Transcript Ch. 12 Alkenes - Home | Tennessee Wesleyan College

Ch. 12 Alkenes
Homework- 12.15, 12.17, 12.19,
12.23, 12.25, 12.27, 12.36, 12.37,
12.41,12.42, 12.43, 12.46
Alkenes
• The organic chemical industry world wide
produces more pounds of ethylene than
any other chemical
• Annual production in the U.S. alone
exceeds 55 billion pounds
• Ethylene occurs only in trace amounts in
nature
• Most is made through thermal cracking of
ethane
Cracking
• In cracking, a saturated hydrocarbon in
converted to an unsaturated hydrocarbon
plus H2
CH3CH3
800-900 oC
(Thermal Cracking)
CH2=CH2
• Alkenes- first class of Unsaturated
hydrocarbons
• Alkenes are compounds that contain one
or more carbon-carbon double bonds
• The simplest alkene is ethylene
120o
H
C
H
H
C
2
sp
H
• Alkynes are unsaturated hydrocarbons
that contain one or more carbon-carbon
triple bonds
• The simplest Alkyne is acetylene
• This is all we will say about alkynes
because they are not widespread in nature
and are not important in biochemistry
Structure
• We have already said the Carbon’s are
sp2 hybridized, have 120 degree bond
angles, and are trigonal planar
• Ethylene is planar or flat
• Most importantly, there is no rotation about
the C-C double-bond of ethylene or any
double bond of an alkene
Cis/Trans Isomerism
• Because of the restricted rotation about
the double bond, an alkene in which each
carbon of the double bond has two
different groups bonded to it shows
cis/trans isomerism
• Cis/Trans Isomerisms-Isomers that have
the same order of attachment of their
atoms but a different arrangement of their
atoms in space due to the presence of
either a ring or a C-C double bond
Examples
H 3C
CH3
C
H 3C
C
C
H
H
Cis-2-butene
H
H
C
CH3
Trans-2-butene
IUPAC Nomenclature
•
•
Alkenes are named using the IUPAC
system
Rules:
1. Find the longest chain that includes both
carbons of the double or triple bond.
Indicate the length of the parent chain by
using a prefix that tell the number of carbon
atoms and the suffix, -ene (drop –ane)
2. Number the chain from the end that gives
the lower set of numbers to the carbons of
the double bond. Designate the position of
the double bond by the number of its 1st
carbon
IUPAC Nomenclature
3. Branched Alkenes are named in a
manner similar to alkanes; substituent
groups are located and named.
Examples:
IUPAC Nomenclature
• For Alkynes, Follow the same rules except
use the Suffix, -yne
• Example:
Skip Common Names Section 12.3B
Cis/Trans Configuration, revisited
• The orientation of the carbon atoms of the
parent chain determines whether an
alkene is cis or trans
• If the carbons of the parent chain are on
the SAME side of the double bond, the
Alkene is cis
• If they are on OPPOSITE sides, it is a
trans alkene
• The cis/trans notation
goes in front of the
name
• Examples:
Naming Cycloalkenes
• In naming cycloalkenes, we number the
carbon atoms of the ring double bond 1
and 2 in the direction that gives the
substituent encountered first the lower
number
• Examples:
Dienes, Trienes, and Polyenes
• Name alkenes that contain more than one
double bonds as alkadienes, alkatrienes,
and so on
• We refer to those that contain several
double bonds more generally as polyenes
• Examples:
• Earlier, we said that compounds with 1 CC double bonds that can have cis/trans
isomers, has two, one cis, one trans
• For an Alkene with n C-C double bonds
that can have cis/trans isomers, 2n
isomers are possible
• Example: 2,4-heptadiene
Physical Properties
• Alkenes and Alkynes are nonpolar
compounds and the only intermolecular
forces between them are London forces
• So they have very similar physical
properties to Alkanes
• Example: liquid at RT, float on H2O,
insoluble in H2O
Addition Reactions of Alkenes
• The most characteristic reaction of
Alkenes is addition to the C-C double bond
• The double bond is broken and in its place
single bonds form to two new atoms or
groups of atoms
• Almost all addition reactions result in
products that are more stable (have lower
energy) than the reactants
Types of Addition Reactions
Hydrochlorination
Bromination
H
C C
+ H-Cl
C C
C C
+ Br2
Hydrogenation
Hydration
H
C C
Br
Cl
+ H2O
C C
C C
C C
H
OH
+ H2
Br
H
C C
Addition of Hydrogen Halides
• Hydrogen Halides = H-F,H-Cl, H-Br, H-I
• Example
H
H2C
CH2
+ H-Cl
Cl
H2C
CH2
• When 1-propene reacts, we get only one
of the two possible products
H
Cl
+ H-Cl
H
+
Cl
Not Seen
Experimentally
Regioselectivity
• Because we only see one of the possible
products, we say the reaction is
Regioselective
• Regioselective-A reaction in which one
direction of bond forming or bond breaking
occurs in preference to all others.
• This regioselectivity was noted by
Markovnikov who made the following
generalization to help predict the product:
Markovnikov’s Rule
• “In the addition of HX or H2O to an alkene,
the hydrogen will add to the carbon of the
double bond which already has the most
hydrogens.”
• The Halogen goes to the other carbon
1 Hydrogen
H
Cl
+ H-Cl
2 Hydrogens
H
+
Cl
Not Seen
Experimentally
• In these reaction of a hydrogen halide with a
double bond, we see that one of the bonds in
the C-C double bond is broken, the H-X bond
is broken, and new bonds are made between
one of the carbons and the X and between
the other carbon and the Hydrogen
• It is useful for chemist to know how all this
happens.
– Does it happen all at once?
– Is there a certain sequence that occurs?
• How reactions occur is explained by
Reaction Mechanisms.
Reaction Mechanism
• Reaction Mechanism- A step-by-step
description of how a chemical reaction
occurs.
• In a mechanism, we use curved arrows to
show the movement of electrons
• The arrow starts where the electrons are
and points to where they are going
Reaction Mechanism
• Consider the 2-step mechanism for the
addition of H-Cl to 2-butene
Carbocation
• Carbocation- a species containing a
carbon atom with only 3 bonds to it and
bearing a positive charge
• Carbocations are classified as 1o, 2o, and
3o just as we have done previously.
Addition of Water:
Acid-Catalyzed Hydration
• In the presence of an acid catalyst, most
commonly H2SO4, water adds to a C-C
double bond to give an alcohol.
• This is called hydration
• Hydration reactions also follow
Markovnikov’s Rule
Hydration Examples
OH
CH3 CH
CH2
+ H2O
+ H2O
H2SO4
CH3 CH
H2SO4
OH
Hydration reactions have a 3 step mechanism
H
CH2
Addition of Br2 and Cl2
• Adds on Br/Cl to each Carbon of the
double bond.
• Examples:
Br
CH3 CH
CHCH3 + Br2
CH3 CH
Br
CHCH3
Cl
+ Cl2
Cl
Not responsible for mechanism!!
Addition of Hydrogen:
Reduction
• Adds a Hydrogen to each Carbon of the
double bond
• Uses a Metal Catalyst such as Pt, Pd, or
Ni
H
CH3 CH
CHCH3 + H2
H
CH3 CH
CHCH3
or
CH3 CH2 CH2CH3
+ H2
Polymers
• Polymers-Any long chain molecule
synthesized by the bonding together of
many single parts called Monomers
• Examples-
• Know Table 12.2 page 374 (all 9)
– Molecule formula, common name, polymer
name, and use