Chapter 3Alkenes and Alkynes
Alkene: a hydrocarbon that contains one or more carbon-carbon double bonds.ethylene is the simplest alkene.
Alkyne: a hydrocarbon that contains one or more carbon-carbon triple bonds.acetylene is the simplest alkyne.
H
C C
H
H H
H-C C-H
Acetylene(an alkyne)
Ethylene(an alkene)
Structure:The VSEPR model predicts bond angles of
120° about each carbon of a double bond. In ethylene, the actual angles are close to
120°.The VSEPR model predicts bond angles of
180° about each carbon of a triple bond.
In substituted alkenes, angles about each carbon of the double bond may be greater than 120° because of repulsion between groups bonded to the double bond.
H
C C
H
H H
121.7°H3C
C C
H
H H
124.7°
Ethylene Propene
Cis-trans isomerismbecause of restricted rotation about a carbon-
carbon double bond, an alkene with two different groups on each carbon of the double bond shows cis-trans isomerism.
trans-2-Butenemp -106°C, bp 1°C
cis-2-Butenemp -139°C, bp 4°C
HC C
CH3
H3C H
HC C
H
H3C CH3
Nomenclature
To name an alkene; The parent name is that of the longest chain that
contains the C=C. Number the chain from the end that gives the lower
numbers to the carbons of the C=C. Locate the C=C by the number of its first carbon. Use the ending -ene to show the presence of the C=C Branched-chain alkenes are named in a manner
similar to alkanes; substituted groups are located and named.
Examples
CH3CH2CH2CH2CH=CH2 CH3CH2CHCH2CH=CH2
CH3
CH3CH2CHC=CH2CH2CH3
CH2CH3
1-Hexene 4-Methyl-1-hexene 2,3-Diethyl-1-pentene
1 1
1
2 22
3 3
3
4 44
5 5
5
6 6
• Alkynes follow the same rules as for alkenes, but use the ending -yne to show the presence of the triple bond.
CH3CHC CHCH3
CH3CH2C CCH2CCH3
CH3
CH3
3-Methyl-1-butyne 6,6-Dimethyl-3-heptyne
1 1
2 23
3 44 5
6 7
Common names are still used for some alkenes and alkynes, particularly those of low molecular weight.
CH3CH=CH2 CH3C=CH2
CH3
CH2=CH2
IsobutylenePropyleneCommon name:IUPAC name: 2-MethylpropenePropene
EthyleneEthene
CH3C CH CH3C CCH3HC CH
Common name:IUPAC name:
Methylacetylene DimethylacetylenePropyne 2-ButyneEthyne
Acetylene
To name a cycloalkene:number the carbon atoms of the ring double
bond 1 and 2 in the direction that gives the lower number to the substituent encountered first.
number and list substituents in alphabetical order.
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 are named as alkadienes, alkatrienes, and so on.
Those that contain several double bonds are referred to more generally as polyenes (Greek: poly, many).
CH2=CCH=CH2
CH3
CH2=CHCH2CH=CH21,4-Pentadiene 2-Methyl-1,3-butadiene
(Isoprene)1,3-Cyclopentadiene
Physical Properties
Alkenes and alkynes are nonpolar compounds.
The only attractive forces between their molecules are London dispersion forces.
Their physical properties are similar to those of alkanes with the same carbon skeletons.
Alkenes and alkynes are insoluble in water but soluble in one another and in nonpolar organic liquids.
Alkenes and alkynes that are liquid or solid at room temperature have densities less than 1.0 g/mL; they float on water.
Reactions of Alkenes
CC
C C
C C Br2
HCl
H2O
CC H2
C CBr Br
C CH Cl
C CH OH
C CHH
Descriptive Name(s )Reaction
+
+
+
+
bromination
hydrochlorination
hydration
hydrogenation(reduction)
Most alkene addition reactions are exothermic.The products are more stable (lower in
energy) than the reactants.
H
C C
H
HH
+ H C C H
H
H
H
H
H H
one double bondand one single bond
three single bonds
are replaced by+ heat
Just because they are exothermic doesn’t mean that alkene addition reactions occur rapidly.
reaction rate depends on the activation energy
Many alkene addition reactions require a catalyst.
Addition of HX
Addition of HX (HCl, HBr, or HI) to an alkene gives a haloalkane.H adds to one carbon of the C=C and X to the
other.
CH2=CH2 HCl CH2-CH2
ClH
Chloroethane(Ethyl chloride)
Ethylene
+
reaction is regioselective.Markovnikov’s rule: H adds to the less
substituted carbon and X to the more substituted carbon.
CH3CH=CH2 HCl CH3CH-CH2
HClCH3CH-CH2
ClH
1-Chloropropane(not formed)
2-ChloropropanePropene
+
Chemists account for the addition of HX to an alkene by a two-step reaction mechanism.We use curved arrows to show the
repositioning of electron pairs during a chemical reaction.
The tail of an arrow shows the origin of the electron pair (either on an atom or in the double bond).
The head of the arrow shows its new position.Curved arrows show us which bonds break
and which new ones form.
Common Mechanism Steps
Pattern 1: Add a proton
Pattern 2: Take a proton away
Pattern 3: Reaction of an electrophile and a nucleophile to form a new covalent bond
An electrophile is an electron-poor species that can accept a pair of electrons to form a new covalent bond.
A nucleophile is an electron-rich species that can donate a pair of electrons to form a new covalent bond.
Variation on a Pattern: Add a proton to a carbon-carbon double bond
Simplified version:
Addition of HCl to 2-Butene
Step 1: Add a protonreaction of the carbon-carbon double bond
with H+ gives a secondary (2°) carbocation intermediate.
CH3CH=CHCH3 H+ CH3CH-CHCH3
H+
A 2° carbocation intermediate
+
Step 2: Reaction of an electrophile and a nucleophile to form a new covalent bondreaction of the carbocation intermediate with
chloride ion completes the addition.
Cl CH3CHCH2CH3
Cl
CH3CHCH2CH3Chloride
ion2° Carbocationintermediate
- ++
2-Chlorobutane
::::
:
::
Addition of H2O
Addition of water is called hydrationhydration is acid catalyzed, most commonly
by H2SO4.
• hydration follows Markovnikov’s rule; H adds to the less substituted carbon and OH adds to the more substituted carbon.
CH3CH=CH2 H2OH2SO4
CH3CH-CH2
HOH
Propene 2-Propanol+
CH3C=CH2
CH3
H2OH2SO4 CH3C-CH2
CH3
HOH2-Methyl-2-propanol2-Methylpropene
+
Addition of H2O to Propene
Step 1: Add a proton
CH3CH=CH2 H+ CH3CHCH2
H+
A 2° carbocationintermediate
+
Step 2: Reaction of an electrophile and a nucleophile to form a new covalent bond
CH3CHCH3 O-HH
CH3CHCH3
OHH
+ ++
An oxonium ion
:
:
:
Step 3: Take a proton away
CH3CHCH3
OHH
CH3CHCH3
OHH++
+
: :
:
2-Propanol
Addition of Cl2 and Br2 Addition takes place readily at room temp.
reaction is generally carried out using pure reagents, or mixing them in a nonreactive organic solvent
CH3CH=CHCH3 Br2 CH2Cl2CH3CH-CHCH3
Br Br
2,3-Dibromobutane2-Butene
+
Br2CH2Cl2
Br
Br+
1,2-DibromocyclohexaneCyclohexene
Addition of H2--Reduction Virtually all alkenes add H2 in the presence
of a transition metal catalyst, commonly Pd, Pt, or Ni.
HH3C
C C
H CH3
PdCH3CH2CH2CH3
trans-2-Butene
+ H225°C, 3 atm
Butane
Pd+ H2
Cyclohexene Cyclohexane
25°C, 3 atm
The addition of hydrogen to an alkene involving a transition metal catalyst.
Polymerization
• polymer: Greek: poly, many and meros, part• monomer: Greek: mono, single and meros,
part
nCH2=CH2 CH2CH2initiator
Ethylene Polyethylenen(polymerization)
• Show the structure of a polymer by placing parentheses around the repeating monomer unit.
• Place a subscript, n, outside the parentheses to indicate that this unit repeats n times.
The structure of a polymer chain can be reproduced by repeating the enclosed structure in both directions.
CH2CH-CH2CH-CH2CH-CH2CHCH3 CH3 CH3 CH3
CH2CHCH3
The repeating unitPart of an extended polymer chain
n
monomer units shown in red
n
CH2=CH2
CH2=CHCH3
CH2=CHCl
CH2=CCl2
CH2=CHCN
CF2=CF2
CH2=CHC6H5CH2=CHCOOC2H5
CH3
CH2=CCOOCH3
poly(vinyl chloride), PVC;construction tubing
polyacrylonitrile, Orlon;acrylics and acrylatespolytetrafluoroethylene, PTFE;Teflon, nonstick coatings
polystyrene, Styrofoam; insulationpoly(ethyl acrylate); latex paints
poly(methyl methacrylate), Lucite,Plexiglas; glass substitutes
poly(1,1-dichloroethylene); Saran Wrap is a copolymerwith vinyl chloride
MonomerFormula
Common Name
Polymer Name(s) andCommon Uses
ethylene
propylene
vinyl chloride
1,1-dichloro-ethylene
acrylonitrile
tetrafluoro-ethylene
styreneethyl acrylate
methylmethacrylate
polyethylene, Polythene;break-resistant containers
polypropylene, Herculon;textile and carpet fibers
Summary of Topics: Chapter 3
Nomenclature Properties (mp/bp; solubility) Addition reactions (Markovnikov’s rule,
carbocation intermediates, mechanisms) Hydrohalogenation Hydration Halogenation Hydrogenation (reduction)
Polymerization reactions