Reactive Intermediates I — Carbocations & Carbanions
Almost every mechanism in this book passes through a species that cannot be put in a bottle. This part is about the two most important of them. Carbocations explain rearrangements, solvolysis rates and why Friedel–Crafts alkylation so often gives the wrong product; carbanions explain enolate chemistry and most of carbon–carbon bond formation. For each, the same four questions: what shape is it, what makes it stable, how do you make one, and what does it do next. Two layers on every section: a slow, hand-held beginner path and a research-grade advanced/reference path.
The 10 sections in Part 2
- 1Geometry, hybridisation, and where the empty orbital points Free below
- 2The stability ordering — and the reason behind every entry
- 3Making a carbocation, and what it does next
- 4Wagner–Meerwein 1,2-shifts — and predicting which group moves
- 5Ring expansion and ring contraction
- 6Bridged ions — the non-classical debate, and neighbouring-group participation
- 7Geometry, hybridisation and the inversion barrier
- 8What stabilises a carbanion — four mechanisms and one retracted explanation
- 9Enolates and organometallics — carbanions you can actually use
- 10Where this leads — a pointer to Part 3
Geometry, hybridisation, and where the empty orbital points
Free extractSection B.1 of Part 2, reproduced in full from the book — figures and all. No sign-in, no paywall on this section.
Three bonds, one empty orbital, one plane. Everything else in the chapter is a consequence of this picture.
Take a neutral carbon with four bonds and remove one bonding pair — not the atom, the pair, so the group leaves with both electrons. You are left with three bonds and six valence electrons. Three electron domains around a central atom means trigonal planar geometry, sp² hybridisation, and 120° bond angles. The 2p orbital that was not used in hybridisation is left over, and it is empty.
Three consequences follow immediately, and they are worth stating separately because each one is examined on its own.
- The cation is flat. The three substituents and the cationic carbon lie in one plane. A carbocation centre that is prevented from becoming planar is very hard to form.
- A nucleophile can attack from either face. The two lobes of the empty p orbital are identical, so a stereocentre that becomes a free carbocation is destroyed: the product is racemic. In practice, ion pairing biases this, which is B.3.
- The empty p orbital is the LUMO, and it accepts from anything aligned with it. Neighbouring π systems, neighbouring lone pairs and neighbouring σ bonds can all donate into it — provided they are correctly oriented. Orientation, not just identity, is the recurring theme of this whole part.
The bridgehead test — proof that flatness matters
Theory says a carbocation must be planar. The cleanest experimental proof is a molecule that will not let it be. In a small bicyclic cage, the carbon at the bridgehead is held rigidly pyramidal by three bridges; it physically cannot flatten. If planarity is required, such a substrate should refuse to ionise. It does.
| Bridgehead bromide | Cation geometry available | Relative solvolysis rate |
|---|---|---|
| 1-Bromoadamantane | essentially planar at the bridgehead | 1 (reference) |
| 1-Bromobicyclo[2.2.2]octane | modestly pyramidalised | ~10−3 |
| 1-Bromobicyclo[2.2.1]heptane (norbornane) | severely pyramidalised — cannot flatten | ~10−13 or smaller |
1-Chloroadamantane solvolyses readily; 1-chlorobicyclo[2.2.1]heptane does not react at all under the same conditions. Both are tertiary chlorides. Explain. Easy
The spectroscopic and structural evidence that carbocations really look like this, and the quantitative cost of bending one.
The trigonal-planar picture is not an inference from products. Long-lived carbocations can be prepared in superacid media — typically SbF₅ in SO₂ClF or HF–SbF₅ (“magic acid”) at low temperature — where there is no base and no nucleophile strong enough to quench them, and they can then be looked at directly. This was Olah’s programme, and it is what the 1994 Nobel Prize in Chemistry recognised.
Two observables matter most:
- ¹³C chemical shift of the cationic carbon. An sp² carbon carrying a full positive charge and an empty p orbital is enormously deshielded. The tert-butyl cation resonates near δ 335 and the isopropyl cation near δ 320 — compare a ketone carbonyl at ~200 and an ordinary alkene carbon at ~125. That shift is the fingerprint of a genuine, localised, tricoordinate cation.
- X-ray structure. Crystalline salts of stabilised carbocations — the trityl cation, and eventually the 2-norbornyl cation itself — show the cationic carbon planar to within experimental error, with the sum of the three C–C–C angles at 360°.
The complementary number is the cost of pyramidalisation. Because the empty p is non-bonding, bending the cation out of plane converts it into an sp³-like orbital which is lower in energy on its own — but it also destroys the alignment of all the hyperconjugative donors and, more importantly, forces the three σ bonds into compressed angles. The result is a steep penalty. The bridgehead correlation quantifies it: Schleyer and co-workers showed that bridgehead solvolysis rates over a range of more than ten orders of magnitude correlate with molecular-mechanics strain-energy differences between the halide and the cation, with essentially no adjustable electronic parameter. The reactivity of these systems is their strain.
The one place the planar picture is genuinely wrong
Not every carbocation is a tricoordinate carbenium ion. Two families sit outside the picture and both are examined:
- Bridged (non-classical) ions, where a σ bond is shared between two carbons and the “cationic centre” is pentacoordinate. The 2-norbornyl cation is the famous case and gets its own section, B.6.
- Acylium ions, R–C≡O+, where the carbon is sp-hybridised and linear, not trigonal, because the oxygen lone pair has already donated into the empty orbital to give a genuine triple bond. This is not a minor resonance contributor; it is the dominant structure, which is why acylium ions are so well behaved.
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Read the rest of Part 2
The remaining 9 sections of this part — The stability ordering — and the reason behind every entry, Making a carbocation, and what it does next, Wagner–Meerwein 1,2-shifts — and predicting which group moves, Ring expansion and… — and all nine parts of Reaction Mechanisms are part of ChemVidya Full Access, along with the other books, 55 Study Notes and 6,000+ practice questions.
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