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Introduction to Nucleophilic Substitution, Elimination & Addition Reactions

Substitution

An early technique of preparing phenol (the Dow process) involved the reaction of chlorobenzene with a concentrated sodium hydroxide solution at the temperatures above 350 ºC. The major results are diphenyl and phenol ether (see below in the diagram). This apparent nucleophilic substitution reaction is surprising, because aryl halides are usually incapable of reacting by either an SN1 or SN2 pathway.

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The existence of electron-withdrawing groups (like nitro) para and ortho to the chlorine substantially improve the rate of substitution, as displayed in the set of equations shown on the left below. To illustrate this, a third technique for nucleophilic substitution has been proposed. This two-step technique is characterized by initial addition of the nucleophile (water or hydroxide ion) to the aromatic ring, followed by loss of halide anion from the negatively charged intermediate. The sites on which the negative charge is delocalized are colored blue in the diagram, and the capability of nitro, and other electron withdrawing, groups to stabilize the adjacent negative charge accounts for their rate enhancing effect at the para and ortho locations.

Three additional illustrations of aryl halide nucleophilic substitution are shown on the right side. Only the 2- and 4-chloropyridine isomers go through rapid substitution, the 3-chloro isomer is comparatively unreactive. Nitrogen nucleophiles will also react, as evidenced by the make use of Sanger's reagent for the derivatization of amino acids. The resultant N-2,4-dinitrophenyl derivatives are bright yellow crystalline compounds that facilitated analysis of proteins and peptides, a subject for which Frederick Sanger received one of his two Nobel Prizes in chemistry.

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Such type of addition-elimination processes usually take place at sp2 or sp hybridized carbon atoms, opposite to SN1 and SN2 reactions. When applied to aromatic halides, like in the current discussion, this technique is called SNAr. Some unique characteristics of the three common nucleophilic substitution techniques are summarized in the following table.

Mechanism

Number of Steps

Bond Formation Timing

Carbon Hybridization

SN1

Two

After Bond Breaking

Usually sp3

SN2

One

Simultaneous with
Bond Breaking

Usually sp3

SNAr

Two

Prior to Bond Breaking

Usually sp2

Elimination

There is good evidence that the phenol's synthesis from chlorobenzene does not proceed by the addition-elimination mechanism (SNAr) explained above. For an instance, treatment of para-chlorotoluene with sodium hydroxide solution at temperatures above 350 ºC gave an equimolar mixture of meta- and para-cresols (hydroxytoluenes). bromobenzene and Chloro reacted with the very strong base sodium amide (NaNH2 at low temperature (-33 ºC in liquid ammonia) to give good yields of aniline (aminobenzene). Though, ortho-chloroanisole gave exclusively meta-methoxyaniline under identical conditions. These reactions are illustrated by the equations shown below.

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The description for this curious repositioning of the substituent group lies in a distinct two-step technique we can consider as an elimination-addition process. The intermediate in this technique is an unstable benzyne species. In difference to the parallel overlap of p-orbitals in a stable alkyne triple bond, the p-orbitals of a benzyne are entitled ca.120º apart, so the reactivity of this incipient triple bond to addition reactions is greatly improved. In the nonexistence of steric hindrance (top example) equivalent amounts of meta- and para-cresols are obtained. The steric bulk of methoxy group and the ability of its ether oxygen to stabilize an adjacent anion result in a substantial bias in the addition of amide ammonia or anion.

Addition

Even though it does so less readily than simple dienes or alkenes, benzene adds hydrogen at high pressure in the existence of Pt, Pd or Ni catalysts. The result is cyclohexane and the heat of reaction offers evidence of benzene's thermodynamic stability. Substituted benzene rings might also be reduced in this way, and hydroxy-substituted compounds, like resorcinol, catechol and phenol, give carbonyl products resultant from the fast ketonization of intermediate enols. Nickel catalysts are frequently employed for this purpose, as illustrated in the following equations.

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Benzene is more susceptible to radical addition reactions than to electrophilic addition. We have already noticed that benzene does not react with bromine or chlorine in the nonexistence of a catalyst and heat. In the strong sunlight or with radical initiators benzene adds these halogens to give hexahalocyclohexanes. It is worth noting that these identical conditions effect radical substitution of cyclohexane, the important issues in this change of behavior are the pi-bonds array in benzene, which allow addition, and in cyclohexane, the weaker C-H bonds. The addition of chlorine is displayed below in the diagram.

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