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
Lithium
diisopropylamide
Potassium tertbutoxide
Descriptions and explanations of the four
categories.
Nuc/Base Strengths
Strong/strong. In general, good bases are (i)
also good nucleophiles. Therefore, strong
bases such as negatively charged oxygens
and nitrogens will also be strong nucleophiles.
Weak/weak. In general, weak bases are (ii)
also weak nucleophiles. Therefore, weak
bases such as neutral oxygens with a proton
will also be weak nucleophiles.
Weak/weak nuc/bases are usually also the
solvent for their reactions. This makes sense
as they are so weak that you need a lot of
the nuc/base to facilitie the substitution or
elimination reaction.
Weak/strong
* One exception to strong bases also being
strong nucleophiles is for very bulky
nuc/bases.
SN2 reactions are particularly sensitive to the
size of the nuc/base because they proceed via
a crowded transition state.
** Elimination reactions are less sensitive to the
size of the nuc/base since the beta-hydrogen is
sticking out and is easy to access.
*** Therefore, a very bulky (large) nuc/base
can be a weak nucleophile while still being a
strong base.
Strong/weak.
These nuc/bases fall into two general categories
that will reduce their basicity:
(i) Neutral nuc/bases that have lone pairs on
less electronegative atoms such as nitrogen,
sulfur, and phosphorous. These include
amines, thiols and phosphines.
(ii) Negatively charged nuc/bases that are
stabilized by resonance or have a negative
charge on a large atom such as sulfur or
iodine.
Examples:
R-X = 1°
-OH = strong nuc
strong base
Solvent: PAS
R-X = 3°
CH3OH = weak nuc and weak base
Solvent: PPS
R-X = 2°
-CN = strong nuc AND weak base
Solvent: PAS
R-X = 2°
-OCH3 = strong nuc AND strong base
Solvent: PAS
References
1) http://www.chem.sc.edu/faculty/shimizu/333/Chem_333/6a.ii.html
2) http://www.freelance-teacher.com/organic_chemistry_sn2_sn1_e2_e1.pdf