Aromatics (Benzene)

Question 1

What is the molecular formula of

Benzene ?

Answer 1

C₆H₆

Question 2

What structure

of Benzene is readily accepeted?

Answer 2

Resonance Structure

Question 3

What structure

of Benzene is disputed?

because it might suggest that benzene behaves like an alkene

Answer 3

Kekulé’s Structure

Question 4

What are the problems with

Kekulé’s Structure?

because it might suggest that benzene behaves like an alkene

Answer 4

1. Bond Length disparities expected (due to presence of both C-C and C=C bonds)
2. High reactivity expected
3. Irregular molecular shape expected (Benzene is planar)

These are seen to not be the case

Question 5

What is

the hybdrisation of all the carbons in the benzene molecule?

Answer 5

sp² hybridised

In alkenes, only the carbon atoms in the C=C are sp² hybridised

Question 6

What happens to

the unhybridised p_z orbital in the 6 Benzene carbons?

Answer 6

The p_z orbitals overlap at the the top and bottom, creating a ring-like shape

Recall that the p orbitals have a dumb-bell shape, hence the top and bottom overlap

Question 7

Hence,

what are the bond angles about each carbon centre?

Answer 7

120°

Benzene is a planar molecule

Question 8

What is

the carbon-to-carbon bond length in Benzene?

Answer 8

139 pm

The bond length is shorter than the C-C bond but not as short as the C=C bond. The smaller the bond length, the stronger the bond.

Question 9

Is Benzene very reactive?

Answer 9

No

Question 10

What is the reason for the (low) reactivity of Benzene?

Answer 10

It has a resonance stability

Means that the pi electrons are not readily available

Question 11

What are resonance structures?

Answer 11

These are structures that are used to descibe the presence of delocalised e^- in compounds

Due to the delocalised e^- the compounds can take on more than one form depending on their rel. positioning.

Question 12

Why do resonance structures form?

Answer 12

When there is more than one way to represent the double bonds or lone pairs in a molecule

due to the presence of delocalised electrons

Question 13

How does resonance dispute Kekulé’s structure

Answer 13

Under Kekulé’s structure, ‘Benzene’ can either be represented as 1,3,5-hexatriene or 2,4,6-hexatriene. Because these are 2 diff. structures of the same thing, Benzene is given a resonance structure

Question 14

Which is more reactive:

Benzene or a(n) (cyclo)alkene ?

Answer 14

The (cyclo)alkene

Question 15

What is the reason for this difference in reactivity?:

between benzene and a(n) (cyclo)alkene ?

Answer 15

In the benzene ring, the 6 pi electrons (1 donated by each of the 6 carbons) are delocalised, hence its resonance structure. In (cyclo)alkenes, however, the pi electrons are localised in the C=C bond

Localisation increases electron density

Question 16

Which is correct when speaking about delocalised electrons?:

They are:
* at lower energy levels
* at higher energy levels

Answer 16

They are at lower energy levels

This makes them more stable. The lower the energy level of an electron, the closer it is to the nucleus of the atom

Question 17

What kind of reactions

does Benzene undergo?

Answer 17

Electrophilic Substitution Reactions

Question 18

What is the movement of electrons

in the first reacting step of electrophilic substitution reactions in Benzene?

After the electrophile is formed

Answer 18

The electrophile attacks the electron rich benzene ring. 2 pi electrons move from the benzene ring to the electrophile.

Question 19

Generally speaking,

are catalysts needed for Benzene to react?

Answer 19

Yes

Question 20

Name some

Electrophilic Substitution Reactions of Benzene

Answer 20

1. Halogenation
2. Nitration
3. Alkylation
4. Acylation

Question 21

Under what body of reactions

do alkylation and acylation fall?

Answer 21

Friedel-Crafts reactions

Question 22

What are the reaction conditions, (reagents, catalyst & electrophile),

for the reaction of Benzene with a Halogen?

take Chlorine as an example

Answer 22

REAGENTS: Benzene & Chorine gas
CATALYST: Iron (III) / Aluminum Chloride
ELECTROPHILE: Cl⁺ (chloronium ion)

the halide is peculiar to the reacting halogen

Question 23

What will be physically observed

in the reaction of Benzene with Chlorine?

In the presence of catalyst

Answer 23

Dense fumes of white gas (HCl_(g))are seen

Hydrogen halides escape as white fumes when produced

Question 24

What will be physically observed

in the reaction of Benzene with Bromine?

In the presence of catalyst

Answer 24

There will be a decolorisation of the reacting system

To show that benzene reacts with the bromine gas

Question 25

What are the reaction conditions, (reagents, catalyst & electrophile),

for the reaction of Benzene with conc. HNO₃?

This is the Nitration of Benzene

Answer 25

REAGENTS: Benzene & Nitrating mixture (a mix of conc. HNO₃ and conc. H₂SO₄)
CATALYST: conc. H₂SO₄
ELECTROPHILE: NO₂⁺

The conc. H₂SO₄ helps to form the nitronium ion

Question 26

What is the temperature specification

for the nitration of Benzene?

Answer 26

(Below) 55°C

50°C is typically used

Question 27

How does

conc. H₂SO₄ helps to form the nitronium ion?

Answer 27

It donates one of its hydrogen atoms to HNO₃ forming a complex that dissociates to the nitronium ion and water

Question 28

What are the reaction conditions, (reagents, catalyst & electrophile),

for alkylation reactions of Benzene?

Answer 28

REAGENTS: Benzene & an alkylhalide
CATALYST: The Aluminum or Iron (III) salt of the corresponding halide
ELECTROPHILE: The carbocation formed

Question 29

For Friedel-Crafts alkylation,

are the reaction conditions aqueous or anhydrous?

Answer 29

Anhydrous

Question 30

What is there to note

about the halogen placement in the alkyl halide used as a reagent in alkylation?

Answer 30

The carbon atom in Benzene forms a bond with the carbon atom that bears the halogen

Question 31

What are the reaction conditions, (reagents, catalyst & electrophile),

for acylation reactions of Benzene?

Answer 31

REAGENTS: Benzene & an acylhalide
CATALYST: The Aluminum or Iron (III) salt of the corresponding halide
ELECTROPHILE: The acylium ion formed

Question 32

What is the main product

of an acylation reaction of Benzene?

Answer 32

A Ketone

Question 33

What is

an acyl group?

Answer 33

Represented by RC=O(…), the acyl group contains the carbonyl group

Question 34

How do you represent

an acylium ion ?

Answer 34

(RC=O)⁺

Question 35

What is this?:

Cl⁺

Answer 35

Chloronium ion

Question 36

What is this?:

Cl^-

Answer 36

Chloride ion

Question 37

Why must you use the halides in

in Friedel-Crafts alkylation/acylation reactions?

Answer 37

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