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For the conjugate bases shown below, with R = CH3, which sequence gives the order of increasing basic strength?

1. RCOO− < HC≡C− < R < NH2
2. R < HC≡C− < RCOO− < NH2
3. RCOO− < NH2 < HC≡C− < R
4. RCOO− < HC≡C− < NH2 < R

Correct answer: RCOO− < HC≡C− < NH2 < R

Solution

Conjugate-acid acidity: RCOOH (pKa ~5) > HC≡CH (~25) > NH3 (~38) > CH4 (~50). Since stronger acid gives weaker base, increasing base strength is RCOO- < HC≡C- < NH2- < R- (CH3-).

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