The following table gives the results of four electrophilic...

The following table gives the results of four electrophilic substitution reactions of chlorobenzene. (Sections 22.2-22.3) \begin{tabular}{llll} \hline Reaction & $\%$ 2-substitution & \% 3-substitution & $\%$ 4-substitution \\ \hline Chlorination & 39 & 6 & 55 \\ Nitration & 30 & 0 & 70 \\ Bromination & 11 & 2 & 87 \\ Sulfonation & 1 & 0 & 99 \\ \hline \end{tabular} the electrophile could be (a) Draw the structure of the major product of the sulfonation reaction. (b) Draw the structure of the minor product of the nitration reaction. (c) Explain, with reference to the stabilities of the reaction intermediates, why 2-substituted and 4 -substituted products are favoured in all four reactions. (d) Suggest why the four reactions give different ratios of 2-substituted:4-substituted products.

The following table gives the results of four electrophilic substitution reactions of chlorobenzene. (Sections 22.2-22.3)
\begin{tabular}{llll}
\hline Reaction & $\%$ 2-substitution & \% 3-substitution & $\%$ 4-substitution \\
\hline Chlorination & 39 & 6 & 55 \\
Nitration & 30 & 0 & 70 \\
Bromination & 11 & 2 & 87 \\
Sulfonation & 1 & 0 & 99 \\
\hline
\end{tabular}
the electrophile could be
(a) Draw the structure of the major product of the sulfonation reaction.
(b) Draw the structure of the minor product of the nitration reaction.
(c) Explain, with reference to the stabilities of the reaction intermediates, why 2-substituted and 4 -substituted products are favoured in all four reactions.
(d) Suggest why the four reactions give different ratios of 2-substituted:4-substituted products.

Key Concepts

Electrophilic Aromatic Substitution (EAS)

This is a fundamental reaction mechanism where an electrophile replaces a hydrogen atom on an aromatic ring. Understanding EAS involves analyzing the mechanism of formation of a sigma complex (or arenium ion) where the aromaticity is temporarily lost and then regained upon deprotonation. This mechanism is central to rationalizing the outcomes of various substitution reactions on benzene derivatives.

Directing Effects of Substituents

Substituents already attached to an aromatic ring influence the position where new groups are introduced. They are broadly classified as activating or deactivating groups and are further categorized as ortho/para-directing or meta-directing based on their electronic effects. The nature of these groups significantly affects the distribution of products in electrophilic aromatic substitution reactions.

Resonance Stabilization of Reaction Intermediates

The stability of the sigma complex intermediates in electrophilic aromatic substitution reactions is largely governed by resonance effects. When the intermediate carbocation is stabilized by resonance, particularly through delocalization of the positive charge, the formation of that intermediate is favored. This concept explains why certain positions on the aromatic ring are more reactive than others.

Steric Hindrance and Its Influence on Product Distribution

The spatial arrangement of substituents on the aromatic ring can affect the outcome of substitution reactions. Larger substituents may create steric hindrance that disfavors attack at the adjacent positions, thus directing the reaction to less hindered positions on the ring. This concept helps in understanding variations in product ratios even when electronic factors are similar.

Effect of Electrophile Strength and Reaction Conditions

Different electrophiles and reaction conditions can lead to variations in the distribution of substitution products. The nature and strength of the electrophile, as well as factors like temperature and solvent, influence which reaction pathway is favored. These variations are reflected in the differing ratios of substitution patterns observed across various electrophilic aromatic substitution reactions.

Transcript

0:00 Hello everyone.

00:01 Let us see the following question.

00:03 The following table gives the results of four electrophilic substitution reactions of chlorobenzing.

00:09 So let us see the table given in the question.

00:12 So these are the different substitution products, say chlorination product, nitration product, gromination and salponation product.

00:20 At position 2 and 3 and 4, the percentages are also showing different.

00:25 This is at position 2 substitution that is ortho substitution.

00:30 If it is at position 3 that is called meta substitution, and also in position 4 it is called as para substitution.

00:39 So, now that we are talking about the substitution of chlorobenzene.

00:42 Let us see why these substitutions are favorable like this.

00:47 So, this is the benzene and it is having chlorine.

00:50 So, chlorine has the three lone pairs of electrons.

00:53 So, hence, it is able to relocate the pair of electrons in delocalization.

01:00 So, as we see here due to the delocalization, the electron composition comes to be at position two.

01:07 So, at position two as well as position two, sorry, position four electron density increases.

01:14 So, at 2 comma 4 electrons density increases.

01:21 Hence, cl group is called as orthopara activator.

01:30 Orthopara activating group or orthopara activators.

01:34 At the same time, cl has the highly electronegative.

01:38 It is due to highly electronegativity, negative behavior.

01:42 It also shows minus i effect.

01:45 So due to minus i effect, so the electron has a very small tendency to come to position 3.

01:51 This percentage is very less of course.

01:54 So if it is at position 3, it is called as metta.

01:58 It is meta activator.

01:59 It is meta activators.

02:03 So, suppose if we are talking about sulfonation or chlorination, so if it is chlorination, so chlorination happens in the presence of chlorine in the presence of louis acid fecl3, which would generate a electrophile cl plus.

02:19 If it is nitration, if it is nitration, we use a nitrating mixture that is concentrated hno3.

02:27 Hno3 and concentrated h2, s .o4, which would generate a nitrogenium n -o2 plus...

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