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Hammond Postulate: Energy and Transition-State Structure in HCl Addition

Chloride capture is exergonic. The Hammond postulate predicts an early, carbocation-like transition state with a weakly formed C–Cl bond.

Question

Consider the chloride-capture step of electrophilic HCl addition to an alkene: a chloride ion combines with the carbocation intermediate.

  1. Classify this step as exergonic or endergonic.
  2. Decide whether its transition state is more like the carbocation reactant or the alkyl chloride product.
  3. Sketch an approximate structure for this transition state.

Answer

Chloride capture is exergonic, and its transition state is early and carbocation-like. The Hammond postulate relates structural resemblance to closeness in energy along an elementary step.

1. Energy change

The chloride ion supplies an electron pair to the electron-deficient carbon, forming the C–Cl bond of the alkyl chloride. In the textbook mechanism, this capture step goes downhill in free energy: ΔG∘<0\Delta G^\circ<0.

2. Which structure does the transition state resemble?

The transition state lies closer in energy to the carbocation plus chloride reactants than to the lower-energy product. Hammond reasoning therefore predicts a reactant-like structure. The carbon retains substantial positive charge, and chloride retains substantial negative charge. This is the second step only; do not substitute the product-like transition state of the preceding alkene-protonation step.

3. Approximate transition-state sketch

Early chloride-capture transition state: a nearly trigonal carbon attached to three R groups carries partial positive charge; an approaching chloride carries partial negative charge and has a long dashed forming bond to carbon.

The sketch represents [R3Cδ+⋯Clδ−]‡[\mathrm{R_3C^{\delta+}\cdots Cl^{\delta-}}]^\ddagger. Each R denotes an existing carbon substituent or hydrogen; no particular alkene was specified. The long dashed contact is a partly formed C–Cl bond, not a completed single bond. Carbon is still closer to the trigonal carbocation geometry than to the tetrahedral product, and chloride approaches an available face. This is a qualitative drawing, not a calculated geometry or a numerical activation barrier.

Evidence boundary

This is a normalized presentation of OpenStax Organic Chemistry Problem 7-20, selected as a complete representative exercise for the truncated Hammond keyword. It is not a reconstruction of the truncated search post. The conclusion concerns chloride capture in the textbook mechanism; the sketch is qualitative and does not specify bond lengths, a barrier height, or an experimental transition-state structure.

Sources

These references support the concepts and methods used in the explanation above.

Hammond Postulate: Energy and Transition-State Structure in HCl Addition | Verla