Portland State University - - Professor Carl C. Wamser

Chemistry 331 - Fall 1997

Chapter 3 Notes - Alkenes & Alkynes

Alkenes

• C=C double bond
• Cn H2n - general formula
• unsaturated
  not "saturated" with maximum hydrogens
  unsaturated fats have some double bonds (easier to digest)
• note that cycloalkanes also are Cn H2n

Alkene Nomenclature

• parent alkene is the longest continuous carbon chain that includes the double bond
• number from the end that gives the double bond the lower number
• use -ene suffix and only the first number of the double bond
• name substituents as usual

Cycloalkene Nomenclature

• double bond assumed at C1-C2
• number in the direction that gives substituents lower numbers

3-methylcyclohexene

5,5-dimethyl-1,3-cyclopentadiene

Alkene Structure

• C=C double bond is one sigma bond and one pi bond
• sp2 hybridization (trigonal planar)
• pi bond doesn't rotate
• unlike ethane, ethene has no other conformers

Cis-Trans Isomers

• another example of stereoisomers
• 2-butene has two isomers:

trans-2-butene

cis-2-butene

• usually trans is more stable than cis

Cis-Trans Isomers

• an alkene can have cis-trans isomers only when each C in the double bond is attached to 2 different groups

Stereochemistry Designation

cis or trans ?

• note that cis-trans works only for disubstituted alkenes
• E,Z designation is more general

E,Z Designation

• assign priorities to the two groups on each C of the double bond (high, low for each C)
• if the the two high priority groups are on the same side, it is Z
• if the the two high priority groups are on opposite sides, it is E

Sequence Rules

• assign priority based on the atoms directly attached to the carbons of the double bond
• higher priority goes to:
• higher atomic number
• break ties by considering the next bonded atoms

Practice Nomenclature

(E)-3,4-dimethyl-2-octene

(Z)

Alkynes

Structure

• carbon-carbon triple bond
• sp hybridization (linear)
• no cis-trans possibilities
• the two pi bonds are perpendicular
• high electron density
• (usually more reactive than alkenes)

Alkynes - Nomenclature

• -yne suffix (with number)
• rules similar to alkenes
• with both -enes and -ynes, suffix is -en-yne and numbering is from the end closer to a multiple bond
  
• (E)-4-hexen-1-yne