Organic chemistry has two main divisions. One division deals with aliphatic (fatty) compounds, the first compounds you encountered in Organic Chemistry I. The second division includes the aromatic (fragrant) compounds, of which benzene is a typical example

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Aromatic Compound
I. Topics of to understand& handle the following pathways of
CONVERSION& Pre-steps Rx.
1. Alkylbenzene, Alkenylbenzene
-Stability;
**As a RULE;
"Alkenylbenzene that have their side-chain double bond conjugated with
the benzene ring are more stable than those that do not."
-Reactivity;

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*Explanation of the above Rx.
*Mechanism "TO UNDERSTAND AND AVOID MISUSING IN
CONVERSION STEPS"
1. In case of Peroxides
-"Free Radical mechanism"
-Depends mainly on the bond energy as which can reactive the others.

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2. In case of NO Peroxides
-"Ionic mechanism"
2. Oxidation of the side chain
-Using hot alkaline 𝐾𝑀𝑁𝑂4, alkyl, alkenyl, alkynyl and acyl grp.s all
oxidized to -COOH grp.
-For alkylbenzene, 3°alkyl grp. can resist oxidation. GIVE REASON!
---Answer; Need to benzylic hydrogen for alkyl grp. oxidation.

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*Note;
to oxidize the benzene ring itself;

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**Application;
Chemistry of Industrial Styrene
Styrene is one of the most important industrial chemicals—more than
11billion pounds (5billionKg=5millionton) is produced each year.
-How to prepare it ?!
II. Aralkylhalides; Side-Halogen Comp.
Hint; memorize the mentioned comp. in their common names, please!
1. examples;

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2. Synthesis;
- Free radical halogenations of alkylbenzene.
- Addition of HX to alkenylbenzene.
- Reaction of HX with aromatic alcohols.
3. Reactions
Remember the general points of 𝑆 𝑁1& 𝑆 𝑁2.

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-Nucleophilic substitution reactions either 𝑆 𝑁1 or 𝑆 𝑁2.
-1°& 2° benzylic halide typically react via an 𝑆 𝑛2.(NO Elimination)
-3° benzylic halide typically react via an 𝑆 𝑁1. GIVE REASON!
---Answer; due to the resonance stabilized carbocation& steric effect.
-Unlike allylic systems, NO benzylic rearrangement.
GIVE REASON!
---Answer; I would result in loss of aromaticity.
-We can shift from one mechanism to the other by changing reaction
conditions.
1. 𝑆 𝑁2 Rx Mechanism.
1°& 2° benzylic halides typically react via an 𝑆 𝑁2 pathway.
Allylic and benzylic halides exhibit enhanced 𝑆 𝑁2 reactivity thanks to
conjugation of the newly formed and breaking bonds with the adjacent π-
electron system.
2. 𝑆 𝑁1 Rx Mechanism.
3° benzylic halides typically react via an 𝑆 𝑁1 pathway,
A carbocation is formed as a high-energy intermediate, and this species
bonds immediately to nearby nucleophiles.
If the nucleophile is a neutral molecule, the initial product is an "onium"
cation.

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*Summary
-These halides give mainly 𝑆 𝑁2 Rx.
-These halides give either 𝑆 𝑁1 or 𝑆 𝑁2 Rx.
-These halides give mainly 𝑆 𝑁1 Rx.

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*SN reactions of Aralkyl halide
VIP examples;
-Benzaldehyde from Aralkylhalide via hydrolysis of benzylidene.
-Benzoic acid from Aralkylhalide via hydrolysis of benzotrihalides.

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III. Miscellaneous Reactions.
1. Oxidation of benzene ring
2. Reduction of Aromatic comp.
A. Hydrogenation of benzene.
-cyclohexane and cyclohexene cannot be isolated .

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B. Birch Reduction
to reduce benzene ring into 1,4-cyclohexadiene
-Rx.;
-Mechanism;

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IV. Aryl Halides
-Synthesis;
1. Direct Halogenation of arenes via Sigma complex mechanism.
2. Sandmeyer Reaction.
3. Balz-Schiemann Reaction.

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*Diazotisation
The nitrosation of primary aromatic amines with nitrous acid (generated
in situ from sodium nitrite and a strong acid, such as hydrochloric acid,
sulfuric acid, or HBF4) leads to diazonium salts, which can be isolated if
the counterion is non-nucleophilic.
Diazonium salts are important intermediates for the preparation of
halides (Sandmeyer Reaction, Schiemann Reaction), and azo compounds.
-Reactions of Aryl Halides
1. Electrophilic Aromatic Substitution
2. Formation of aryl Grignad reagent and organometalic compounds

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3. 𝑆 𝑛𝑢 𝐴𝑟
-Aromatic nucleophilic reaction do not occur except if there are electon
with drawing groups ortho or para to halogens likeNO2,..et.
GIVE REASON!
---Answer; Due to…
1. The benzene ring of an aryl halide prevents backside
attack in an 𝑆 𝑛2 reaction.

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2. Phenyl cations are very unstable; thus 𝑆 𝑛1 reactions do
not occur.
3. Due to stability of benzene ring and its electron cloud, the carbon-
halogen bonds of aryl and vinylic halides are shorter and stronger than
those of alkyl, allylic, and benzylic halides.
Stronger carbon-halogen bonds mean that bond breaking by either an
𝑆 𝑛1 or 𝑆 𝑛2 mechanism will require more energy.
4.More two points can affect the strength of carbon-hydrogen bond;
A. The carbon of either type of halide is 𝑆𝑝2
hybridized, and there for the
electrons of the carbon orbital are closer to the nucleus than those of an
𝑆𝑝3
-hybridized carbon.
B. Resonance can strengthen the carbon-halogen bond by giving it
double-bond character.
3.1. Activated 𝑆 𝑛𝑢 𝐴𝑟 "Addition-Elimination" mechanism

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Explanation;
Example;
Why EWG?!

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**Activator OR Deactivator**
-We should say activator (ordeactivator ) for either erlectophilic or
nucleophilc reactions.
-Activating and deactivating functions are applied to substituents effects.
"𝑁𝑂2"
-A strong deactivating substituent in electrophilic aromatic substitution.
-A strong activating substituent in nucleophilic aromatic substitution.
3.2. Factors affect 𝑆 𝑛𝑢 𝐴𝑟
1. # and the power of activating groups present at the o-and p-positions
of the leaving group.
-More EWG, less energy used.

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2.Leaving grp. type
-Due to the size of the atom itself which affect its ability to withdrawing
electron "electronegativity".
-You find that the opposite of that concept you have to apply in the simple
nucleophilic substitution.
3. EWG position effect
m-Fluoronitrobenzene reacts with sodium methoxide 105 times more
slowly (Consider no reaction) than do its ortho and para isomers.
Due to Non-Stabilized Meisinheimer intermediate (the –ve charge is
restricted to carbon in all resonance forms (Carbon has low EN).
The concept that Meisinheimer intermediate is a reactive intermediate so
the mentioned position of nito- and fluro- make the negative charge
restricted onto the stabilized carbon of the resonance system make the
system energy increase so the rate of the reaction decrease.

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𝐻3
𝑵𝑶 𝟐
*Remember;