Compound	pKa	Hybridization at Carbon
Ethane	50	sp³
Ethylene	44	sp²
Terminal alkyne (acetylene)	25	sp

Why hybridization affects C–H acidity: an sp orbital has more s-character (50%) than sp² (33%) or sp³ (25%). Electrons in an orbital with more s-character sit closer to the nucleus, so the conjugate base (a carbanion) is more stable when the negative charge sits in an sp orbital. This is why a terminal alkyne C–H (Chapter 9) is acidic enough to be deprotonated by strong bases such as sodium amide, while an alkane or alkene C–H is not.

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