Exams › JEE Main › Chemistry
In which of the following pairs is the indicated (central) carbon-carbon single bond of GREATER strength in the first species than in the second?
- CH3-CH2-Br vs CH3-CH2-Cl
- CH3-CH=CH-Br vs CH3-CH(Br)-CH3
- CH3-COCl vs CH3-CH2-Cl
- CH2=CH-CH=CH2 (the central C2-C3 bond) vs CH2=CH-CH2-CH3 (the C2-C3 bond)
Correct answer: CH2=CH-CH=CH2 (the central C2-C3 bond) vs CH2=CH-CH2-CH3 (the C2-C3 bond)
Solution
The central C2-C3 single bond of 1,3-butadiene is an sp2-sp2 bond with extra strength from conjugative (partial double-bond) character, whereas the corresponding C2-C3 bond in 1-butene (CH2=CH-CH2-CH3) is an sp2-sp3 bond with no conjugation. Higher s-character plus partial pi delocalisation makes the butadiene central bond shorter and stronger.
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