Orientation in Benzene and Directive Influence or Orienting

Orientation in Benzene and Directive Influence or Orienting Effect of Substituents, Isomerism and Derivatives

(E.S.R. of Monosubstituted Benzene)

Isomerism in Monosubstituted Benzene

All the six hydrogen atoms of benzene are identical and equivalent. Thus replacement of any H atom of benzene always results in the same one monosubstituted product (C6H5–G) and thus only one monosubstitution benzene is possible (e.g. one nitrobenzene, one chlorobenzene etc) and no isomerism evolves on monosubstitution.

MSBs have five replaceable hydrogens. A second substituent (Y) can occupy any of the remaining five positions. On disubstitution, when one hydrogen atom of monosubstituted benzene (C6H5–G) is replaced by second substituent Y (atom or group of atoms) to give disubstituted benzene (C6H4GY), isomerism appears. The second substituent (Y) may be situated on adjacent (2 and 6), alternate (3 and 5) or opposite (4) carbon atoms. Thus only three disubstituted products are possible as positions 1, 2 and 1, 6 are equivalent. Similarly 1, 3 and 1, 5 are equivalent.

Effect of Substituents on further substitution

1. Positions 2 and 6 are same (ortho product)

2. Positions 3 and 5 are same (meta product)

3. Position 4 is para position

Ortho position or compounds

The monosubstituted benzene (C6H5–G) having the second substituent at adjacent positions (2 or 6) are called ortho compounds abbreviated as –o.

Meta position or compounds

The monosubstituted benzene (C6H5–G) having the second substituent at alternate position (3, 5) are called meta compound abbreviated as –m.

Para position or compounds

The monosubstituted benzene (C6H5–G) having the second substituent at opposite position (4) are called Para compound abbreviated as –p.

For example; Xylene exists in three isomeric forms:

Two types of influences of Substituents

A substituent (G) already present on the benzene ring exercises two types of influences on disubstitution

1. Directive effect or orientation effect (direct 2nd substituent to specific carbon in the aromatic ring)

2. Activity effect (can activate or deactivate the ring)

1. Directive effect or orientation effect or Orienting Effect of Substituents

The first substituent (G) may direct the next incoming substituent (Y+ or E+) to ortho, meta or para positions, depending on the nature of the first substituent. This is called directive effect or orientation effect (orient means to arrange).

The influence of a substituent already present in the benzene ring which determines the position to be taken by the second entering group in the ring is called Directive Influence or Orienting Effect of the Substituents.

It is the ability of a group already present in the benzene ring to direct the incoming group to a particular position.

Orient means “To determine the position of”.

The process of assigning the position of second incoming group in the monosubstituted benzene (C5H5–G) is called Orientation.

When mono substituted benzene undergoes an electrophilic attack, the rate of reaction and the site of attack vary with the functional group already attached to it. Some groups increase the reactivity of benzene ring and are known as activating groups while others which decrease the reactivity are known as deactivating groups. We further divide these groups into two categories depending on the way they influence the orientation of attack by the incoming electrophile. Those which increase the electron density at ”ortho” and “para” positions are known as ortho-para directors while those which increase the electron density at “meta” position are known as meta directors.

There are two types of substituents which produce directive effect are,

(i) Those which direct the incoming group to ortho- and para-positions simultaneously (Neglecting meta all together).

(ii) Those which direct the incoming group to meta-position only (Neglecting ortho- and para-positions all together).

2. Activity effect

The substituent already present may activate or deactivate the benzene ring towards further substitution. These effects are called the activity effects.

The substituent already present can increase or decrease the rate of further substitution, i.e., it either activates or deactivates the benzene ring towards further substitution. These effects are called activity effects.

DIRECTIVE EFFECTS OF SUBSTITUENTS

In monosubstituted benzene (C6H5-G), there are five available hydrogen atoms. Of these two are ortho, two meta and one para to G. If substitution was random, on purely statistical basis, the proportion of the disubstituted products would be

Ortho = 2/5 of the total or 40%

meta = 2/5 of the total or 40%

Para = 1/5 of the total or 20%

But, this distribution is never observed actually. The proportion of the products formed, in fact, is determined by the nature of the first substituent already present on the ring rather than on any mathematical probability.

Types of Directing Groups

Directing groups are of two types:

1. Ortho-para directors

2. Meta directors

1. Ortho-Para Directing Substituents
1. These are the substituents which direct the second substituent to ortho and para positions. Certain substituents direct the 2nd entrant predominately to the ortho and para positions simultaneously (the meta product is either not formed or formed in less amount). They are called ortho-para directors.

2. These substituents enhance the reactivity of ring and they are activating groups (except halide groups).

3. All o,p-directors have non-bonding electron pair on the key atom except alkyl or aryl groups.


2. Meta Directing Substituents
1. These are the substituents which direct the second substituent to meta position.

2. These substituents reduce the activity of ring and they are deactivating groups.


Mixed Classification of Directing Groups
For the sake of convenience, directing groups are divided into three different classes with regard to whether they are activating or deactivating and whether they are o-p-directing or m-directing.

(a). o-p-directing and activating groups
(b). o-p-directing and deactivating groups
(c). m-directing and deactivating groups

Atoms or groups that make the benzene molecule more reactive by increasing the ring's electron density are called activating groups. Activating groups serve as ortho‐para directors when they are attached to a benzene ring, meaning that they direct an incoming electrophile to the ortho or para positions.

An atom or group that makes the benzene molecule less reactive by removing electron density from the ring acts as a deactivating group. Deactivating groups direct incoming electrophiles to the meta position. You can further classify activating and deactivating groups or atoms as strong, moderate, or weak in their directing influence.

This table lists some typical activating and deactivating groups by the order of their strength.

Making Polysubstituted Benzene

Benzene containing more than one substituents is called polysubstituted benzene. The synthesis of polysubstituted product of benzene depends on the already present substituent. If the already present substituent is activator, the polysubstituted product is formed easily.

For example

1. When benzene alkylation of benzene occur in the presence of Lewis acid, we obtain alkylbenzene but the reaction does not stop at the stage of monoalkylation because alky group is activator group which increases the reactivity of benzene, therefore we obtain polysubstituted of alkyl benzene

2. When nitration of benzene occurs in the presence of H2SO4 at 60°C, nitrobenzene is obtained. But the further nitration will not occur easily as NO2 group is deactivator. It needs high temperature (100°C) and for dinitrobenzene and trinitrobenzene fuming nitric acid and sulphuric acid is required. The second nitro group further deactivates the benzene ring, so more drastic conditions are required for trinitration.

Nitration of toluene into TNT 3. when nitration of toluene occurs in the presence of H2SO4 at 25°C, two isomeric ortho and para- nitrotoluene is obtained as methyl group is o-p-director. the further nitration of both isomers gives same trisubstituted product i.e. dinitrobenzene (as one ortho and one para are vacant). The further nitration gives trinitrotoluene (as one ortho is vacant).