangements (different connectivity of atoms). They are different compounds with different physical and chemical properties. 2. Isomers of (butane): (a) n-Butane — a straight chain: . All four carbon atoms are connected in a continuous linear chain. (b) Isobutane (2-methylpropane) — a branched chain: . Three carbons form the main chain with one carbon as a methyl branch on the second carbon. Both have formula but their boiling points and other properties differ because the branching affects the shape and intermolecular forces. 3. Chains, branches, and rings — building blocks of organic diversity: (a) Straight chains — carbon atoms linked linearly (e.g., methane → ethane → propane → butane → pentane → hexane in Table 4.2). (b) Branched chains — one or more carbon atoms branch off from the main chain (e.g., isobutane). The possibility of branching increases rapidly with the number of carbon atoms (pentane has 3 isomers). (c) Ring (cyclic) structures — carbon atoms arranged in closed loops. Saturated cyclic: cyclohexane (), a hexagonal ring with single bonds. Unsaturated cyclic: benzene (), a hexagonal ring with alternating single and double bonds. 4. The combinatorial effect — chains, branches, and rings, plus single/double/triple bonds, plus the variety of functional groups (alcohols, aldehydes, ketones, carboxylic acids, etc.) attached at different positions — generates the millions of known carbon compounds.
Key Points
- Structural isomers: same molecular formula, different arrangement/connectivity of atoms
- has 2 isomers: n-butane (straight chain) and isobutane (branched chain)
- Pentane () has 3 structural isomers
- Carbon chains: straight (e.g., n-butane), branched (e.g., isobutane), or cyclic (e.g., cyclohexane, benzene)
- Cyclohexane () is a saturated cyclic compound; benzene () is an unsaturated cyclic compound
Related Questions
Q5
Distinguish between saturated and unsaturated carbon compounds with the help of examples — ethane (), ethene (), and ethyne (). Draw their electron dot structures. Which type is more reactive and why?
Q3