Unit 5- Topic 18

Question 1

Physical properties of benzene

Answer 1

Every aromatic compound has benzene - every aliphatic compound is those who do are not aromatic
Benzene is an arene
Is a colourless liquid
Boiling temperature of 80
It is insoluble in water
Toxicity - carcinogen
Molecular formula: C6H6 it is highly unsaturated

Question 2

what 5 problems where encountered in the discovery of benzene

Answer 2

-the three C=C should mean that benzene decolourises bromine water in an addition reaction, however it doesn’t. a substitution reaction occurs this suggests that there are no C=C present.
-in dibromobenzene, four isomers should exist in Kekule structures but only three were known to exist. This suggests that the bonds between the carbon atoms in the benzene ring are the same, not different
-X-ray diffraction show a value of C-C in benzene as an intermeadiate between the C-C and C=C in cyclohexene. this suggests all the carbon carbon bonds are all the same
-thermochemical data about enthalpy changes show benzene has a lower enthalpy change than expected
-infra-red spectra shows absoptions which are typical of an aromatic C=C stretch

Question 3

what does the unexpected thermochemical data about enthalpy changes show about benzene

Answer 3

the enthalpy change of cyclohexene (-120) and the enthalpy change of cyclohexa-1, 4-diene (-239) indicate that the enthalpy change for adding 1 mol of H2 to 1 mol of C=C bond is -120.
-Kekule’s structures would be the treble of cyclohexene. so -120 x 3, -360
-the actual benzene value is much lower.

Question 4

what did all the problems with Kekule’s structure demonstrate about benzene

Answer 4

after the sigma bonds are formed, each carbon atom has one electron in a p-orbital, there is a formation of one large pi-bond made up of all six electrons. these six electrons form a delocalised pi-bond. this means that: there are no individual C=C bonds, so there is no addition reaction with bromine. There are three isomers because there is no difference in the arrangement of electrons when the bromine atoms are on adjacent carbon atoms. When charge is spread around in a species, there is more stability.

Question 5

why does a substitution occur between benzene and bromine water

Answer 5

because substitution preserves the stability of the delocalised electrons in the pi bond

Question 6

reaction of hydrogenation of benzene

Answer 6

benzene + hydrogen -> cyclohexane (C6H12)
conditions: heating under pressure with nickel catalyst

Question 7

reaction of combustion of benzene

Answer 7

benzene + oxygen -> carbon dioxide + water
observation: smoky flame

Question 8

bromination of benzene

Answer 8

benzene + bromine -> bromobenzene + hydrogen bromide
conditions: heated under reflux with halogen carrier catalyst (AlCl3, AlBr3, FeBr3 (iron (III) fillings are suitable since it reacts to form iron (III) bromide)
reaction: substitution

Question 9

nitration of benzene

Answer 9

benzene + concentrated nitric acid -> nitrobenzene + water
conditions: warming with concentrated sulfuric acid as a catalyst. Carried out between 50º and 60ºC. At lower temps the reaction is too slow and at higher it can form dinitrobenzene and trinitrobenzene
reaction: substitution

Question 10

what does the alkylation and acylation of benzene have in common

Answer 10

-using a reagent represented by XY, one of the hydrogen atoms in benzene is substituted by Y, other product is HX
-a catalyst is needed- aluminium chloride, iron(III) chloride and iron(III) bromide can also work
-anhydrous conditions are needed because water would react with the catalyst and sometimes also with the organic product

Question 11

alkylation of benzene

Answer 11

reaction: substitution of one hydrogen atoms of benzene by an alkyl group
benzene + halogenoalkane -> alkylbenzene (methylbenzene) + hydrogen chloride
conditions: anhydrous conditions, aluminium chloride catalyst

Question 12

acylation of benzene

Answer 12

reaction: substitution of one of the hydrogen atoms in benzene by an acyl group`
benzene + acyl chloride -> ketone (phenylketone) + hydrogen chloride
conditions: anhydrous conditions, aluminium chloride catalyst

Question 13

sulfonation of benzene

Answer 13

reaction: replacement of a hydrogen atom by an -SO3H group
benzene + sulfuric acid -> benzenesulfonic acid + water
conditions: warmed with fuming sulfuric acid at 40ºC for 20 to 30 minutes
if the question asks about sulfuric acid use H2SO4 if it asks about SO3 as the electrophile just substitute SO3 in the sulfuric acid REACTANT

Question 14

how to make fuming sulfuric acid

Answer 14

made by dissolving sulfur trioxide in concentrated sulfuric acid

Question 15

what does all the electrophilic substitution reactions of benzene have in common

Answer 15

-the benzene ring is electron rich due to the delocalised electrons in the pi bond allows it to attract electrophiles.
-using Y+ to represent electrophile. as Y+ approaches the delocalised pi bond, it attracts two of the six electrons, formig a covalent bond so giving an intermeadiate species with a positive charge- one carbon is joined to both H and Y
-this intermeadiate lacks stability , so in the next step the H leaves as H+ and the electrons join the four to restore the delocalised pi-bopnd

Question 16

electrophilic substitution of bromination of benzene

Answer 16

Step 1: formation of electrophile= AlBr3 + Br2 -> Br+ + AlBr4 -
step 2: electrophilic attack
step 3:formation of aromatic product
step 4: regeneration of the catalyst= AlBr4 - + H+ -> AlBr3 + HBr

Question 17

electrophilic substitution of nitration of benzene

Answer 17

Step 1: formation of electrophile (nitric acid acts as a base) =
HNO3 + H2SO4 -> NO2+ + HSO4 - + H2O
step 2: electrophilic attack
step 3:formation of aromatic product
step 4: regeneration of the catalyst= HSO4 - + H+ -> H2SO4

Question 18

electrophilic substitution of alkylation of benzene

Answer 18

Step 1: formation of electrophile= AlCl3 + CH3Cl -> CH3+ + AlCl4 -
step 2: electrophilic attack
step 3:formation of aromatic product
step 4: regeneration of the catalyst= AlCl4 - + H+ -> AlCl3 + HCl

Question 19

electrophilic substitution of acylation of benzene

Answer 19

Step 1: formation of electrophile= AlCl3 + CH3COCl -> CH3CO+ + AlCl4 -
step 2: electrophilic attack
step 3:formation of aromatic product
step 4: regeneration of the catalyst= AlCl4 - + H+ -> AlCl3 + HCl

Question 20

what is phenol

Answer 20

a hydroxyl group joined to the benzene ring. You can expect it to undergo electrophilic substitutions reaction due to the benzene ring, it has the physical properties of alcohols

Question 21

bromination of phenol

Answer 21

phenol + bromine water -> 2,4,6-tribromophenol + hydrogen bromide
conditions: at room temperature without a catalyst
observations: bromine water decolourises and organic white precipitate is formed.
reaction: substitution reaction occurs three times as there are three bromine atoms in the organic product

Question 22

why does bromination of phenol occur more readily than bromination of benzene

Answer 22

-the lone pair of electrons in the pz-orbital on the oxygen atom of the OH group interacts with the delocalised pi electrons in the benzene ring in phenol.
-the electron density above and below the ring of atoms is increased making the molecule much more reactive towards electrophiles
-bromine molecules are polarised as they approach the benzene ring in phenol. eventually the Br-Br bond breaks and the Br+ electrophile attacks the benzene ring in phenol