Aromantic compounds

Question 1

what is the molecular formula of benzene?

Answer 1

C6H6

Question 2

when is phenyl used?

Answer 2

you get a phenyl group C6H5 by removing hydrogen from a benzene ring and the group will always be attached to something else e.g phenol,phenylethanone etc.

Question 3

Explain Kekule’s ideas but the issues behind it

Answer 3

benzene was cyclic and had alternate single and double bonds
Issues:
1.benzene doesn’t undergo electrophilic addition reactions with bromine-alkenes do
2.Benzene has relatively low reactivity unlike alkenes.
3.Benzene has a regular planar structure with equal C-C bond lengths (C=C bonds shorter than C-C)

Question 4

Evidence against Kekule: reaction with bromine

Answer 4

Benzene doesn’t undergo electrophilic addition with bromine like alkenes do. Bromine water remains orange unless it undergoes electrophilic substitution with the usage of a halogen carrier such as Fe,FeBr3,AlBr3 etc. to make Br+ more reactive

Question 5

Evidence against Kekule: Bond length

Answer 5

Benzene has a regular structure with equal C-C bond lengths i.e. its planar. However Kekule didn’t have a regular structure with C=C bonds being shorter than C-C bonds.

Question 6

Evidence against Kekule: Enthalpy of hydrogenation

Answer 6

its less exothermic than expected giving benzene greater stability. Therefore Kekule is wrong because he predicted a more exothermic enthalpy of hydrogenation

Question 7

Explain structure of delocalised Benzene model

Answer 7

• planar cyclic molecule
• triganol planar 120 degrees around each C atom
• 3 sigma bonds per C atom the 4th electron being in a p-orbital
• p-orbitals overlap sideways above and below plane of ring forming pi bond.
• 6 pi electrons delocalised over 6 C atoms leading to further stability
• benzene doesn’t undergo addition reactions as stability would be lost.

Question 8

Why are higher temperatures not used in nitration?

Answer 8

multi substitution would occur

Question 9

Why are Friedel-Crafts reactions important?

Answer 9

leads to C-C bond formation

Question 10

Why do phenols not react with metal carbonates?

Answer 10

Phenol isn’t acidic enough.

Question 11

Comparison of alkene, benzene and phenol: electrons

Answer 11

alkene- pi electrons localised between 2 C atoms + high electron density
benzene-6 pi electrons delocalised over 6 C atoms + lower electron density than alkenes
phenol-6 pi electrons delocalised over 6 C atoms + lone pair in p-orbital on O delocalised into ring activating it + higher electron density than benzene

Question 12

Comparison of alkene, benzene and phenol: attracting electrophiles

Answer 12

alkene-polarises Br2 molecule and attracts it
benzene-only weakly polarises Br2 molecule needs halogen carrier to make powerful electrophile Br+
phenol-polarises Br2 molecules and attracts it

Question 13

Comparison of alkene, benzene and phenol: type of reaction

Answer 13

alkene-electrophilic addition colour change orange to colourless
benzene-electrophilic substitution
phenol-electrophilic substitution colour change orange to colourless and formation of white precipitate 2,4,6-tribromophenol

Question 14

define activating group

Answer 14

lone pair delocalised into benzene ring activating it higher pi electron density of C 2 and 4

Question 15

define deactivating group

Answer 15

withdraw electron density from benzene ring deactivating it. higher pi electron density at C 3

Question 16

define aromatic compound

Answer 16

contains at least one benzene ring.

Question 17

define arene

Answer 17

a hydrocarbon containing at least one benzene ring.

Question 18

explain electrophilic substitution: halogenation

Answer 18

OE:benzene+Br2(AlBr3+heat and under reflux)–>benzene bromine + HBr
generation of electrophile=Br+AlBr3–>AlBr4- + Br+
regeneration of the catalyst:
H+ + AlBr4- –> AlBr3 + HBr

Question 19

electrophilic substitution:nitration

Answer 19

OE:benzene+HNO3(H2SO4/50 degrees)–>Benzene NO2 + H2O
generation of electrophile=HNO3+H2SO4–>NO2+ +HSO4- + H2O
regeneration of catalyst=H+ + HSO4- –> H2SO4

Question 20

electrophilic substitution: Friedel-crafts alkylation

Answer 20

OE:benzene+bromoethane(AlBr3+heat and under reflux)–>ethanebenzene+HBr
generation of electrophile=CH3CH2Br+AlBr3=AlBr4- + CH3CH2+
regeneration of catalyst=H+ + AlBr4- = AlBr3+HBr

Question 21

electrophilic substitution: Friedel-Crafts acylation

Answer 21

OE:benzene+bromoethanone(AlBr3+heat and under reflux)=benzeneethanone+HBr
generation of electrophile=CH3COBr+AlBr3=AlBr4- + CH3CO+
regeneration of catalyst=H+ + AlBr4-= AlBr3 + HBr

Question 22

How to answer relative resistance exam Q’s

Answer 22

-alkene pi e- localised between 2C atoms
-benzene 6 pi e- delocalised over 6C atoms
-phenol lone pair of e- on O atom delocalises into benzene ring
-benzene lower pi e- density than both alkene/phenol
-therefore benzene can’t attract electrophiles as strongly
(in terms of reaction mention electron density because higher density=attracts more strongly)