topic 18a: benzene

Question 1

aliphatic

Answer 1

straight or branched

Question 2

aromatic/arene

Answer 2

contains a ring

Question 3

descrieb the bonding in benzene

Answer 3

• each c atom is bonded to 2 other c atoms and one H atom, by SINGLE SIGMA BONDS
• this leaves one unused electron on each c atom in a p orbital
• the 6 p electrons are in a delocalised ring structure above and below the plane of carbon atoms, forming pi bonds by overlapping

Question 4

problem with kekule model

Answer 4

• c-c bonds should all be the same legnth

Question 5

bond angle

Answer 5

120

Question 6

describe the enthalpies of hyration and how this disproves the double bond model?

Answer 6

• cyclohexene + h2 = cyclohexane with delta h -120
• 1,3,5 cyclohexatriene + 3H2 gives 3x the delta h
• BUT FOR BENZENE, the delta h is less negative
• shows that the 6 pi electrons are DELOCALISED and not arranged in 3 double bonds

Question 7

benzene stability?

Answer 7

• v stable
• delocalisation energy

Question 8

why doesnt benzene undergo addition but alkenes do?

Answer 8

• for benzene this would involve breaking the delocalised electron structure which makes it stable
• whereas in alkenes, the pi bond is LOCALISED

Question 9

what has priority a halogen or alkyl group

Answer 9

HALOGEN

Question 10

Benzene safety

Answer 10

toxic

Question 11

methyl benzene vs benzene

Answer 11

• less toxic
• reacts more readily, as methyl group has positive inductive effect; pusehs electrons into the delocalised ring system, increasing electron density so more attractive to electrophiles

Question 12

combustion of benzene equation

Answer 12

C6H6 + 7.5 O2 = 6CO2 + 3H2O

Question 13

benzene state

Answer 13

liquid

Question 14

what kind of flame does benzene combust with and why

Answer 14

• very sooty
• LOW C:H RATIO

Question 15

benzene + bromine reagent and conditions

Answer 15

• Br2 reagent
• FeBr3 catalyst
• FeBr3 +Br2 = FeBr4- + Br+

Question 16

benzene and bromine formula

Answer 16

c6h6 + br2 = c6h5Br + HBr

Question 17

benzene and chlorine reagent and condition

Answer 17

• AlCl3 + Cl2 = AlCl4- + Cl+
• OR FeCl3

Question 18

nitration of benzene reagent and condition

Answer 18

• conc nitric acid and conc sulfuric acid (CATALYST)
• 60 degrees

Question 19

name of mechanism for halogenation and nitration of benzene

Answer 19

electrophilic substitution

Question 20

equation of formation of electrophile for nitration

Answer 20

HNO3 + 2H2SO4 = NO2+ + 2HSO4- + H3O+

Question 21

reformation of AlCl3 catalyst

Answer 21

H+ + AlCl4- = AlCl3 + Hcl

Question 22

reformation of sulfuric acid catalyst

Answer 22

H+ + HSO4- = H2SO4

Question 23

equaion of nitration of benzene

Answer 23

C6H6 + NO2+ = C6H5NO2 + H+

Question 24

hydrogenation of benzene

Answer 24

• benzene to cyclohexane
• h2
• nickel catalyst

Question 25

friedel crafts alkylation reagents and conditions

Answer 25

- haloalkane - ANHYDROUS AlCl3 catalyst - heat under refluc

Question 26

formation of electrophile eg ethyl group for freidel crafts alkylation

Answer 26

Ch3Ch2Cl + AlCl3 = CH3CH2+ + AlCl4-

Question 27

regenratin of catalyst after friedel crafts alkylation

Answer 27

H+ + AlCl4- = AlCl3 + HCl

Question 28

what does friedel crafts ACTYLATION do

Answer 28

adds a c=och3

Question 29

friedel crafts acylation conditions and reagents

Answer 29

- ACYL CHLORIDE - anhydrous AlCl3 catalyst - heat under reflux

Question 30

formation of electrophile in friedel crafts ACYLATION

Answer 30

CH3COCl + AlCl3 = AlCl4- + CH3CO+

Question 31

in nitration, which of the 2 acids acts as a base

Answer 31

- HNO3 acts as a base as H2SO4 is a stronger acid so protonates HNO3

Question 32

why can phenol act as a weak acid

Answer 32

can donate the H from the OH

Question 33

why is phenol much more reactive than benzene

Answer 33

- lone pairs on oxygen atom - delocalises into ring, interacting with delocalised pi system - increases electron density of ring - more susceptible to attack by electrophile

Question 34

why is the phenol ion very stable and therefore a good acid

Answer 34

- when the h+ leaves you get the o- - the charge can delocalise into the delocalised pi electron structure so the ion is more stable

Question 35

phenol + bromine

Answer 35

- bromine water decolourises - white ppt formed if 2,4,6 tribromophenol

Question 36

phenol vs benzene, bromonation (6)

Answer 36

-both electrophilic substitution -benzene forms bromobenzene (monosub) phenol forms 2,4,6 tribromophenol (triple sub) - benzene needs FeBr3 catalyst, phenol doesnt - phenol room temp, benzene heat under refluc - benzene less reactive than phenol - lone pair on o dissociates into ring, into delocalised pi system - increases electron density in ring so more susceptible to attack by electrophiles

Question 37

proton NMR of benzene

Answer 37

- 1 peak as all protons equivalent - delta 7-8

Question 38

important to note in mechanisms

Answer 38

- draw arrow frmo the ring to the POSITIVE ATOM (eg the c+ for alkylation)

Question 39

alcohol to alkene

Answer 39

concentrated phosphoric acid

Question 40

aldehyde/ketone to alcohol

Answer 40

- LiAl4 in dry ether

Question 41

if you form intramolecular forces then...

Answer 41

- weaker IMFs - lower bp

Question 42

why must fume cupboard stay above safety line (2)

Answer 42

- above is when exhaust system not strong enough to draw in fues - to toxic fumes escape into lab

Question 43

what combusts with a black smoky flame

Answer 43

- arenes - alkenes - low h:c ratio

Question 44

why is phenyl methanol less reactive than phenol (4)

Answer 44

- lone pair on O delocalises into ring - increases e- density of ring - in phneol the -OH is directly attached to ring so directly overlaps with pi electrons - but in phenylmethanol is too far away