benzene

Question 1

what is benzene?

Answer 1

a cyclic, planar molecule with general formula C6H6

Question 2

describe the bonding in benzene

Answer 2

• 4 valent electrons
• each bonded to 2 carbons and 1 hydrogen
• lone electrons are in p orbitals
• p orbitals stick out and above plane
• lone electrons combine to form a delocalised electron

Question 3

describe the C-C bonds in benzene

Answer 3

• same bond length due to delocalised electron structure
• 139pm
• smaller than normal C-C but bigger than double bonds

Question 4

describe the kekule benzene structure

Answer 4

6 carbon cyclic ring
double bonds on every other carbon

Question 5

describe the delocalised electron benzene structure

Answer 5

6 carbon cyclic ring with a circle

Question 6

how is benzene stability measured?

Answer 6

comparing enthalpy change of hydrogenation in benzene and cyclohexa-1,3,5-triene

Question 7

what is the theoretical enthalpy change of hydrogenation in benzene and why?

Answer 7

-360kjmol-1
cyclohexene = 120

Question 8

what is the experimental enthalpy change of hydrogenation in benzene?

Answer 8

-208kjmol-1

Question 9

what makes benzene more stable than the theoretical value?

Answer 9

• energy is required to break bonds and released to form bonds
• suggests more energy is required to break benzene bonds
• more stable due to delocalised electron structure

Question 10

name the family of chemicals to which benzene belongs

Answer 10

arenes

Question 11

what term is given to molecules that don’t contain a benzene ring?

Answer 11

alicyclic

Question 12

name 4 compounds which contain a benzene ring and examples

Answer 12

• azo = dyes
• TNT = explosives
• aspirin = painkillers
• TCP = antiseptics

Question 13

what is the difference in enthalpy between the kekule and true benzene structure known as?

Answer 13

delocalisation enthalpy

Question 14

how is delocalisation enthalpy accounted for?

Answer 14

delocalised pi bond system makes the molecule more stable

Question 15

if benzene had a kekule structure what would it look like?

Answer 15

irregular hexagon

Question 16

if all C-C bond angles are 120, what would the shape of the benzene molecule look like?

Answer 16

regular hexagon

Question 17

how many electrons are present in the pi orbital?

Answer 17

6

Question 18

describe some properties of benzene

Answer 18

• colourless
• flammable
• found in crude oil and petrol
• a carcinogen which can cause cancer

Question 19

state 3 pieces of evidence which reject the kekule model

Answer 19

• lack of reactivity
• lengths of C-C bonds
• hydrogenation enthalpies

Question 20

summarise how the lack of benzene reactivity rejects the kekule model

Answer 20

• the presence of the double C bond should decolourise bromine
  -but benzene does not undergo electrophilic addition

Question 21

summarise how the lengths of C-C bonds rejects the kekule model

Answer 21

• bonds are 0.139nm
• between that of a single and double bond

Question 22

summarise how the hydrogenation enthalpies reject the kekule model

Answer 22

• it would be expected to have a hydrogenation enthalpy 3x that of cyclohexene
• less energy is produced than expected