alkenes and alkynes

• Alkene


  A hydrocarbon that contains one or more carbon-carbon double bonds

  
• Ethylene


  The simplest alkene

  
• Alkyne


  A hydrocarbon that contains one or more carbon-carbon triple bonds

  
• Acetylene


  The simplest alkyne
• Alkenes


  • The VSEPR model predicts bond angles of 120° about each carbon of a double 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

  
• 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

  
• Naming alkenes


  1. The parent name is that of the longest chain that contains the C=C

  2. Number the chain from the end that gives the lower numbers to the carbons of the C=C

  3. Locate the C=C by the number of its first carbon

  4. Use the ending -ene to show the presence of the C=C

  5. Branched-chain alkenes are named in a manner similar to alkanes in which substituted groups are located and named

  
• Naming alkynes


  Follow the same rules as for alkenes, but use the ending -yne to show the presence of the triple bond

  
• Common names


  Still used for some alkenes and alkynes, particularly those with low molecular weight

  
• Naming cycloalkenes


  1. 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

  2. It is not necessary to explicitly number the position of the double bond in a cycloalkene as in linear alkenes

  3. Number and list substituents in alphabetical order

  
• Alkadienes, alkatrienes, polyenes


  Alkenes that contain more than one double bond

  
• Alkenes and alkynes


  • 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
  • Insoluble in water but soluble in one another and in nonpolar organic liquids
  • Liquid or solid at room temperature have densities less than 1.0 g/mL; they float on water
• Addition reactions of alkenes


  • Most are exothermic
  • The products are more stable (lower in energy) than the reactants
  • Reaction rate depends on the activation energy
  • Many require a catalyst
• Addition of HX (HCl, HBr, or HI) to an alkene


  1. H adds to one carbon of the C=C and X to the other

  2. Reaction is regioselective

  3. Markovnikov's rule: H adds to the less substituted carbon and X to the more substituted carbon
• Mechanism for addition of HX to an alkene


  Step 1: Add a proton

  Step 2: Reaction of an electrophile and a nucleophile to form a new covalent bond
• Proton transfer


  An acid is a proton donor and a base is a proton acceptor
• Electrophile


  An electron-poor species that can accept a pair of electrons to form a new covalent bond
• Nucleophile


  An electron-rich species that can donate a pair of electrons to form a new covalent bond
• Addition of HCl to 2-Butene


  Step 1: Add a proton to form a secondary carbocation intermediate

  Step 2: Reaction of the carbocation intermediate with chloride ion to complete the addition
• Addition of H2O (hydration)


  Step 1: Add a proton

  Step 2: Reaction of an electrophile and a nucleophile to form a new covalent bond

  Step 3: Take a proton away
• 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
• Addition of Cl2 and Br2


  Addition takes place readily at room temperature

  Reaction is generally carried out using pure reagents, or mixing them in a nonreactive organic solvent

  Addition of Br2 is a useful qualitative test for the presence of a carbon-carbon double bond
• Addition of H2 (reduction)


  Virtually all alkenes add H2 in the presence of a transition metal catalyst, commonly Pd, Pt, or Ni
• Polymerization


  The single most important reaction of alkenes from the perspective of the organic chemical industry

  Polymer: Any long-chain molecule synthesized by bonding together many single parts, called monomers

  Monomer: A single part that is bonded together to form a polymer
• Polymer structure


  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

  
• 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