Organic 2 Dr. Thoraya A.Farghaly

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.

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.

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

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

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

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

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, …..).

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

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

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-)

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

Compounds with special common names Polysubstituted benzene

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

1) Halogenation

2) Nitration

3) Sulfonation 4) Friedel Crafts Alkylation

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)+

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 ?

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.

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

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

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.

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.

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