Thursday, February 21, 2008

Alkenes & elimination it is important

Chapter 5 Notes: Alkene Structure, Elimination Reactions

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
  • alkynes - triple bonds
  • combinations - e.g., enynes
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-
  • 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
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 unambiguously only for disubstituted alkenes
  • cis and trans can be used to follow the structure of the parent chain in some cases
  • E,Z designation is more general and always works
E,Z Designation
  • assign priorities to the two groups on each C of the double bond
    ( one high priority, one low priority 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
Priority Rules
  • consider just each atom bonded to the C
  • higher atomic number is higher priority
  • for a tie, consider each atom bonded next in turn
  • for a double bond, consider it like two identical atoms
Practice Nomenclature
(E)-3,4-dimethyl-2-octene
(Z)
Relative Stabilities
  • greater substitution leads to greater stability
  • tetrasubstituted > trisubstituted > disubstituted > monosubstituted
  • trans typically more stable than cis
Elimination Reactions - Preparation of Alkenes
  • dehydrogenation (loss of H2 from an alkane)
  • dehydration (loss of H2O from an alcohol)
  • dehydrohalogenation (loss of HX from an alkyl halide)
Dehydration
  • regioselectivity - Zaitsev Rule (prefer more stable alkene)
  • stereoselectivity
  • mechanism - carbocation intermediate
  • reactivity: 3° > 2° > 1°
  • carbocation rearrangements - hydride shifts, alkyl shifts
Dehydrohalogenation
  • base-promoted elimination
  • E2 mechanism - concerted, bimolecular
  • stereoselectivity - anti elimination
  • E1 elimination for 3° alkyl halides in absence of base

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