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
- common names for groups: vinyl, allyl
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
(Ea ~ 60 kcal/mol)
- unlike ethane, ethene has no other conformers
Alkene Polarity
- C=C double bond slightly polar
- sp2 carbon more electronegative than sp3 carbon
- propene
0.3 D
- chloroethene 1.4 D
- trans-1-chloropropene 1.7 D
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
- alcohol + strong acid --> alkene
- regioselectivity - Zaitsev Rule (prefer
more stable alkene)
- reactivity: 3° > 2° > 1°
(follows stability of carbocations formed)
- mechanism - E1 with carbocation intermediate
- carbocation rearrangements - hydride shifts, alkyl shifts
- 2° carbocations often rearrange to 3°
- e.g., 2,2-dimethylcyclohexanol --> 1,2-dimethylcyclohexene
- 1° alcohols rearrange without forming a 1° carbocation
- e.g., 1-butanol goes to 2-butyl cation
- ethanol dehydrates via E2 mechanism (no 1° carbocation)
Dehydrohalogenation
- base-promoted elimination
from alkyl halide
- regioselectivity - Zaitsev Rule (prefer more stable alkene)
- E2 mechanism - concerted, bimolecular
- stereoselectivity - anti elimination
- H and X removed MUST be anti to one another
- in cyclohexanes, both H and X MUST be axial
- anti elimination favors pi bond formation in transition state
- e.g., compare cis & trans 1-bromo-2-methylcyclohexane
- cis - forms 1-methylcyclohexene (Zaitsev) major product
- trans - forms only 3-methylcyclohexene