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A compound contains a benzene ring with: a terminal alkyne substituent (-C(triple bond)C-H, terminal proton H_A); a benzylic -CH2- side chain (proton H_B); a -CH(COOH)- group attached to the ring (carboxylic proton on COOH labeled H_C, and the benzylic C-H on the same carbon labeled H_D). What is the correct decreasing order of acidity?

1. H_C > H_D > H_B > H_A
2. H_C > H_D > H_A > H_B
3. H_D > H_C > H_B > H_A
4. H_C > H_A > H_D > H_B

Correct answer: H_C > H_A > H_D > H_B

Solution

H_C is on -COOH (most acidic, pKa ~3). H_A is the terminal alkyne proton (sp carbon, pKa ~25). H_D is on a benzylic carbon also alpha to -COOH (doubly activated, but still less acidic than sp C-H). H_B is a simple benzylic C-H (least acidic, pKa ~43). Order: H_C > H_A > H_D > H_B.

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