4.2 Aromacity

Question 1

molecular and skeletal formula of benzene

Answer 1

C6H6, hexagon w circle inside

Question 2

How can the structure of benzene be represented? which model is more suitable?

Answer 2

hexagon w circle inside and hexagon w three lines the first model is more suitable as the second model does not demonstrate the delocalised electron system

Question 3

describe the bonding in benzene

Answer 3

benzene has six carbon atoms each forming a single covalent bond to the carbons next to it and a hydrogen atom. this means each carbon atom has one unpaired electron in the outer p-orbital. these unpaired electrons combine to form a delocalised ring of electrons.

Question 4

explain the delocalised electron system of benzene in terms of the types of bonds involved

Answer 4

the p orbital on each carbon atom in benzene overlaps with the p orbitals either side, this series of overlaps produces a pi bond system. these electrons are spread out over the whole carbon ring are described as delocalised.

Question 5

what is the shape and bond angle of benzene?

Answer 5

benzene is planar, regular hexagon and the shape around each carbon atom in trigonal planar, bond angle 120

Question 6

what is the general name for compounds containing a benzene ring?

Answer 6

arenes or aromatic compounds

Question 7

why does benzene not undergo electrophilic addition reactions?

Answer 7

benzene’s delocalised ring of electrons is a region of high electron density attracting electrophiles,
the ring of electrons is very stable so benzene does not undergo electrophilic addition as this would break up the delocalised ring of electrons.

Question 8

what are the two common electrophilic substitution mechanisms for benzene?

Answer 8

friedel-crafts acylation
nitration of benzene

Question 9

how is the acyl chloride electrophile made stronger for friedel-crafts acylation?

Answer 9

an acyl chloride is reacted with AlCl3 which accepts a lone pair of electrons from the acyl chloride, increasing the polarisation in the acyl chloride forming a carbocation which is a much stronger electrophile able to react with the benzene ring.

Question 10

give the equation of the reaction between aluminium chloride and ethanoyl chloride

Answer 10

AlCl3 + CH3COCl -> CH3C+O + AlCl4-

Question 11

draw electrophilic substitution mechanism for rctn between benzene and methanoyl chloride

Answer 11

initially: HCOCl + AlCl3 -> HC+O + AlCl4-
benzene w H arrow out of circle to H-C+doublebondO intermediate w Harrown in and H-CdoublebondO -> benzene w H-CdoublebondO + H+

Question 12

Explain how AlCl3 behaves as a catalyst in the friedel-crafts acylation of benzene

Answer 12

AlCl3 initially reacts with acyl chloride to produce a reactive carbocation intermediate and AlCl4-, at the end of the mechanism a hydrogen ion is released fron the benzene ring whihc reacts with the AlCl4- to produce HCl and AlCl3, so AlCl3 does not get used up during the reaction acting at a catalyst

Question 13

what are the conditions required for friedel-crafts acylation reactions?

Answer 13

the reactants should be heated under reflux in a non aqueous solvent

Question 14

why are electrophiles attreacted to benzene?

Answer 14

electrophiles are electron pair acceptors so are attracted to areas of high electron density, benzenes ring of delocalised electrons, is an area of high electron density attracting electrophiles

Question 15

give the reactants and catalyst required for the nitration of benzene:

Answer 15

reactants: benzene, conc nitric acid
catalyst: conc sulfuric acid

Question 16

how is the electrophile, NO2+, prduced for the nitration of benzene?

Answer 16

conc nitric and sulfuric acid react to forn NO2+ intermediate
HNO3 + H2SO4 -> H2NO3+ + HSO4-
H2NO3+ -> NO2+ + H2O

Question 17

draw the electrophilic substitution mechanism for the nitration of benzene

Answer 17

benzene w H arrow out to NO2+ intermediate then end w nitrobenzene + H+

Question 18

how does sulfuric acid act as a catalyst in the nitration of benzene?

Answer 18

initially the sulfuric acid rects to produce the NO2+ electrophile: HNO3 + H2SO4 -> H2NO3+ + HSO4-
H2NO3+ -> NO2+ + H2O
at the end of the mechanism a hydrogen ion is released from the benzene reacting with the HSO4- ion to reproducemH2SO4, which doesnt get used up so acts as a catalyst

Question 19

what condition ensures mononitration will occur in the nitration of benzene?

Answer 19

mononitration is when only one NO2 group is added to the benzene, occurs if temp is kept below 55C

Question 20

Draw the electrophilic substitution mechanism for the reaction between benzene and chlorine

Answer 20

same as others but Cl+ end w cl=hlorobenzene + H+

Question 21

Why is the C-Cl bond stronger in benzene than in a chloroalkane?

Answer 21

the lone pair of electrons on chlorineatom is delocalised over the benzene ring which strengthens the carbon chlorine bond

Question 22

why is chlorobenzene unreactive towards nucleophiles?

Answer 22

the C-Cl bond in the benzene is stronger than halogenoalkanes making it harder to break and so it is not easily attacked by nucleophiles