Alkenes

Alkenes/Alkynes Compounds that contain carbon and hydrogen Alkenes have a double bond General formula of C n H 2n Alkynes have a triple bond General formula of C n H 2n-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.

Name the following

Arrangement of Alkenes Carbons containing the double bond are sp 2 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.

Homework P ,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 + H 2 O 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 (HOSO 3 H)

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 + Cl 2  Cl-C-C-Cl Mechanism will not be considered

Homework 3.41ace,42a-d,46