aromatic chem

Question 1

molecular formula of benzene

Answer 1

c6h6

Question 2

give 3 reasons on why benzene is not an alkene

Answer 2

1)reactivity with bromine -it would decoulourise but it doesn’t as benzene only reacts with br2 with a catalyst and high temp

2)enthalpy of hyrogenation is less exothermic than expected(compared to cyclohexatriene) because benzene is more stable

3)benzene is less reactive than an alkene .undergoes substitution reaction and not addition in order to keep the ring of delocalised electrons

Question 3

what shape, bond angle and bond length is in a benzene molecule

Answer 3

planar
120 bond angle
bond lengths are all equal

Question 4

what’s the bonding in benzene

Answer 4

-each carbon has 3 covalent bonds
-1 spare electron in the p orbital and these overlap( to form pi bond)
-delocalisation

Question 5

what’s the stability in benzene ( hydrogenation)

Answer 5

expected enthalpy of hydrogenation is -360 kjmol-1
enthalpy for benzene is -208kjmol-1. less exothermic than expected
benzene is more stable( than alkenes)

Question 6

physical properties of arenes

Answer 6

Benzene is colourless liquid at room temp
benzene has a boiling point at 353k.similiar to that of hexane (354)

Question 7

why does benzene have a higher melting point than hexane

Answer 7

benzene has a melting point of 279k.this is a lot higher than hexane. this is because being a planar molecule can stack together very well in the solid state.this makes the van Der Waals IMFS harder to break

Question 8

the reactivity of aromatic compounds

Answer 8

the ring is in an area of high electron density due to the delocalisation.this means they can react with electrophiles

the aromatic ring is very stable.energy has to be put in to break the ring system.this means in almost all reactions the ring remains intact at the end of the reaction

Question 9

combustion of arenes

Answer 9

arenas are rarely burned because they are extremely useful substances In the pharmaceutical industry so burning them is wasteful

as arenas are unsaturated, they tend to undergo incomplete combustion .this means you don’t get as much energy out from burning the molecule as could be ,they tend to burn with a smoky flame as solid particles form

Question 10

Reagents for nitration, forming the electrophile

Answer 10

conc h2so4
conc hno3

Question 11

write an equation to show how the electrophile is generated in nitration

Answer 11

H2SO4 + HNO3 = HSO4- + H2O + NO2+

Question 12

an equation to show the sulphuric acid catalyst being reproduced

Answer 12

HSO4- + H+ = H2SO4

Question 13

uses of nitrated arenes

Answer 13

used to produce explosive

Question 14

what is required for friedel-crafts acylation

Answer 14

acyl chloride

Question 15

give the reagents and general equation for forming the electrophile in Frieda-crafts acylation

Answer 15

reagents- An acylchloride and AlCl3 catalyst

general equation- RoCl + AlCl3. —>. RCO+ + AlCl4-

Question 16

the reactivity of aromatic compounds

Answer 16

1)the ring is in an area of high electron density due to the delocalisation.this means arenes react with electrophiles

2)the aromatic ring is very stable.this means alot of energy is required to break the ring system.this means almost all reactions the ring remains intact at the end of the reaction.

Question 17

what are arenes used for

Answer 17

very useful in pharmecutical industry do burning them is wasteful

as they are unsatrurated ,tend to through incomplete combustion .tend to burn w very smoky flame as solid carbon particles form