aromatic compounds

Question 1

what is the formula for benzene?

Answer 1

C6H6

Question 2

what are the properties of benzene?

Answer 2

1) colourless,sweet smelling flammable liquid
2) found in crude oil
3) classified as carcinogen

Question 3

what was kekule model of benzene?

Answer 3

6 membered rings of carbon atoms joined by alternating single and double bonds which is reactive

Question 4

what are the evidence to disprove kekule model?

Answer 4

1) lack of reactivty in benzene
2) length of carbon-carbon bonds
3) hydrogenation enthalpies

Question 5

how does lack of reactivity of benzene disprove Kekule model?

Answer 5

if benzene had double bonds it should decolourise bromine in an electrophilic addition reaction however it does not undergo this reaction under normal conditions

Question 6

how does length of carbon bonds in benzene disprove kekule model?

Answer 6

using X-ray diffraction, bond lengths can be measured. All bonds in benzene were 0.139nm which is halfway between single and double bond

Question 7

how does hydrogenation enthalpies of benzene disprove kekule model?

Answer 7

since there are 3 double bonds in kekule model, there should have the enthalpy change of hydrogenation 3x of cyclohexene(-120x3 =-360kj).
However, the actual enthalpy is -208kj so 152 less energy is produced

Question 8

what does the actual enthalpy of hydrogenation for benzene indicate?

Answer 8

actual structure of benzene is more stable than kekule model

Question 9

what is the delocalised model of benzene?

Answer 9

benzene is a planar cyclic hydrocarbon containing 6 hydrogen and carbon atoms
each carbon atom uses 3 of its 4 electrons therefore the remaining electron in the p-orbital in plane of bonded carbon-hydrogen atoms overall with adjacent p-orbitals to form a ring of electron density ( pi bonds)

Question 10

what is a substituent

Answer 10

when a benzene ring is attached to alkyl chain with a functional group or to an alkyl chain with 7 or more carbon atom ( phenyl)

Question 11

what are the 3 exceptions to substituents?

Answer 11

benzoic acid, phenylamine , benzaldehyde

Question 12

can benzene undergo electrophilic substitution

Answer 12

yes , a hydrogen atom is replaced

Question 13

what is the nitration of benzene?

Answer 13

benzene reacts with nitric acid to form nitrobenzene

Question 14

what are the conditions for nitration of benzene

Answer 14

catalysed by sulfuric acid and heated to 50 degrees

Question 15

what happens if benzene is heated over 50 degrees

Answer 15

further substitution occurs forming dinitrobenzene

Question 16

what is nitrobenzene used for

Answer 16

dyes,pharmaceutical, pesticides

Question 17

what is the electrophile in nitration of benzene

Answer 17

NO2+, nitronium ion formed by reaction of sulfuric acid and nitric acid

Question 18

what is the equation for formation of nitronium

Answer 18

HNO3+ H2S04 —- NO2+ + HSO4- + H20

Question 19

how does nitration occur?

Answer 19

nitronium ion accepts a pair of electrons from benzene ring to form a dative covalent bond. The intermediate is unstable and breaks down to form nitrobenzene and the H+ ion forming a stable benzene ring
The H+ ion reacts with HS04 to form H2S04 again

Question 20

what catalyst is needed for halogenation of benzene?

Answer 20

a halogen carrier

Question 21

what are some examples of halogen carrier?

Answer 21

AICL3, FECL3, FEBr

Question 22

what are the conditions for bromination of benzene?

Answer 22

room temperature and pressure in the presence of a halogen carrier

Question 23

what is the electrophile for bromination of benzene

Answer 23

Br+

Question 24

how does bromination of benzene occur?

Answer 24

Bromonium ion accepts a pair of electrons from the benzene ring to form a dative covalent bond. The intermediate is unstable and breaks down to form bromobenzen and a H+ion. H+ ion reacts with FeBr4- to regenerate FeBr3 catalyst

Question 25

what is the equation for formation of bromonium ion

Answer 25

Br2+ FeBr3 —– Br+ + FeBr4-

Question 26

what is the equation for regeneration of FeBr3

Answer 26

H+ + FeBr4+ ——– FeBr3 + HBr

Question 27

what is the alkylation of benzene?

Answer 27

substitution of a hydrogen atom in benzene ring by an alkyl group

Question 28

what is an acylation reaction?

Answer 28

When benzene reacts with acyl chloride in presence of ALCL3 to form an aromatic ketone

Question 29

how does alkenes decolourise bromine by electrophilic addition?

Answer 29

The Pi bond in alkene contains localised electrons above and below plane with high electron density. The localised electrons in pi bond induces a dipole in the non-polar bromine molecule making a positive and negative bromine atom. The positive bromine atom allows bromine molecule to act as an electrophile

Question 30

what are phenols

Answer 30

an OH group attached to an aromatic ring

Question 31

why are phenols less soluble in water than alcohols

Answer 31

due to presence of non-polar benzene ring

Question 32

what are the products of dissociation of phenol

Answer 32

phenoxide ion and proton

Question 33

what is the product of bromination of phenol

Answer 33

a white precipitate of 2,4,6 tribromophenol. Does not require a halogen carrier

Question 34

what is the product of phenol with sodium hydroxide

Answer 34

sodium phenoxide and water

Question 35

what are the products of nitration of phenol

Answer 35

2, nitrophenol and water
4,nitrophenol and water

Question 35

why is phenol more reactive than benzene

Answer 35

lone pair of electron from oxygen p-orbital of OH group is donated into the Pi system which increases electron density therefore attracts electrophiles more readily

Question 36

what are the directing group for NH2

Answer 36

positions 2 or 4

Question 37

what is the directing group for NO2

Answer 37

position 3

Question 38

what does ortha,meta and para mean

Answer 38

ortha = position 2
meta = position 3
para = position 4