C6. Aromaticity

Question 1

Å =

Answer 1

1 x 10-10 m

Question 2

C-C bond length?

Answer 2

1.54Å

Question 3

C=C bond length?

Answer 3

1.34 Å

Question 4

bond length in benzene?

Answer 4

1.39 Å

Question 5

Describe reactivity of benzene

Answer 5

-Benzene does not react with many electrophiles that add to double bonds in alkenes.
-When benzene does react with an electrophile, it undergoes substitution, rather than addition (uncommon)

Question 6

Describe the enthalpy of hydrogenation of benzene

Answer 6

-We would expect benzene to have 3 x the enthalpy of hydrogenation compared to cyclohex-1-ene.
-Benzene is 30 kcal mol-1 more stable than we would expect.
-Aromaticity makes benzene more stable.

Question 7

orbital description of benzene?

Answer 7

-The carbon atoms in bezene are sp2 hybridised.
-σ-bonded framework formed of C sp2 orbitals
-Unhybridised 2p orbitals are arranged perpendicular to the plane
-p orbital overlap leads to extended π system. (one note for diagram)

Question 8

what is needed for a molecule to be aromatic? Huckel’s rules

Answer 8

-must be cyclic
-must be planar
-must be completely conjugated
-must have (4n+2) electrons

Question 9

Describe the anti-aromaticity of cyclooctatetrene

Answer 9

-The 6π electrons make cyclooctatetrene anti-aromatic
-This anti-aromaticity destabilises the molecule and forces it to adopt a non-planar geometry.
-Bond lengths are unequal and the molecule is now non-planar and non-aromatic

Question 10

how do you encourage aromaticity for cyclooctatetrene

Answer 10

treat with powerful oxidising agent. This will now demonstrate 4n+2 rule (one note)

Question 11

Describe heterocyclic aromaticity (pyridine)

Answer 11

-Nitrogen atoms can be tricky to assign electrons to in aromatic rings.
-The nitrogen in pyridine is sp2 hybridised. Whereas the nitrogen in pyrrole is “sp3” hybridised (it looks sp3 hybridised but is in fact sp2). (one note)

Question 12

how many p electrons does pyridine- like nitrogens and pyrrole- like nitrogens contribute to the pi system?

Answer 12

-Pyridine-like nitrogens contribute ONE p electron to the π system.
-Pyrrole-like nitrogens contribute TWO p electrons to the π system.

Question 13

what makes a molecule NON- aromatic?

Answer 13

-not cyclic
-not planar
-not completely conjugated

Question 14

what makes a molecule ANTI- aromatic?

Answer 14

If the molecule has 4n electrons and meets the other criteria, it is anti-aromatic and destabilised