Chemistry topic 18

Question 1

Define aliphatic

Answer 1

straight or branched chain organic substances

Question 2

Define aromatic/arene

Answer 2

Includes one or more ring of 6 carbon atoms with delocalised bonding

Question 3

Describe the structure of benzene

Answer 3

Benzene has 6 carbon atoms bonded in a planar ring structure with delocalised electrons.

Each carbon atoms has 3 sigma bonds; 2 carbon atoms and 1 hydrogen atoms.

Question 4

What is the formula of benzene?

Answer 4

C6H6

Question 5

Why does benzene undergo electrophilic substitution?

Answer 5

The delocalised π system gives the benzene molecule stability and it also gives it an area of high electron density.

Question 6

Why does benzene undergo electrophilic substitution?

Answer 6

The delocalised π system gives the benzene molecule stability and it also gives it an area of high electron density.

Question 7

Explain the evidence for benzene structure

1. EA

Answer 7

1. Benzene does not undergo electrophilic addition:
   Unlike alkenes, in benzene, there is no c=c bond present,(Kukules structure). Therefore, it does not decolourise bromine. The π system gives stability to the structure as it is not broken.

Question 8

Explain the evidence for benzene structure

2. TD S

Answer 8

2. Benzene has thermodynamic stability:
   The actual value of enthalpy for benzene is less exothermic than the theoretical enthalpy value for cyclo-1,3,5-triene. The difference (152 KJ mol-1) is known as the delocalisation energy,

Question 9

Explain the evidence for benzene structure

3. BL

Answer 9

Evidence suggests that all the C-C bonds are the same length.

Question 10

What reaction does benzene undergo?

Answer 10

benzene undergoes electrophilic subsitution

Question 11

What reaction do alkenes undergo?

Answer 11

Alkenes undergo electrophilic addition

Question 12

Compare the electron density in alkenes and benzenes

Answer 12

Benzene has delocalised π electrons spread around the 6 carbon atoms, whereas alkenes have localised π electrons shared between 2 carbon atoms.

As a result, benzenes have a lower electron density, whereas alkenes have a higher electron density

Question 13

Compare the stability of benzene and alkenes

Answer 13

Benzene is more stable and is unlikely to be destabilised by an addition reaction that will ‘break’ the π ring.

But alkenes are less stable and are highly likely to undergo an addition reaction to achieve stability

Question 14

Compare how benzene and alkenes react with bromine

Answer 14

In benzene there is no reaction with Br as electron density is not great enough to create a dipole in Br2 molecule.

Alkenes react with Br2 as the molecule is readily polarised by the high electron density in c=c bond. This creates an electrophile