Aryl Halides
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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

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Aryl halides bonding physical properties and reactions

Aryl Halides - Bonding, Physical Properties and Reactions

Nucleophilic Aromatic Substitution of Chlorobenzene

Nucleophilic Aromatic Substitution: Addition-Elimination


Aryl halides bonding physical properties and reactions

What’s the Difference Between Ar- and Ph-?


Bonding in aryl halides

Bonding in Aryl Halides


Aryl halides bonding physical properties and reactions

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.


Aryl halides bonding physical properties and reactions

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)


Aryl halides bonding physical properties and reactions

Resonance Picture

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


Sources of aryl halides

Sources of Aryl Halides


Aryl halides bonding physical properties and reactions

Preparation of Aryl Halides

Halogenation of arenes (Section 12.5)

electrophilic aromatic substitution


Physical properties of aryl halides

Physical Properties of Aryl Halides


Aryl halides bonding physical properties and reactions

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


Aryl halides bonding physical properties and reactions

Reactions Involving Aryl Halides

Electrophilic aromatic substitution (Section 12.14)

halide substituents are ortho-para directing & deactivating


Aryl halides bonding physical properties and reactions

Substitution Reactions Involving Aryl Halides

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


Aryl halides bonding physical properties and reactions

Nucleophilic Substitution of Chlorobenzene

This reaction does not proceed via SN2……..


Aryl halides bonding physical properties and reactions

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

Why is Chlorobenzene Unreactive?


Aryl halides bonding physical properties and reactions

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


Aryl halides bonding physical properties and reactions

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.


Aryl halides bonding physical properties and reactions

What is the Mechanism of This Reaction?


Nucleophilic substitution in nitro substituted aryl halides

Nucleophilic Substitution inNitro-Substituted Aryl Halides


Aryl halides bonding physical properties and reactions

Nucleophilic Aromatic Substitution (SNAr)?


Aryl halides bonding physical properties and reactions

1.0

7 x 1010

2.4 x 1015

too fast to measure

The More Electron-Deficient the

Haloarene, the Faster the Substitution


Aryl halides bonding physical properties and reactions

Direct Displacement Doesn’t Occur!


Aryl halides bonding physical properties and reactions

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


Aryl halides bonding physical properties and reactions

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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