See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Step 1 — Identify the molecular formula and connectivity of each structure. Structure (a): Wedge-dash representation of a carbon chain. The central carbon bears H, H, CH3, CH3, and C2H5 as substituents — but a carbon can only have four bonds. Reading carefully: this is a sawhorse/wedge-dash of a two-carbon unit. The left carbon has H (wedge), H, CH3 (dash) and is bonded to the right carbon which has CH3, C2H5 (dash). This gives 3-methylpentane: CH3-CH(CH3)-CH2-CH2-CH3 or more precisely it depicts a specific conformation of 3-methylpentane... Actually re-reading: the structure shows a single stereocenter with substituents H, H, CH3, C2H5 — this is 2-methylbutane (isopentane) or a segment thereof. Most likely (a) is a specific conformation of 3-methylpentane shown as a wedge-dash formula. Structure (b): Newman projection. Front carbon: C2H5, H3C, H. Back carbon: H, H, CH3. This represents a staggered or eclipsed conformation of 3-methylpentane (the C2-C3 bond Newman projection with ethyl and methyl groups). Structure (c): Wedge-dash with two CH3 groups on wedges and two CH3 groups on dashes and one H — this is 2,3-dimethylbutane with the central C-C bond shown, with each carbon bearing two methyls and one H. Structure (d): Sawhorse drawing with CH3, CH3 on front carbon and C2H5, H on back carbon — this is another conformation of 3-methylpentane. Structure (e): Newman projection with CH3, H, CH3 on front and CH3, H3C, H on back — front carbon has two methyls and one H; back carbon has two methyls (H3C and CH3) and one H. This gives a carbon with substituents (CH3)2CH- on each side, which is 2,3-dimethylbutane. Step 2 — Group by constitutional identity. Group 1 (3-methylpentane, C6H14, branched): structures (a), (b), (d) all appear to represent 3-methylpentane in different conformations/representations. Group 2 (2,3-dimethylbutane, C6H14, different branching): structures (c) and (e) both represent 2,3-dimethylbutane. Step 3 — Answer A: Which are identical in all respects? (c) and (e): Both are 2,3-dimethylbutane. Structure (c) is a wedge-dash and (e) is a Newman projection of the same compound in the same configuration/conformation — they represent identical molecules. (b) and (d): Both are Newman projection and sawhorse of 3-methylpentane in the same conformation — they are identical representations of the same molecule in the same arrangement. Answer A: (c & e) and (b & d). Step 4 — Answer B: Which are conformational isomers? Conformational isomers have the same connectivity and configuration but differ only by rotation about single bonds. (a) and (b) are both 3-methylpentane but in different conformations (different Newman/wedge-dash representations showing different rotational states). (a) and (d) are also both 3-methylpentane in different conformations. Answer B: (a & b) or (a & d). Step 5 — Answer C: Which are constitutional isomers? Constitutional isomers have the same molecular formula but different connectivity. 3-Methylpentane [(a),(b),(d)] and 2,3-dimethylbutane [(c),(e)] are both C6H14 but with different carbon skeletons, so any pairing of one from each group gives constitutional isomers: (a & c), (a & e), (b & c), (c & d), (d & e). Answer C: (a & c) or (a & e), (b & c), (c & d), and (d & e). Therefore, the correct answer is A-(c&e),(b&d); B-(a&b) or (a&d); C-(a&c) or (a&e), (b&c), (c&d) and (d&e).