Alkenes

Intro to alkenes

• An alkene is a hydrocarbon that has a carbon-carbon double bond

• Alkene carbons are sp2 hybridized:

Ethene (aka ethylene)

http://wps.prenhall.com/wps/media/objects/340/348272/Instructor_Resources/Chapter_07/Text_Images/FG07_00-02UN.JPG

Common alkenes

Polyethylene Polystyrene Biological molecules

Degree of unsaturation

• Alkenes are unsaturated– Additional hydrogens could be added to them

(with a catalyst)

• Calculating elements of unsaturation:

# 𝑜𝑓 𝑢𝑛𝑠𝑎𝑡𝑢𝑟𝑎𝑡𝑖𝑜𝑛𝑠 = +,-+-./0 /1+

– C = # of carbons – N = # of nitrogens– H = # of hydrogens – X = # of halogens

Degree of unsaturation

• Q: Calculate the number of elements of unsaturation in C6H12

• Q: Give five examples of structures with this formula (C6H12)

• Q: How many degrees of unsaturation are in C6H3NClBr ?

Alkenes Rings Alkynes (next unit)

# unsaturations 1 1 2

Nomenclature

• Use the root name of the longest chain containing the double bond

• Change the “-ane” ending to “-ene”• Use numbers to specify the location of the

double bond (if required)

Nomenclature

• Alkenes as substituents:Methylene

Vinyl

Allyl

Phenyl

E/Z isomers

• How do we describe the difference between these two molecules?

• An alkene is Z if the highest priority groups are on the “Zame Zide”

(E)-pent-2-ene (Z)-pent-2-ene

E/Z isomers

• Name the following compound:

Stability of alkenes

• Zaitsev’s rule: More substituted double bonds are usually more stable.

Name Structure -ΔH° (kJ/mol)

Ethene 137

Cis-but-2-ene 120

Trans-but-2-ene 116

2,3-dimethylbut-2-ene 111

Higher EnergyLess stable

Lower EnergyMore stable

Cycloalkenes

• Cis-cycloalkenes can be of any ring size• Trans-cycloalkenes are only stable for 8-

membered rings and larger

Cis-cyclodecene Trans-cyclodecene

Cycloalkenes

• Bredt’s Rule: A bridged bicyclic compound cannot have a double bond at a bridgehead position unless one of the rings contains at least eight carbon atoms.

Doesn’t work Acceptable

Preparation of alkenes

• Elimination of alkyl halides:

• E2 mechanism (one-step reaction)

Preparation of alkenes

• To promote elimination over substitution:– Use a bulky alkyl halide (tertiary cannot undergo

SN2 substitution)– Use a bulky base (poor nucleophiles)

Preparation of alkenes

• The E2 reaction is stereospecific (different stereoisomers of the reactant give different stereoisomers of the product)

• Ex.