module 6 - organic chemistry and analysis

Question 1

benzene is a major feedstock used in which industries?

Answer 1

polymers
pharmaceuticals
dyes
pesticides
explosives

Question 2

what is the empirical and molecular formula of benzene

Answer 2

empirical - CH
molecular = C6H6

Question 3

what did kekule propose?

Answer 3

that benzene was a cyclic structure containing three alternating carbon-carbon double bonds

Question 4

what did kekule think was the reason for benzene’s low reactivity

Answer 4

due to the rapid equilibrium between two isomers
- changing position of double bonds make reactions difficult

Question 5

what were the three problems with kekule’s model

Answer 5

1. resistance to reaction
2. bond length
3. enthalpy change of hydrogenation

Question 6

problems with kekule’s model:
resistance to reaction

Answer 6

addition does not occur - product no longer contains a benzene ring
- substitution occurs - still contains benzene ring

Question 7

problems with kekule’s model:
bond lengths

Answer 7

C-C and C=C have different bond lengths so benzene hexagon would be irregular

benzene has a different bond length (between c=c and c-c)

Question 8

problems with kekule’s model:
enthalpy change of hydrogenation

Answer 8

benzene can be saturated with hydrogen to form cyclohexane
- cyclohexene = -120kJmol-1
- kekule’s model (cyclohex-1,3,5-triene) would expect to have an enthalpy of -360kJmol-1

benzene has an enthalpy of -208kJmol-1

more stable bonds than alkenes

Question 9

how do we know benzene has more stable bonds than alkenes

Answer 9

because it has a higher bond enthalpy of hydrogenation

Question 10

describe the delocalised model of Benzene

Answer 10

1. each carbon creates 3 covalent sigma bonds
2. the remaining electron is found in a p-orbital at a right angle above and below the carbon atom
3. the p-orbitals overlap (sideways) evenly creating a ring of delocalised pi- electrons above and below the structure

Question 11

naming benzene - when is it a “benzene” root name

Answer 11

when there is:
- an alkyl group (methyl etc)
- a halogen (bromo etc)
- a nitro group (NO2)
- a carboxylic acid (COOH)

Question 12

naming benzene -
when is it the prefix phenyl

Answer 12

most other functional groups
(apart from alkyls, halogens, nitro groups and carboxylic acids)

Question 13

define electrophile

Answer 13

an electron pair acceptor

Question 14

why does benzene resist electrophilic addition

Answer 14

because it would disrupt the delocalised Pi- ring
- product would be less stable

Question 15

what reaction does benzene undergo instead of electrophilic addition and why

Answer 15

electrophilic substitution by replacing a hydrogen for another group
because it retains the energetically stable delocalised Pi-electron ring

Question 16

describe the process of electrophilic substitution

Answer 16

1. electrophile attracted to ring due to high e- density
   - a pair of e- from the ring are donated to electrophile to form covalent bond
2. an unstable intermediate forms with incomplete ring structure and positive charge
   - bond with hydrogen undergoes heterolytic fission to donate electrons to the ring structure
3. the stable ring structure returns leaving the substituted product and a H+ ion

Question 17

what are the prefix and suffix given to benzene containing compounds

Answer 17

prefix = phenyl
suffix = benzene

Question 18

why is the electrophile attracted to the benzene ring

Answer 18

attracted to the delocalised Pi-electrons due to the high electron density

Question 19

what does the nitration of benzene do to the structure

Answer 19

adds an -NO2 to the ring

Question 20

what are the reagent, catalyst and conditions for nitration of benzene

Answer 20

reagent = conc. nitric acid (HNO3)
catalyst = conc. sulfuric acid (H2SO4)
conditions = 50-55 degrees Celsius

Question 21

what temperature does nitration of benzene occur at and why

Answer 21

50-55 degrees Celsius
because multiple substitutions occur at higher temps

Question 22

why is sulfuric acid a catalyst in the nitration of benzene?

Answer 22

because it is used up during electrophile formation and then reforms at the end of reaction

Question 23

what are the 3 stages of nitration of benzene

Answer 23

1. Nitronium ion (NO2+) formation
2. electrophile attack
3. catalyst reformation

Question 24

how do Friedel-Crafts reactions work?

Answer 24

use halogen carriers (Alx3, FeX3 or Fe) as catalysts to substitute R-Cl groups to rings via
- halogenation - sub a halogen
- alkylation - sub an alkyl using haloalkanes
- acylation - sub a ketone using acyl chlorides

Question 25

what conditions are all Friedel-Crafts reactions performed under?

Answer 25

under reflux and in anhydrous conditions

Question 26

describe electrophile formation in Friedel-Crafts reactions

Answer 26

the chlorinated group is polarised by the catalyst and generates the electrophile (R+ or R delta+)

Question 27

what ion acts as an electrophile during nitration of benzene

Answer 27

Nitronium

Question 28

what are phenols

Answer 28

aromatic compounds that have an alcohols hydroxyl group attached DIRECTLY to the ring

Question 29

what is the simplest formula for a phenol

Answer 29

C6H5OH

Question 30

describe the structure of phenols

Answer 30

oxygen bonded to the benzene has two lone pairs of electrons in p-orbitals
as the ring has low electron density, one of the lone pairs of e- in the oxygen p-orbital overlaps into ring structure - becomes delocalised
increasing the ring’s electron density allowing electrophilic substitution to occur more easily

therefore phenol is more reactive than benzene

Question 31

what are the properties of phenol

Answer 31

• p-orbital overlap into the ring weakens the O-H bond so phenol can donate H+ and form phenoxide

means that phenol solutions are acidic

Question 32

why are phenols weak acids

Answer 32

because the OH bond is weakened by the overlap of the P-orbital into the ring

Question 33

what is the name of the negative ion that phenols form?

Answer 33

phenoxide ion