See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: The length of a C=C double bond is influenced by the degree of substitution (hyperconjugation and steric/electronic effects) and the hybridization-related bond character. More alkyl substituents on the doubly bonded carbons increase electron density via hyperconjugation, but more importantly, increased substitution causes greater steric repulsion and also increases the s-character contribution considerations. However, the dominant effect here is that greater alkyl substitution around the double bond leads to more hyperconjugative electron donation into the pi bond, which slightly weakens (and thus lengthens) the pi bond. Additionally, with more substituents, the C=C bond acquires more single-bond character due to increased hyperconjugation from the attached alkyl groups, elongating the bond. Step 1: Identify the substitution pattern around the C=C in each compound. (a) CH3-CH=CH2 (propene): one methyl substituent on the double bond carbons (monosubstituted alkene) (b) CH3-CH=CH-CH3 (2-butene): two methyl substituents (disubstituted alkene) (c) CH3-C(CH3)=C(CH3)-CH3 (2,3-dimethyl-2-butene): four methyl substituents (tetrasubstituted alkene) (d) CH2=CH2 (ethene): no substituents (unsubstituted alkene) Step 2: Apply the principle. As the number of alkyl groups attached to the doubly bonded carbons increases, hyperconjugation increases, donating more electron density and causing slight bond elongation. The tetrasubstituted alkene (option c) has the maximum number of alkyl groups (four methyl groups) around the double bond. Step 3: Rank bond lengths. Ethene (d) < propene (a) < 2-butene (b) < 2,3-dimethyl-2-butene (c). Thus, option (c) has the longest C=C double bond. Why other options fail: - (d) ethene has no alkyl substituents, so minimal hyperconjugation and shortest C=C bond. - (a) propene has only one alkyl substituent, slightly longer than ethene but shorter than di- or tetrasubstituted. - (b) 2-butene has two alkyl substituents, longer than (a) and (d) but shorter than (c). Therefore, the correct answer is C.