3.3.10 Aromatic Chemistry

Question 1

what is an arene

Answer 1

hydrocarbon baced of benzene

Question 2

benzene molecular formular

Answer 2

C6H6
hexagon with circle in middle

Question 3

where did arenes first come from

Answer 3

sweet smelling oils that is why they are called aromatic compounds

Question 4

what is a phenyl

Answer 4

a benzene that has lost a hydrogen
C6H5

Question 5

Benzene important structure and bonding point s

Answer 5

• flat benzene ring
• unsaturated - but doesnt undergo addition reactions
• bond length in-between c-c and c=c

Question 6

cyclohexane

Answer 6

C6H12
has 3 double bonds

Question 7

benzenes delocalization and stability explained

Answer 7

• each carbon creates 3 covalent bonds
• carbon has the capacity for 4 bonds means leaves a delocalized electron
• 6 delocalized electrons per benzene
• the 4th electron is in the p orbital - as the p orbitals overlap leads to the areas of delocalized electrons
• means there is an equal region of electron density above and below the carbon ring

Question 8

what is benzenes stability sometimes called

Answer 8

benzenal stability

Question 9

thermochemical evidence for the stability of benzene

Answer 9

• cyclohexene to cyclohexane’s enthalpy change of hydration is - 120 kjmol-1
• means you would assume theoretically if - benzene has 3 double bonds like in cyclohexa-1,3,5 triene that bond enthalpy = -360kjmol-1
• benzene actually is -208kjmol-1

Question 10

reasoning behind the -152kjmol-1 difference in enthalpy

Answer 10

• less exothermic than expected
• means takes more energy to break bonds in molecule
• because it is more stable from the delocalized electrons

Question 11

what is bonds breaking and making endo or exo

Answer 11

bond breaking - endothermic - pos
bond making - exothermic - neg

Question 12

models for benzene

Answer 12

• kekule model - original - with alternating double and single bonds
• Pauling’s delocalized model - overcomes all problems and explains non polarity

Question 13

evidence against the first benzene model explained

Answer 13

1. thermodynamics
   - less exothermic than expected as takes more energy to break bonds as more stable
2. reactivity
   - should undergo electrophilic addition but actually doesnt as to stable needs a catalyst
   - does undergo substitution
3. bonds length
   - kekule c-c = 0.154 , c=c = -o.34
   - real
   0.140 not single or double
4. isomers
   - is kekule should have 4 isomers but only has 3 that actually exist

Question 14

reactivity of benzene

Answer 14

• ring is an area of high electron density - means it is attacked by electrophiles
• ring is very stable - means energy has to be put in to break ring - ring remains intact in arenes

Question 15

physical properties of benzene

Answer 15

• colorless liquid at room temperature
• boils at 353K freezes 297K
• bp is similar to hexanes
• mp is not similar to hexanes its higher - because benzene is flat so packs together well in solid state
• non polar

Question 16

what is a halogen and a benzene called (eg chlorine)

Answer 16

chlorobenzene

Question 17

nitrate (NO2) and benzene name

Answer 17

nitrobenzene

Question 18

alkyl group and benzene name (eg methane)

Answer 18

methylbenzene

Question 19

primary amine (NH2) and benzene name

Answer 19

phenylamine

Question 20

alkene and benzene name (eg ethene)

Answer 20

phenylethene

Question 21

ketone and a benzene name

Answer 21

phenylethanone

Question 22

ester and benezne

Answer 22

phenylethanoate

Question 23

alcohols and benzene name

Answer 23

phenol

Question 24

cobxylic acid and benzene name

Answer 24

benzenecarboxylic acid - benzoic acid

Question 25

combustion of arenes

Answer 25

- even with complete combustion there is still smoke - as high carbon : hydrogen - produces soot

Question 26

basic principle of the electrophilic substitution reaction mechanism

Answer 26

1. arrow coming form benzene to electrophile 2. broken ring in middle with + and a hydrogen and the electrophile attached on outside 3. arrow from hydrogen into the cicle 4. complete circle and a H+ on outside (electrophile still attached ) 5. arrow form catalysts ion to the H+ 6. new molecule has bene created and catalysts has been regenerated SAME STEPS FOR ALL REACTIONS

Question 27

Nitrigenation of benzene

Answer 27

1. conditions - (conc H2SO4 + concHNO3 -> H3O+ , NO2+ HSO4- - reflux - less than 50 degrease for one substitution 2. creating the electrophile the hydrogenating mixture electrophile = NO2+ catalyst ion = HSO4- 3. electrophilic substitution reaction 4. regenerating catalyst HSO4- + H+ -> H2SO4

Question 28

Acylation of benzene

Answer 28

1. conditions - reflux - less than 60 degrese - acyl chloride - halogen carrier 2. creating the electrophile - acyl chloride + AlCl3 -> R - c = 0+ + Al Cl4 - 3. electrophilic substitution reaction 4. regenerating catalyst AlCl4- + H+ -> HCl + AlCl3

Question 29

Alkylation

Answer 29

1. conditions - room temp - haloalkane - halogen carrier - ionic aq (high volatility, less toxic waste gases ) 2. creating the electrophile haloalkane + AlCl3 -> CH+ + AlCl4- 3. electrophilic substitution reaction 4. regenerating catalyst AlCl4- + H+ -> HCl + AlCl3