CHM030 3.1: Alkenes Structure and Reactivity

• sometimes called olefins, hydrocarbons that contain carbon-carbon double bond, C=C occur abundantly in nature
• Properties of Alkenes and Alkynes

  their physical properties are similar to those of alkanes with the same carbon skeletons
• Properties of Alkenes and Alkynes

  they are nonpolar compounds
• Properties of Alkenes and Alkynes

  the only attractive forces between their molecules are very

  weak London dispersion forces (Van der Waal’s forces)
• Properties of Alkenes and Alkynes

  Alkenes and alkynes that are liquid at room temperature have densities less than 1.0 g/mL (they float on water)
• Properties of Alkenes and Alkynes

  Soluble in nonpolar solvents (kerosene, hexane, CHCl3 , CCl4 )

  
  

  Insoluble in water
• Properties of Alkenes and Alkynes

  Low boiling point and melting point
• Degree of Unsaturation

  • ADD no. of HALOGENS to no. of HYDROGENS (X is equivalent to H)
  • SUBTRACT no. of NITROGENS to no. of HYDROGENS
  • OXYGENS are not counted (oxygen links to H)
  • Formula: [2(no. of carbons)+no. of halogens]-no. of nitrogens/2
• Electronic Structure of Alkenes

  carbon atoms in a double bond are sp2-hybridized

  • Three equivalent orbitals at 120 º separation in plane
  • Fourth orbital is atomic p orbital

  occupied π (pi) orbital prevents rotation about σ (sigma) bond

  • π (p orbitals are parallel)
  • broken π bond after rotation (p orbitals are perpendicular)
• Cis-Trans Isomers of Alkenes

  • The cis isomer has the two methyl groups on the same side of the double bond
  • The trans isomer has the methyl groups on opposite sides.
• Cis-Trans Isomers of Alkenes

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

  • If one of the double-bond carbons is attached to two identical groups cis-trans isomerism is not possible.
• Sequence Rules: The E,Z Designation

  Cahn-Ingold Prelog

  • 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

  Rule 1: rank according to atomic number

  Rule 2: if the first substituents are a tie, proceed to others and compare until something has a higher atomic number

  • do not combine, always compare

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

  Cis alkenes are less stable than trans alkenes due to steric (spatial) interference (steric strain) between the large substituents on the same side of the double bond

  • trans-isomers are more stable than cis-isomers
  • compare heat given off on hydrogenation: △H°
  • less stable isomer is higher in energy and gives off more heat
  • tetrasubstituted > trisubstituted > disubstituted > monosusbtituted
  • hyperconjugation stabilizes alkyl
• Organohalogens 


  Halogens (X: F, Cl, Br, I) replaces hydrogen
• Formula for saturated acyclic compound

  CnH2n+2
• Degree of Unsaturation 


  Number of multiple bonds or rings