Acids and bases

Almost every organic reaction is an electron pair moving from somewhere rich to somewhere poor. Acid–base chemistry is the simplest case, and pKa is how it is measured: lower pKa means stronger acid, and each unit is a factor of ten.

The useful question is never "is this acidic?" but "how stable is the anion left behind?" An acid is strong exactly when its conjugate base is comfortable.

Four things stabilise that anion, in roughly this order of importance:

• Atom — a negative charge is happier on a more electronegative atom, and happier still on a bigger one. (pKa: CH₄ ~50, NH₃ 38, H₂O 15.7, HF 3.2.)
• Resonance — spreading the charge over two oxygens is why acetic acid (pKa 4.8) beats ethanol (pKa 16) by eleven orders of magnitude.
• Induction — nearby electronegative atoms pull charge away. Trichloroacetic acid is pKa 0.7.
• Hybridisation — an sp carbon holds a lone pair closer to the nucleus, which is why terminal alkynes (pKa about25) can be deprotonated by bases such as amide, while ordinary alkane C–H bonds require much stronger conditions.