alkenes

• Alkenes


  Hydrocarbons that contain a carbon–carbon double bond, C = C

  
• Alkenes


  • Ethylene - plant hormone that induces ripening in fruits
  • Turpentine oil - made from the resin of certain pine trees, used as medicine
  • β-carotene - an orange pigment in some fruits and vegetables, precursor of vitamin A
• Alkenes and alkynes


  • Their physical properties are similar to those of alkanes with the same carbon skeletons
  • They are nonpolar compounds
  • The only attractive forces between their molecules are very weak London dispersion forces (Van der Waal's forces)
  • Alkenes and alkynes that are liquid at room temperature have densities less than 1.0 g/mL (they float on water)
  • Soluble in nonpolar solvents (kerosene, hexane, CHCl3, CCl4)
  • Insoluble in water
  • Low boiling point and melting point

  
• Degree of unsaturation


  • Relates molecular formula to possible structures
  • Number of multiple bonds or rings

  
• Formula for saturated acyclic compound is CnH2n+2

  
• Each ring or multiple bond replaces 2 H's
• Organohalogens


  • Halogen replaces hydrogen
  • C4H6Br2 and C4H8 have one degree of unsaturation
• Oxygen atoms - if connected by single bonds, these don't affect the total count of H's
• Compounds with the same degree of unsaturation can have many things in common and still be very different
• If C-N Bonds Are Present

  • Nitrogen has three bonds
  • So if it connects where H was, it adds a connection point
  • Subtract one H for equivalent degree of unsaturation in hydrocarbon

  
• Degree of Unsaturation


  1. Count pairs of H's below CnH2n+2

  2. Add number of halogens to number of H's (X equivalent to H)

  3. Don't count oxygens (oxygen links H)

  4. Subtract N's - they have two connections
• Electronic Structure of Alkenes

  • Carbon atoms in a double bond are sp2-hybridized
  • Three equivalent orbitals at 120º separation in plane
  • Fourth orbital is atomic p orbital
  • Occupied π orbital prevents rotation about σ-bond
  • Rotation prevented by π bond - high barrier, about 268 kJ/mole in ethylene

  
• Cis-Trans Isomers of Alkenes

  • The cis isomer has the two methyl groups on the same side of the double bond, and the trans isomer has the methyl groups on opposite sides
  • Because bond rotation can not occur, the two but-2-enes can not spontaneously interconvert and are different chemical compounds

  
• Cis-Trans Isomers of Alkenes

  • Cis alkenes are less stable than their trans isomers due to steric (spatial) interference (steric strain) between the large substituents on the same side of the double bond
• Cis–trans isomerism is not limited to disubstituted alkenes. It also occurs whenever each double-bond carbon is attached to two different groups.

  
• If one of the double-bond carbons is attached to two identical groups cis–trans isomerism is not possible.

  
• Cis-trans isomers are possible only when both carbons are bonded to two different groups

  
• Sequence Rules: The E,Z Designation

  • Assign a priority to the substituents on each carbon of the double bond
  • E (German: entgegen, opposite) - higher priority groups are on opposite sides of the double bond
  • Z (German: zusammen, together) - higher priority groups are on the same side of the double bond
• Sequence Rules: The E,Z Designation

  1. Rule 1: Considering the double-bond carbons separately, look at the atoms directly attached to each carbon and rank them according to atomic number. The atom with the higher atomic number has the higher ranking, and the atom with the lower atomic number (usually hydrogen) has the lower ranking.

  2. Rule 2: If a decision can not be reached by ranking the first atoms in the substituents, look at the second, third, or fourth atoms away from the double-bond carbons until the first difference is found.

  3. Rule 3: Multiple-bonded atoms are equivalent to the same number of single-bonded atoms.

  
• Cis alkenes are less stable than trans alkenes
• Tetrasubstituted alkenes are more stable than trisubstituted, which are more stable than disubstituted, which are more stable than monosubstituted
• Hyperconjugation stabilizes alkyl groups in alkenes
• Addition of H2 to Alkynes


  1. Alkynes are converted into alkanes by reduction with two (2) molar equivalents of H2 over a palladium catalyst

  2. The reaction proceeds through an alkene intermediate

  3. The reaction can be stopped at the alkene stage if the right catalyst is used

  4. The catalyst most often used for this purpose is the Lindlar catalyst, a specially prepared form of palladium metal

  5. Hydrogenation occurs with syn stereochemistry, so alkynes give cis alkenes when reduced

  
• Addition of HX to Alkynes

  1. Alkynes give electrophilic addition products on reaction with HCl, HBr, and HI

  2. The regioselectivity of addition to a monosubstituted alkyne usually follows Markovnikov's rule

  3. The H atom adds to the terminal carbon of the triple bond, and the X atom adds to the internal, more highly substituted carbon

  4. The reaction can usually be stopped after addition of 1 molar equivalent of HX to yield a vinylic halide

  5. An excess of HX leads to formation of a dihalide product

  
• Addition of X2 to Alkynes

  1. Bromine and chlorine add to alkynes to give dihalide addition products with anti stereochemistry

  2. Either 1 or 2 molar equivalents can be added

  
• Addition of H2O to Alkynes

  1. Addition of water takes place when an alkyne is treated with aqueous sulfuric acid in the presence of mercuric sulfate catalyst

  2. Markovnikov regioselectivity is found for the hydration reaction, with the H attaching to the less substituted carbon and the OH attaching to the more substituted carbon

  3. The enol rearranges to a more stable ketone isomer

  4. A mixture of both possible ketones results when an internal alkyne is hydrated

  5. Only a single product is formed when a terminal alkyne is hydrated

  
• Formation of Acetylide Anions


  1. When a terminal alkyne (R―C ≡ C―H) which are weakly acidic is treated with a strong base such as sodium amide, NaNH2, the terminal hydrogen is removed and an acetylide anion is formed

  2. The presence of an unshared electron pair on the negatively charged alkyne carbon makes acetylide anions both basic and nucleophilic

  3. Acetylide anions react with alkyl halides such as bromomethane to substitute for the halogen and yield a new alkyne product

  4. Terminal alkynes can be prepared by reaction of acetylene with sodium amide, NaNH2

  5. Internal alkynes can be prepared by reaction of a terminal alkyne with sodium amide, NaNH2

  6. The one limitation to the reaction of an acetylide anion with an alkyl halide is that only primary alkyl halides, RCH2X, can be used