Alkenes

Alkenes/Alkynes

• Compounds that contain carbon and hydrogen

• Alkenes have a double bond• General formula of CnH2n

• Alkynes have a triple bond• General formula of CnH2n-2

• Properties are similar to alkanes– Nonpolar, Flammable, Low density

• Both are unsaturated

Multiple Double Bonds

• Some compounds have multiple double bonds

• Alternating double bonds – conjugated

C=C-C=C

• Continuous double bonds – cumulated

C=C=C

• Random double bonds – isolated

C=C-C-C=C

Nomenclature

• Similar to alkane rules

• Alkenes end in –ene, Alkynes in –yne– 2 double bonds –diene, 3 –triene

• Longest chain must include the multiple bond and numbered so it gets the lowest number

• If the multiple bond is equidistant from each end start closest to branch point

Cont.

• Position of multiple bond must be numbered in the name (use lower number)

• If there is a double and triple bond, double bond get priority

• Cyclic alkenes named in similar way.

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Arrangement of Alkenes

• Carbons containing the double bond are sp2 hybridized

• All bonds are planar

• Contain a sigma and pi bond

• No free rotation about that bond– Cannot be rotamers

Hybridization in Ethene

Geometric Isomerism in Alkenes

• Since double bonds cannot rotate cis- and trans- isomers are possible

• Cis- groups are on the same side of the double bond

• Trans- are on opposite sides

• Different compounds with different boiling points and reactivities

• Can the following have cis/trans isomerism? If so draw and name.

• Can the following have cis/trans isomerism? If so draw and name.

• Can the following have cis/trans isomerism? If so draw and name.

• Can the following have cis/trans isomerism? If so draw and name.

Homework

• P.112 34,35a-d,38a-c

Reactions of Alkenes

• Alkenes react by electrophilic addition

– Double bond is destroyed

– Both parts of reactant are added

• Alkane is formed

– Electrophile - Electron lover / electron poor–H+ is an excellent Electrophile / Lewis Acid

– Nucleophile – Nucleus lover / electron rich–Bonds are electron rich / Lewis Base

Addition of HCl to Ethene

C=C + HCl ?

Electrophilic Addition Mechanism

• Electrons move from electron rich to electronpoor. Nucleophile to electrophile

• Double bond is nucleophile / H+ is electophile

• A carbocation is formed

– An electron deficient carbon with three bonds

• Very reactive

• Electophile

• Draw the mechanism for the reaction

C=C + HCl

• Predict the product and draw the mechanism for the reaction

C=C + H2O H+

• Predict the product for the reaction

C-C-C=C-C + HBr

Markivnikovs Rule

• "when an unsymmetrical alkene undergoes addition with E-Nu, then the electrophile, E, adds to the carbon of the alkene that has the greater number of hydrogen substituents, and the nucleophile, Nu, to the carbon of the alkene with the fewer number of hydrogen substituents"

Explanation

• Rule has to do with the carbocation intermediate

• Reactions proceed to form the most stable carbocation

• The more carbons that are attached the more stable the carbocation

• Given names of primary, secondary, tertiary

Carbocation Examples

• Primary = 1º

• Secondary = 2º

• Tertiary = 3º

• Order of stability 3º > 2º > 1º

• Predict the product and draw the mechanism for the reaction

C-C-C=C-C + HBr

• Predict the product and draw the mechanism for the reaction of ethylcyclopentene with sulfuric acid (HOSO3H)

• Predict the product and draw the mechanism for the reaction of methlycyclopropene with water and an acid catalyst

Addition of Halogens

• Halogens can also be added added to alkenes

• Dihalide is formed

C=C + Cl2 Cl-C-C-Cl

• Mechanism will not be considered

Homework

• 3.41ace,42a-d,46