The mechanism of a reaction is a step-by-step description of how that reaction is thought to occur. In microscopic detail, chemical reactions involve multiple steps, including encounter equilibria, bond-making and bond-breaking, diffusion steps and re-hybridization ("heavy atom rearrangement"). In undergraduate organic chemistry, mechanisms are generally simplified to include only bond-making and breaking steps and "curved arrows" are generally utilized to indicate the flow of electrons within these steps. Although these simplified mechanisms are only approximations, they are useful since they allow a clearer understanding of how a given reaction proceeds, and allow reactions to be organized by mechanisms, making the study of organic chemistry simpler and more logical. The process of "pushing electrons" using curved arrows requires a few simple conventions:"Pushing Electrons;"
Representing Reaction Mechanisms
Thus, homolytic cleavage of a carbon-bromine bond (a radical process) is shown below in (a) and the heterolytic cleavage is shown in (b).
The heterolytic cleavage (b) leads to the formation of a pair of ions, which is indicated in the mechanism by the positive and negative charges. Two examples of this type of heterolytic cleavage reaction which are discussed in elementary organic chemistry are the E1 and SN1 reaction mechanisms. Both of these reactions are unimolecular (the "1") meaning that only one of the reactant molecules is present in the rate-limiting transition state. The prefixes on these general mechanisms refer to elimination ("E") and substitution, nucleophilic ("SN").
Treating the SN1 reaction first, the rate of reaction of 2-methyl-2-propanol with HCl to give 2-chloro-2-methylpropane has been found to be independent of the concentration of chloride anion in the reaction mixture, suggesting that only the alcohol is present in the rate-limiting transition state and that reaction with chloride anion is fast. Thus, the rate-limiting step most likely involves acid-catalyzed loss of the hydroxyl group to give an intermediate carbocation (the rate-limiting step is the slowest step in a given reaction sequence). Reaction of the carbocation with chloride anion is then very fast, to give the final product.
The major bond-making and bond-breaking steps in this process can be represented by the mechanism shown below.
The protonation step is a rapid equilibrium process (described by the Ka for the protonated alcohol). The slow, or rate-limiting step is the breaking of the carbon-oxygen bond, and the fast step is the addition of chloride anion. Chloride anion concentration does not affect the rate of the reaction since the step involving the nucleophilic attack occurs after the rate-limiting step.
An example of a unimolecular elimination reaction (E1) is shown in the scheme below.
In this reaction, 2-chloro-2-methylpropane reacts with a base to lose HCl and form the alkene, 2-methylpropene. Again, the rate of this reaction is found to be independent of the concentration of the base that is used, requiring that the only reactant in the rate-limiting transition state is the alkyl halide and that loss of the proton from the carbocation is very fast.
In the mechanistic representation of this reaction, the chlorine is shown to depart with a pair of electrons to give the carbocation and chloride anion. To show the process of elimination, the electron pair from the base is shown removing the hydrogen from the carbon adjacent to the carbocation. Concurrent with this, the electron pair from the carbon-hydrogen bond is shown moving into the space between the carbocation and the carbon, to form a carbon-carbon double bond; again, this process must be fast relative to the spontaneous loss of chlorine from the starting material in order for the reaction to be independent of the concentration of the base.
Implied in the mechanism, although not generally discussed, is the re-hybridization of the CH2 group and the overlap of the adjacent p-orbitals to form the alkene p -system. There are also major changes in the solvation of the (neutral) starting material as it forms ions, and further changes as the neutral alkene is produced. While these steps are generally ignored, examples exist where these types of processes are partially, or largely, rate-limiting.
Both of the examples described above are multiple step reactions involving a high-energy intermediate (the carbocation). These types of reactions are said to occur stepwise, and generally one step in the sequence is slow and is rate-limiting. In a concerted process, all of the reactant molecules are present together in the rate-limiting transition state and all of the bond-making and bond-breaking steps occur simultaneously.
Two examples of these types of concerted processes are the SN2 and the E2 reaction mechanisms. An example of an SN2 process is shown below:
The rate of the reaction of 2-chloropropane with hydroxide anion is dependent on both the concentration of the alkyl halide and on the concentration of hydroxide anion (the reaction is bimolecular; the "2" in SN2). This means that both of these must be present together in the rate-limiting transition state. The process of substitution within this transition state can be shown using curved arrows as shown above. The electron pair on hydroxide anion is shown to attack to carbon bearing the chlorine (the leaving group) at the same time that the carbon-chlorine bond is breaking. In one simultaneous, coupled, movement the bond from the oxygen to the carbon has formed, the bond from the carbon to the chlorine has undergone heterolytic cleavage, and the central carbon has undergone stereochemical inversion (S-2-chlorobutane has formed R-2-butanol). A concerted reaction such as this does not have a "high energy" intermediate (such as the carbocation, described for the SN1 process), but occurs in a single step through a concerted transition state.
The second common example of a concerted bimolecular process is the E2 elimination.
In an E2 elimination reaction, the rate is dependent on the concentration of the alkyl halide and on the concentration of the base. Again, both of these must be together in the rate-limiting transition step, and the electron flow in this transition state is typically drawn as shown above.
In a coupled, concerted process, the electron pair on the base removes the proton from the a -carbon at the same time as the electron pair is moving into the space between the carbons at the same time as the carbon-chlorine bond is breaking. Again, there is no intermediate, only the concerted transition state.
The reaction mechanisms shown in the animations include examples of concerted bimolecular processes, along with examples of highly complex multiple step reactions. For all of these examples, however, the mechanism can be simplified and shown as a discrete set of electron movements using the curved arrow approach. In those examples where the rate-limiting transition state is well-characterized, it is shown as part of the animation. For a more detailed discussion of each of the reactions in this section, please see the discussion in your text.