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  Reactions of Substituted Benzenes Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Reactions of Benzene Derivatives

When substituted benzene compounds go through electrophilic substitution reactions of the type discussed above, two related characteristics must be referred:

I. The first is the relative reactivity of the compound compared with benzene itself. Experiments have displayed that substituents on a benzene ring can affect reactivity in a profound manner. For an instance, a methoxy or hydroxy or substituent increases the rate of electrophilic substitution about ten thousand fold, as depicted by the case of anisole in the virtual demonstration (above). In contrast, a nitro substituent decreases the ring's reactivity by roughly a million. This deactivation  or activation  of the benzene ring in the direction of electrophilic substitution may be correlated with the electron donating or electron withdrawing influence of the substituents, same as measured by molecular dipole moments. In the diagram we notice that electron donating substituents (blue dipoles) activate the benzene ring toward electrophilic attack, and electron withdrawing substituents (red dipoles) deactivate the ring (make it less reactive to electrophilic attack).

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The effect a substituent exerts on the reactivity of a benzene ring may be described by the interaction of two effects:

The first is the inductive effect of the substituent. Several elements other than metals and carbon have a significantly greater electronegativity than hydrogen.

Subsequently, substituents where nitrogen, halogen and oxygen atoms form sigma-bonds to the aromatic ring exert an inductive electron withdrawal that deactivates the ring. 

The second effect is the effect of conjugation of a substituent function with the aromatic ring. This conjugative interaction assists electron pair donation or withdrawal, to or from the benzene ring, in a way distinct from the inductive shift. Like do nitrogen, oxygen and the halogens, if the atom bonded to the ring has one or more non-bonding valence shell electron pairs, electrons may flow into the aromatic ring by p-π conjugation (resonance), like in the middle picture. At last, polar double and triple bonds conjugated with the benzene ring may withdraw electrons, like within the right-hand diagram. Note: in the resonance illustrations all the contributors are not displayed. In both cases the charge distribution in the benzene ring is greatest at sites para and ortho to the substituent.

In the example of the oxygen and nitrogen activating groups shown in the top row of the previous picture, electron donation by resonance dominates the inductive effect and these compounds display exceptional reactivity in electrophilic substitution reactions. Even though halogen atoms have non-bonding valence electron pairs which participate in p-π conjugation their strong inductive effect predominates, and compounds like chlorobenzene are less reactive than benzene. The three illustrations on the left of the bottom row (in the same diagram) are illustrations of electron withdrawal by conjugation to polar double or triple bonds, and in these examples the inductive effect further improves the deactivation of the benzene ring. Alkyl substituents like methyl increase the nucleophilicity of aromatic rings in similar way as they act on double bonds.

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II. The second issue that becomes significant in reactions of substituted benzenes concerns the site at which electrophilic substitution take place. Because a mono-substituted benzene ring has two corresponding ortho-sites, two equivalent meta-sites and a unique para-site, three possible constitutional isomers might be formed in such type of a substitution. The supposed statistical mixture of isomeric products would be 40% ortho, 40% meta and 20% para, If reaction occurs equally well at all available sites. Again we find that the nature of the substituent affects this product ratio in a dramatic fashion. Methoxybenzene's (anisole) Bromination is extremely fast and gives chiefly the para-bromo isomer, accompanied by 10% of the ortho-isomer and only a trace of the meta-isomer. Bromination of nitrobenzene needs strong heating and produces the meta-bromo isomer as the major product.

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Some additional illustrations of product isomer distribution in other electrophilic substitutions are given in the table below. It is significant to notice here that the reaction conditions for these substitution reactions are not identical, and must be adjusted to fit the reactivity of the reactant C6H5-Y. The high reactivity of anisole, for an instance, requires that the first two reactions be conducted under very mild conditions (low temperature and little or no catalyst). The nitrobenzene reactant in the third instance is extremely unreactive, so rather harsh reaction conditions must be employed to complete that reaction.

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