module 6: benzene, aldehydes and ketones

Question 1

outline the first benzene model

Answer 1

Kekulé suggested benzene was a planar carbon atom ring with alternating single and double bonds
- he then adapted this to say the benzene molecule was constantly flipping between two isomers by switching the single and double bonds

we now know this is wrong but it is a useful way to draw benzene for reaction mechanisms

Question 2

outline the current benzene model

Answer 2

the delocalised model;
says the p-orbitals of all 6 carbon atoms overlap and create a pi system
the system is made of one ring-shaped electron cloud above and below the 6-C plane

Question 3

why do we know Kekulé’s benzene structure was wrong and the delocalised model is correct?

Answer 3

1. Kekulé’s suggested that 3 bonds would have the C-C bond length and 3 would have the C=C bond length

But, X-ray diffraction studies show that all Carboncarbon benzene bonds to be the same length, between that of a single/double

2. cyclohexene (one C=C) has a -120kJmol-1 enthalpy change when hydrogenated
   so if Kekulé was correct the expected benzene enthalpy change when hydrogenated is -360kJmol-1

But, the experimental enthalpy is just -208kJmol-1 which is much less exothermic
the difference is because the delocalised electron ring makes benzene more stable

Question 4

name this

Answer 4

(chloromethyl)benzene

Question 5

name this

Answer 5

phenylamine

Question 6

name this

Answer 6

nitrobenzene

Question 7

name this

Answer 7

phenylethene

Question 8

name this

Answer 8

phenylethanone

Question 9

name this

Answer 9

phenylethanoate

Question 10

name this

Answer 10

phenol

Question 11

what groups activate 3 and 5 on the benzene ring

Answer 11

NO2, acids, esters

Question 12

what groups activate 2, 4 and 6 on the benzene

Answer 12

CH3, OH, NH2

Question 13

when and how is benzene made more reactive

Answer 13

lone pair from O of OH is partially delocalised into the ring

when it has OH, CH3 or NH2, these groups donate electron density into the ring, increasing reactivity

These groups activate attack by electrophiles and direct the incoming electrophile to attack the 2 and/or 4 positions

Question 14

when is benzene made less reactive

Answer 14

when it has electron withdrawing substituents (NO2, acids, esters)

These groups deactivate attack by electrophiles and direct the incoming electrophile to attack the 3 and 5 positions

Question 15

outline the general benzene reaction mechanism

Answer 15

Question 16

outline the nitration of benzene

Answer 16

1. generate electrophile (NO2+)
   H2SO4 + HNO3 <–> NO2+ + H2SO4- + H2O
   NO2+ will then carry out an electrophilic attack on the benzene ring
   - The nitrating mixture of HNO3 and H2SO4 is refluxed with the arene at 25-60˚C

Question 17

outline halogenation of benzene

Answer 17

aromatic compounds will react with halogens in the presence of a metal halide carrier
- iron(III) bromide
- aluminium chloride
AlCl3 + Cl2 → AlCl4- + Cl+
FeBr3 + Br2 → FeBr4- + Br+
metal halide acts as a carrier and creates an electrophile (X+), it’s regenerated at the end

Question 18

outline Friedel-Crafts acylation

Answer 18

an acyl group (alkyl group containing a carbonyl, C=O group is substituted into the benzene ring (doesnt have to be methylbenzene)

Question 19

outline and explain the shape and structure of cyclohexene

Answer 19

• each carbon has 4 covalent bonds
• 4/6 Cs attempt a tetrahedral shape with a 109.5˚C bond angle
• 2/6Cs form a pi bond and attempt a trigonal planar shape with a 120˚C bond angle

Question 20

what is formed when phenol is reacted with bromine at room temp

Answer 20

2,4,6-tribromophenol
a white precipitate

Question 21

why is phenol able to act as a weak acid

Answer 21

phenol can donate the H in the OH group
C6H5OH <—> C6H5O- + H+

the phenoxide ion is stable because the lone pair of electrons delocalised with the electron cloud on the whole ion

Question 22

how can you differentiate between aldehydes and ketones

Answer 22

ToLLens’ reagent - ammoniacal silver nitrate

aLdehyde - goes from colourless to a silver mirror (because it is oxidised into a carboxylic acid and silver ions are reduced into metallic silver)
ketone - stays colourless

Question 23

oxidation of primary alcohols

Answer 23

heat with acidified potassium dichromate
if apparatus is set up to distill of as it forms; an aldehyde is formed
if not; a carboxylic acid forms (heated under reflux)

Question 24

oxidation of secondary alcohols

Answer 24

forms ketones

Question 25

what reaction type do most carbonyl compounds undergo

Answer 25

nucleophilic addition

Question 26

how is 2,4-dinitrophenylhydrazine used

Answer 26

aka 2,4-DNPH
it can tell you if a solution is an aldehyde/ketone

a condensation reaction occurs
if an aldehyde/ketone is present: a deep orange precipitate forms that can be purified by recrystallisation

The melting point of the formed precipitate can then be measured and compared to literature values to find out which specific aldehyde or ketone had reacted with 2,4-DNPH

Question 27

what is the mechanism for the reaction of carbonyl groups with NaBH4

Answer 27

H- ion comes from sodium boroHYDRIDE
nucleophilic addition reaction

Question 28

what does NaBH4 react with and how

Answer 28

only carbonyl groups (C=O), aldehydes and ketones
they’re are reduced to alcohols via a nucelophiluc addition reaction

Question 29

what is a monobasic acid

Answer 29

one mole of acid donates 1 mole of H+