pKa as a Predictive Tool in Organic Chemistry
One tabulated number decides which side a proposed step favours. Comparing the pKa of the acid formed with the acid consumed tells you which side an equilibrium sits on, and the reasons behind the numbers predict cases no table lists.
What a learner can do afterwards
- Compares two pKa values to say whether a proposed proton transfer is favourable and by how many orders of magnitude
- Explains an acidity difference using element, charge, hybridisation, induction and delocalisation in that order
- Predicts which of several protons in a molecule is removed first and names the base strong enough to do it
- Ranks the basicity of a set of nitrogen compounds and justifies the ranking
1 · Read
Bronsted transfers are proton swaps scored by pKa. The equilibrium always favours the weaker acid and weaker base, so compare the values: the side with the larger pKa wins. Each unit of gap shifts things by one order of magnitude, so a gap of 5 means an equilibrium constant of one hundred thousand to one. Small pKa means strong acid, since pKa is minus log Ka.
To judge a proton transfer and pick the stronger base, compare both pKa values from the same medium on the ladder. The side with the larger pKa wins the equilibrium. A larger gap means a larger K, not certain conversion.
When values differ, explain why in a fixed order: element, charge, hybridisation, induction, then delocalisation. A proton on oxygen beats one on carbon, a charged acid beats a neutral cousin, and sp beats sp2 beats sp3 since s character hugs electrons close. Electronegative neighbours pull through induction, and a spread out anion wins through delocalisation. The ketone alpha proton beats an alkane proton for exactly that last reason: its anion parks charge on oxygen.
Read the same ladder backwards to rank bases. The stronger the conjugate acid, the weaker the base. Aliphatic amines, anilines, and neutral amides land worlds apart because their conjugate acids differ wildly in stability. Weakest to strongest, the order is neutral amides, then anilines, then aliphatic amines.
Compare the pKa values, count the gap in tens, and explain the gap in order.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.