Aromatic Comp. Lec.5

Dr.M_T The 3rd Vision Academy 01156281369

I

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 𝑆𝑁1& 𝑆𝑁2.


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-Nucleophilic substitution reactions either 𝑆𝑁1 or 𝑆𝑁2.

-1°& 2° benzylic halide typically react via an 𝑆𝑛2.(NO Elimination)

-3° benzylic halide typically react via an 𝑆𝑁1. 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. 𝑆𝑁2 Rx Mechanism.

1°& 2° benzylic halides typically react via an 𝑆𝑁2 pathway.

Allylic and benzylic halides exhibit enhanced 𝑆𝑁2 reactivity thanks to

conjugation of the newly formed and breaking bonds with the adjacent π-

electron system.

2. 𝑆𝑁1 Rx Mechanism.

3° benzylic halides typically react via an 𝑆𝑁1 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 𝑆𝑁2 Rx.

-These halides give either 𝑆𝑁1 or 𝑆𝑁2 Rx.

-These halides give mainly 𝑆𝑁1 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. 𝑆𝑛𝑢𝐴𝑟

-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 𝑆𝑛2 reaction.


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2. Phenyl cations are very unstable; thus 𝑆𝑛1 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

𝑆𝑛1 or 𝑆𝑛2 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 𝑆𝑛𝑢𝐴𝑟 "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 𝑆𝑛𝑢𝐴𝑟

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

𝑵𝑶𝟐

*Remember;

Science

Aromatic Comp. Lec.5