3.3.10 Aromatic chemistry

Question 1

How did Kekule propose the structure of benzene to be?

Answer 1

cyclohexa-1,3,5-triene
six carbon ring of alternating single and double rings

Question 2

What 3 things prove Kekule’s theory wrong?

Answer 2

1. Unlike alkenes, benzene does not undergo electrophilic addition (e.g. it does not decolourise
   bromine water).
2. X-ray analysis of solid benzene shows a regular hexagon with all the bonds the same length. This
   length is intermediate between a typical single and double bond.
3. DH associated with hydrogenating the three unsaturated C=C bonds would be expected to be 3 x that for hydrogenating the one double bond of cyclohexene (-120 kJ mol-1). However, the
   experimental value of -208 kJ mol-1 is much smaller than the predicted value of -360 kJ mol-1
   .

Question 3

Is cyclohexa-1,3,5-triene or benzene more stable?

Answer 3

benzene

Question 4

Why is benzne more stable than cyclohexa-1,3,5-triene?

quantitatively

Answer 4

Question 5

ΔΗ=

Answer 5

Bonds broken - bonds made

Question 6

Why is benzne more stable than cyclohexa-1,3,5-triene?

words

Answer 6

• ΔΗ=bonds broken-bonds made
• ΔΗ= enthalpy of hydrogenation for each molecule
• **bonds broken (strength)= ΔΗ+ bonds made **
• bonds made = what would be left after hydrogenating = cyclohexane, so 6 x C-H bonds formed = 6 x 413= 2478
• therefore bonds broken in cyclohexa-1,3,5-triene= 2478 + -360 =2,118 kJ mol -1
• therefore bonds broken in benzene= 2478 + -208 =2270 kJ mol -1
• therefore more energy required to break bonds in benzene than cyclohexa-1,3,5-triene