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Elimination Reactions
C C
X Y
C C + X Y
Dehydrohalogenation (-HX) and Dehydration (-H2O) are the main types of elimination reactions.
Dehydrohalogenation (-HX)
strong
base
X = Cl, Br, I
+ " "C C
X
H XC C
H
E1 mechanism
1)
++ Br
_slow
+
2)..
:
+fast
O.. +O
C C
Br
C C
H
C C
HC C
H
H H
H
H
H
rate determining step
E2 mechanism
..:..
__
+
+ Br_
..:
concerted mechanism
H O
C C
Br
H
H O
H
C C
Orientation of elimination: Zaitsev’s Rule
• In reactions of removal of hydrogen halides from alkyl halides or the removal of water from alcohols, the hydrogen which is lost will come from the more highly-branched b-carbon.
A. N. Zaitsev -- 1875 C C C C
H
H
H H
X
H
H
HH
CH3
Less branchedMore branched
Product formed from previous slide
C
C CC
H
HH
H
HCH3
HH
Comparing Ordinary and Bulky Bases
CH3 C CH CH3
Br
NaOC2H5
C2H5OHheat
C CHCH3 CH3
CH3 C CH CH3
Br
KOC(CH3)3
(CH3)3COHheat
C CHCH3 CH2
Major
H
CH3 CH3
CH3
H
CH3
Major
H
1-butene: watch out for competing reactions!
H3C CH2 CH2 CH2 Br
KOCH3
Non-bulky
SN2
H3C CH2 CH2 CH2 O-CH3
H3C CH2 CH CH2
bulky baseKO-t-butyl
E2
Substitution or Elimination?
Factors to Consider:
1. How Basic is the Nucleophile?
2. Steric Hindrance at Reacting Carbon
3. Steric Hindrance at Nucleophile
Summary of Reactivity
Reactions of Benzene
KMnO4 oxidation no reaction
Br2/CCl4 addition no reaction
HI addition no reaction
H2/Ni reduction no reaction
Reagent Cyclohexene Benzene
Heats of hydrogenation and combustion are far lower than they should be.
cyclohexene + H2, Ni cyclohexane + 28.6 Kcal/mole
1,3-cyclohexadiene + 2 H2, Ni cyclohexane + 55.4 Kcal/mole
(predicted value = 2 X 28.6 = 57.2 Kcal/mole)
benzene + 3 H2, Ni, heat, pressure cyclohexane + 49.8 Kcal/mole
(predicted value = 3 X 28.6 = 85.8 Kcal/mole)
Heat of hydrogenation for benzene is 36 Kcal/mole lower than predicted!
Chapter 16
Unusual Stability
Facts about benzene:
a) Formula = C6H6
b) Isomer number:
one monosubstituted isomer C6H5Y known
three disubstituted isomers C6H4Y2 known
c) Benzene resists addition reaction, undergoes substitution reactions.
d) Heats of hydrogenation and combustion are far lower than they should be.
e) From X-ray, all of the C—C bonds in benzene are the same length and intermediate in length between single and double bonds.
Nomenclature for benzene:
monosubstituted benzenes:
Special names:
CH3 NH2 OH
CO2H SO3H
toluene aniline phenol
benzoic acid benzenesulfonic acid
Disubsituted benzenes:
ortho- meta- para-
1,2- 1,3- 1,4-
Electrophilic Aromatic Substitution (Aromatic compounds) Ar-H = aromatic compound
1. Nitration
Ar-H + HNO3, H2SO4 Ar-NO2 + H2O
2. Sulfonation
Ar-H + H2SO4, SO3 Ar-SO3H + H2O
3. Halogenation
Ar-H + X2, Fe Ar-X + HX
4. Friedel-Crafts alkylation
Ar-H + R-X, AlCl3 Ar-R + HX
+HO-NO2 + H2SO4 H2O-NO2 + HSO4
-
+ +H2O-NO2 H2O + NO2
H2SO4 + H2O HSO4- + H3O+
HNO3 + 2 H2SO4 H3O+ + 2 HSO4- + NO2
+
nitration
H
NO2
H
NO2
H
NO2
H
NO2
resonance
Mechanism for nitration:
1) HONO2 + 2 H2SO4 H3O+ + 2 HSO4- + NO2
+
2) + NO2+
H
NO23)
RDS
NO2 + H+
H
NO2
Mechanism for sulfonation:
1) 2 H2SO4 H3O+ + HSO4- + SO3
2) + SO3
RDS
H
SO3-
3)H
SO3-
SO3- + H+
4) SO3- SO3H+ H3O+ + H2O
Mechanism for halogenation:
1) Cl2 + AlCl3 Cl-Cl-AlCl3
2) + Cl-Cl-AlCl3RDS
H
Cl+ AlCl4
-
3)H
Cl+ AlCl4
- Cl + HCl + AlCl3
Mechanism for Friedel-Crafts alkylation:
1) R-X + FeX3 R + FeX4-
2) + RRDS
3)
H
R
H
R+ FeX4
- R + HX + FeX3
Substituent groups on a benzene ring affect electrophilic aromatic substitution reactions in two ways:
1) reactivity
activate (faster than benzene)
or deactivate (slower than benzene)
2) orientation
ortho- + para- direction
or meta- direction
Effect of Substituent
-CH3
activates the benzene ring towards EAS
directs substitution to the ortho- & para- positions
-NO2
deactivates the benzene ring towards EAS
directs substitution to the meta- position
Common substituent groups and their effect on EAS:
-NH2, -NHR, -NR2
-OH-OR-NHCOCH3
-C6H5
-R-H-X-CHO, -COR-SO3H-COOH, -COOR-CN-NR3
+
-NO2
incr
easi
ng r
eact
ivit
y
ortho/para directors
meta directors
