arenes

Question 1

implications of pi electron cloud delocalisation

Answer 1

1. each carbon atom in benzene has an unhybridised 2p orbital that is perpendicular to the hexagonal plane of carbon atoms
2. since all these unhybridised 2p orbitals are adjacent and parallel to one another, they perform a side-on overlap with each other to produce a delocalised pi electrn cloud
3. this causes all C-C bond lengths to be equal, creating a planar shape while giving all C-C bonds partial ddouble bond character
4. this also prevents benzene from undergoing any of the typical addition reactions that alkenes show, as all C-C bonds are resonance stabilised preventing electrophiles from reacting with benzene

Question 2

reagents and conditions from bromination

Answer 2

1. bromine liquid, solid aluminium bromide as a lewis acid catalyst, warm
   2, bromine liquid, solid iron bromide as a lewis acid catalyst, warm
2. bromine liquid, solid iron or aluminium, warm
3. all reactions RTP in stabilising conditions

Question 3

why are aluminium or iron bromide called the lewis acid catalysts

Answer 3

1 aluminium and iron in these species are electron deficient and can accept a lone pair of electrons from the halogen, generating a strong electrophile with a full positive charge
2. aluminium and iron bromide are regenerated after the reaction

Question 4

conditions and reagents for nitration

Answer 4

1. concentration nitric acid, concentrated sulfuric acid as a bronsted lowry acid catalyst, maintained at 50 degrees celcius
2. mantained at 30 degrees celcius in stabilising conditions

Question 5

why does sulfuric acid act as a bronsted-lowry acid catalyst in nitration?

Answer 5

1. sulfuric acid is a stronger bronsted-lowry acid than nitric acid hence protonates nitric acid, generating a strong electrophile NO2+
2. sulfuric acid is regenerated after the reaction

Question 6

reagents and conditions for friedel-crafts alkylation

Answer 6

1. chloroalkane, aluminium chloride as lewis acid catalyst, warm
2. RTP in stabilising conditions

Question 7

observations for chlorination and bromination

Answer 7

1. in chlorination, decolourisation of greenish-yellow chlorine gas and white fumes of HCl
2. in bromination, decolourisation of reddish-brown bromine liquid and white fumes of HBr

Question 8

observations for nitration

Answer 8

1. yellow oily liquid formed

Question 9

observations for friedel-crafts alkylation

Answer 9

1. white fumes of HCl

Question 10

desccribe inductive effect in benzene ring

Answer 10

1. the inductive effect arises from the polarisation of electron density in a bond due to the eletronegativity of nearby atoms
2. electron-withdrawing groups inductively withdraw electron density from the benzene ring
3. electron-donating groups inductively donate electron density into the benzene ring via the sigma bond

Question 11

describe delocalisation effect in the benzene ring

Answer 11

1. electron-donating groups have a lone pair of electrons in the p orbital of the atom directly joined to the benzene ring, and can therefore donate the lone pair of electrons into the benzene ring via delocalisation
2. electron-withdrawing groups form a double/triple bond to electronegative atoms, and electron density in the ring is drawn away

Question 12

reagents and conditions for side-chain oxidation to give benzoic acid

Answer 12

1. potassium manganate, dilute sulfuric acid, heat

Question 13

observations for side-chain oxidation to give benzoic acid

Answer 13

1. purple potassium manganate decolourises and white precipitate of benzoic acid is formed

Question 14

reagents and conditions for side chain oxidation to give sodium benzoate

Answer 14

1. potassium manganate, dilute sodium hydroxide, heat

Question 15

observations for side-chain oxidation to give sodium benzoate

Answer 15

1. purple potassium manganate decolourises and brown precipitate of manganese dioxide is formed

Question 16

reagents and conditions to convert sodium benzoate to benzoic acid

Answer 16

1. dilute sulfuric acid

Question 17

distinguishing test between alkenes and arenes

Answer 17

1. add bromine in CCl4 dropwise with shaking to 1 cm3 of each compound in separate test tubes. bromine remains reddish brown in benzene, but reddish brown bromine decolourises in alkenes
2. add aqueous bromine dropwise with shaking to 1 cm3 of each compound in separate test tubes. bromine remains yellow orange in bezene, but yellow orange bromine decolourises in alkenes