What are Deactivating Groups?
In organic chemistry, a deactivating group (or electron withdrawing group) is a functional group attached to a benzene molecule that removes electron density from the benzene ring, making electrophilic aromatic substitution reactions slower and more complex relative to benzene. Depending on their relative strengths, deactivating groups also determine the positions (relative to themselves) on the benzene ring where substitutions must take place; this property is therefore important in processes of organic synthesis.
What are Meta Directing Groups?
If the relative yield of the ortho product and that of the para product are higher than that of the meta product, the substituent on the benzene ring in the monosubstituted benzene is called an ortho, para directing group. If the opposite is observed, the substituent is called a meta directing group. Example: Nitro group is a meta directing group.
What is Directive Effect?
For Ortho-para Directors
The resonance theory explains why some substituents are ortho-para and others are meta-directing. Let's look at some phenol resonance forms where the -OH group is already attached to the benzene ring.
-OH Group Attached to Benzene Ring
Directive Effect
On the oxygen-atom of the -OH group attached to the ring, there are two nonbonding electron pairs. The interaction with the p-system distributes one of these into the ring. The ortho and para positions in resonance forms have a higher electron density than the meta positions. As a result of the electron delocalization, the resonance hybrid has negative charges in the ortho and para positions. Regardless of its nature, the electrophile (E+) would naturally gravitate toward these electron-rich centres.
Electrophile Attracted Towards Electron-rich Centres
Are All Meta Directing Groups Deactivating or Not Deactivating?
They are meta-directing not because they stabilise the intermediate for meta-substitution: far from it. Instead, they destabilise the:
o,p-substitution intermediates in relation to the meta intermediate.
In general, o,p-directing groups stabilise o,p substitution intermediates relative to the meta intermediate.In comparison to the meta intermediate, m-directing groups destabilise the intermediates for o,p substitution.
What is Benzene?
Benzene is the most fundamental organic, aromatic hydrocarbon. Benzene is a basic petrochemical and a natural component of crude oil. It is a colourless liquid with a gasoline-like odour. In nature, benzene is highly toxic and carcinogenic. Its primary application is in the manufacture of polystyrene.
Although benzene is a naturally occurring substance produced by volcanoes and forest fires and found in many plants and animals, it is also a major industrial chemical derived from coal and oil. Benzene is a clear, colourless liquid when pure. Benzene is used in the manufacturing of other chemicals, as well as some plastics, detergents, and pesticides. It is also found in gasoline.
Important Questions
1. How will you account for the structure of benzene?
In benzene, all six carbon atoms are SP2 hybridised. Each carbon atom's two SP2 hybrid orbitals overlap with the SP2 hybrid orbitals of adjacent carbon atoms to form six C-C sigma bonds in the hexagonal plane. Each carbon atom's remaining SP2 hybrid orbital overlaps with a hydrogen atom's s-orbital to form six C-H sigma bonds. Each carbon atom now has one hybridised p-orbital perpendicular to the ring plane. C-atoms' unhybridized p-orbitals are close enough to form a bond via lateral overlap.
2. N – pentane has higher boiling point than neopentane but the melting point of neopentane is higher than that of n – pentane.
The surface area and vander wall forces of attraction in neopentane are much weaker than in n-pentane due to the presence of branches. As a result, the boiling point of neopentane is lower than that of n-pentane. The melting point is determined by the arrangement of molecules in the crystal lattice. Because neopentane is more symmetrical than n-pentane, it packs much more tightly in the crystal lattice than n-pentane and thus has a much higher m.p than n-pentane.
Conclusion
Deactivating groups are substituents that decrease the rate of a reaction.
The groups which direct the incoming group to meta position are called meta-directing groups.
Meta acts as a Deactivating Group because they don’t tend to donate electrons.
Benzene is the most fundamental organic, aromatic hydrocarbon.
Benzene is also cyclohexatriene.
Multiple Choice Questions
1. The bond length of c-c bond in benzene is
(a) 1.45 A
(b) 1.38 A
(c) 1.33 A
(d) 1.23 A
Answer: (b)
2. Which of the following reagents does not react with benzene?
(a) Concentrated H2SO4
(b) HNO3 / H2SO4
(c) Iodoethane in the presence of iron
(d) Sodium hydroxide solution
Answer: (d)
3. CHO group on benzene nucleus
(a) Activates ring
(b) Deactivates ring
(c) Does not affect the ring
(d) None of the above
Answer: (b)
4. Identify deactivating and meta directing groups from the following
(a) -CHO
(b) -NH2
(c) -OH
(d) -OCH3
Answer: (a)
FAQs on Identify Deactivating and Meta Directing Groups
1. Where is Benzene commonly found?
Benzene is a carcinogenic substance found in a wide range of modern products and industries. It is formed by natural processes as well as human activities. Benzene is a common industrial chemical. Benzene is found in crude oil and is a significant component of gasoline. Plastics, resins, synthetic fibres, rubber lubricants, dyes, detergents, drugs, and pesticides are all made from it. Volcanoes and forest fires naturally produce benzene.
2. Why is CL-Group ortho para directing but ring deactivating?
Given chlorine's high electronegativity, the Cl- substituent (like the other halogen substituents) has an electron-withdrawal effect on the aromatic ring, reducing electron density and making the ring less nucleophile, deactivating it. However, the halogen groups contain some lone pairs of nonbonding electrons that can be added to the pi bonds, causing carbocation resonance. This effect overcomes the group's inductive effect, which destabilised the ortho-carbocation with chlorine, bromine, and iodine. As shown in the diagram below, the resonance stabilises only the ortho and para carbocations, directing the substitution.
3. Why is methylbenzene more reactive than ethyl benzene, towards electrophilic substitution?
Probably the Ethyl group has a greater +I effect than the Methyl group, Ethyl Benzene is more electron rich and thus more reactive than Methyl Benzene. However, you must also consider the hyper-conjugation effect of the alkyl substituents, as well as the fact that the hyper-conjugation effect outweighs the inductive effect. Because the Ethyl group has two alpha hydrogens and the Methyl group has three alpha hydrogens, the Methyl group's hyperconjugation effect dominates the Ethyl group, and Methyl Benzene is more reactive overall.
