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