Aromatic Compounds, Carbonyls and Acids Flashcards
Which homologous series is benzene in?
The arene homologous series
What is the molecular formula of benzene?
C₆H₆
What is the empirical formula of benzene?
CH
What is the state of benzene at room temperature?
Liquid
What was the structure of Kekule’s benzene model? Also draw an image.
- A six-membered carbon ring with alternating single and double bonds between the carbon atoms.
- One isomer is made, when one group is added to benzene.
- Three structural isomers are made when two groups were added to benzene.
Problems with Kekule’s model?
3 pieces of experimental evidence:
1) Unlike alkenes, benzene is resistant to addition reactions.
2) Enthalpy of hydrogenation of benzene shows that benzene is much more stable than predicted.
3) All six carbon bonds in the benzene are the same length.
Why was the fact that benzene is resistant to addition reactions a problem with the model?
The C-C double bonds in Kekule’s model suggest that it benzene will be able to undergo similar reactions to alkene (which has the functional group C=C). However benzene cannot undergo addition reactions.
How did Kekule try to justify the reason that benzene cannot undergo addition reactions?
He said that the double and single bonds in his benzene model changed position in a very fast equilbrium.
Why was the enthalpy of hydrogenation of benzene a problem with Kekule’s model?
Kekule’s model of benzene has the same structure as cyclo-1,3,5-hexatriene so benzene was predicted to have the same enthalpy of hydrogenation as this molecule, which was -360kjmol-1. However, the actual enthalpy of hydrogenation of benzene was calculated to be -208kjmol-1. Benzene was less exothermic, meaning it was more stabel than expected.
Why was the simialr bond length of benzene a problem with Kekule’s model.
X-ray diffraction techniques have shown that C=C double bonds are shorter than C-C single bonds, so Kekule’s model suggest that there should be three shorter bonds and three longer bonds, but in reality the length of all six bonds in benzene is the same.
