27 aromatic chemistry

Question 1

what does the ring in benzenes skeletal formula represent?

Answer 1

cloud of delocalised electrons

Question 2

what is the name for kekule’s model?

Answer 2

cyclohexatriene

Question 3

what is the difference in reactions with Br2 between benzene and cyclohexatriene?

Answer 3

kekule model: orange to colourless
benzene: only undergoes substitution

Question 4

what is the enthalpy of hydrogenation of cyclohexatriene and benzene?

Answer 4

kekules model: 360 kj mol -1
benzene: 208 kj mol -1

Question 5

using the enthalpy of hydrogenation, how is benzene more stable than cyclohexatriene?

Answer 5

because the difference in enthalpy change of benzene and cyclohexatriene is 152 kj mol-1 more stable
benzene being less exothermic

Question 6

how long are the bonds in benzene?

Answer 6

length in between single and double bond

Question 7

why can kekule’s model undergo addition but benzene can’t?

Answer 7

benzene is less reactive than alkenes

Question 8

why does benzene undergo substitution reactions rather than addition reactions?

Answer 8

to maintain the ring of delocalised electrons

Question 9

what do the overlapped p orbitals form?

Answer 9

extended pi electron system

Question 10

what does benzene look like at room temp?

Answer 10

colourless liquid

Question 11

is benzene’s boiling point similar or different to hexane’s? why?

Answer 11

similar

Question 12

is benzene’s melting point similar or different to hexane’s? why?

Answer 12

benzene is planar so can stack when solid- harder to break VDW forces

Question 13

what mechanism does benzene undergo?

Answer 13

electrophilic substitution

Question 14

what are the two types of electrophilic substitution that benzene undergoes?

Answer 14

acylation
nitration

Question 15

describe the overall bonding in benzene

Answer 15

1. each carbon has 3 covalent bonds
2. electrons in p orbitals overlap to form a pi cloud
3. delocalisation of electrons

Question 16

describe the overall shape of benzene

Answer 16

1. planar
2. hexagonal, 6 carbons, 180 bond angle
3. C-C equal length, between length of single and double bond

Question 17

describe the overall stability of benzene

Answer 17

1. expected enthalpy change of hydrogenation of cyclohexatriene -360
2. benzene less exothermic at -208
3. benzene more stable

Question 18

what is the catalyst used in acylation?

Answer 18

AlCl3/aluminium chloride

Question 19

what is the electrophile in acylation?

Answer 19

RC+O

Question 20

how do you form RC+O for acylation?

Answer 20

RCOCl + AlCl3 -> RC+O AlCl4-

Question 21

what replaces a H on benzene during acylation?

Answer 21

RCO

Question 22

how many curly arrows are used in acylation and where do they go to and from?

Answer 22

2 curly arrows
1- from benzene ring to C+ (RC+O)
2- from H to + in benzene ring

Question 23

how do you reform the catalyst in acylation?

Answer 23

AlCl4- + H+ -> AlCl3 + HCl (g)

Question 24

what is the electrophile in nitration reaction?

Answer 24

NO2+

Question 25

what are the reagents for nitration?

Answer 25

conc H2SO4 and conc HNO3

Question 26

what is the catalyst in nitration?

Answer 26

conc H2SO4

Question 27

how do you form NO2+ for nitration?

Answer 27

H2SO4 + HNO3 -> NO2+ + H2O + HSO4-

Question 28

how many curly arrows are used in nitration and where do they go to and from?

Answer 28

2 arrows
1- from ring to N+
2- from C-H bond to + in ring

Question 29

what replaces a H during nitration?

Answer 29

NO2