Question

A chemical reaction that shows aromaticity. How does aromaticity influence this reaction? —>Examples: Does it affect physical properties of chemical behavior, will it make it easier or harder for the reactants to undergo the reaction, will it help determine the products that are formed in the reaction?

Answer 1

The remarkable stability of the unsaturated hydrocarbon benzene has been discussed in an earlier chapter. The chemical reactivity of benzene contrasts with that of the alkenes in that substitution reactions occur in preference to addition reactions, as illustrated in the following diagram (some comparable reactions of cyclohexene are shown in the green box).

A demonstration of bromine substitution and addition reactions is helpful at this point. A virtual demonstration may be initiated by clicking here.

Many other substitution reactions of benzene have been observed, the five most useful are listed below (chlorination and bromination are the most common halogenation reactions). Since the reagents and conditions employed in these reactions are electrophilic, these reactions are commonly referred to as Electrophilic Aromatic Substitution. The catalysts and co-reagents serve to generate the strong electrophilic species needed to effect the initial step of the substitution. The specific electrophile believed to function in each type of reaction is listed in the right hand column.

To summarize, when carbocations are formed one can expect them to react further by one or more of the following modes: 1. The cation may bond to a nucleophile to give a substitution or addition product.   2. The cation may transfer a proton to a base, giving a double bond product.   3. The cation may rearrange to a more stable carbocation, and then react by mode #1 or #2.

S_N1 and E1 reactions are respective examples of the first two modes of reaction. The second step of alkene addition reactions proceeds by the first mode, and any of these three reactions may exhibit molecular rearrangement if an initial unstable carbocation is formed. The carbocation intermediate in electrophilic aromatic substitution (the benzenonium ion) is stabilized by charge delocalization (resonance) so it is not subject to rearrangement. In principle it could react by either mode 1 or 2, but the energetic advantage of reforming an aromatic ring leads to exclusive reaction by mode 2 (ie. proton loss).