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Which of the following groups exhibits a +I (electron-donating inductive) effect?

1. A phenyl group attached through a single bond
2. A cyclohexyl group attached through a single bond
3. A p-tolyl (methyl-substituted benzene) group attached through a single bond
4. A vinyl group (-CH=CH2)

Correct answer: A cyclohexyl group attached through a single bond

Solution

Inductive effects depend on the electronegativity/hybridisation of the attached carbon. A saturated alkyl or cycloalkyl carbon is sp3 and is a weak electron donor (+I). In contrast, the carbon of a phenyl, tolyl (aryl), or vinyl group is sp2; sp2 carbons hold electrons more tightly (greater s-character, higher effective electronegativity) and therefore act as electron-withdrawing (-I) groups. Hence only the cyclohexyl group shows a +I effect.

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