Electrocyclic
In a third class of additions, both portions of the attacking reagent combine simultaneously with the substrate. Reactions of this kind sometimes retain predominantly electrophilic or predominantly nucleophilic character, as can be shown by structural and environmental effects. In a number of important cases, however, quite different behaviour is observed. For example, the addition of cyclopentadiene to 1,4-benzoquinone follows second-order kinetics and proceeds at nearly the same rate in the gas phase and in solvents of widely differing polar character.
In this equation the polygons represent rings of carbon atoms (one at each corner), with double bonds between certain atoms as shown. Therefore, there must be little development of charge in the transition state, and the formation of the two new single bonds and the accompanying electronic movements must be well synchronized. A large number of such reactions are known; they are characterized by a remarkable stereospecificity (stereochemical specificity), controlled in part by steric effects and in part by the stereo-electronic characteristics of the combining double-bond systems.
Homolytic
Additions by free-radical mechanisms also are well known. They replace the concomitant polar additions most easily when homolytic (decomposition of a compound into two neutral atoms or radicals) fission of the reagent can be readily catalyzed and when the radicals produced as intermediates sustain chain processes. Addition of hydrogen bromide to olefins falls into this class. Equations 1–4 describe the main part of the sequence; reactions 2 and 3 are repeated many times before reaction 4 or some other reaction intervenes to break the chain. As a result, one act of initiation results in many molecules of product.
The reaction can give an orientation of substituents opposite to that found in electrophilic addition, which in the above example would produce CH3CH(Br)CH3, and in suitable cases it can be just as stereospecific.
Elimination reactions
Elimination reactions can be treated formally as the reverse of additions. The simplest examples of this class of reactions are the olefin-forming 1,2-eliminations—that is, eliminations of substituents from adjacent carbon atoms—but eliminations to give other types of double bonds are equally well known. Again, 1,3-eliminations—eliminations of substituents from carbon atoms separated by a third carbon—give compounds with three-membered rings of carbon atoms (cyclopropanes). Furthermore, the so-called conjugate eliminations occur when one or more double bonds are inserted between carbon atoms bearing the substituents that are eliminated; the result of such eliminations is a system of alternating double and single bonds (a conjugated system). Finally, there also are fragmentation reactions, in which two small fragments are lost from the organic molecule. Of these reaction types, only the 1,2-eliminations will be discussed here, it being understood that examples of the mechanisms may be found, as appropriate, in other types of elimination reactions.
Concerted, bimolecular
Concerted bimolecular eliminations are characterized by second-order kinetics; they occur readily with powerful nucleophiles. A favoured stereochemical course (trans-elimination) involves a particular geometry, as shown, which requires that in the starting material the eliminated units be situated on opposite sides of the molecule.
The olefinic product then must have the particular structure shown, rather than that of its geometric isomer. The relative extent to which the various bonds are formed and broken in the transition state varies considerably with the substrate.
Stepwise, bimolecular
If removal of the electrophilic fragment precedes the loss of the nucleophile, the reaction becomes stepwise and involves a carbanionic intermediate.
Reaction by this path, which sometimes can be characterized by exchange of protons between the solvent and the starting material, is less stereospecific than the reaction by the concerted mechanism. This lessened stereospecificity is caused by the carbanion intermediate’s not maintaining the rigid geometry characteristic of the concerted mechanism.
Stepwise, unimolecular
A carbonium ion produced by heterolysis (decomposition of a compound into oppositely charged particles or ions) may lose a proton, thereby effecting a 1,2-elimination reaction:
Such eliminations, which generally accompany nucleophilic substitutions, are promoted by electron release to the carbonium ion centre. The loss of the proton usually occurs in such a way as to predominantly give the thermodynamically more stable of the alternative products.
Cyclic
Some cyclic eliminations are fully concerted, but in others the loss of a nucleophilic or of an electrophilic component can be dominant. For example, the gas-phase pyrolysis (destructive heating) of alkyl halides shows the orientation and structure effects characteristic of unimolecular stepwise elimination reactions in solution. In such cases, the transition state (shown below), though still cyclic and preserving the stereochemistry, must involve greater stretching of the carbon-chlorine than of the carbon-hydrogen bond.