Benzene + Aromatic Compounds

Question 1

What are aromatic compounds?

Answer 1

• contains 1 or more benzene rings

Question 2

What are arenes?

Answer 2

• hydrocarbons containing 1 or more benzene rings
• simplest arene is benzene

Question 3

What are the problems w Kekule’s structure?

Answer 3

• benzene doesn’t decolourise bromine water, so doesn’t react like alkenes (is resistant to addition reactions)
• double bonds are stronger + shorter than single bonds but in benzene all C-C bond lengths are equal (between double + single) so it is a regular hexagon
• enthalpy of hydrogenation is less exo. that expected suggesting benzene is more stable than Kekule’s structure suggests

Question 4

What is the structure of benzene (delocalised model of benzene)?

Answer 4

• each C atom is bonded to 2 other C atoms + 1 H atom (these 3 bonds are σ-bonds)
• this forms a planar hexagonal ring w a trigonal planar shape around each C
• this leaves a 4th e- on each C in a p-orbital above + below the plane of C atoms
• these p-orbitals overlap sideways in both directions to form a delocalised π bonding system
• the 6 π e-s are now spread out above + below ring, + are delocalised so there’s less repulsion between them making benzene more stable

Question 5

What is a phenyl group?

Answer 5

• C6H5

Question 6

What is the type of mechanism for benzene?

Answer 6

• electrophilic substitution

Question 7

Describe electrophilic substitution of benzene.

Answer 7

• elecrophile is attracted to e- rich π-system
• electrophile accepts a pair of π-e-s from ring
• unstable intermediate formed (π-system temporarily disrupted)
• H+ leaves, putting 2 e-s back into ring

Question 8

What are the reagents + conditions for the nitration of benzene?

Answer 8

• conc HNO3
• in presence of conc H2SO4 cat.
• reflux at 50°C

Question 9

What is reflux?

Answer 9

• continuous process of evaporation + condensation to prevent volatile reagents escaping

Question 10

Write an equation for the nitration of benzene.

Answer 10

• C6H6 + HNO3 —> C6H5NO2 + H2O

Question 11

What are the reagents + conditions for the halogenation of benzene?

Answer 11

• a halogen in presence of a halogen carrier cat. (e.g. AlCl3, FeCl2 or iron metal)

Question 12

Write an equation for the halogenation of benzene.

Answer 12

• C6H6 + Cl2 —> C6H5Cl + HCl

Question 13

What catalyst would be suitable for the halogenation of benzene?

Answer 13

• AlBr3

Question 14

What are the types of Freidel-Crafts reactions?

Answer 14

• alkylation
• acylation

Question 15

What is alkylation?

Answer 15

• when a hydrogen is replaced by an alkyl group

Question 16

What are the reagents + conditions for Friedel-Crafts alkylation reactions?

Answer 16

• a haloalkane
• in presence of a halogen carrier cat.

Question 17

What is acylation?

Answer 17

• when a hydrogen is replaced by an acyl group

Question 18

What is an acyl group?

Answer 18

• an alkyl group (R) attached to a C=O

Question 19

What are the reagents + conditions for Freidel-Crafts acylation reactions?

Answer 19

• an acyl chloride
• in presence of a halogen carrier catalyst
• reflux (e.g. 60°C for 30mins)
• anhydrous conditions (no water)

Question 20

Why are Freidel-Craft reactions important in synthesis?

Answer 20

• bc a new C-C bond is formed to aromatic ring

Question 21

What are halogen carrier catalysts?

Answer 21

• catalyst that takes a halogen atom from reagent + ‘carries’ it by forming a covalent bond to halogen

Question 22

Describe the reactivity of Benzene.

Answer 22

• doesn’t decolourise bromine unless a catalyst (e.g. AlBr3) is present bc it’s less reactive
• has a delocalised π system
• has a lower e- density, so polarises electrophiles less strongly, so is less attractive to electrophiles + so is less reactive
• reacts by electrophilic substitution, bc it keeps delocalised π system intact (addition would permanently disrupt it + so extra stability would be lost)

Question 23

Describe the reactivity of Alkene.

Answer 23

• decolourises bromine bc it’s more reactive
• has a localised π-bond
• has a higher e- density, so polarises electrophiles more strongly, so is more attractive to electrophiles + so is more reactive
• reacts by electrophilic addition bc localised π-bond can break + so be added to

Question 24

What are the physical properties of phenols?

Answer 24

• phenol contains H-bonding
• is a solid at room T°C
• is slightly soluble in water (benzene ring is non-polar)

Question 25

What is a phenol?

Answer 25

- a v weak acid (partially ionises releasing a few H+ ions)

Question 26

What are the reagents + conditions for the reaction of phenol w bromine?

Answer 26

- bromine water at room T°C

Question 27

What is observed when phenol reacts w bromine?

Answer 27

- decolourises bromine water - forms an insoluble white ppt. (2,4,6-tribromophenol

Question 28

What are the reagents + conditions for the reaction of phenol w nitric acid (HNO3)?

Answer 28

- dilute HNO3 at room T°C

Question 29

What 2 products form when a phenol reacts w nitric acid?

Answer 29

- 2-nitrophenol - 4-nitrophenol

Question 30

Describe the reactivity of phenol.

Answer 30

- phenol is more reactive than benzene - decolourises bromine - it also reacts w dilute HNO3 w/o need for a cat. - this is bc phenol has a lone pair of e-s in a p-orbital on the O atom that is able to overlap w delocalised π bonding system - the lone pair is drawn into ring, leading to a higher e- density in ring (it is activated) - electrophile is attracted more strongly + polarised more strongly

Question 31

What has a 2,4-directing effect?

Answer 31

- a benzene ring w a -OH or -NH2 group attached - bc they’re both e- donating groups

Question 32

What has a 3-directing effect?

Answer 32

- a benzene ring w a -NO2 group attached - bc it is an e- withdrawing group