aromatic compounds

Question 1

aromatic definition

Answer 1

a molecule which is stabilised by electron delocalisation in a ring

Question 2

what is the molecular formula of benzene

Answer 2

C6H6

Question 3

outline the structure and bonding in the kekule model of benzene

Answer 3

a 6 carbon ring with alternating single and double bonds, with 6 C-C sigma bonds and 3 C-C pi bonds formed by p-orbital overlap
in this model the double bonds have localised electrons giving them high electron densities

Question 4

what is the difference between a sigma bond and a pi bond

Answer 4

sigma bonds are formed by the head on overlap of orbitals, creating a region of electron density diectly between atoms

pi bonds are formed by the sideways overlap of p-orbitals, creating regions of electron density above and below plane

Question 5

outline the structure and bonding in the delocalised model of benzene

Answer 5

a 6 carbon ring with each C atom having a spare electron in a p-orbital, which overlap and spread around the carbon ring to form a ring of delocalised electron density in planes above and below the ring

Question 6

what are the bond angles in benzene

Answer 6

120

Question 7

what 3 pieces of evidence are used to disprove the kekule model

Answer 7

lack of reactivity - benzene does not decolourise Br(aq) in the dark at room temperature, less reactive compared to normal alkenes

thermodynamic stability - the enthalpy of hydrogenation of benzene is much less exothermic than 3x the enthalpy of hydrogenation of cyclohexene

bond lengths - despite C-C and C=C bonds having different lengths normally, all bonds in benzene are the same length, intermediate between C-C and C=C bonds

Question 8

what makes benzene so stable

Answer 8

the delocalised ring of electrons spread out across 6 carbon ring

Question 9

how does the delocalised model explain benzenes lack of reactivity

Answer 9

electron density between each C is lower than between a C=C in an alkene, so it has less attraction to electrophiles and is less polarising, therefore less reactive

Question 10

how does the delocalised model explain benzene being less exothermic than expected

Answer 10

delocalised ring of electron stabilises benzene, this thermodynamic stability causes the hydrogenation of benzene to be less exothermic than expected when compared to other alkenes

Question 11

how does the delocalised model explain benzenes bond lengths

Answer 11

no alternating C=C bonds so all bonds in benzene are the same length (intermediate between C-C and C=C bonds)

Question 12

by what mechanism does benzene normally react + why

Answer 12

electrophilic substitution

benzene undergoes substitution rather than addition as this allows the stable system of electron delocalisation to be maintained

Question 13

what is needed for benzene to react with most compounds

Answer 13

a catalyst

Question 14

compare the reactivity of benzene and phenol

Answer 14

phenol is more reactive due to the OH group which is activating as lone pair on O is delocalised onto ring, this means it is more polarising ans to it has greater attraction to electrophiles and can react with electrophiles faster or without catalysts

Question 15

what is needed for benzene to react with halogens, haloalkanes or acyl chlorides

Answer 15

halogen carrier catalysts
e.g. AlCl3, FeBr3

Question 16

compare the reactivity of benzene and cyclohexene

Answer 16

electron density between each C in benzene is less than an alkene, whereas in the C=C in cyclohexene the electrons are localised, so it has a greater electron density making it more polarising and increasing its attraction towards electrophiles , meaning it is more reactive and can react with electrophiles faster or without catalysts

Question 17

why are conditions in reactions involving benzene and nucleophiles often very harsh

Answer 17

this is to overcome the high activation energies of the reactions, as arenes normally repel nucleophiles

Question 18

outline the physical properties of phenol

Answer 18

relatively high melting + boiling points, as it can form hydrogen bonds due to OH

phenol is very water soluble due to its OH group, which allows it to form hydrogen bonds with water

Question 19

give the formula for phenol

Answer 19

C6H5OH

Question 20

when phenol reacts with Br(aq) what is observed

Answer 20

the solution will decolourise and a white precipitate will be formed
this precipitate is 2,4,6-tribromophenol

Question 21

when phenol reacts with Na what is observed

Answer 21

effervescence and bubbling

Question 22

how can phenol act as an acid

Answer 22

due to its OH group, it is able to donate a proton, forming a phenoxide ion - C6H5O-
they are weak acids so they do not fully dissociate

Question 23

in comparison to alcohols and carboxylic acids, how acidic is phenol

Answer 23

carboxylic acids are more acidic than phenols, which are more acidic than alcohols

• alcohols don’t readily react with NaOH - phenols can
• carboxylic acids readily react with Na2CO3 - phenols cannot

Question 24

activating group definition

Answer 24

groups than can increase the reactivity of the benzene ring, as they are electron donating, as they often have a lone pair of atoms bonded to the ring which increases electron density

Question 25

deactivating group definition

Answer 25

groups with decrease the reactivity of the benzene ring, as they are electron withdrawing, as they often have atoms with double bonds bonded to the ring

Question 26

give examples of activating groups

Answer 26

NH2 NHR OH OR C6H5 R

Question 27

give examples of deactivating groups

Answer 27

COR COOR SO3H CHO COOH CN NO2 NR3+ halogens

Question 28

how do the reactions of benzene differ with activating or deactivating groups

Answer 28

if activating groups are present reactions will be faster, and conditions will be more mild compared to benzene, no catalyst is needed and multiple substitutions may occur

if deactivating groups are present reactions will be slower and conditions will be harsher compared to benzene, catalysts are often needed and multiple substitutions are less likely

Question 29

which groups are 2,4 directing

Answer 29

NH2 NHR OH OR C6H5 R halogens

basically all activating groups + halogens

Question 30

which groups are 3 directing

Answer 30

COR COOR SO3H CHO COOH CN NO2 NR3+

basically all deactivating groups except halogens

Question 31

give the reagent and conditions needed to go from benzene to cyclohexene - hydrogenation

Answer 31

reagent - H2
conditions - Ni catalyst

Question 32

give the reagent and conditions needed to go from benzene to C6H5-CO-R - acylation
then from C6H5COR to C6H5-CH(OH)-R - reduction to alcohol

Answer 32

reagent - acyl chloride
conditions - AlCl3 or FeCl3
- products are benzaldehyde or phenyl(methan..)one
- byproduct = HCl

reagent - NaBH4
conditions - aqueous

Question 33

give the reagent and conditions needed to go from benzene to nitrobenzene or dinitrobenzene - nitration

Answer 33

reagent - conc HNO3
conditions - conc H2SO4 and temp of approx 55C
- byproduct = H2O

reagent - conc HNO3
conditions - conc H2SO4 and a temp of approx 70C
- byproduct = H2O

Question 34

give the reagent to go from nitrobenzene to phenylamine - reduction

Answer 34

reagent - conc HCl and Sn and excess NaOH
- byproduct = H2O

Question 35

give the reagent to go from phenylamine to C6H5-NHCO-R

Answer 35

reagent - acyl chloride
conditions - AlCl3 or FeCl3
- byproduct = HCl

Question 36

give the reagent and conditions to go from benzene to methylbenzene - alkylation

Answer 36

reagent - haloalkane
conditions - AlCl3 or FeCl3
- byproduct = HCl