C25 - Aromatic compounds

Question 1

What is benzene

Answer 1

C6H6

Question 2

What are properties of benzene

Answer 2

Colourless
Sweet smelling
Highly flammable
Liquid
Carcinogen
Aromatic

Question 3

What are the two models of benzene

Answer 3

Kekule’s model
Delocalised model

Question 4

What is an arene

Answer 4

Aromatic (containing one or more benzene rings) hydrocarbon

Question 5

What is Kekule’s model

Answer 5

Hexagonal carbon ring, joined by alternate double and single bonds

Question 6

Evidence to disprove Kekule’s model

Answer 6

-Lack of reactivity of benzene: it does not undergo electrophilic addition reactions or decolourise bromine suggesting lack of double bonds
-Lengths of C-C bond lengths: bond lengths should be alternating but they are uniform
-Hydrogenation enthalpies: enthalpy should be three times cyclohexane, but isn’t

Question 7

What is the delocalised model of benzene

Answer 7

-Planar, cyclic hexagonal hydrocarbon (C6H6)
-Each C atom uses its available 4 electrons in bonding to 2 other C atoms and 1 hydrogen atom
-Each C atom has 1 electron in a p-orbital, perpendicular to the plane
-Adjacent p-orbital overlap sideways, above and below plane to form ring of e- density
-Overlapping of p-orbitals creates system of pi-bonds, 6 delocalised e- in system of pi-bonds

Question 8

Naming aromatic compounds with one substituent group

Answer 8

-Alkyl groups, halogens, nitro groups are prefixes
-Benzene substituent to alkyl chain (phenyl is prefix)

Question 9

What is benzoic acid

Answer 9

Question 10

What is phenylamine

Answer 10

Question 11

Naming aromatic compounds with more than one substituent group

Answer 11

Alkyl group takes priority (alkylbenzene becomes parent chain)

Question 12

What is benzaldehyde

Answer 12

Question 13

Benzene electrophilic substitution

Answer 13

H atom on benzene ring is replaced by another atom/group of atoms (electrophile)

Question 14

Nitration of benzene

Answer 14

C6H6 + HNO3 –> C6H5NO2 + H2O
Conditions- H2SO4 + 50C

Question 15

What happens if nitration of benzene occurs at a higher temp

Answer 15

Further substitution, forming dinitrobenzene (1,3)

Question 16

Mechanism of nitration of benzene

Answer 16

Question 17

What does the halogenation of benzene require

Answer 17

Require halogen carrier catalyst (can be generated in situ)
Eg. AlCl3, FeCl3, AlBr3, FeBr3

Question 18

Bromination of benzene

Answer 18

C6H6 + Br2 —> C6H5Br + HBr
Conditions- RTP, FeBr3/AlBr3

Question 19

Bromination of benzene mechanism

Answer 19

Question 20

Chlorination of benzene

Answer 20

C6H6 + Cl2 —> C6H5Cl + HCl
RTP, FeCl3/AlCl3

Question 21

Alkylation of benzene

Answer 21

Benzene + Haloalkane —> Alkyl benzene + HCl
Conditions - RTP, AlCl3

Question 22

Acylation

Answer 22

Benzene + acyl chloride —> Aromatic ketone + HCl
Conditions - AlCl3 RTP

Question 23

Do alkenes decolourise bromine

Answer 23

Yes

Question 24

What happens in the reaction of cyclohexene and bromine

Answer 24

Pi bond in alkene contains localised electrons above and below the plane of the two carbon atoms in a bond
Produces region of high e- density
Localised e- in pi bond induce a dipole in non-polar bromine
Slightly +ve bromine atom enables the bromine molecule to act like a dipole

Question 25

Does benzene react with bromine

Answer 25

No, unless a halogen carrier catalyst is present Delocalised electrons spread below and above the plane Lower electron density Cannot polarise non polar bromine molecule

Question 26

What is phenol?

Answer 26

Organic chemical containing hydroxyl group bonded to aromatic ring

Question 27

Are phenols more or less soluble than alcohol

Answer 27

Less due to the presence of non-polar benzene ring

Question 28

What happens when phenol is dissolved in water

Answer 28

Partially dissociates forming phenoxide ion and proton

Question 29

Is phenoxide a strong or weak acid

Answer 29

Weak, it partially dissociates in water

Question 30

Phenols acidity compared to alcohols an Carboxylic acids

Answer 30

More acidic than alcohols, less acidic than carboxylic acids

Question 31

What’s Ka

Answer 31

Dissociation constant

Question 32

Reactivity of ethanol, phenol and carboxylic acids

Answer 32

-Ethanol does not react with NaOH (strong base) or NaCO3 (weak base) -Phenols react with NaOH (strong base) but not NaCO3 (weak base) -Carboxylic acids react with NaOH (strong base) and NaCO3 (weak base)

Question 33

What reaction can be used to distinguish between phenol and a carboxylic acid

Answer 33

Reaction with NaCO3 Phenol won't react Carboxylic acid reacts to produce CO2 gas

Question 34

Reaction of phenol with sodium hydroxide

Answer 34

C6H5OH + NaOH --> C6H5NaO + H2O

Question 35

What type of reactions does phenol usually undergo

Answer 35

Electrophilic substitution -Milder conditions than benzene

Question 36

Bromination of phenol

Answer 36

C6H5OH + 3Br2 —> C6H2OHBr2 + H2O Conditions - RTP, no catalyst

Question 37

Nitration of phenol

Answer 37

C6H5OH + HNO3 (dilute) —> C6H4OHNO2 + H2O Conditions - RTP, no catalyst

Question 38

Reactivity of phenol vs benzene

Answer 38

Bromine and nitric acid react more readily with phenol Phenol reacts with dilute nitric acid rather than conc

Question 39

Why is phenol more reactive than benzene

Answer 39

Lone pair of e- on p-orbital of O atom is donated into pi system Inc electron density, attracts electrophiles more strongly

Question 40

What happens when phenols undergo further substitution with nitric acid

Answer 40

Form two isomers: 2-nitrophenol 4-nitrophenol

Question 41

Reaction of phenylamine and bromine

Answer 41

Occurs more rapidly than benzene

Question 42

Reaction of nitrobenzene and bromine

Answer 42

Occurs more slowly than benzene

Question 43

What does it mean to activate

Answer 43

A group causes the aromatic ring to react more readily with an electrophile

Question 44

What does it mean to deactivate

Answer 44

A group causes the aromatic ring to react less readily with an electrophile

Question 45

What is a directing effect

Answer 45

Different groups attached to benzene rings cause further substituents to attach at a specific location, affecting whether it’s an an activating or deactivating group

Question 46

Which directing groups are activating groups

Answer 46

2- and 4- directing groups (orthodo and para directors) EXCEPT halogens

Question 47

Which directing groups are deactivating

Answer 47

3 directing groups (meta directing)

Question 48

Directing group table

Answer 48

Question 49

Bond angles in benzene vs 1,3,5-tricyclohexene

Answer 49

Both 120