3.3.10 Aromatic Chemistry (A2)

Question 1

what is benzene’s formula and structure?

Answer 1

C6H6, planar hexagon with delocalised system

Question 2

what is another name for arenes? how did this come about?

Answer 2

aromatic compounds, as first found in sweet smelling dyes

Question 3

what is the most common type of reaction for benzene?

Answer 3

substitution (of a H for a different functional group)

Question 4

what is the shape of benzene?

Answer 4

flat, regular hexagon, bond angle 120 degrees

Question 5

what is the bond length between adjacent C atoms?

Answer 5

intermediate between C-C and C=C
1.39 Å (angstrommes)

Question 6

what happens to the 4th electron in the p orbital of each C atom in benzene?

Answer 6

delocalises, forming rings of delocalised electron density above/below the planar hexagon

Question 7

what is the effect on benzene’s stability of the rings of electron density?

Answer 7

makes it very stable, gives it aromatic stability

Question 8

what is the thermochemical evidence that benzene is more stable than cyclohexa-1,3,5-triene?

Answer 8

hydrogenation of cyclohexene = -120kJ/mol
therefore cyclohexa-1,3,5-triene = -360kJ/mol
benzene hydrogenation = -208kJ/mol so benzene is 152kJ/mol more stable

Question 9

why else (other than thermochemistry) is cyclohexa-1,3,5-triene not a suitable model for benzene?

Answer 9

would not be symmetrical (C=C shorter then C-C) but benzene is
would easily undergo nucleophilic addition reactions across the double bond - benzene does not
would form two isomers on the addition of Br2 or similar - benzene does not

Question 10

what is the appearance of benzene at 298K?

Answer 10

colourless liquid

Question 11

why does benzene have a relatively high melting point?

Answer 11

close packing of flat hexagonal molecules when solid

Question 12

is benzene soluble in water? why?

Answer 12

no
non-polar

Question 13

dangers of benzene?

Answer 13

carcinogen

Question 14

How do you name compounds containing a benzene ring?

Answer 14

-benzene

Question 15

why is benzene attacked by electrophiles?

Answer 15

high electron density above/below ring due to delocalised electrons (system)

Question 16

what is delocalisation energy and what is the effect of this on benzene’s reactions?

Answer 16

large amount of energy that is needed to break the aromatic delocalised system apart.
results in the aromatic delocalised system almost always staying intact

Question 17

what is seen when benzene is combusted? why?

Answer 17

“dirty” (smoky) flame due to soot from unburnt carbon
due to the high C:H ratio (1:1)

Question 18

which ion is used to nitrate benzene?

Answer 18

NO2+ (+ charge is on the nitrogen)
nitronium ion

Question 19

how is the NO2+ ion generated?

Answer 19

conc. H2SO4 and conc. HNO3
H2SO4 + HNO3 –> H2NO3+ + HSO4-
H2NO3+ –> H2O + NO2+
overall: H2SO4 + HNO3 –> HSO4- + NO2+ + H2O

Question 20

how is the H2SO4 catalyst regenerated in the nitration of benzene?

Answer 20

HSO4- + H+ –> H2SO4 (H+ from benzene ring)

Question 21

what are the uses of nitrated arenes?

Answer 21

production of explosives eg. TNT
(1-methyl-2,4,6-trinitrobenzene) - releases lots of heat and gas on explosion
To make aromatic amines that are used for industrial dyes

Question 22

what type of catalyst is used for a Friedel-Crafts reaction?

Answer 22

A halogen carrier eg. AlCl3

Question 23

what is happening when AlCl4- is formed in terms of electrons?

Answer 23

chlorine atom’s lone pair of electrons is forming a co-ordinate bond to Al

Question 24

How is the AlCl3 catalyst reformed?

Answer 24

AlCl4- + H+ –> HCl + AlCl3 (H+ from benzene)

Question 25

in cyclic compounds, what might happen if two double bonds are next to each other?

Answer 25

C=C bonds are close in proximity, so electrons in p-orbitals can partially delocalise and move between the two C=C bonds

Question 26

what effect would electrons in p-orbitals moving between the two C=C double bonds have on the stability of the molecule and its enthalpy of hydrogenation?

Answer 26

makes the molecule more stable; makes enthalpy of hydrogenation more positive