lecture 4

Question 1

what is the formula for unit of saturation?

Answer 1

(2C +2 + N - H -X) /2

Question 2

what is the problem with Kekule structure

Answer 2

1. bond length: C-C bond in benzene has different bond
   length from single bond and double bond
2. bond energy: the heat of hydrogenation is -209 kj/mol which is different from three c=c bond
3. reactivity: the double bonds in benzene don’t react like typical alkenes

Question 3

how many pi-MOs does benzene have?

Answer 3

3 pi bonding MOs

3pi antibonding MOs

Question 4

what is the criteria for aromaticity?

Answer 4

1. monocyclic
2. planar
3. 4n+2 pi-electrons (n=0,1,2,3,4,…)

Question 5

which MO would pi-electrons in aromatic compounds fill in?

Answer 5

aromatic compounds have pi-electrons only in pi-bonding MOs.

Question 6

why aromatic compounds are more stable than anti aromatic compounds?

Answer 6

bond order is directly proportional to the stability of compound. in aromatic compounds bond order is maximum, because all electrons go in bonding orbitals and are all paired.