CH27 Aromatic Chemistry

Question 1

Benzen formula

Answer 1

C6H6

Question 2

Another name for arenes

Answer 2

Aromatic compounds - first found in sweet-smelling dyes

Question 3

Most common type of reaction of benzene

Answer 3

Substitution - H for different functional group

Question 4

Shape of benzene

Answer 4

Flat, regular hexagon

120 degree bond angle

Question 5

What happens to 4th electron in p orbital of each C atom

Answer 5

Delocalises to form rings of electron density above and below hexagon

Question 6

Effect on stability of rings of electron density

Answer 6

Makes benzene very stable

Question 7

Thermochemical evidence that benzene is more stable than cyclohexa-1,3,5-trien

Answer 7

Hydrogenation of cyclohexene = -120 kJ mol-1
Hydrogenation of cyclohexa-1,3,5-triene = -360 kJ mol-1
Hydrogenation of benzene = -208 kJ mol-1

Question 8

Why else is cyclohexa-1,3,5-triene not a suitable model for benzene

Answer 8

Would not be symmetrical - benzene is
Easily undergo addition reactions across double bond - benzene would not
Would form 2 isomers on the addition of Br2 - benzene does not

Question 9

Appearance of benzene at 298K

Answer 9

Colourless liquid

Question 10

Why does benzene have high MP

Answer 10

Close packing of flat hexagonal molecules when solid

Question 11

Is benzene soluble in water - why

Answer 11

No - non polar

Question 12

Dangers of benzene

Answer 12

Carcinogen

Question 13

Naming benzene containing compounds

Answer 13

-benzene

phenyl-

Question 14

Why is benzene attacked by electrophiles

Answer 14

High electron density above and below ring due to delocalised electrons

Question 15

What is delocalisation energy

Answer 15

The large amount of energy required to break aromatic ring part

Question 16

What is seen when benzene is combusted - why

Answer 16

Smoky flames due to soot from unburnt carbon - high C:H ratio

Question 17

Which ion is used to nitrate benzene

Answer 17

NO2+

Nitronium ion

Question 18

How is nitronium ion generated

Answer 18

Conc H2SO4 and Conc HNO3
H2SO4 + HNO3 –> H2NO3+ + HSO4-
H2NO3+ –> H2O + NO2+

Question 19

How is H2SO4 catalyst regenerated

Answer 19

HSO4- + H+ –> H2SO4

Question 20

Uses of nitrated arenes

Answer 20

Production of TNT

Make aromatic amines for industrial dyes

Question 21

How do substituents with a positive inductive effect affect further substitution

Answer 21

Release electrons into delocalised ring increasing electron density making further reactions more likely

Question 22

How do substituents with a negative inductive effect affect further substitution

Answer 22

Remove electrons from delocalised electron ring, decreasing electron density making further substitution reactions less likely

Question 23

What type of catalyst used for Friedel-Crafts reaction

Answer 23

A halogen carrier

AlCl3

Question 24

Equation used to acylate benzene

Answer 24

AlCl3 + RCOCl –> AlCl4 - + RCO+

RCO+ can attack benzene

Question 25

What is happening when AlCl4 - in terms of electrons

Answer 25

Cl atom’s lone pair is forming a coordinate bond to Al

Question 26

How is AlCl3 catalyst reformed

Answer 26

AlCl4 - + H+ –> HCl + AlCl3

Question 27

How could you use Freidel-Crafts mechanism to add a methyl group to a benzene ring

Answer 27

Use halogenoalkane and

AlCl3 to create an electrophile

Question 28

If considering cyclic compounds - what might happen if 2 double bonds are next to each other

Answer 28

electrons in p orbitals can partially delocalise and move between 2 C=C bonds