Alkene Structure and Preparations

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 (145)	1.10 (104)
acetylene	sp	1.21 (196)	1.06 (128)

• 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