Chemistry 20
Chapter 3
Alkenes and Alkynes
Hydrocarbons
Large family of organic compounds
Composed of only carbon and hydrogen
Saturated hydrocarbons
Unsaturated hydrocarbons
Alkanes
H
Alkenes, Alkynes
& Aromatics
H
C-C
C=C
H
C
C
C
C
C
H
CC C
H
Alkenes and Alkynes
Saturated compounds (alkanes):
Have the maximum number of hydrogen
atoms attached to each carbon atom.
Unsaturated compounds:
Have fewer hydrogen atoms attached to
the carbon chain than alkanes.
• Containing double bond are alkenes.
CnH2n
• Containing triple bonds are alkynes.
CnH2n-2
Thermal cracking
CH3-CH3
800-900°C
CH2=CH2 + H2
120°
124.7°
121.7°
H
H H H3 C 180° H
H
C C
C C
C CC-H
H-C
H H H H
Ethylene
Ethylene
H
H
Prop
ene e
Acetylen
(an alken e)
(an alk yn e)
Naming Alkenes & Alkynes
Using the IUPAC alkane names:
Alkene names change the end to -ene.
Alkyne names change the end to -yne
STEP 2
Number the carbon atoms starting
from the end nearest a double or triple bond.
STEP 3
Give the location and name of each
substituent (alphabetical order) as a
prefix to the name of the main chain.
Give the location
for double and
triple bond
Naming Alkenes & Alkynes
CH2= CH ─ CH2─ CH3
1
3
2
4
CH3─ CH=CH─ CH3
1
2
3
2
3
2
3
2-Methyl-2-butene
1
CH3─ CC ─ CH3
1
2-Butene
4
CH3
|
CH3─ CH=C─CH3
4
1-Butene
4
2-Butyne
Naming Alkenes & Alkynes
CH3─ CH2─ CC ─ CH3
5
4
3
2
2-Pentyne
1
CH3
CH3─ CH2─ C=CH ─ CH3
5
4
3
2
3-Methyl-2-pentene
1
CH2 – CH3
CH3─ CH2─ C=CH ─ CH3
5
4
3
2
1
3-Ethyl-2-pentene
Conformation
Under normal condition, no rotation is possible about the
carbon-carbon double and triple bond.
Flat or planar molecules
Cis & Trans Stereoisomers
H
H
H
C=C
H3C
CH3
C=C
CH3
C 4H 8
H3C
H
C 4H 8
cis-2-Butene
trans-2-Butene
The same molecular formula and the same connectivity of their atoms
but a different arrangement of their atoms in space.
mp & bp of cis < mp & bp of trans
Naming of Cycloalkenes
Number the carbon atoms of the ring double bond 1 and 2
in the direction that gives the substituent lower number.
List substituents in alphabetical order.
No location for C = C
6
4
3
5
4
1
2
3-Meth ylcyclop entene
(not 5-methylcyclopen ten e)
1
5
2
3
4-Ethyl-1-meth ylcyclohexen e
(not 5-ethyl-2-methylcyclohexene)
Dienes, Trienes, and Polyenes
Alkenes that contain more than one double bond.
diene, triene, …
Alkane name: -ne
1
2
4
CH2 =CHCH2 CH=CH2
1,4-Pentadien e
1
2 3
CH3
CH 2 =CCH=CH2
2-Meth yl-1,3-b utadiene
(Isoprene)
1
2
3
1,3-Cyclopentad iene
Chemical properties of Alkenes & Alkynes
More reactive than Alkanes
Addition of Hydrogen (Hydrogenation-Reduction)
Addition of Hydrogen Halides (Hydrohalogenation)
Addition of water (Hydration)
Addition of Bromine & Chlorine (Halogenation)
Chemical properties of Alkenes & Alkynes
Addition reactions
Double bond is broken and two new single bonds are formed.
–C = C –
– C – C–
Exothermic reactions
Products are more stable (have the lower energy).
Chemical properties
More reactive than Alkanes
1. Hydrogenation (Reduction):
• A hydrogen atom adds to each carbon atom of a double
bond.
• A catalyst such as platinum or palladium is used (Transition metals).
H H
│ │
H–C=C–H + H2
Ethene
Pt
Pt
H
H
│
│
H– C – C– H
│
│
H
H
Ethane
Chemical properties
1. Hydrogenation (Reduction):
Chemical properties
More reactive than Alkanes
2. Hydrohalogenation:
• A hydrogen halide (HCl, HBr, or HI) adds to alkene to
give haloalkane.
H H
H
H
│ │
│
│
H–C=C–H + HCl
Ethene
H– C – C– H
│
│
H
Cl
Chloroethane
Chemical properties
2. Hydrohalogenation:
- reaction is regioselective.
- Markovnikov’s rule: H adds to double bonded carbon that
has the greater number of H and halogen adds to the other carbon.
CH3 CH=CH2 + HCl
Prop ene
Cl H
CH3 CH-CH2
H Cl
CH3 CH-CH2
2-Ch loroprop ane
1-Chlorop ropan e
(not formed)
The rich get richer!
Chemical properties
3. Hydration (addition of water):
• Water adds to C=C to give an alcohol.
• Acid catalyst (concentrated sulfuric acid).
• A regioselective reaction (Markovnikov’s rule).
CH3 CH=CH2
Propene
CH3
CH3 C=CH2
+
+
2-Methylp ropen e
H2 O
H2 O
H2 SO4
OH H
CH3 CH-CH2
2-Propan ol
CH3
H2 SO4
CH3 C-CH2
HO H
2-Methyl-2-prop anol
Chemical properties
More reactives than Alkanes
4. Halogenation:
• A halogen atom adds to each carbon atom of a double
bond.
• Usually by using an inert solvent like CH2Cl2.
H H
│ │
CH3–C=C–CH3 + Cl2
2-Butene
CH2Cl2
H
H
│
│
CH3– C – C– CH3
│
│
Cl
Cl
2,3-dichlorobutane
Polymerization
Polymer: a long-chain molecule produced by bonding together
many single parts called monomers.
The most important reactions of alkenes in industry.
nCH2 =CH2
initiator
CH2 CH2 n
(polymerization)
Ethylene
Polyethylene
Polymerization
Polypropene (Polypropylene)
monomer un its show n in red
Polymerization
n
Propene
CH3
CH3
CH3
CH3
CH2 CH-CH2 CH-CH2 CH-CH2 CH
CH3
CH2 CH
Part of an extended polymer chain
The repeating un it
Monomer
n
Naming of polymers
Prefix “ploy-” + name of the monomer
Propene
Polypropene
If the name of monomer consists of two words:
Its name is enclosed in parentheses.
Cl
Vinyl chloride
n
Polymerization
Cl
Poly(Vinyl chloride)
PVC
Polymers in our life
Polyethylene
• Low-density polyethylene (LDPE):
Inexpensive
– A highly branched polymer; polymer chains do not pack well and the
London dispersion forces between them are weak.
– Softens and melts above 115°C.
– Approximately 65% is used for the production of films (also used for
packaging and trash bags).
• High-density polyethylene (HDPE):
– Only minimal chain branching; chains pack well and the London
dispersion forces between them are strong.
– It has a higher melting point than LDPE and it is stronger.
– It can be blow molded to squeezable jugs and bottles.
Recycling
Cod e
1 PET
Polymer
Common Uses
poly(eth ylen e
terephthalate)
soft drin k bottles, hous ehold
ch emical b ottles , films, textile fib ers
2 HD PE
high-den sity
polyethylen e
milk and w ater jugs, grocery bags,
sq ueezable bottles
3 V
poly(vinyl
ch loride), PV C
low -density
polyethylen e
sh amp oo bottles, pip es, show er curtains,
vinyl siding, w ire in sulation, floor tiles
sh rin k w rap , tras h and grocery bags,
san dw ich b ags , squ eeze bottles
5 PP
polypropylene
6 PS
polystyrene
7
all other p lastics
plas tic lids, clothin g fibers, bottle cap s,
toys , diaper linings
styrofoam cu ps, egg carton s, disp os able
uten sils, packaging materials, app lian ces
various
4 LD PE