Unimolecular nucleophilic substitution reactions proceed by a two-stage mechanism in which heterolysis precedes reaction with the nucleophile. The following equation is a typical example:
in which the symbols are the same as in earlier equations, with the addition of delta plus (δ+) and delta minus (δ−), which indicate partial positive and negative charges, respectively. The significant consideration in this reaction mechanism is the initial separation of the bromide ion (by way of a transition state showing partial separation of the ion) to give a free positively charged organic ion (carbonium ion). This step is the rate-determining step of the reaction, and, because it involves only a molecule of the substrate, the reaction is unimolecular. The second stage of the reaction is the interaction of the intermediate carbonium ion with the nucleophile to give the products of the reaction.
The unimolecular reaction is characterized experimentally by first-order kinetics—i.e., by a rate that depends only on concentration of the substrate (and not the nucleophile), by the absence of effects of steric hindrance, by powerful facilitation of the reaction by the presence of electron-releasing groups attached to the reaction centre, and by variable, and often diagnostic, stereochemistry. Inversion of stereochemical configuration (change from one configuration to the mirror-image configuration) is frequently encountered, accompanied by racemization (production of both mirror images). The extent of racemization depends upon the life of the intermediate carbonium ion, with longer-lived ions leading to more extensive racemization (due to the fact that the symmetrical ion is exposed to attack from either side).
In an important group of structures, a group not formally involved in the overall reaction interacts with a carbonium ion centre to form an intermediate, which then reacts with the nucleophile to give a product of the same stereochemical configuration as the starting material. This behaviour can be represented by the equation
In the first demonstrations of this behaviour, the participating group (G) was a carboxylate anion group, which can be represented in chemical symbols as
Subsequent investigations revealed numerous examples involving other substituents, and the phenomenon is now commonly described as neighbouring-group participation.
A frequent consequence of reaction through intermediates having carbonium ionic character is that some of the products have rearranged skeletal structures. In this equation the symbol Cl represents a chlorine atom.
The fundamental difference between the transition states in the bimolecular and unimolecular mechanisms is the degree of covalent bonding between the nucleophile and the substrate in the transition state. In the unimolecular mechanism such bonding is negligible; in the bimolecular case it has essentially reached the half-bond status. In borderline situations the matter is difficult to resolve, a number of intermediate cases being known, and there has been much controversy as to the validity of the distinction between the bimolecular and the unimolecular mechanisms. Experimentally, however, clear examples of each class have been established.
Nucleophilic substitution at unsaturated carbon centres
Unsaturated carbon centres—including those involving ordinary carbon-carbon double bonds and those involving the extended cyclic systems of alternate single and double bonds known as aromatic rings—are not easily attacked by nucleophilic reagents unless they have been denuded of electrons by electron-attracting substituents. A two-stage process that includes addition of the nucleophile followed by expulsion of a negatively charged (anionic) group is the course normally taken for substitutions at aromatic centres. The presence of the aromatic ring enforces the geometry of the product, and the reaction is favoured by electron-withdrawing groups, such as the nitro (―NO2) group, which help to accommodate the negative charge on the intermediate. An example of this type of reaction is the displacement of fluoride ion from 2,4-dinitrofluorobenzene by nucleophiles such as ethoxide ion.
In this equation fluorine atoms are indicated by the chemical symbol F; nitro groups (consisting of one nitrogen and two oxygen atoms) are indicated by the symbols ―NO2; normal benzene rings (of six carbon atoms, each of which carries a single hydrogen atom) are indicated by regular hexagons with circles in them; and benzene rings containing disrupted electronic structures are indicated by hexagons with partial dotted circles.
Substitution reactions at ordinary double bonds (olefinic bonds) also take place by a two-stage process. When the two stages in the reaction occur synchronously or in very quick succession, the product has the same geometrical relationship that existed in the starting material. If, however, the anionic intermediate has sufficient lifetime, rotation about the new carbon-carbon single bond can precede loss of the negatively charged group, resulting in production of two products of differing molecular geometry—that is, products in which the substituents are differently situated with respect to the double bond.
If the intermediate anion takes up a hydrogen ion (proton) and then loses hydrogen and halogen simultaneously (concerted elimination), the reaction is then said to be following an addition-elimination sequence. Examples of such reactions are known, particularly in situations in which the double bond includes an atom other than carbon. In aromatic systems the reverse situation, in which elimination occurs, followed by addition, also is found. Finally, unimolecular mechanisms of substitution also are known to take place at particularly activated unsaturated centres. For example:
in which the symbol Ar represents a benzene ring or other aromatic system.