Additions to C=C and C≡C
Every addition in this part is a competition between two things: which end of the double bond the electrophile attacks, and which face it attacks from. Get both right and you have the product. This part explains where Markovnikov’s rule comes from rather than asking you to memorise it, shows why bromination gives anti addition while hydrogenation gives syn, and covers the reagents — borane, mercury, peracid — whose whole purpose is to give the product the simple mechanism would not. Two layers on every section: a slow, hand-held beginner path and a research-grade advanced/reference path.
The 10 sections in Part 6
- 1The electrophilic addition mechanism and its carbocation intermediate Free below
- 2Markovnikov selectivity, derived from cation stability — and the rearrangements that prove the cation is real
- 3Bridged ions: halonium, mercurinium, and the anti stereochemistry they enforce
- 4Hydroboration–oxidation — concerted, syn, and anti-Markovnikov
- 5Oxymercuration–demercuration and catalytic hydrogenation
- 6Oxidative additions: epoxidation, dihydroxylation and ozonolysis
- 7Radical addition of HBr and the peroxide effect
- 8Conjugate addition to electron-poor alkenes, and the 1,2/1,4 question
- 9Alkynes — the same mechanisms with a worse intermediate
- 10Where this goes next
The electrophilic addition mechanism and its carbocation intermediate
Free extractSection F.1 of Part 6, reproduced in full from the book — figures and all. No sign-in, no paywall on this section.
Two steps, one intermediate. Learn this skeleton once and it carries two-thirds of Part F.
Take propene and hydrogen bromide, in a solvent that does not interfere — acetic acid, or dichloromethane. Nothing about HBr looks like an electrophile until you remember that it is a strong acid: the H–Br bond is polarised Hδ+–Brδ− and the proton is the electrophilic end.
Step 1. The π electrons of propene reach out and grab the proton. The curly arrow starts in the middle of the C=C bond and ends on the hydrogen. A second arrow runs from the H–Br bond onto bromine, because bromine must take both electrons of that bond with it — otherwise carbon would end up with five bonds. Two arrows, two consequences: a new C–H σ bond, and a carbocation on the carbon that did not get the proton.
Step 2. The bromide ion — a perfectly good nucleophile, and conveniently right there — donates a lone pair to the empty p orbital of the cation. One arrow. Product.
Mechanism — electrophilic addition of HBr to propene
- Alkene attacks the electrophile. The π electrons of C=C attack the Hδ+ of H–Br. Simultaneously the H–Br bonding pair collapses onto bromine. This is the rate-determining step.
- A carbocation is formed on whichever alkene carbon did not receive the proton. It is sp² hybridised, trigonal planar, with an empty p orbital perpendicular to the plane — exactly the species from Part 3, with all its stability preferences and all its willingness to rearrange.
- Bromide captures the cation. A lone pair on Br⁻ attacks the empty p orbital. Because that orbital has two lobes, attack from either face is possible, so if the cation carbon is a stereocentre the product is close to racemic — the advanced layer explains why it is usually not exactly racemic.
Notice what the mechanism does not say. It does not say which carbon becomes the cation — that is F.2. It does not say which face bromide attacks — and for an open, planar cation the honest answer is both, more or less equally. Those two silences are the whole content of the rest of this part.
The mechanism predicts that more substituted alkenes should react faster, because alkyl groups both raise the π-orbital energy and stabilise the developing cation. That prediction is worth a number. Acid-catalysed hydration — the same mechanism with H₃O⁺ as the electrophile and water as the nucleophile — spans an extraordinary reactivity range as you add methyl groups to the double bond:
| Alkene | Cation formed on protonation | Relative rate of acid-catalysed hydration |
|---|---|---|
| CH₂=CH₂ (ethene) | primary, CH₃CH₂⁺ | 1 |
| CH₃CH=CH₂ (propene) | secondary, (CH₃)₂CH⁺ | of order 10⁶ |
| (CH₃)₂C=CH₂ (2-methylpropene) | tertiary, (CH₃)₃C⁺ | of order 10¹¹ |
The rate law is not what the simple picture predicts; the intermediate is usually not a free ion; and the ‘first step’ is really two.
The two-step scheme above is correct as bookkeeping and wrong as a description of what a molecule does. Three refinements matter at postgraduate level, and all three appear in CSIR-NET Part C questions dressed as ‘which of the following is consistent with the observed kinetics’.
1. Hydrogen halide additions are commonly third order, not second
In non-polar and weakly polar solvents the addition of HX to alkenes obeys
— second order in the acid. The reason is that a bare proton is not transferred to a hydrocarbon in benzene or in hexane; the energy cost of separating H⁺ and X⁻ is prohibitive. A second molecule of HX assists, hydrogen-bonding to the departing halide so that the leaving group is the far better stabilised HX₂⁻ rather than X⁻. In polar hydroxylic solvents the solvent does that job and the kinetics revert towards second order overall. The lesson is general and worth carrying into every other part of the book: an order in the rate law counts molecules in or before the rate-determining transition state; it does not tell you they are all doing chemistry.
2. The first ‘step’ passes through a π complex
Before any bond is made or broken, the electrophile and the alkene form a loose π complex — a charge-transfer association in which the alkene HOMO donates into an empty or antibonding orbital of the electrophile without significant rehybridisation of carbon. For halogens these complexes are directly observable: mixing an alkene with Br₂ or I₂ produces an immediate colour change and a new charge-transfer band in the UV, long before any product appears. The π complex is a shallow minimum on the way to the transition state, not the transition state itself.
This matters because it explains why the ‘electrophile’ need not be positively charged. Br₂ is neutral and has no low-lying empty orbital in the ordinary sense; what it offers is the σ* orbital of the Br–Br bond, which is low enough in energy to accept electron density from an alkene π orbital. Filling σ* weakens Br–Br, which is exactly what the mechanism needs.
3. Ion pairs, not free ions
When the proton has been transferred, the halide does not wander off into the solvent and come back. In most solvents the pair R⁺ X⁻ is born as a contact ion pair and collapses within a few molecular vibrations. This is why HX additions are usually far from cleanly racemic even though the cation is planar: whichever face the proton was delivered to is the face on which the halide is now sitting, and collapse of the intimate pair delivers a measurable excess of syn addition. Adding a common-ion salt, or a more polar solvent, or anything else that separates the pair, moves the product towards true racemisation.
Combining these three refinements gives a picture that is more honest and no harder to draw: π complex → proton transfer within an assisted, solvated assembly → ion pair → collapse. For examination purposes the two-step skeleton is what you draw; this paragraph is what you say when the question asks you to comment on the kinetics or the stereochemical outcome.
One diagnostic follows from all of this and distinguishes electrophilic addition from every radical alternative in F.7: cationic additions are strongly accelerated by electron-donating substituents and strongly retarded by electron-withdrawing ones, giving a steeply negative Hammett ρ⁺ for ring-substituted styrenes, whereas a radical addition on the same substrates gives ρ near zero. If a question hands you a substituent-effect table, read the sign of the slope first — it reports the charge on the reacting carbon in the transition state before you have drawn a single arrow.
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The remaining 9 sections of this part — Markovnikov selectivity, derived from cation stability — and the rearrangements that prove the cation is real, Bridged ions: halonium, mercurinium, and the anti stereochemistry they… — 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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