Module 7 Reactivity of Benzene

Question 1

Why can’t benzene undergo addition reactions?

Answer 1

Aromatic rings are remarkably stable

Aromatic rings are stable enough that they do not undergo such reactions

Question 2

Structure of Benzene

Answer 2

• Resonance (bonds aren’t as electron rich)
• every carbon is sp2 hybridized
• all carbons have overlapping p-orbitals

Question 3

What is Aromaticity?

Answer 3

Aromatic (fragrant) is the term used to molecules that exhibit this type of stability.

Question 4

Aromatic compound must meet two criteria:

Answer 4

1) A fully conjugated ring with overlapping p-orbitals (all carbons are sp2)
2) Have an ODD number of e- pairs or 4n+2 total π electrons where n=0, 1, 2, 3, 4, etc.

Question 5

Do all fully conjugated rings have aromatic stability?

Answer 5

No!

Some fully conjugated cyclic compounds react like normal alkenes

Not all the p-orbitals overlap,

Question 6

Aromatice Heterocycles?

Answer 6

Rings with heteroatoms (atoms other than C or H) can be aromatic. The heteroatoms are sp2-hybridized so that a lone pair can be placed in the remaining p orbital, which forms a molecular orbital with the p orbitals of the other atoms.

Question 7

If the lone pair is necessary for aromaticity, then the lone pair will not be ______.

Answer 7

basic

Question 8

when a benzene ring is named as a substituent it is called a ______ group. If the benzene ring is attached to a CH2, the entire group is a _____ group.

Answer 8

phenyl and benzyl.

Question 9

Cations and anions on the benzylic carbon can be stabilized by ____

Answer 9

resonance.

Question 10

Reactions at the Benzylic Position (Na2CR2O7, H2SO4, H2O) or (KMnO4, H2O heat)

Answer 10

Benzylic positions are readily oxidized by chromic acid.

the benzylic position needs at least one proton attached to undergo oxidation.

-the proton is replaced with carboxylic acid.

Question 11

Can Aromatic rings and alkyl groups be oxidized the same way? (benzylic position)

Answer 11

No. Oxidation of the benzylic position is very specific to that position and needs at least one H to be oxidized.

Question 12

Two other reactions at the benzylic position.

Answer 12

SN1/SN2

E1/E2

Question 13

Why can’t benzene react like alkenes in an addition reaction?

Answer 13

Pi electrons in the benzene ring are not as nucleophilic, it is aromation and electrons are delocalized.

Benzene can react with a super-duper electrophile, something so electron defficient that even delocalized electrons of the aromatic benzene rings will attack.

Question 14

Super-Duper Electrophile

Answer 14

In the presence of a super-duper electrophile a benzene double bond can attack it.

Question 15

EAS Mechanism

Answer 15

1) Generate super-duper electrophile
2) Double Bond attacks the electrophile
3) Generation of a carbocation
4) Base deprotonates the carbon bonded to the electrophile
5) double bond reforms

This reaction is favoured because it regenerates the extra stable aromatic ring.

We substitiute the hydrogen for the electrophile.

Question 16

Halogentation (Br2, FeBr3)

Answer 16

-FeBr3 activates the Br2 (LAC) making it even more electron poor.

Lewis acids are an electron pair acceptor, the electron pairs of bromine will attack the lewis acid forming a new intermediate with an electron deficient bromine, that pulls electron densitry away from the adjacent bromine.

generating the bromine electrophile.

Question 17

Halogentation (Br2, AlBr3)

Answer 17

The mechanism of bromination would be the same as when FeBr3 is used

Question 18

Halogenation (Cl2, AlCl3)

Answer 18

Very Similar Reaction to bromine.

Question 19

Halogenation (Cl2, AlCl3)

Answer 19

Very Similar Reaction to bromine.

Question 20

Sulfonation (Fuming H2SO4 contains SO3 gas)

Answer 20

Reversible, allows us some control if we want add multiple groups on.

Equilibrium in sulfuric acid where one acts as a base and the other as an acid, the protonated SO4, has a good leaving group, H2O leaves forming the SO3 electrophile.

Question 21

Nitration (HNO3, H2SO4)

Answer 21

It is believed a nitronium ion is the active electrophile.

Nitric acid acts as a base and deprotonates sulfuric acid, generating a good leaving group, when the water leaves we get our electrophile.

Question 22

Making Amines

Answer 22

A nitro group can be reduced to form an amine

1) Fe or Zn, HCL
2) NaOH

Question 23

Friedle-Crafts Alkylation (Alkyl Halide, AlCl3)

Answer 23

Chlorine is electron withdrawing (very deficient).

this generates a carbocation in an alkyl group.

The halide leaving group must be attached to an sp3 hybridized carbon

Question 24

We can also alkylate using an alkene and _ _ _ _

Answer 24

H3PO4

Phosphoric acid protonated the alkene to generate a carbocation which acts as the electrophile.

Carbocation rearrangements will occur.

Question 25

Friedel-Crafts Acylation (AlCl3, Acyl group)

Answer 25

Acylation and alkylation both form a new carbon-carbon bond

The active electrophile is an acylium ion

Acylium ions are resonance stabilized, and not subject to rearrangement: