After perfect this section, you should be may be todraw the resonance contributors because that the carbocation intermediary formed during the reaction that a offered monosubstituted benzene derivative with any of the electrophiles discussed in this chapter. Classify every of the substituents provided in objective 2 of ar 16.4 as being one of two people meta or ortho/para directing. Classify each of the substituents provided in target 2 of ar 16.4 as being ortho/para directing activators, ortho/para directing deactivators, or meta directing deactivators. Suspect the product or products formed indigenous the reaction of a provided monosubstituted benzene derivative through each that the electrophiles questioned in this chapter. Explain, by illustration the resonance contributors because that the intermediate carbocation, why the electrophilic substitution of an alkyl benzene outcomes in a mixture of mostly ortho- and also para- substituted products. Define why the electrophilic substitution the phenols, amines and also their derivatives proceeds much more rapidly 보다 the electrophilic substitution that benzene itself. Explain, by illustration the resonance contributors for the intermediary carbocation, why meta substitution predominates in electrophilic aromatic substitution reactions carried out ~ above benzene derivatives containing one of the substituents R3N+, NO2, CO2H, CN, CO2R, COR or CHO. Explain why electrophilic aromatic substitution of benzene derivatives containing one of the substituents listed in target 7, above, proceeds more slowly than the electrophilic substitution of benzene itself. Explain, by drawing the resonance contributors for the intermediary carbocation, why the electrophilic fragrant substitution of halobenzenes produce a mixture of greatly ortho- and also para-substituted products. Describe why the electrophilic aromatic substitution of halobenzenes proceeds an ext slowly than does the electrophilic substitution that benzene itself. Use the principles developed in this chapter to guess in i beg your pardon of the 3 categories provided in objective 3, above, a formerly unencountered substituent must be placed.
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Make specific that you deserve to define, and also use in context, the crucial terms below.steric impact steric hindrance
As you observed in ar 16.4, a substituent ~ above a benzene ring deserve to be one activator or a deactivator. At the exact same time, a substituent can likewise be a meta director or one ortho/para director. Of the four possible combinations, only three room known—there room no meta directing activators.
If you look at the data because that the nitration that toluene, friend will check out that the productivity of o‑nitrotoluene is 63% and that the p‑nitrotoluene is 34%. Statistically, we need to expect to acquire twice as lot ortho product together para product, due to the fact that the former is produced by attack at one of two people of two carbon atoms whereas the last is created by strike at only one carbon atom (see figure below).
In this instance, the observed ortho/para proportion is virtually 2:1, together we could expect. However, if we examine the ortho/para ratio uncovered in the nitration of a number of other arenes, we view that this is not always the case. Keep in mind that the data for the nitration of toluene provided in the table below differ indigenous those gift elsewhere. The sport may an outcome from a difference in temperature, reaction conditions or reagent, and emphasizes the suggest that that is the trends which are important, no the numbers themselves.
<Source: these data were taken from the audiocassette Some organic Reaction Pathways, by Peter Sykes. London: Educational techniques Subject Group, The mslsec.comical Society, 1975.>
Table \(\PageIndex1\): Nitration the arenes
The table over shows united state that as the size of the alkyl substituent already present in the ring increases, strike at the ortho position becomes an ext difficult, and the portion of ortho isomers in the mixture of assets decreases. This is an instance of a steric effect—an impact caused by the dimension of the substituent—and we would say that as the size of the alkyl group increases, assault at the ortho place becomes less favorable together a result of steric hindrance. Note that the dimension of the electrophile can also be a variable in identify the ortho/para ratio: the larger the electrophile, the much less able it is to assault at the ortho position, particularly if the substituent currently present in the ring is itself rather bulky.
When drawing the resonance contributors to the carbocation formed during an electrophilic aromatic substitution, be afflicted with in mind the those of the type
are an especially important, since in such frameworks each atom own a finish octet the electrons.
Note that, as carry out the hydroxyl and also amino groups, the halogens have actually an inductive electron-withdrawing effect and also a resonance electron-releasing result on a benzene ring. The difference in actions during electrophilic substitutions occurs because, through the hydroxyl and also amino groups, the resonance effect is much higher than the inductive effect, whereas v the halogens, there is a much finer balance. In the situation of the latter, the inductive result reduces the overall reactivity, however the resonance effect method that this reduction is felt less at the ortho and para positions than at the meta position.
Examples the activating groups in the family member order from the many activating group to the the very least activating:
-NH2, -NR2 > -OH, -OR> -NHCOR> -CH3 and other alkyl groups
with R together alkyl teams (CnH2n+1)
Examples of deactivating teams in the family member order from the many deactivating to the least deactivating:
-NO2, -CF3> -COR, -CN, -CO2R, -SO3H > Halogens
with R together alkyl groups (CnH2n+1)
The stimulate of reactivity amongst Halogens from the more reactive (least deactivating substituent) come the the very least reactive (most deactivating substituent) halogen is:
F> Cl > Br > I
The bespeak of reactivity of the benzene rings towards the electrophilic substitution as soon as it is substituted through a halogen groups, adheres to the stimulate of electronegativity. The ring the is substituted v the most electronegative halogen is the many reactive ring (less deactivating substituent) and also the ring the is substituted through the the very least electronegative halogen is the least reactive ring (more deactivating substituent), when we to compare rings through halogen substituents. Additionally the dimension of the halogen results the reactivity that the benzene ring that the halogen is enclosed to. As the size of the halogen increase, the reactivity that the ring decreases.
The direction of the reaction
The activating team directs the reaction come the ortho or para position, which means the electrophile substitutes because that the hydrogen the is on carbon 2 or carbon 4. The deactivating group directs the reaction to the meta position, which means the electrophile substitutes because that the hydrogen that is ~ above carbon 3 with the exception of the halogens which are deactivating groups however direct the ortho or para substitution.
By looking at the device above, we deserve to see exactly how electron donating groups direct electrophilic substitution come the ortho and para positions. Since the extra electron thickness is localized top top the ortho and para carbons, this carbons are more likely to react through the electrophile.
Inductive impacts of alkyl teams activate the direction the the ortho or para substitution, i m sorry is once s electrons gets moved toward the ring.
Deactivating team (meta directors)
The deactivating groups deactivate the ring by the inductive effect in the presence of an electronegative atom that withdraws electron density away native the ring.
Alkyl groups are inductively donating, thus are activators. This resulsts in o/p assault to form a tertiary arenium carbocation which speeds up the reaction.
The O-CH3 team is one ortho, para director
The methoxy team is an example of groups that room ortho, para directors by having and also oxygen or nitrogen nearby to the aromatic ring. This very same activation is current with alcohols, amines, esters and also amides (with the oxygen or nitrogen attached come the ring, no the carbonyl).
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Groups v an oxygen or nitrogen attached come the aromatic ring space ortho and also para directors since the O or N deserve to push electrons right into the ring, make the ortho and para positions much more reactive and stabilizing the arenium ion the forms. This reasons the ortho and para assets to form faster than meta. Generally, the para product is preferred due to the fact that of steric effects.