Title: Chapter 23 Aryl Halides
1Chapter 23Aryl Halides
223.1Bonding in Aryl Halides
3Aryl 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.
4Table 23.1 CH and CCl Bond Dissociation
Energies of Selected Compounds
Bond EnergykJ/mol (kcal/mol)
X H
X Cl
CH3CH2X
sp3
410 (98)
339 (81)
sp2
452 (108)
368 (88)
sp2
469 (112)
406 (97)
5Aryl 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. - Because the carbon-halogen bond is stronger, aryl
halides react more slowly than alkyl halides when
carbon-halogen bond breaking is rate determining.
623.2Sources of Aryl Halides
7Preparation of Aryl Halides
- Halogenation of arenes (Section 12.5)
- The Sandmeyer reaction (Section 22.17)
- The Schiemann reaction (Section 22.17)
- Reaction of aryl diazonium salts with iodide ion
(Section 22.18)
823.3Physical Properties of Aryl Halides
9Physical Properties of Aryl Halides
- resemble alkyl halides
- all are essentially insoluble in water
- less polar than alkyl halides
? 1.7 D
? 2.2 D
1023.4Reactions of Aryl HalidesA Review and a
Preview
11Reactions of Aryl Halides
- Electrophilic Aromatic Substitution (Section
12.14) - Formation of aryl Grignard reagents (Section
14.4) - We have not yet seen any nucleophilic
substitution reactions of aryl halides.
Nucleophilic substitution on chlorobenzene occurs
so slowly that forcing conditions are required.
12Example
(97)
13Reasons for Low Reactivity
- SN1 not reasonable because
- 1) CCl bond is strong therefore, ionization
to a carbocation is a high-energy process - 2) aryl cations are less stable than alkyl
cations
14Reasons for Low Reactivity
- SN2 not reasonable because ring blocks attack of
nucleophile from side opposite bond to leaving
group
1523.5Nucleophilic Substitution inNitro-Substitute
d Aryl Halides
16But...
- nitro-substituted aryl halides do
undergonucleophilic aromatic substitution readily
CH3OH
NaOCH3
NaCl
85C
(92)
17Effect of nitro group is cumulative
- especially when nitro group is ortho and/orpara
to leaving group
1.0
too fast to measure
18Kinetics
- follows second-order rate law rate karyl
halidenucleophile - inference both the aryl halide and the
nucleophile are involved in rate-determining
step
19Effect of leaving group
- unusual order F gt Cl gt Br gt I
X
Relative Rate
F
312
Cl
1.0
Br
0.8
I
0.4
NaOCH3, CH3OH, 50C
20General Conclusions About Mechanism
- bimolecular rate-determining step in
whichnucleophile attacks aryl halide - rate-determining step precedes carbon-halogenbond
cleavage - rate-determining transition state is stabilized
byelectron-withdrawing groups (such as NO2)
2123.6The Addition-Elimination Mechanismof
Nucleophilic Aromatic Substitution
22Addition-Elimination Mechanism
- Two step mechanism
- Step 1) nucleophile attacks aryl halide and
bonds to the carbon that bears the
halogen (slow aromaticity of ring lost in
this step) - Step 2) intermediate formed in first step
loses halide (fast aromaticity of ring
restored in this step)
23Reaction
CH3OH
NaOCH3
NaF
85C
(93)
24Mechanism
Step 1
- bimolecular
- consistent with second-order kinetics first
order in aryl halide, first order in nucleophile
25Mechanism
Step 1
26Mechanism
- intermediate is negatively charged
- formed faster when ring bears electron-withdrawing
groups such as NO2
H
H
H
H
NO2
27Stabilization of Rate-Determining Intermediateby
Nitro Group
28Stabilization of Rate-Determining Intermediateby
Nitro Group
29Mechanism
Step 2
H
H
H
H
NO2
30Mechanism
Step 2
H
H
fast
H
H
31Leaving Group Effects
F gt Cl gt Br gt I is unusual, but consistentwith
mechanism
- carbon-halogen bond breaking does not occuruntil
after the rate-determining step - electronegative F stabilizes negatively charged
intermediate
3223.7Related Nucleophilic AromaticSubstitution
Reactions
33Example Hexafluorobenzene
(72)
- Six fluorine substituents stabilize negatively
charged intermediate formed in rate-determining
step and increase rate of nucleophilic aromatic
substitution.
34Example 2-Chloropyridine
NaOCH3
CH3OH
50C
- 2-Chloropyridine reacts 230,000,000 times faster
than chlorobenzene under these conditions.
35Example 2-Chloropyridine
- Nitrogen is more electronegative than carbon,
stabilizes the anionic intermediate, and
increases the rate at which it is formed.
36Example 2-Chloropyridine
N
Cl
- Nitrogen is more electronegative than carbon,
stabilizes the anionic intermediate, and
increases the rate at which it is formed.
3723.8The Elimination-Addition Mechanismof
Nucleophilic Aromatic SubstitutionBenzyne
38Aryl Halides Undergo Substitution WhenTreated
With Very Strong Bases
KNH2, NH3
33C
(52)
39Regiochemistry
- new substituent becomes attached to eitherthe
carbon that bore the leaving group orthe carbon
adjacent to it
NaNH2,NH3
33C
40Regiochemistry
- new substituent becomes attached to eitherthe
carbon that bore the leaving group orthe carbon
adjacent to it
41Regiochemistry
NaNH2, NH3
33C
42Same result using 14C label
(48)
(52)
43Mechanism
Step 1
44Mechanism
Step 1
- compound formed in this step is called benzyne
45Benzyne
- Benzyne has a strained triple bond.
- It cannot be isolated in this reaction, but is
formed as a reactive intermediate.
46Mechanism
Step 2
47Mechanism
Step 2
- Angle strain is relieved. The two sp-hybridized
ring carbons in benzyne become sp2 hybridized in
the resulting anion.
48Mechanism
Step 3
NH2
49Mechanism
Step 3
NH2
50Hydrolysis of Chlorobenzene
- 14C labeling indicates that the high-temperature
reaction of chlorobenzene with NaOH goes via
benzyne.
NaOH, H2O
395C
(54)
(43)
5123.9Diels-Alder Reactions of Benzyne
52Other Routes to Benzyne
- Benzyne can be prepared as a reactive
intermediate by methods other than treatment of
chlorobenzene with strong bases. - Another method involves loss of fluoride ion from
the Grignard reagent of 1-bromo-2-fluorobenzene.
53Other Routes to Benzyne
FMgBr
54Benzyne as a Dienophile
- Benzyne is a fairly reactive dienophile, and
gives Diels-Alder adducts when generated in the
presence of conjugated dienes.
55Benzyne as a Dienophile
Br
F
(46)
5623.10m-Benzyne and p-Benzyne
571,3- and 1,4-Dehydrobenzene
- Benzyne can exist as 1,3-Dehydrobenzene or
m-Benzyne and 1,4-Dehydrobenzene or p-Benzyne.
58End of Chapter 23