Chem 334 - Fall 1999
Organic Chemistry I
Dr. Carl C. Wamser

Chapter 5 Notes: Alkene Structures

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

Unsaturated Compounds

• alkenes - polyenes
  dienes, trienes, etc.
• arenes
  aromatic rings (Chapter 19)
• alkynes
  triple bonds (Chapter 10)
• combinations
  e.g., enynes

Degrees of Unsaturation

• alkanes: Cn H2n+2
  each pi bond or ring requires removal of 2 H
  Cn H2n+2-2U
• U = degrees of unsaturation
  = total number of pi bonds + rings
  = 1/2 ( #H - 2 #C - 2 )
• examples: C8 H12
  solve for U = 3
  possible structures:

Bonding Trends

Bond Lengths in Angstroms (Bond Strengths in kcal/mole)

Compound	Hybrid	C-C bond	C-H bond
ethane	sp3	1.54 (88)	1.11 (98)
ethylene	sp2	1.34 (172)	1.10 (104)
acetylene	sp	1.21 (230)	1.08 (125)

• multiple bonds are stronger
• shorter bonds are stronger

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 -en- instead of -an- ( the infix )
• use 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

• use the same sequence rules as for designating R and S
• assign priorities to the two groups on each C of the double bond
  ( one high, one 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

Practice Nomenclature

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

(Z)

Terpenes

• natural products made from units of isoprene (5-carbons)
• see numerous examples in your text