Effect of Substituents on Reactivity-Orientation, Chemistry tutorial

Effect of Substituents on Reactivity and Orientation:

Benzene structures only one monosubstituted product via the electrophilic substitution. Let us see what occurs when we bear out an electrophilic substitution on substituted benzene.

Studies have given that the substituents influence the reactivity and the orientation of the benzene ring. Three possible disubstituted products, via, ortho, para and Meta can effect. Such 3 products aren't formed at random; rather, a following substitution already connected to the benzene ring generally express the position of the 2nd substituent.

There are 2 kinds of substituents - one is electron-donating group, such as - NR2, - OH, - OR, - NHCOR, and alkyl group, other is electron - withdrawing group which includes. 

                      O       O      O  

                      ||       ||     ||

Halogens - CH, - CH, - COR, - CN, - NO2, - NR2. Now we will learn the consequence of substituents on reactivity and orientation.

Effect of Substituents on Reactivity:

To evaluate the rates of electrophilic substitution in benezene, methyl substituted benzene (methyl benzene or toluene) and nitro substituted benzene (nitrobenzene) are contrasted in a reaction, say nitration. It is originate that nitration of methyl benzene (toluene) is more simplistic than benzene whereas nitration of nitrobenzene is more hard. In order terms, benzene ring appears to be activated in toluene and deactivated in nitrobenzene. Let us see if we can illustrate this on the origin of intermediate carbocation structured

1257_methyl group.jpg

In the case of methylbenzene (II), the methyl group, that is an electron-donating group, tends to neutralize the echargeon, the carbocation; this dispersal of the charge stabilizes the carbocation thus leading to faster reaction than benzene.

In case of nitrobenzene (III), the NO2 group that is an electron-withdrawing group, tends to intensify the positive charge and destabilize the carbocation. Due to this result, the rate of the reaction is slower than in benzene. Reactivity in electrophilic aromatic substitution based, then, upon the affinity of a substituent group to liberate or remove electrons. A group that liberates electrons activates the ring; a group that extracts electrons deactivates the ring. Therefore, the arrange of reactivity of the above compounds towards electrophilic substitution reaction is:

636_electrophilic substitution.jpg

Effect of Substituents on Orientation

The 2nd effect of a substituent is to direct the position of the incoming substituent. Therefore, for instance, nitration of phenol provides ortho and para-nitro phenol as major products.

1924_nitrophenol.jpg

On the other hand, nitration of nitrobenzene yields meta dinitrobenzene as a major product that is this

1034_dinitrobenzene.jpg

Illustrates that different substituents have diverse result on substitution reaction. Therefore substituents can be classified into 3 groups, that is

  • Ortho and para-directing activator
  • Meta-directing deactivator (No metadirecting activators are known.)
  • Ortho and para-directing deactivator

Table: List several of the groups in each category

1846_List some of the groups.jpg

Before we try to account for the orientation in electrophilic substitution, we should illuminate our classifications of activating and deactivating. Activating groups activate all the positions of the ring. They are ortho and para directors since they activate ortho and para position much more than they do the meta position. Likewise, deactivating groups deactivates all positions in the ring. They are meta directors since they deactivate the ortho and para position more than they deactivate the meta position. Therefore, the consequence of any group, whether activating or deactivating, is toughest at the ortho and para positions.

Ortho and para-directing activator:  To comprehend the orientation consequence of the substituents, we have to 1st written all the possible resonance forms of the charged intermediates for each of the 3 probable reaction courses. Reactions of NO2 at the para, meta and ortho positions of phenol provide the intermediates through the giving resonance structures.

a) Ortho-Attack

56_Ortho-Attack.jpg

b) Meta-Attack

914_Meta-Attack.jpg

c) Para-Attack  

1767_Para-Attack.jpg

In case of ortho and para attacks, structures IV and X correspondingly illustrate that the unshared electron pair of oxygen delocalize the positive charge on the carbocation and, therefore, 4 resonance formations are feasible. In the case of meta attack, because the charge and not be delocalized onto the -OH group, the carbocation that is formed has only 3 resonance structures. Therefore, carbocation formed via ortho and para attacks are more stable and, therefore, ortho and para introphenols are the main products.

Now acquire the instance of electrophilic aromatic substitution on alkyl substituted benzene ring. Let us examine the possible resonance structures of carbocation formed through the hit of the electrophile, NO2, on toluene.

1. Ortho-Attack

179_Ortho-Attack 2.jpg

2. Meta-Attack

1139_Meta-Attack2.jpg

3. Para-Attack

611_Para-Attack2.jpg

As indicated above, in formations III and VIII, resulting from ortho and para attack correspondingly, the positive charge is placed on the carbon atom to that the methyl group is connected. Since that structure has tertiary carbocation quality, it is more constant than the others, in that the positive charge is at a secondary carbocation. On the other side, meta attack generates an intermediate in which no one of the resonance structure formations advantages from this tertiary carbocation stabilization. Therefore, electrophilic attack on a carbon placed ortho or para to methyl group leads to a cationic in-between that is more constant than the one derived through attack at the meta carbon. Substitution at ortho-and para-position is, hence, desirered to meta-substation.

This can as well be illustrated on the basis of inductive consequence. The carbocation III and VIII shaped through the ortho-and para-attack correspondingly, is stabilized through inductive result of methyl group and, hence, formed in major amount.

Meta-directing deactivators: We can affect comparable arguments to meta-directing groups. Such groups are all electronegative groups with no an unshared electron pair on an atom adjacent to the benzene ring. In all such cases, the benzene ring would be deactivated. Let us take the instance of nitration of nitrobenzene. The possible resonance structures of the carbocation formed are:

a) Ortho-attack

2066_ortho attack 3.jpg

b) Meta-attack

1723_meta-attack3.jpg

c) para-attack

1125_para-attack3.jpg

In every the 3 cases, carbocations structured have 3 resonance structures. But the structure III and VIII effecting from ortho and para correspondingly are extremely unfavorable since the positive charge is placed directly on the carbon carrying the electron withdrawing group. A severe electrostatic repulsive interaction between the carbocation and the positive end of the NO2+ group strongly disfavors this carbocation.

The carbocations shaped via meta attack, have no these form through analogous charges on adjacent atoms. Hence, its transition state is the most stable, and attack at meta-position is favored.

Ortho and Para Directing Deactivator: Halogens are abnormal in their consequence on electrophilic aromatic substitution. They are deactivating until now ortho and para directing. For understanding the orientation, consider the attack of electrophile at ortho, meta and para locations of chlorobenzene.

a) Ortho-Attack

1447_Ortho-Attack4.jpg

b) Meta-attack

2163_Meta-Attack4.jpg

c) Para-attack

1043_para-attack4.jpg

In structures III and IX, effecting from ortho and para attack, there is a positive charge on carbon bearing the halogen atom. Throughout its inductive consequence, chlorine withdraws electrons, making this structure unstable. But there is an additional feature that one should not forget. It is identified that halogen can share alone pair of electrons and accommodate the positive charge, as given away in structures IV and X, for ortho and para attack, correspondingly. Such formations are moderately stable. No these structure is possible whenever the electrophile attacks on meta position. Structures IV (in ortho attack) and X (in para attack) outweigh the instability rendered through structures III and IX. Hence, attack at ortho and para position is favored.

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