الشريحة 1

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Transcript الشريحة 1

Organic A
Chapter 8
Alkenes (I)
By Prof. Dr.
Adel M. Awadallah
Islamic University of Gaza
Alkenes and Alkynes
Hydrocarbons (contain only carbon and hydrogen)
a) Saturated: (Contain only single bonds)
Alkanes (CnH2N + 2 )
Cycloalkanes (CnH2N )
b) Unsaturated: contain
Alkenes: double bonds (,,,CnH2N)
Alkynes: triple bonds ((CnH2N - 2)
Aromatic: benzene like compounds
Facts about double and triple
bonds
H
o
H
180 o
bond angle
109.5
120
bond length
154 pm
134 pm
121 pm
rotation
possible
restricted
restricted
geometry
tetrahedral
triagonal planer
linear
Hypridization
sp3
Bond Length in Benzene
sp2
sp
139 pm (plannar, sp2 hypridized)
A pi bond is one in which the electrons in the p orbitals are held
above and below the plane of the molecule.
The sigma bond is stronger than the pi bond.
A double bond is formed from a sigma bond and a pi bond, and so it
is stronger than a single bond.
Physical Properties
• Physical properties:
•
non-polar or weakly polar
•
no hydrogen bonding
•
relatively low mp/bp
•
water insoluble
~ alkanes
• Importance:
•
common group in biological molecules
•
starting material for synthesis of many plastics
The Chemistry of Vision
The more substituted alkene will form
• Saytzeff orientation:
•
In dehydrohalogenation the preferred product is the alkene that has
the greater number of alkyl groups attached to the doubly bonded
carbon atoms
•
(the more substituted alkene will form)
• Ease of formation of alkenes:
• R2C=CR2 > R2C=CHR > R2C=CH2, RCH=CHR > RCH=CH2 >
CH2=CH2
• Stability of alkenes:
• R2C=CR2 > R2C=CHR > R2C=CH2, RCH=CHR > RCH=CH2 >
CH2=CH2
•
•
•
•
•
CH3CH2CHCH3 +
Br
sec-butyl bromide
KOH(alc) 
CH3CH2CH=CH2
1-butene 19%
+
CH3CH=CHCH3
2-butene
81%
RCH=CH2
RCH=CHR
Mechanisms of Elimination
E2 with concentrated base 3>2>1
second order
rate = K[RX][B]
Mechanisms of Elimination
E1 with dilute or weak base
3>2
first order
rate = K[RX]
• Order of reactivity in E2: 3o > 2o > 1o
• CH3CH2-X

• CH3CHCH3 
•
3 adj. H’s
CH3CH=CH2
6 adj. H’s & more stable
alkene
X
•
CH3
• CH3CCH3 
•
CH2=CH2
X
CH3
CH=CCH3
9 adj. H’s & most stable
alkene
Evidence for the E2 mechanism
1) second order
2) No Rearrangement
3) Show a large hydrogen isotope effect
Primary hydrogen isotope effect:
A bond to hydrogen (protium) is broken faster than a bond
to deuterium (D) KH / KD = 5 - 8
This means that the breaking of hydrogen is in the rate
determining step
Evidence for the E2 mechanism
The Absence of Hydrogen Exchange
The carbanion mechanism
(E1cB elimination unimolecular of the conjugate base)
Run the reaction until about half the substrate had been
converted into alkene. Unconsumed 2-phenylethyl bromide
was recovered. It contained no deuterium. So, the reaction
was not acompanied by hydrogen exchange. This rules out
the carbanion mechanism
Evidence for the E2 mechanism
The Element Effect (is the breaking of the C-X
bond in the rate determining step????)
Strength of the bond
R-F > R-Cl > R-Br > RI
Reactivity toward SN2, SN1, E2 and E1
R-I > R-Br > R-Cl > R-F
So, R-X bond breaking is in the rate
determining step
E1 Mechanism
Elimination, unimolecular
a)
b)
c)
d)
e)
f)
g)
E1 •
RX: 3o > 2o > 1o •
rearragement possible  •
may yield mixtures
 •
Saytzeff orientation •
element effect •
no isotope effect •
rate = k [RW] •
The E1 reaction: Orientation
Elimination vs. substitution
Substitution is generally the main reaction, but,
E1 Elimination occurs more with 3 > 2 >1
CH3
CH3
H3C
EtOH / H2O
oC
CH3
CH3
H3C
80
Br
CH3
+
OH
CH2
H3C
19%
H
CH3
H3C
Br
80 oC
H
H
EtOH / H2O
CH3
H3C
OH
+
H3C
CH2
5%
2. dehydration of alcohols:
a)
b)
c)
d)
e)
f)
ROH: 3o > 2o > 1o
acid is a catalyst
rearrangements are possible 
mixtures are possible 
Saytzeff
mechanism is E1
80 oC
Mechanism of Dehydration (E1)
Dehydration (Rearrangement)
E1 Mechanism, Rearrangement
Synthesis of 1-butene from 1-butanol:
• CH3CH2CH2CH2-OH + HBr
•
SN2
•
•
•
•
 CH3CH2CH2CH2-Br
E2
 KOH(alc)
CH3CH2CH=CH2
only!
• To avoid the rearrangement in the dehydration of the alcohol the
alcohol is first converted into an alkyl halide.