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Chapter 12 Alkenes and Alkynes
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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.

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Bonding in Unsaturated Hydrocarbons

• The two bonds in a double bond are different
• one bond is a sigma (?) bond these are very
  strong typically are not broken in a reaction
• the other bond is a pi (p) bond these involve
  sideways overlap of p-orbitals and are weaker
  than ? bonds (not as much orbital overlap)
• Alkenes are flat and have a trigonal planar shape
  around each of the two Cs in a double bond

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Bonding in Unsaturated Hydrocarbons

• The three bonds in a triple bond are different
• A triple bond consists of one ? bond and two p
  bonds
• the two p bonds are orthogonal (perpendicular)
• Alkynes are linear around each of the two Cs in
  the triple bond
• Because alkenes and alkynes have p bonds, which
  are much weaker than ? bonds, they are far more
  chemically reactive than alkanes notice the
  number of reactions they undergo

Bonding picture similar to alkenes
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VSEPR

• VSEPR models predict bond angles of 120 about
  each carbon of a double bond. In ethylene, the
  actual angles are close to 120.
• 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.

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IUPAC Naming - Alkenes

• To name an alkene
• The parent name is that of the longest chain that
  contains the CC (sound familiar?).
• Number the chain from the end that gives the
  lower numbers to the carbons of the CC.
• Locate the CC by the number of its first carbon.
• Use the ending -ene to show the presence of the
  CC
• Branched-chain alkenes are named in a manner
  similar to alkanes in which substituted groups
  are located and named.

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Common Names
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Examples
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More than 1 double bond?
Same idea for alkynes
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Examples
1-butene
2,3-dimethyl-1-hexene
1,4-pentadiene
2,3,3-trifluoro-1-octene
Some texts put the 1 between hex and ene
hex-1-ene
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Cis/Trans

• Cis-trans isomerism
• Because 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.

What about three or four different groups? Its
EZ.
Actually, youll use EZ for simple alkenes as
well.
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EZ system (NIB)

• Because everything attached to the carbon-carbon
  double bond is different, there aren't any
  obvious things which you can think of as being
  "cis" or "trans" to each other. We need to
  assign a priority to each group attached to the
  double bond.
• Cahn-Ingold-Prelog - You look first at the atoms
  attached directly to the carbon atoms at each end
  of the double bond - thinking about the two ends
  separately. The atom which has the greater atomic
  number is given the higher priority.

E is for entgegen (opposite) and Z is for
zusammen (together)
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How its done
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Examples
(Z)-3-methyl-2-pentene (3-methyl-cis-2-pentene)
(E)-1-bromo-1-chloropropene
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Cycloalkenes

• 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.
• Note that it is not necessary to explicitly
  number the position of the double bond in a
  cycloalkene as in linear alkenes.
• Number and list substituents in alphabetical
  order.

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IUPAC Naming - Alkynes

• To name an alkyne
• Same as naming alkenes but use yne to denote the
  presence of a triple CC bond. Book does not go
  over many reactions or names we will.

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Examples
Try 11, 13, 15, 17, 19, and 21 from the
book Worksheet (Alkenes and Alkyne naming)
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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. Geometry
  around the double or triple bond is different
  however.
• 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.

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Addition Mechanisms
These are common reaction that alkenes undergo
however writing reaction in the fashion only
tells part of the story. A mechanism is a way of
describing what is observed experimentally
maps of reactions.
NIB section
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MECHANISM
STEP-BY-STEP ACCOUNT OF WHAT HAPPENS
X-
X
step 1
step 2
C
C
C
C
C
C

E
E

E
intermediates are formed during a reaction but
are not products
Intermediate
Notice the formation of a carbon without 4 bonds!
This is a carbocation.
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Hyperconjugation
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CARBOCATION STABILITY
HYPERCONJUGATION

• electrons in an adjacent
• C-H s bond help to stabilize
• the positive charge of the
• carbocation by proximity
• (overlap)

R
C
R
C
H
R
R
C
H
R
R
Most stable
Least stable
2
gt
gt



tertiary
secondary
primary
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Can you recognize the following carbocations? 1o,
2o, 3o
2 o
2 o
1 o
2 o
2 o
3 o
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Addition of Hydrogen Halides Markovnikovs rule
The rich get richer.
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Markovnikovs Rule

• In the ionic addition of an acid to the
  carbon-carbon double bond of an alkene, the
  hydrogen of the acid attaches itself to the
  carbon atom which already holds the greater
  number of hydrogens.
• Them that has, gets!
• The richer get richer! (V. W.
  Markovnikov -- 1838 - 1904)

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Example
H from H-X adds to this carbon
C
H
C
H
3
2
C
l
HCl
Anion adds to the more substituted carbon
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SOME ADDITION Reaction EXAMPLES
only major product is shown
C
H
C
H
3
3
C
l
HCl
C
H
C
H
3
2
C
l
HCl
C
H
C
H
C
H
C
H
3
2
HCl
C
l
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Mechanisms

• We will now spend a week or so going over
  reaction mechanisms and predicting products for a
  few addition reactions. You will need to take
  notes as there are no slides for these reactions.
  Well also discuss carbocation rearrangements
  and hydrogen shifts.

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Why are alkenes and alkynes important?

• Polymers technology
• Found in nature
• Pharmaceuticals

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Polymers

• Movie

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Poly(mer)
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Biopolymers
Cellulose, Starch, DNA, RNA, and Proteins
http//www.youtube.com/watch?vhLTFiekwFy8feature
related
Well talk more about these in the biochemistry
section of the class
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Terpenes

• Terpene A compound whose carbon skeleton can be
  divided into five-carbon units identical with the
  carbon skeleton of isoprene.
• Terpenes illustrate an important principle of the
  molecular logic of living systems.
• In building large molecules, small subunits are
  bonded together by a series of enzyme-catalyzed
  reactions and then chemically modified by
  additional enzyme-catalyzed reactions.

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Examples
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Sight driven by Cis/Trans Isomerism
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End of Chapter 12

• Please review the notes on mechanisms.
• Exam 3 will cover all of chapter 12 naming,
  mechanisms as well. A large part will be
  reactions and mechanisms.
• Understand the basics of polymers we will
  revisit this later in the year.