Summarizes the nature and rections of Alkenes, Dienes and alkynes

1. Presented by:
JanineV. Samelo
BSChem2

2.  Alkenes are a family of hydrocarbons
(compounds containing carbon and hydrogen
only) containing a carbon-carbon double bond.
 General formula: CnH2n
 Example:
 Functional group = carbon-carbon double bond
 sp2 hybridization => flat, 120o bond angles
 Shape => trigonal planar
 σ bond & π bond => H2C=CH2
 C2H4 ethylene
ethene C2H4
propene C3H6
C C
H
H H
H

3.  Structural isomerism
 All the alkenes with 4 or more carbon atoms in them
show structural isomerism.This means that there
are two or more different structural formulae that
you can draw for each molecular formula.
 For example, with C4H8, it isn't too difficult to come
up with these three structural isomers:

4.  Geometric (cis-trans) isomerism
 The carbon-carbon double bond doesn't allow any
rotation about it.That means that it is possible to
have the CH3 groups on either end of the molecule
locked either on one side of the molecule or
opposite each other.
 These are called cis-but-2-ene (where the groups
are on the same side) or trans-but-2-ene (where
they are on opposite sides).
 Cis-but-2-ene is also known as (Z)-but-2-ene; trans-but-2-ene is also known as (E)-but-
2-ene.

5. Boiling Points
 The boiling point of each alkene is very similar to
that of the alkane with the same number of carbon
atoms. Ethene, propene and the various butenes
are gases at room temperature. All the rest that
you are likely to come across are liquids.
 It has a boiling point which is a small number of
degrees lower than the corresponding alkane.The
only attractions involved areVan derWaals
dispersion forces, and these depend on the shape
of the molecule and the number of electrons it
contains. Each alkene has 2 fewer electrons than
the alkane with the same number of carbons.

6. Solubility
 Alkenes are virtually insoluble in water, but
dissolve in organic solvents.
 non-polar or weakly polar
 no hydrogen bonding
 relatively low mp/bp ~ (similar to alkanes)
 water insoluble
Importance:
 common group in biological molecules
 starting material for synthesis of many plastics

7. 1.Parent chain = longest continuous carbon chain
that contains the C=C.
alkane => change –ane to –ene
• prefix a locant for the carbon-carbon double bond
using the principle of lower number.
2.Alphabetize, name the substituents, etc.
3.If a geometric isomer, use E/Z (or cis/trans) to
indicate which isomer it is.

8. Synthesis of Alkenes
Alkyl halides are dehydrohalogenated with
base to form alkenes. Alcohols are
dehydrated with heat and acid to form
alkenes.The product with the most
number of carbons attached to the carbon-
carbon double bond is formed in the higher
yield.

9.  Polymers of Alkenes
Ethylene is polymerized to polyethylene, which is used
for bags, films, and bottles.
 Propylene is polymerized to polypropylene, which is
used for plastics.
 Styrene is polymerized to polystyrene, which is used for
plastics, plastic cups, and foam insulation.
 Methyl α-methacrylate is polymerized to polymethyl α-
methacrylate, which is used for plexiglass and Lucite
paints.
 Acrylonitrile is polymerized to polyacrylonitrile, which is
used as Orlon or Acrylan fibers.
 Tetrafluoroethylene is polymerized to
polytetrafluoroethylene, which is used asTeflon.
 Vinyl chloride is polymerized to polyvinyl chloride,
which is used in plastics, films, and plumbing.
 Vinylidene chloride is polymerized to polyvinylidene
chloride, which is used in Saran.

10. addition reactions.
 Example
 The rather exposed electrons in the pi bond are
particularly open to attack by things which carry
some degree of positive charge.These are
called electrophiles

11. 3. dehalogenation of vicinal dihalides
| | | |
— C — C — + Zn  — C = C — + ZnX2
| |
X X
example:
CH3CH2CHCH2 + Zn  CH3CH2CH=CH2 +
ZnBr2 Br Br
 Not generally useful as vicinal dihalides are usually
made from alkenes. May be used to “protect” a
carbon-carbon double bond.

12. 1. dehydrohalogenation of alkyl halides
| | | |
— C — C — + KOH(alc.)  — C = C — + KX + H2O
| |
H X
a) RX: 3o > 2o > 1o
b) no rearragement
c) may yield mixtures
d) Saytzeff orientation
e) element effect
f) isotope effect
g) rate = k [RX] [KOH]
h) Mechanism = E2

13.  rate = k [RX] [KOH] => both RX &
KOH in RDS
 R-I > R-Br > R-Cl “element effect”
=> C—X broken in RDS
 R-H > R-D “isotope effect”
=> C—H broken in RDS
  Concerted reaction: both the C—X and C—H
bonds are broken in the rate determining step.

14.  Mechanism = elimination, bimolecular E2
 One step! “Concerted” reaction.
base:
C
W
C
H
C C + H:base + :W
RDS

15.  CH3CHCH3 + KOH(alc)  CH3CH=CH2
Br
isopropyl bromide propylene
 CH3CH2CH2CH2-Br + KOH(alc)  CH3CH2CH=CH2
n-butyl bromide 1-butene
 CH3CH2CHCH3 + KOH(alc)  CH3CH2CH=CH2
Br 1-butene 19%
sec-butyl bromide +
CH3CH=CHCH3
2-butene 81%

16. .
Hydrocarbon containing two
carbon-carbon double bonds
Alkadienes
Isolated dienes
1,4-pentadiene
1,5-Cyclo-
octadiene
Separated by one
or more sp3-C atom.
Conjugated dienes:
1,3-butadiene
1,3-cyclo-
hexadiene
Double bonds and
single bonds alternate
along the chain.
Cumulated dienes
Allene
The C atom is common
for two double bonds
C C C C C
C C C C
H2C C CH2
Hydrocarbon containing two
carbon-carbon double bonds

17. .
cis,cis –2,4-hexadiene
(2Z,4Z)-2,4-hexadiene
(2Z,4E)-2,4-hexadiene
cis,trans-
C C
C C
CH3
H3C
H
HH
H
C C
C C
CH3
H3C
H
H
H
H
•Suffix: ne diene
• Cis-trans isomers:

18. . 4 C atoms are sp2- hybridized.
C2-C3 σbond: sp2-sp2overlap
C
H
H
C
C
C
H
HH
H
1,3-Butadiene:
πbond：2p-2p overlap
C2-C3 partially overlap
by 2p-2p orbital
4 C atoms are coplanar
C
H
H
C
C
C
H
HH
H
4 πelectrons are delocalized
over 4 C atomsC
Delocalization of πelectrons
lowers the energy.

19. .
Two possible planar conformation of
1,3-butadiene:
s-Cis conformation s-Trans conformation

20.  Conjugated dienes have enhanced stability as compared
to molecules without conjugated double bonds due to
resonance. In general, this makes them slightly less
reactive than other types of alkenes in general and
dienes specifically. However, many reactions proceed
through high-energy cation or radical intermediates; in
these cases the resonance stabilization of the
intermediate allyl species makes conjugated dienes more
reactive than non-conjugated dienes or simple alkenes.
 Hydrobromination:
 Example:
 Butadiene + HBr--> 3-bromobutene (LowTemperature) +
1-bromobut-2-ene (HighTemperature) + 1-bromobutene
(Not Observed)

21.  Diels-Alder Reaction
 One of the most important of all diene reactions is
the Diels-Alder Reaction, in which a conjugated
diene reacts with an dienophile to form a
cyclohexene.
Requirements:The diene must be able to access the
s-cis conformation for the reaction to take place.
 Example:
H2C CH C
O
H H2C CH C
O
OCH2CH3 H2C CH C N
HC C COOCH3

22.  - A Dienophile must contain a double or triple
bond.Typically, an electron withdrawing
group is conjugated to the dienophile to make
it electron-poor (nitriles, ketones, and esters
are common electron withdrawing groups).
Because the reaction is highly stereospecific,
the configuration of the dieneophile will
determine the relative stereochemistry of the
cyclohexene product.
 The Diels-Alder reaction occurs most
effectively with an electron-poor dieneophile
and an electron-rich diene ('normal demand').
'Inverse demand' Diels-Alder reactions can
also be carried out, in which the dienophile is
electron-rich and the diene electron-poor.
A species which likes to
attack Dienes

23.  Alkynes contain carbon-carbon triple bonds.
 The carbon in an alkyne is sp, has a bond angle of
180o, and a linear shape. A carbon-carbon triple
bond contains one sigma bond and two pi
bonds.
 A terminal alkyne contains at least one hydrogen
attached to the carbon-carbon triple bond. An
alkyne that is not terminal contains two alkyl
groups attached to the carbon-carbon triple
bond.

24.  Synthesis of Alkynes
Vicinal dihalides are dehydrohalogenated twice
with base to form alkynes. Geminal dihalides are
dehydrohalogenated twice with base to form
alkynes.
 Reactions of Alkynes
Hydrogen halide adds across a triple bond, via
Markovnikov addition and with anti or syn
addition, to form dihaloalkanes. Halogen adds
across a triple bond to form a
tetrahaloalkane. Hydrogen adds across a triple
bond to make an alkane.

25.  Alkynes are compounds which have low
polarity, and have physical properties that are
essentially the same as those of the alkanes
and alkenes.
 They are insoluble in water.
 They are quite soluble in the usual organic
solvents of low polarity (e.g. ligroin, ether,
benzene, carbon tetrachloride, etc.).
 They are less dense than water.
 Their boiling points show the usual increase
with increasing carbon number.
 They are very nearly the same as the boiling
points of alkanes or alkenes with the same
carbon skeletons.

26.  Alkynes Preparation
 The carbon-carbon triple bond of the alkynes is
formed in the same way as a double bond of the
alkenes, by the elimination of atoms or groups
from two adjacent carbons.
W X W X
HC - CH ==> HC = CH ==> HCCH
X X
Alkane Alkene Alkyne
The groups that are eliminated and the reagents
used are essentially the same as in the preparations
of alkenes.

27. H3C C C CH3
xs H2
Pt
CH3CH2CH2CH3
H2/Pd/BaSO 4
Quinoline
(Lindlar's Catalyst)
H3C
C
H
C
H
CH3
Na NH 3 (liq)
H3C
C
H
C
H
CH3

28. H3C C C H
HX
H3C C C H
H
+
X-
H3C C C H
H
X
Markovnikov
addition
a vinyl halide
H3C C C H
H
X
HX
H3C C C H
H
HX
+ X
-
a heteroatom
stabilized carbocation
H3C C C H
H
HX
+
H3C C C H
H
X H
X
a geminal dihalide

29.  Keto-enol
tautomerization

30.  Formation ofVicinal tetrahalides
C C
an alkyne
X2
C C
X X
X X
a vicinal tetrahalide
carbon
tetrachloride

31. R C CH
Na
R C C Na
+
R C C Na
+ + R'CH 2 X R C C CH2R' + NaX
+ H2
Terminal
Alkyne
Methyl
or
Primary
Alkyl
Halide
 Alkynyl Anion Synthesis of Alkynes via
Bimolecular Nucleophilic Substitution

32.  Six-membered Ring formation
(4+2)
 Diene + Dienophile