Transcript Chapter 3 Alkenes and Alkynes

Chapter 3
Alkenes 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
H
C
C
H
H
Ethylene
(an alken e)
H-C
C-H
Acetylen e
(an alk yn e)

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
121.7°
H
124.7°
H3 C
H
C C
C C
H
H
Ethylene
H
H
Prop ene

Cis-trans isomerism
 because
of restricted rotation about a carboncarbon double bond, an alkene with two
different groups on each carbon of the double
bond shows cis-trans isomerism.
H
H
H
C C
H3 C
CH3
C C
CH3
cis -2-Butene
mp -139°C, b p 4°C
H3 C
H
t rans-2-Butene
mp -106°C, bp 1°C
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

5
6
5
6
2
4
3
1
5
4
2
4
3
1-Hexene
1
2
1
CH3
CH3 CH2 CH2 CH2 CH=CH2
3
CH3 CH2 CHCH2 CH=CH2
4-Meth yl-1-h exene
CH2 CH3
CH3 CH2 CHC=CH2
CH2 CH3
2,3-D ieth yl-1-pen tene
•
Alkynes follow the same rules as for alkenes,
but use the ending -yne to show the presence
of the triple bond.
1
CH3 CHC CH
CH3
2
4
3
3-Methyl-1-butyne
CH3
CH3 CH2 C CCH2 CCH3
CH3
1
2 3
6,6-Dimethyl-3-heptyne
4
5
6
7

Common names are still used for some
alkenes and alkynes, particularly those of
low molecular weight.
CH3
IUPAC name:
Common name:
CH2 =CH2
Ethene
Ethylene
CH3 CH= CH2
Prop ene
Prop ylen e
CH3 C=CH2
2-Meth ylp ropen e
Is ob utylene
HC CH
CH3 C CH
CH3 C CCH3
IUPAC n ame: Ethyn e
Propyne
2-Butyne
Common name: Acetylen e Methylacetylene Dimeth ylacetylen e

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.
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 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 =CHCH2 CH=CH2
1,4-Pentadien e
CH3
CH2 =CCH=CH2
2-Meth yl-1,3-b utadiene
(Isoprene)
1,3-Cyclopentad iene
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
D escriptive N ame(s )
Reaction
C C
C C
C C
C C
H Cl
C C
hydrochlorin ation
+ H2 O
H OH
C C
hydration
+ Br2
Br Br
C C
bromination
+ H
2
H H
C C
+
HCl
hydrogenation
(red uction)

Most alkene addition reactions are
exothermic.
 The
products are more stable (lower in
energy) than the reactants.
H
H
C C
+ H H
H
H
one double bond
and one single bond
H H
are replaced by
H C C H + heat
H H
three single bonds
 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.
H Cl
CH2 =CH2
Ethylene
+
HCl
CH2 -CH2
Chloroethane
(Ethyl chloride)
 reaction
is regioselective.
 Markovnikov’s rule: H adds to the less
substituted carbon and X to the more
substituted carbon.
CH3 CH=CH2 + HCl
Prop ene
Cl H
CH3 CH-CH2
2-Ch loroprop ane
H Cl
CH3 CH-CH2
1-Chlorop ropan e
(not formed)

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 proton
 reaction
of the carbon-carbon double bond
with H+ gives a secondary (2°) carbocation
intermediate.
CH3 CH=CHCH3 + H+
H
CH3 CH-CHCH3
A 2° carbocation intermediate
+

Step 2: Reaction of an electrophile and a
nucleophile to form a new covalent bond
 reaction
of the carbocation intermediate with
chloride ion completes the addition.
:
: :
:Cl :
Ch loride
ion
+
+
CH3 CHCH2 CH3
2° Carb ocation
in termediate
: Cl :
CH3 CHCH2 CH3
2-Chlorobutan e
Addition of H2O

Addition of water is called hydration
 hydration
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.
CH3 CH=CH2
Propene
CH3
CH3 C=CH2
+
H2 O
+
2-Methylpropene
H2 O
H2 SO4
OH H
CH3 CH-CH2
2-Propan ol
CH3
H2 SO4
CH3 C-CH2
HO H
2-Methyl-2-propanol
Addition of H2O to Propene

Step 1: Add a proton
CH3 CH=CH2 + H+
H
CH3 CHCH2
A 2° carb ocation
intermediate
+

Step 2: Reaction of an electrophile and a
nucleophile to form a new covalent bond
CH3 CHCH3
:
+
+
:O-H
H
:
H
H
O+
CH3 CHCH3
An oxonium ion

Step 3: Take a proton away
H
O+
CH3 CHCH3
:
:
H
:OH
+
CH3 CHCH3 + H
2-Propan ol
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
CH3 CH=CHCH3
+
Br2
2-Butene
+ Br2
Cycloh exene
CH2 Cl2
CH2 Cl2
Br Br
CH3 CH-CHCH3
2,3-D ibromobu tan e
Br
Br
1,2-Dib romocyclohexan e
Addition of H2--Reduction

Virtually all alkenes add H2 in the presence
of a transition metal catalyst, commonly
Pd, Pt, or Ni.
H3 C
H
C
+ H2
C
H
CH3
trans-2-Buten e
+ H2
Cycloh exene
Pd
25°C, 3 atm
CH3 CH2 CH2 CH3
Butane
Pd
25°C, 3 atm
Cyclohexan e

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
initiator
nCH2 =CH2
CH2 CH2 n
(polymerization)
Ethylene
Polyethylene
•
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.
monomer un its show n in red
n
CH 3
CH3
CH 3
CH3
CH2 CH-CH2 CH-CH2 CH-CH2 CH
CH3
CH 2 CH n
Part of an extended polymer chain
The repeating un it
Monomer
Formula
CH2 =CH2
CH2 =CHCH3
CH2 =CHCl
CH2 =CCl2
CH2 =CHCN
CF2 =CF2
Common
N ame
ethylene
Polymer N ame(s ) and
Common Uses
polyethylen e, Polyth ene;
break-resistan t containers
propylene
polypropylene, Hercu lon;
textile and carp et fib ers
vinyl chlorid e poly(vinyl ch loride), PV C;
cons truction tubing
1,1-dichloropoly(1,1-d ichloroethylene);
ethylene
Saran Wrap is a cop olymer
w ith vinyl chlorid e
acrylon itrile
polyacrylonitrile, Orlon;
acrylics and acrylates
tetrafluoropolytetrafluoroethylene, PTFE;
ethylene
Teflon , nonstick coatin gs
CH2 =CHC6 H5
CH2 =CHCOOC2 H5
styrene
ethyl acrylate
polystyrene, S tyrofoam; insulation
poly(eth yl acrylate); latex paints
CH2 =CCOOCH3
CH3
methyl
methacrylate
poly(methyl methacrylate), Lucite,
Plexiglas; glass s ubs titu tes
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