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Aryl Halides - Bonding, Physical Properties and Reactions Nucleophilic Aromatic Substitution of Chlorobenzene Nucleophilic Aromatic Substitution: Addition-Elimination. What’s the Difference Between Ar- and Ph-?. Bonding in Aryl Halides. Aryl Halides.

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Aryl Halides - Bonding, Physical Properties and Reactions

Nucleophilic Aromatic Substitution of Chlorobenzene

Nucleophilic Aromatic Substitution: Addition-Elimination




Aryl Halides

Aryl halides are halides in which the halogen is attached directly to an aromatic ring.

Carbon-halogen bonds in aryl halides are shorter and stronger than carbon-halogen bonds in alkyl halides.


H2C

CHX

sp2

452 (108)

368 (88)

sp2

469 (112)

406 (97)

X

Dissociation Energies of Selected Compounds

Bond Energy:kJ/mol (kcal/mol)

X = H

X = Cl

sp3

410 (98)

339 (81)


Resonance Picture

C-X bonds in aryl halides have more double bond character than C-X bonds in alkyl halides



Preparation of Aryl Halides

Halogenation of arenes (Section 12.5)

electrophilic aromatic substitution



resemble alkyl halides

are essentially insoluble in water

less polar than alkyl halides

Cl

Cl

 2.2 D

 1.7 D

Physical Properties of Aryl Halides


Reactions of aryl halides a review and a preview
Reactions of Aryl Halides:A Review and a Preview


Reactions Involving Aryl Halides

Electrophilic aromatic substitution (Section 12.14)

halide substituents are ortho-para directing & deactivating


Substitution Reactions Involving Aryl Halides

Nucleophilic substitution on chlorobenzene occurs so slowly that forcing conditions are required.


Nucleophilic Substitution of Chlorobenzene

This reaction does not proceed via SN2……..


the SN2 is not reasonable because the aromatic ring blocks back-side approach of the nucleophile. Inversion is not possible.

Why is Chlorobenzene Unreactive?


SN1 Also Unlikely:

Aryl Cations are Highly Unstable

SN1 not reasonable because:

C—Cl bond is strong; therefore, ionization to a carbocation is a high-energy process

aryl cations are highly unstable


SN1 Reaction is Possible with Very

Powerful Leaving Groups such as Dinitrogen

This is a unique case: halides are not good enough leaving groups for this process to occur.



Nucleophilic substitution in nitro substituted aryl halides
Nucleophilic Substitution inNitro-Substituted Aryl Halides



1.0

7 x 1010

2.4 x 1015

too fast to measure

The More Electron-Deficient the

Haloarene, the Faster the Substitution



Kinetics of Nucleophilic Aromatic Substitution

follows second-order rate law:

rate = k [aryl halide] [nucleophile]

inference:

both the aryl halide and the nucleophile are involved in rate-determining step


Reaction Rate Depends on X: I > Br > Cl > F

Effect of Leaving Group Upon Rate of SN2

During SN2 reactions, the C-X bond breaks during the rate-determining step

C-F (485 kJ/mol), C-Cl (327 kJ/mol)

C-Br (285 kJ/ml, C-I (213 kJ/mol)


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