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Date: 5-8-2018
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Date: 29-5-2017
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Date: 23-6-2016
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Of the four possible combinations, only three are known—there are no meta directing activators.
If you look at the data for the nitration of toluene, you will see that the yield of o‑nitrotoluene is 63% and that of p‑nitrotoluene is 34%. Statistically, we should expect to obtain twice as much ortho product as para product, because the former is produced by attack at either of two carbon atoms whereas the latter is produced by attack at only one carbon atom (see Figure 1.1, below).
Figure 1.1: Proportions of o‑nitrotoluene and p‑nitrotoluene produced by the nitration of toluene
In this instance, the observed ortho/para ratio is almost 2:1, as we might expect. However, if we study the ortho/para ratio found in the nitration of a number of other arenes, we see that this is not always the case. Note that the data for the nitration of toluene given in the table below differ from those presented elsewhere. The variation may result from a difference in temperature, reaction conditions or reagent, and emphasizes the point that it is the trends which are important, not the numbers themselves.
Substrate | % ortho | % para | ortho/para ratio |
---|---|---|---|
toluene | 58 | 37 | 1.57:1 |
ethylbenzene | 45 | 49 | 0.92:1 |
isopropylbenzene | 30 | 62 | 0.48:1 |
tert-butylbenzene | 1 | 73 | 0.22:1 |
[Source: These data were taken from the audiocassette Some Organic Reaction Pathways, by Peter Sykes. London: Educational Techniques Subject Group, The Chemical Society, 1975.]
Table 1.1: Nitration of arenes
The table above shows us that as the size of the alkyl substituent already present in the ring increases, attack at the ortho position becomes more difficult, and the percentage of ortho isomers in the mixture of products decreases. This is an example of a steric effect—an effect caused by the size of the substituent—and we would say that as the size of the alkyl group increases, attack at the ortho position becomes less favourable as a result of steric hindrance. Note that the size of the electrophile can also be a factor in determining the ortho/para ratio: the larger the electrophile, the less able it is to attack at the ortho position, particularly if the substituent already present in the ring is itself quite bulky.
When drawing the resonance contributors to the carbocation formed during an electrophilic aromatic substitution, bear in mind that those of the type
are particularly important, because in such structures each atom possesses a complete octet of electrons.
Note that, as do the hydroxyl and amino groups, the halogens have an inductive electron-withdrawing effect and a resonance electron-releasing effect on a benzene ring. The difference in behaviour during electrophilic substitutions arises because, with the hydroxyl and amino groups, the resonance effect completely swamps the inductive effect, whereas with the halogens, there is a much finer balance. In the case of the latter, the inductive effect reduces the overall reactivity, but the resonance effect means that this reduction is felt less at the ortho and para positions than at the meta position.
Substituted rings are divided into two groups based on the type of the substituent that the ring carries:
Examples of activating groups in the relative order from the most activating group to the least activating:
-NH2, -NR2 > -OH, -OR> -NHCOR> -CH3 and other alkyl groups
with R as alkyl groups (CnH2n+1)
Examples of deactivating groups in the relative order from the most deactivating to the least deactivating:
-NO2, -CF3> -COR, -CN, -CO2R, -SO3H > Halogens
with R as alkyl groups (CnH2n+1)
The order of reactivity among Halogens from the more reactive (least deactivating substituent) to the least reactive (most deactivating substituent) halogen is:
F> Cl > Br > I
The order of reactivity of the benzene rings toward the electrophilic substitution when it is substituted with a halogen groups, follows the order of electronegativity. The ring that is substituted with the most electronegative halogen is the most reactive ring ( less deactivating substituent ) and the ring that is substituted with the least electronegatvie halogen is the least reactive ring ( more deactivating substituent ), when we compare rings with halogen substituents. Also the size of the halogen effects the reactivity of the benzene ring that the halogen is attached to. As the size of the halogen increase, the reactivity of the ring decreases.
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"عادة ليلية" قد تكون المفتاح للوقاية من الخرف
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ممتص الصدمات: طريقة عمله وأهميته وأبرز علامات تلفه
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الأمين العام للعتبة العسكرية المقدسة يستقبل شيوخ ووجهاء عشيرة البو بدري في مدينة سامراء
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