1. Organic 2
Dr. Thoraya A.Farghaly

3. Kekule `s structure of benzene Postulates:
Benzene is a cyclic hexagonal planer structure of six carbon atoms
with alternating single and double bonds.
Benzene can be considered as 1,3,5-cyclohexatriene.
Benzene is existed in two resonating structures.
In the light of the proposed structure of benzene, it should give two
ortho disubsitituted isomers and in reality only one isomer is observed.

4. The two ortho substituents are
attached to one single bond
The two ortho substituents are
attached to one double bond
Resonance theory
“The benzene ring can be represented by two equivalents resonating
structures and the truly structure is the hybrid one”
The hybrid structure can explain the formation of single ortho
disubstituted isomer.

5. The resonance theory also explains why the six carbon-carbon bonds are equivalents; the value of each c-c bond is shorter than normal single bond and longer than the double bond

6. The resonance theory also explains the high stability of benzene ring due to the delocalization of pi-electrons.

7. Delocalization of pi-electrons
Molecular orbital description
Delocalization of pi-electrons
Criteria for Aromaticity
The features of Aromatic compounds
Cyclic and contain conjugated system of p electrons
Contain an odd no. of pairs of p electrons

8. Huckel’s Rule The aromatic compounds must contain (4n+2) p electrons
(n = integer number, 0, 1, 2, …..)
Other Aromatic systems

9. The features of Antiaromatic compounds
Cyclic, planar, conjugated system of p-electrons
(double, single, double, single, ……….)
contain (4n) p-electrons (n = integer number 1, 2, 3, …..).

10. Annulenes: Monocyclic hydrocarbons with conjugated system.
A prefix in brackets indicates the number of carbons in the ring.
If the compounds did not fulfill the requirements of
aromatic or antiaromatic characters it will be Non-aromatic
Compounds

11. Nomenclature of monosubstituted benzene
Name of substituent followed by the word of benzene

12. When benzene ring is a substituent, it is called phenyl group
Nomenclature of disubstituted benzene
If the two substituents in positions 1,2 they named as ortho (o-)
If the two substituents in positions 1,3 they named as meta (m-)
If the two substituents in positions 1,4 they named as para (p-)

13. If the two substituents are different, they are listed in
alphabetical order. The group with lower alphabet take
position no. 1

14. Compounds with special common names
Polysubstituted benzene

15. Reactions of benzene It undergoes electrophilic substitution reactions rather than addition reactions in order to preserve the delocalized π system. A) Electrophilic substitution reactions:
These reactions are accelerated by lewis acids to generate the electrophile

16. 1) Halogenation

17. 2) Nitration

18. 3) Sulfonation
4) Friedel Crafts Alkylation

19. Limitation of Friedel Crafts Reaction
If the alkyl group is long (at least 3-carbons) it will carry out rearrangement. Primary carbocation (R-CH2)+ will convert into secondary carbocation (R2CH)+. Also, the secondary carbocation (R2CH)+ will convert into tertiary carbocation (R3C)+

20. strong Electron-withdrawing groups (EWG) because these groups
2) Friedel-Crafts reaction could not occurred when the benzene ring have
strong Electron-withdrawing groups (EWG) because these groups
decrease the electron density on the ring.
Q: How do you account for fact that:
benzene in the presence of AlCl3 react with isobutyl bromide to yield tert-butylbenzene ?

21. 3) Friedel-Crafts reaction could not occurred when the benzene ring have
NH2- NHR- or NR2 groups because the lone pair of electron on nitrogen
atom of these groups will react with Lewis acid (AlCl3) and the group
will become strong EWG so the reaction will not proceeded.

22. 5) Friedel Crafts Alkylation
Acylbenzenes have many uses in organic chemistry as precursors for the preparation of n-alkylbenzenes withought rearrangement

23. Strarting from benzene, show how can you prepare
i) iso-propyl benzene ii) n-propyl benzene

24. Effect of substituent on the reactivity of benzene ring
Electron donating group such as NH2, OH, OR and Cl
will increases the electron density on benzene ring so the rate of electrophilic substitution reaction will increase.
While, Electron withdrawing group such as C=O, NO2, SO3H and COOH will decreases the electron density on benzene ring so the rate of electrophilic substitution reaction will decrease.

25. Effect of substituent on the orientation in benzene ring
Electron-Donating Group (ortho and para directing group)
will increase the electron density on the benzene ring specially on
ortho and para positions.
Electron-Withdrawing Group (meta directing group) will decrease the electron density on the benzene ring specially on ortho and para positions.

27. Q:How can you prepare the following compounds from benzene
(assume that a pure para isomer can be separated from an ortho, para mixture)?
1) p-nitrotoluene
2) p-bromonitrobenzene
3) p-bromobenzoic acid
4) m-bromobenzenesulfonic acid
5) p-chlorophenol