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					                    Chapter 12: 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-C      C-H
                   H       H
                   Ethylene         Acetylen e
                  (an alken e)     (an alk yn e)
                        Chapter 12: Alkenes and Alkynes

Cis-trans isomerism: Double Bond = No Free rotation!!!

• 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.

   H           H                              H           CH3
         C=C                                       C=C
  H3 C     H                                H3 C      H
      cis-2-Buten e                             t rans-2-Bu ten e
    mp -139°C, bp 4°C                          mp -106°C, b p 1°C
                           Chapter 12: Alkenes and Alkynes

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
in which substituted groups are located and named.
                                                                    5               1
                                                                        4       2
   6       4       2              6       4         2
       5       3       1              5         3       1

                                              CH3                      CH2 CH3
CH3 CH2 CH2 CH2 CH=CH2        CH3 CH2 CHCH2 CH=CH2           CH3 CH2 CHC=CH2
                                                                          CH2 CH3
       1-Hexene                4-Meth yl-1-h exene           2,3-D ieth yl-1-pen tene
                                   Chapter 12: Alkenes and Alkynes

To name an alkyne:
Follow the same rules as for alkenes, but use the ending -yne to show
the presence of the triple bond.
                                    1                           CH3               3   4
                              2                                               2           5
   CH3 CHC CH             3                      CH3 CH2 C CCH2 CCH3                          6   7
       CH3                                                      CH3
           3-Methyl-1-b utyne                       6,6-D imeth yl-3-hep tyn e

 Common names are still used for some alkenes and alkynes,
 particularly those with low molecular weight.
                  CH2 =CH2        CH3 CH= CH2       CH3 C=CH2
  IUPAC name:      Ethene           Prop ene     2-Meth ylp ropen e
 Common name:     Ethylene         Prop ylen e      Is ob utylene
                HC CH     CH3 C CH          CH3 C CCH3
  IUPA C n ame: Eth yn e    Propyne           2-Butyne
 Common name: Acetylen e Methylacetylene D imeth ylacetylen e
                             Chapter 12: Alkenes and Alkynes

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.

                     4                                             1
                         3                                 5
                 1       2                                     3

     3-Meth ylcyclop entene                 4-Ethyl-1-meth ylcyclohexen e
   (not 5-methylcyclopen ten e)           (not 5-ethyl-2-methylcyclohexene)
                      Chapter 12: Alkenes and Alkynes

•Alkenes that contain more than one double bond are named as
alkadienes, alkatrienes, and so forth.
•Those that contain several double bonds are referred to more
generally as polyenes (Greek: poly, many).

 CH2 =CHCH2 CH=CH2         CH2 =CCH=CH2
    1,4-Pentadien e    2-Meth yl-1,3-b utadiene         1,3-Cyclopentad iene

 Alkenes and alkynes physical properties are very similar to the
 alkanes properties
                          Chapter 12: Alkenes and Alkynes

  Terpene: A compound whose carbon skeleton can be divided into
  five-carbon units identical with the carbon skeleton of isoprene.

          CH3              head        C            tail
                                    1 2    3   4
    CH2 =C-CH=CH2                  C-C-C-C
2-Methyl-1,3-b utadiene          Isoprene unit
     (Is oprene)

  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.
                           Chapter 12: Terpenes

                                                   formin g th is
head                                               bond makes
                                  OH               the ring

       Myrcene         Geraniol         Limonen e                 Menth ol
       (Bay oil)      (Ros e and         (Lemon                 (Peppermint)
                    other flow ers)    an d oran ge)


                Farnes ol                         Vitamin A (retinol)
          (Lily-of-th e valley)
                          Chapter 12: Alkenes and Alkynes

The most common reaction is addition to the double bond.

    Reaction                                       D escriptive N ame(s )
                                     H Cl
      C C      +   HCl               C C           hydrochlorin ation

                                     H OH
      C C      +   H2 O              C C           hydration

                                     Br Br
      C C      +   Br2               C C           bromination

                                     H H
      C C      +   H2                C C           hydrogenation
                                                   (red uction)
                         Chapter 12: Reactions

1.   Combustion (but we already spoke about it in chp.11)

2.   Addition to the double bond.

3.   Polymerization
                        Chapter 12: Alkenes and Alkynes

Most alkene addition reactions are exothermic.
• the products are more stable (lower in energy) than the
       H         H                                         H H
                               are replaced by
           C C       + H H                                HC C H +        heat
       H      H                                           H H
       one d ou ble bond                            three single bond s
      and on e single bond
• 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 (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    +   HCl           CH2 -CH2
              Ethylene                    Chloroethan e
                                         (Ethyl chloride)

• Reaction is regioselective. One direction of bond forming (or
  bond breaking) occurs in preference to all other directions.
• Markovnikov’s rule: H adds to the less substituted carbon and X
  to the more substituted carbon.
                                   Cl H               H Cl
   CH3 CH=CH2 + HCl             CH3 CH-CH2        CH3 CH-CH2
      Propene                 2-Chloropropane   1-Chlorop ropane
                                                  (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
Step 1
 • Reaction of the carbon-carbon double bond with H+ gives a
   secondary (2°) carbocation intermediate, a species containing a
   carbon atom with only three bonds to it and bearing a positive
     CH3 CH=CHCH3 + H+                      CH3 CH-CHCH3
                                    A 2° carbocation intermediate
Step 2
 • Reaction of the carbocation intermediate with chloride ion
   completes the addition.

                                               : Cl :
          -           +
       : :

      :Cl :   +   CH3 CHCH2 CH3              CH3 CHCH2 CH3
   Chloride       2° Carb ocation           2-Chlorobutan e
     ion           in termediate
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.

                                            OH H
                               H2 SO4
      CH3 CH=CH2    +   H2 O            CH3 CH-CH2
        Propene                         2-Propan ol

           CH3                               CH3
                                H2 SO4
       CH3 C=CH2    +   H2 O             CH3 C-CH2
                                           HO H
    2-Methylp ropen e                2-Methyl-2-prop anol

Step 1   CH3 CH=CH2 + H+                CH3 CHCH2
                                      A 2° carb ocation
                                        H       H

              +                            O+

Step 2   CH3 CHCH3    +   :O-H         CH3 CHCH3
                           H         An oxonium ion

          H       H

             O+                  ::OH
Step 3   CH3 CHCH3            CH3 CHCH3 + H+
                              2-Propan ol
                        +Cl2 / +Br2
Addition takes place readily at room temp.
 • Reaction is generally carried out using pure reagents, or mixing
   them in a nonreactive organic solvent.
                                                   Br Br
       CH3 CH=CHCH3     +   Br2                CH3 CH-CHCH3
                                   CH2 Cl2
          2-Butene                           2,3-D ibromobu tan e
                     + Br2
                                  CH2 Cl2
         Cycloh exene                1,2-Dib romocyclohexan e

• Addition of Br2 is a useful qualitative test for the presence of a
  carbon-carbon double bond.
• Br2 has a deep red color; dibromoalkanes are colorless.
Virtually all alkenes add H2 in the presence of a transition metal
catalyst, commonly Pd, Pt, or Ni.
     H3 C           H
            C   C       + H2                   CH3 CH2 CH2 CH3
                               25°C, 3 atm
         H      CH3                                Butane
      trans-2-Buten e

                        + H2
                               25°C, 3 atm
            Cycloh exene                     Cyclohexane
         < Wilhelm Normann invented what
         he called fat hardening. Yoday
         known as “Hydrogenation”

“The process of hydrogenation
adds hydrogen atoms to cis-
unsaturated fats, eliminating
double bonds and making them
into partially or completely
saturated fats. However, partial
hydrogenation converts a part of   Coronary heart disease
cis-isomers into trans-
unsaturated fats instead of
hydrogenating them completely “.
From the perspective of the organic chemical industry, the single
most important reaction of alkenes is polymerization:
 • polymer: Greek: poly, many, and meros, part; any long-chain
   molecule synthesized by bonding together many single parts,
   called monomers.
 • monomer: Greek: mono, single and meros, part.
            monomer un its show n in red

            CH3    CH3    CH3   CH3                 CH3
        CH2 CH-CH2 CH-CH2 CH-CH2 CH             CH2 CH n
       Part of an extended polymer chain   The repeating un it
         polypropene (polypropylene)
Low-density polyethylene (LDPE):
 • A highly branched polymer; polymer chains do not pack well and
   London dispersion forces between them are weak.
 • Softens and melts above 115°C.
 • Approximately 65% of all LDPE is used for the production of films
   for packaging and for trash bags.
High-density polyethylene (HDPE):
 • Only minimal chain branching; chains pack well and London
   dispersion forces between them are strong.
 • Has higher melting point than LDPE and is stronger
 • Can be blow molded to squeezable jugs and bottles.

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