Chapter 25 - Aromatic Compounds Flashcards
What was Kekule’s model of the benzene
It would have been called cyclohexa-1,3,5-triene.
It would have alternate double bonds in the closed ring.
What were the 3 pieces of evidence that challenged Kekule’s model?
- X-ray diffraction analysis
- Enthalpy of hydrogenation analysis
- Bromine water testing.
Explain the X-Ray diffraction analysis
It showed that the bonds between C atoms were all the same length of 0.139nm.
Rather than C-C being longer than C=C.
This means Benzene was a flat, planar molecule and also a perfect hexagonal shape.
Explain the Hydrogenation experiments
If he was correct, the enthalpy value of benzene should be 3* the value of cyclohexenes (as it had 3 double bonds)
Benzene’s enthalpy was actually a lot lower.
This meant that it was more stable and unreactive, this was unusual because double bonds are usually very reactive and a site for activity.
Bromine water testing
It can decolourise any alkene,
But when added to benzene, it doesn’t decolourise it.
Explain the Delocalised model
The electrons can move to any carbon, and not just the ones adjacent to it. The lone electrons alternate top half and bottom to minimise the electron repulsion and spread out the negative charge. Hence, the pi bond is spread out over all 6 atoms of carbon, above and below the plan of the C atoms.
What is a resonance structure
Snapshots of benzene in time highlighting which pair of carbon atoms have the electron density to attract other particles.
Carboxylic acid
COOH
If dominant: Benzoic acid
Ester
O=C-O-R
If dominant: -oate
If dominated: #-alkyoxy- #oxo
Acyl Chloride
O=C-Cl
If dominant: -oyl chloride
If dominated: #-chloro-#-oxo-R
Amide Group
O=C-NH2
If dominant: Amide
If dominated: #-amino-#-oxo
Nitrile
-C=-N
If dominant: -nitrile/ -cyanide.
If dominated: cyano-
Aldehyde
O=C-H
If dominant: -al
If dominated: formyl-
Ketones
R- C=O
If dominant: -one
If dominated: oxo-
Alcohol
-OH
If dominant: -ol
If dominated: hydroxy-
