aromatic chemistry

Question 1

a c=c bond is made up of … and …

Answer 1

sigma bond
pi bond

Question 2

what is a sigma bond

Answer 2

a covalent bond formed from simple end on overlap of atomic orbitals

Question 3

what is a Pi bond

Answer 3

a covalent bond formed from sideways on overlap of p orbitals

Question 4

what is the general length of c-c bond

Answer 4

0.15 nm

Question 5

what is the general length of c=c bond

Answer 5

0.13nm

Question 6

what is the bond length in buta-1,3-diene

Answer 6

between 0.13-0.15 nm

Question 7

is buta-1,3-dienes bond length different than what we expect and why

Answer 7

p orbitals on 1 and 2 carbons could overlap to form a π bond. p orbitals on 3 and 4 carbons could also overlap to from a π bond
therefore we might expect a bond length of double bonds between 1+2 and 3+4 but single bond length between 2+3

however all four π orbitals on each carbon overlap to form a delocalised π electron system that contains 4 electrons and spans across all 4 carbon atoms hence a general length between 0.13 and 0.15 nm

Question 8

what is the empirical formula of benzene

Answer 8

CH

Question 9

what is the molecular formula of benzene

Answer 9

C6H6

Question 10

describe kekules benzene

Answer 10

double bonds and single bonds interchangeably between carbons
C6H6
cyclic

Question 11

what is the evidence against kekules benzene

Answer 11

-reactions with bromine water
-bond lengths
-isomers of 1,2-dichlorobenzene
-enthalpies of hydrogenation

Question 12

describe why reactions with bromine water disproved kekules benzene

Answer 12

cyclohexene has a c=c bond and so decolourises bromine water
benzene doesn’t decolourise bromine water so implies no double bonds

Question 13

describe why bond lengths disproved kekules benzene

Answer 13

all bond lengths in benzene are the same and are 0.14 nm. this implies they are a mixture of double and single bonds but not strictly either

Question 14

describe why isomers of 1,2-dichlorobenzene disproved kekules benzene

Answer 14

if kekules structure was correct there should be two isomers of this but there is only one

Question 15

describe why enthalpies of hydrogenation disproved kekules benzene

Answer 15

the hydrogenation of cyclohexene gives us -120 kJ and so we can predict that the hydrogenation of kekules benzene would be -360 kJ

however the actual hydrogenation is -208 which means that benzene is 152 kJ more stable than suggested as it only releases -208kJ compared to the 360 kJ suggested

Question 16

what is enthalpy of hydrogenation

Answer 16

the enthalpy change when one mole of unsaturated compound is saturated by reacting it with hydrogen under standard conditions

Question 17

what causes benzene to be more stable than kekule suggested

Answer 17

the delocalisation of the 6 π electrons around all the carbon atoms in its ring structure

Question 18

describe the actual structure of benzene

Answer 18

each atom forms 3 sigma bonds with 2 neighbouring carbons and a hydrogen
the unbonded p orbitals on each carbon atom then overlap with the p orbitals on neighbouring carbons to form a network of delocalised π electrons above and below the plane of carbon atoms
the delocalised π bond contains 6 electrons (1 from each carbon)

Question 19

what is an electrophile

Answer 19

electron pair acceptors

Question 20

why does the benzene ring attract electrophiles

Answer 20

high density of electrons attracts