chapter 3 alkenes and alkynes
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Chemistry 20. Chapter 3 Alkenes and Alkynes. Hydrocarbons. Large family of organic compounds Composed of only carbon and hydrogen. Saturated hydrocarbons Alkanes. Unsaturated hydrocarbons Alkenes, Alkynes & Aromatics. C = C. C C. C - C. Alkenes and Alkynes. - PowerPoint PPT PresentationTRANSCRIPT
Chapter 3
Alkenes and Alkynes
Chemistry 20
Hydrocarbons
Large family of organic compounds
Composed of only carbon and hydrogen
Saturated hydrocarbons
Alkanes
Unsaturated hydrocarbons
Alkenes, Alkynes & Aromatics
C - C C = C C CC
CC
C
CC
H
H
H
H
H
H
A Kekulé structureshowing all atoms
A Kekulé structureas a line-angle formula
Saturated compounds (alkanes): Have the maximum number of hydrogenatoms attached to each carbon atom.
Unsaturated compounds: Have fewer hydrogen atoms attached tothe carbon chain than alkanes.
• Containing double bond are alkenes.
CnH2n
• Containing triple bonds are alkynes.
CnH2n-2
Alkenes and Alkynes
Thermal cracking
CH3-CH3
800-900°CCH2=CH2 + H2
H
C C
H
H H
121.7°H3C
C C
H
H H
124.7°
Ethylene Propene
120°
H
C C
H
H H
H-C C-H
Acetylene(an alkyne)
Ethylene(an alkene)
180°
Using the IUPAC alkane names:
Alkene names change the end to -ene.
Alkyne names change the end to -yne
Naming Alkenes & Alkynes
Give the locationfor double and
triple bond
STEP 3Give the location and name of eachsubstituent (alphabetical order) as aprefix to the name of the main chain.
STEP 2Number the carbon atoms starting
from the end nearest a double or triple bond.
CH2= CH ─ CH2─ CH3 1-Butene
CH3─ CH=CH─ CH3 2-Butene
CH3
|CH3─ CH=C─CH3 2-Methyl-2-butene
CH3─ CC ─ CH3 2-Butyne
Naming Alkenes & Alkynes
1 2 3 4
1 2 3 4
4 3 2 1
1 2 3 4
CH3─ CH2─ CC ─ CH3 2-Pentyne
CH3
CH3─ CH2─ C=CH ─ CH3 3-Methyl-2-pentene
CH2 – CH3
CH3─ CH2─ C=CH ─ CH3 3-Ethyl-2-pentene
Naming Alkenes & Alkynes
5 4 3 2 1
5 4 3 2 1
5 4 3 2 1
Conformation
Under normal condition, no rotation is possible about the carbon-carbon double and triple bond.
Flat or planar molecules
Cis & Trans Stereoisomers
C = CHH
CH3H3C
C = C
H
H CH3
H3C
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
C4H8 C4H8
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
1 2
3
4
5
1
2
3
4
5
6
3-Methylcyclopentene(not 5-methylcyclopentene)
4-Ethyl-1-methylcyclohexene(not 5-ethyl-2-methylcyclohexene)
Dienes, Trienes, and Polyenes
Alkenes that contain more than one double bond.
Alkane name: -ne diene, triene, …
CH2=CCH=CH2
CH3
CH2=CHCH2CH=CH21,4-Pentadiene 2-Methyl-1,3-butadiene
(Isoprene)1,3-Cyclopentadiene
12 4 1 2 3
1
2 3
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.
Exothermic reactions
–C = C – – C – C–
Products are more stable (have the lower energy).
• 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 H
│ │ Pt │ │
H–C=C–H + H2 H– C – C– H
│ │
H H
Ethene Ethane
Chemical properties
1. Hydrogenation (Reduction):
Pt
More reactive than Alkanes
Chemical properties
1. Hydrogenation (Reduction):
• A hydrogen halide (HCl, HBr, or HI) adds to alkene to
give haloalkane.
H H H H
│ │ │ │
H–C=C–H + HCl H– C – C– H
│ │
H Cl
Ethene Chloroethane
Chemical properties
2. Hydrohalogenation:More reactive than Alkanes
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.
CH3CH=CH2 HCl CH3CH-CH2
HClCH3CH-CH2
ClH
1-Chloropropane(not formed)
2-ChloropropanePropene
+
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).
CH3CH=CH2 H2OH2SO4
CH3CH-CH2
HOH
Propene 2-Propanol+
CH3C=CH2
CH3
H2OH2SO4 CH3C-CH2
CH3
HOH2-Methyl-2-propanol2-Methylpropene
+
• A halogen atom adds to each carbon atom of a double bond.
• Usually by using an inert solvent like CH2Cl2.
H H H H
│ │ │ │
CH3–C=C–CH3 + Cl2 CH3– C – C– CH3
│ │
Cl Cl
2-Butene 2,3-dichlorobutane
Chemical properties
4. Halogenation:
CH2Cl2
More reactives than Alkanes
Polymerization
Polymer: a long-chain molecule produced by bonding together many single parts called monomers.
nCH2=CH2 CH2CH2initiator
Ethylene Polyethylenen(polymerization)
The most important reactions of alkenes in industry.
CH2CH-CH2CH-CH2CH-CH2CHCH3 CH3 CH3 CH3
CH2CHCH3
The repeating unitPart of an extended polymer chain
n
monomer units shown in red
n
Polymerization
Propene
Monomer
Polymerization
Polypropene (Polypropylene)
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.
ClCl
nPolymerization
Vinyl chloride Poly(Vinyl chloride) PVC
Polymers in our life
• Low-density polyethylene (LDPE):
– 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.
Polyethylene
Inexpensive
3 V
5 PP
6 PS
Code Polymer Common Uses
1 PET poly(ethyleneterephthalate)
soft drink bottles, householdchemical bottles, films, textile fibers
2 HDPE high-densitypolyethylene
milk and water jugs, grocery bags, squeezable bottles
poly(vinylchloride), PVC
shampoo bottles, pipes, shower curtains, vinyl siding, wire insulation, floor tiles
4 LDPE low-densitypolyethylene
shrink wrap, trash and grocery bags, sandwich bags, squeeze bottles
polypropylene plastic lids, clothing fibers, bottle caps, toys, diaper linings
polystyrene styrofoam cups, egg cartons, disposable utensils, packaging materials, appliances
7 all other plastics various
Recycling