Aromatics

Question 1

benzene’s formula

Answer 1

C6H8

Question 2

what is another name for arenes and what does it mean

Answer 2

aromatic compounds as first found in sweet smelling dyes

Question 3

what is the most common type of reaction of benzene

Answer 3

substitution (of H for a different functional group)

Question 4

what is the shape of benzene

Answer 4

flat
regular hexagon
bond angle = 120

Question 5

what is the bond length between adjacent C atoms in benzene

Answer 5

intermediate between C-C and C=C

Question 6

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

Answer 6

delocalises to form rings of electron density above and below the hexagon, forming rings of delocalised electron density above/below the hexagon

Question 7

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

Answer 7

makes benzene very stable, even though it is unsaturated (aromatic stability)

Question 8

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

Answer 8

hydrogenation of cyclohexane = -120 kJmol-1
cyclohexa-1,3,5-triene = -360 kJmol-1
benzene hydrogenation = -208 kJmol-1
so benzene is 152kJmol-1 more stable

Question 9

why else would cyclohexa-1,3,5-triene not a suitable model for benzene

Answer 9

• would not be symmetrical (C=C shorter than C-C), but benzene is
• would easily undergo addition reactions across the double bonds whereas benzene does not
• would form two isomers on the addition of Br2 or similar whereas 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 and why

Answer 12

no because its nonpolar

Question 13

dangers of benzene

Answer 13

carcinogen

Question 14

how do you name compounds containing a benzene ring

Answer 14

-benzene, or phenyl- can designate position on ring using numbers if there is more than one substituent

Question 15

why is benzene attacked by electrophiles

Answer 15

high electron density above/below ring due to delocalised electrons

Question 16

what is the 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 ring apart. results in the aromatic ring almost always staying intact

Question 17

what is seen when benzene is combusted

Answer 17

smoky flames due to soot from unburnt carbon
this is because of the high carbon:hydrogen ratio

Question 18

which ion (name and formula) is used to nitrate benzene

Answer 18

NO2 +
(+ charge on the nitrogen)
nitronium ion or nitryl cation

Question 19

how is NO2+ ion generated

Answer 19

concentrated H2SO4 and concentrated 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

overall equation for the nitration of benzene

Answer 21

C6H6 + HNO3 –> C6H5NO2 + H2O

Question 22

what are the uses of nitrated arenes

Answer 22

production of explosives e.g. TNT releases lots of heat and gas on explosion

to make aromatic amines that are used for industrial dyes

Question 23

how do substituents with a positive inductive effect affect further substitution

Answer 23

they release electrons into the delocalised electron ring, increasing the electron density and making further substitution reactions more likely/quick

direct substituents to the 2,4,6 positions

Question 24

how do substituents with a negative inductive effect affect further substitution

Answer 24

remove electrons from the delocalised electron ring, decreasing the electron density and making further substitution reactions less likely/quick

direct substituents to the 3,5 position

Question 25

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

Answer 25

a halogen carrier

Question 26

write an equation to form an electrophile that could be used to acylate benzene starting with AlCl3 and RCOCl

Answer 26

AlCl3 + RCOCl –> AlCl4- + RCO+
RCO+ can attack benzene (+ on the C)

Question 27

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

Answer 27

chlorine atom’s lone pair of electrons is forming a coordinate bond to Al

Question 28

how is the AlCl3 catalyst reformed

Answer 28

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

Question 29

how could you use a Friedel-Crafts mechanism to add a methyl group to a benzene ring

Answer 29

use a halogenoalkane and AlCl3 to create an electrophile that can attack benzene

Question 30

if you are considering cyclic compounds, what might happen if two double bonds are next to each other

Answer 30

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