1. Chapter 10.1 & 20.1: Organic Reactions
ALKENES

2. Alkenes
Unsaturated hydrocarbons with at least one double bond present.
Ex: Methene Ethene Propene But-1-ene Pent-1-ene
Reactivity is higher than that of alkanes
C=C double bond is less energetically stable than C-C single bond
C=C has a bond energy of 612 kJ/mol
C-C has bond energy of 348 kJ/mol
Energy of two single bonds > the energy of one double bond
Activation energy for alkene addition reactions is relatively low (i.e. they are more reactive than alkanes)

3. Alkenes undergo electrophilic addition RXNs
The 120° trigonal planar structure of Carbon in C=C makes it available for attack
 bonds are weaker & not on plane of bond, so easier to break
 bond is area of e- density so electrophiles are attracted
Electrophile: species that is e- deficient or becomes e- deficient in presence of a  bond (has partial or full + charge)

4. Alkenes undergo electrophilic addition RXNs
Types of electrophilic addition RXNs
Addition of:
Hydrogen (hydrogenation) = alkene  alkane
Halogens = alkene  dihalogenoalkane
Hydrogen halide = alkene  halogenoalkane
Water (hydration) = alkene  alcohol
Alkene + alkene (polymerization) = alkenes  polythene

5. Alkenes: Addition RXNs – hydrogenation
Addition of hydrogen to make an unsaturated molecule into a saturated molecule
Requires: a nickel catalyst and heat

6. Alkenes: Addition RXNs – hydrogenation
RXN is the basis for production of margarine from vegetable oils
Vegetable oils often long chain polyunsaturated (multiple double bonds) fats
As hydrogen is added to oils, # of double bonds decreases and the chain takes on more regular shape
Increases melting point = solid

7. Alkenes: Addition RXNs - Halogenation
Double bond of an alkene is broken and two new bonds are formed
Ex: bromination of ethene
Ethene is bubbled through Room Temp
Ethene is colorless
Bromine is brown
1,2-dibromoethane is colorless
RXN = brown + colorless  colorless

8. Alkenes: Addition RXNs - Halogenation
Bromine (non-polar) becomes polarized when gets close to e- rich double bond
Br becomes electrophile as gets +
Heterolytic split of Br2 forms Br+ and Br-
Slow step: carbocation intermediate

9. Alkenes: Addition RXNs - Halogenation
Carbocation is unstable
Reacts quickly with Br- forming 1,2-dibromoethane
If reaction with C2H4 and Br2 also includes Cl-
Products formed are BrH2CCH2Br and BrH2CCH2Cl
Shows that the initial Br+ mechanism is correct
No ClH2CCH2Cl forms

10. Alkenes: Addition RXNs - Halogenation
Both carbons in the double bond get one bromine
This reaction is simple test for the presence of unsaturated hydrocarbons
Bromine (dark brown color) will add “across” double bonds and will be consumed
Thus, the brown color gets lighter.
If bromine is added to an unknown substance and the color fades, it indicates the presence of unsaturated hydrocarbons

11. Alkenes: Addition RXNs – hydrogen halides
Halogen halide reativity: HI> HBr > HCl
Due to decreasing strength of H—X bond
When ethene is bubbled through conc. aqueous solution of HBr at RT  bromoethane

12. Alkenes: Addition RXNs – hydrogen halides
Reaction mechanism is similar to halogen
H—Br splits heterolytically to form H+ and Br-
Slow step: H+ is electrophile attacks double bond
Br- reacts with carbocation to form bromoethane

13. Alkenes: Addition RXNs – hydrogen halides Unsymmetric addition
Propene + hydrogen bromide
Theoretically results in 2 products: which one is true?
The answer comes from which carbocation is most stable

14. Alkenes: Addition RXNs – hydrogen halides Unsymmetric addition
Any asymmetric hydrogen halide will result in predictable product
Markovnikov’s Rule: the Hydrogen will attach to the carbon that is already bonded to the greater number of hydrogens
More general: the more electropositive part of the reacting species bonds to the least highly substituted carbon atom in the alkene

15. Alkenes: Addition RXNs – hydration
Addition of water to an alkene to form an alcohol
Requires: heat with steam and conc. sulfuric acid catalyst
Slow step: H+ and HSO4- ions are added across the double bond
Hydrolysis (“cutting of water”) where OH- replaces HSO4- and H2SO4 is reformed

16. Alkenes: Addition RXNs - polymerization
Addition polymerization
Polymers are long chain molecules formed by the joining together of a large number of repeating units, called monomers.
Addition polymerization involves the joining together of monomers containing double bonds:

17. Alkenes: Addition RXNs - polymerization
Two other common examples of addition polymerization
PVC: Polychloroethene (also called polyvinyl chloride)
Polypropene

18. Alkenes: Addition RXNs - polymerization
Two other common examples of addition polymerization
PTFE: Poly(tetrafluoroethene) (also called Teflon)

19. Alkenes: Addition RXNs - polymerization

20. Alkenes undergo electrophilic addition RXNs
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