Why does benzene undergo electrophilic substitution

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This is what you need to understand for the purposes of the electrophilic substitution mechanisms:. Benzene, C 6 H 6 , is a planar molecule containing a ring of six carbon atoms each with a hydrogen atom attached. Benzene resists addition reactions because that would involve breaking the delocalisation and losing that stability.

Benzene is represented by this symbol, where the circle represents the delocalised electrons, and each corner of the hexagon has a carbon atom with a hydrogen attached. Because of the delocalised electrons exposed above and below the plane of the rest of the molecule, benzene is obviously going to be highly attractive to electrophiles - species which seek after electron rich areas in other molecules.

Species: A useful word which can mean any particle you want it to mean - an atom, a molecule, an ion or a free radical. The electrophile will either be a positive ion, or the slightly positive end of a polar molecule.

If you aren't sure what a polar molecule is, read about electronegativity and polar bonds before you go on. The delocalised electrons above and below the plane of the benzene molecule are open to attack in the same way as those above and below the plane of an ethene molecule. However, the end result will be different. If benzene underwent addition reactions in the same way as ethene, it would need to use some of the delocalised electrons to form bonds with the new atoms or groups. This would break the delocalisation - and this costs energy.

Note: You can read about electrophilic addition to ethene if you are interested. Instead, it can maintain the delocalisation if it replaces a hydrogen atom by something else - a substitution reaction. The hydrogen atoms aren't involved in any way with the delocalised electrons. In most of benzene's reactions, the electrophile is a positive ion, and these reactions all follow a general pattern. As a result, it is highly attractive to electron deficient species i.

Therefore, it undergoes electrophilic substitution reactions easily. Nucleophiles are electron-rich. Hence, they are repelled by benzene.

Hence, benzene undergoes nucleophilic substitutions with difficulty. The mass of an electron is 9. If its K. Calculate the wavelength of an electron moving with a velocity of 2. Determine the empirical formula of an oxide of iron which has A sample of drinking water was found to be severely contaminated with chloroform, CHCl 3 , supposed to be carcinogenic in nature. The level of contamination was 15 ppm by mass. Density of a gas is found to be 5. What will be its density at STP?

Describe the bulk preparation of dihydrogen by electrolytic method. What is the role of an electrolyte in this process? Chlorine is used to purify drinking water. Excess of chlorine is harmful. The excess of chlorine is removed by treating with sulphur dioxide. Present a balanced equation for this redox change taking place in water. Would you expect the first ionization enthalpies for two isotopes of the same element to be the same or different?

Justify your answer. What are the oxidation number of the underlined elements in each of the following and how do you rationalise your results? 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, and 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.

A two-step mechanism has been proposed for these electrophilic substitution reactions. In the first, slow or rate-determining, step the electrophile forms a sigma-bond to the benzene ring, generating a positively charged benzenonium intermediate. In the second, fast step, a proton is removed from this intermediate, yielding a substituted benzene ring. The following four-part illustration shows this mechanism for the bromination reaction.

Also, an animated diagram may be viewed. These may be viewed repeatedly by continued clicking of the "Next Slide" button. This mechanism for electrophilic aromatic substitution should be considered in context with other mechanisms involving carbocation intermediates. To summarize, when carbocation intermediates are formed one can expect them to react further by one or more of the following modes:.

The cation may bond to a nucleophile to give a substitution or addition product. The cation may transfer a proton to a base, giving a double bond product. The cation may rearrange to a more stable carbocation, and then react by mode 1 or 2.

S N 1 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. When substituted benzene compounds undergo electrophilic substitution reactions of the kind discussed above, two related features must be considered:. The first is the relative reactivity of the compound compared with benzene itself. Experiments have shown that substituents on a benzene ring can influence reactivity in a profound manner.

For example, a hydroxy or methoxy substituent increases the rate of electrophilic substitution about ten thousand fold, as illustrated 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 activation or deactivation of the benzene ring toward electrophilic substitution may be correlated with the electron donating or electron withdrawing influence of the substituents, as measured by molecular dipole moments.

In the following diagram we see 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. The influence a substituent exerts on the reactivity of a benzene ring may be explained by the interaction of two effects:.

The first is the inductive effect of the substituent. Most elements other than metals and carbon have a significantly greater electronegativity than hydrogen. Consequently, substituents in which nitrogen, oxygen and halogen atoms form sigma-bonds to the aromatic ring exert an inductive electron withdrawal, which deactivates the ring left-hand diagram below. The second effect is the result of conjugation of a substituent function with the aromatic ring.

This conjugative interaction facilitates electron pair donation or withdrawal, to or from the benzene ring, in a manner different from the inductive shift.

Finally, polar double and triple bonds conjugated with the benzene ring may withdraw electrons, as in the right-hand diagram. Note that in the resonance examples all the contributors are not shown. In both cases the charge distribution in the benzene ring is greatest at sites ortho and para to the substituent. In the case of the nitrogen and oxygen activating groups displayed in the top row of the previous diagram, electron donation by resonance dominates the inductive effect and these compounds show exceptional reactivity in electrophilic substitution reactions.

The three examples on the left of the bottom row in the same diagram are examples of electron withdrawal by conjugation to polar double or triple bonds, and in these cases the inductive effect further enhances the deactivation of the benzene ring. Alkyl substituents such as methyl increase the nucleophilicity of aromatic rings in the same fashion as they act on double bonds.