Benzene

Question 1

What are arenes?

Answer 1

Organic hydrocarbons containing one or more benzene rings.

Question 2

Electrophilic substitution of Benzene catalyst.

Answer 2

Anhydrous FeBr₃

Made by adding iron filings to the benzene and bromine

Question 3

How does the elecrtophilic substitution occur?

Answer 3

The elecrtophile is created when FeBr₃ polarises the Br₂ forming a dative bond with the FeBr₃ by donating a lone pair of electrons from one Br making the other one partially+ producing an elecrtophile. The Br+ and electron rich benzene ring then attract each other.

Question 4

Catalyst in these elecrtophilic substitution reactions are called?

Answer 4

Halogen carriers (FeBr₃ ,AlCl₃, FeCl₃)

Question 5

Which positions on the benzene ring are activated by electron donating species.

Answer 5

2 and 4

Question 6

The 2 and 6 positions in substituted arenes are…..?

Answer 6

Equivalent.

Question 7

Why is the carbon-halogen bond in halogenoarenes stronger than the equivalent bond in halogeno alkane?

Answer 7

Because one of the lone pairs of the halogen atom overlaps slightly with the pi bonding system in the benzene giving the bond a partial double bond character.

Question 8

Catalyst used when Cl₂ reacts with methylbenzene.

Answer 8

AlCl₃. (If Cl is not in excess then two different compounds are formed, but when in excess one product is formed).

Question 9

When chlorine reacts with methylbenzene using UV light.

Answer 9

Conditions-the Cl is passed into boiling methylbenzene in presence of UV light.
•There is no sub. into the ring in excess Cl, eventually all three hydrogen atoms in the methyl will be replaced by Cl.

Question 10

In Nitration of benzene how is the electrophile (NO₂ ion or nitronium) formed?

Answer 10

From a mixture of concentrated nitric acid and concentrated sulfuric acid.

Question 11

Conditions of nitration.

Answer 11

Reflux at 55⁰C .

Question 12

How does the electrophile affect the benzene ring?

Answer 12

The ring donates a pair of electrons to the electrophile thus disrupting it’s ring of electrons so there are now four π bonding electrons and + charge is spread over 5 carbon atoms.

Question 13

Since NO₂ is electron-withdrawing …

Answer 13

It deactivates positions 2 and 4 on the benzene ring . So when a nitro group is bonded to benzene any further substitution takes place at position 3 and 5

Question 14

Uses of Friedel-Crafts reaction. (Alkylation or acylation)

Answer 14

Manufacture of detergents or reactants needed to make plastics.

Question 15

Formula of Phenol.

Answer 15

C₆H₅OH

Question 16

How does phenol look?

Answer 16

A crystalline solid.

Question 17

Unusual oxidation in alkylarenes

Answer 17

Alkanes are not usually oxidized by KMnO₄, but in alkylarenes the alkane side oxidises to form carboxylic acid E.g methylbenzene produces benzoin acid when refluxed with alkaline KMnO₄ and then with dilute sulfuric acid.

Question 18

Why is the melting point of phenol relatively high?

Answer 18

This is because of the hydrogen bonding between it’s molecules.

Question 19

Phenol dissolves in alkaline solution because…

Answer 19

It is a weak acid.

Question 20

Phenol reacts vigorously with sodium to form…

Answer 20

Hydrogen gas (given off) and sodium phenoxide.

Question 21

Why does phenol react more readily with electrophiles?

Answer 21

This is because the overlap of the lone pair of electrons with the ring increases it’s electron density making the benzene ring more open to electrophilic attack. (Activates 2 &4)

Question 22

In nitration of phenol what are the conditions?

Answer 22

Catalyst-dilute HNO₃
Temp- r.t.p
Here two different compounds are formed.
( if Conc HNO₃ was used then 2,4,6-trinitrophenol is formed , the sixth position is activated)