Nomenclature of Alkenes and Cycloalkenes Alk ene s (Olefins)

1. Chapter 3Alkenes and Alkynes :Nomenclature, Properties and SynthesisElimination Reactions of Alkyl Halides

2. Nomenclature of Alkenes and Cycloalkenes • Alkenes (Olefins) • Alkenes have the general formula CnH2n where n = 2,3,… • contain at least one carbon-carbon double bond • Alkenes are named by finding the longest chain containing the double bond and changing the name of the corresponding parent alkane from -ane to -ene • The compound is numbered to give one of the alkene carbons the lowest number × × Chapter3

3. The (E)-(Z) System for Designating Alkene Diastereomers • The Cahn-Ingold-Prelog convention is used to assign the groups of highest priority on each carbon • If the group of highest priority on one carbon is on the same side as the group of highest priority on the other carbon the double bond is Z (zusammen) • If the highest priority groups are on opposite sides the alkene is E (entgegen) Chapter3

4. Relative Stabilities of Alkenes • Generally cis alkenes are less stable than trans alkenes because of steric hinderance Chapter3

5. Overall Relative Stabilities of Alkenes • The greater the number of attached alkyl groups (i.e. the more highly substituted the carbon atoms of the double bond), the greater the alkene’s stability Chapter3

6. Consider the two alkenes 2-methyl-1-pentene and 2-methyl-2-pentene and decide which would be most stable. 2-methyl-1-pentene 2-methyl-2-pentene trisubstituted disubstituted More stable Less stable Chapter3

7. Synthesis of Alkenes via Elimination Reactions • 1- Dehydrohalogenation: is elimination (loss) of HX from an alkyl halide using strong base (KOH) Chapter3

8. Zaitsev’s Rule: Formation of the Most Substituted Alkene is Favored with a Small Base • Some hydrogen halides can eliminate to give two different alkene products • Zaitzev’s Rule: when two different alkene products are possible in an elimination, the most highly substituted (most stable) alkene will be the major product • This is true only if a small base such as ethoxide is used Chapter3

9. 2- Acid Catalyzed Dehydration of Alcohols • Recall that elimination is favored over substitution at higher temperatures • Typical acids used in dehydration are sulfuric acid and phosphoric acid • The temperature and concentration of acid required to dehydrate depends on the structure of the alcohol • Primary alcohols are most difficult to dehydrate, tertiary are the easiest Chapter3

10. Alkynes have the general formula CnH2n-2 where n = 2,3,4,… • Contain at least one CC triple bond. • Alkynes are named by finding the longest chain containing the triple bond and changing the name of the corresponding parent alkane from -ane to -eye • The long chain is number to give the carbon atoms of the triple bond the lower possible number. Nomenclature of Alkynes 1 2 3 4 ethyne (acetylene) 4-methyl-1-pentyne 5 2-methyl-3-hexyne 2,5-dimethyl-3-hexyne Chapter3

11. Synthesis of Alkynes by Elimination Reactions • Alkynes can be obtained by two consecutive dehydrohalogenation reactions of a vicinal dihalide Chapter3

12. The Acidity of Terminal Alkynes • Recall that acetylenic hydrogens have a pKa of about 25 and are much more acidic than most other C-H bonds • The relative acidity of acetylenic hydrogens in solution is: • Acetylenic hydrogens can be deprotonated with relatively strong bases (sodium amide is typical) • The products are called alkynides Chapter3

13. Replacement of the Acetylenic Hydrogen Atom of Terminal Alkynes • Sodium alkynides can be used as nucleophiles in SN2 reactions • New carbon-carbon bonds are the result • Only primary alkyl halides can be used or else elimination reactions predominate Chapter3

14. Hydrogenation of Alkenes • Hydrogen adds to alkenes in the presence of metal catalysts • Heterogeneous catalysts: finely divided insoluble platinum, palladium or nickel catalysts • This process is called a reduction or hydrogenation • An unsaturated compound becomes a saturated (with hydrogen) compound Chapter3

15. Hydrogenation: The Function of the Catalyst • The catalyst provides a new reaction pathway with lower DG‡ values Chapter3

16. In heterogeneous catalysis the hydrogen and alkene adsorb to the catalyst surface and then a step-wise formation of C-H bonds occurs • Both hydrogens add to the same face of the alkene (a syn addition) • Addition to opposite faces of the double bond is called anti addition Chapter3

17. Hydrogenation of Alkynes • Reaction of hydrogen using regular metal catalysts results in formation of the alkane • Syn Addition of Hydrogen: Synthesis of cis-Alkenes • Lindlar’s catalyst produces cis-alkenes from alkynes Chapter3

18. Anti Addition of Hydrogen: Synthesis of trans-Alkenes • A dissolving metal reaction which uses lithium or sodium metal in low temperature ammonia or amine solvent produces trans-alkenes • Net anti addition occurs by formal addition of hydrogen to the opposite faces of the double bond Chapter3

19. Alkenes and Alkynes II: Addition Reactions

20. Introduction: Additions to Alkenes • Generally the reaction is exothermic because one p and one s bond are converted to two s bonds Chapter3

21. In addition reactions the alkene changes from a nucleophile in the first step to an electrophile in the second Chapter3

22. Addition of Hydrogen Halides to Alkenes: Markovnikov’s Rule • Addition of HBr to propene occurs to give 2-bromopropane as the major product • Markovnikov’s Rule (Original): addition of HX to an alkene proceeds so that the hydrogen atom adds to the carbon that already has the most hydrogen atoms Chapter3

23. Mechanism for hydrogen halide addition to an alkene Chapter3

24. Addition of HBr to 2-methylpropene gives only tert-butyl bromide • Modern Statement of Markovnikov’s Rule: In the ionic addition of an unsymmetrical reagent to a double bond, the positive portion of the adding reagent attaches itself to a carbon atom of the double bond so as to yield the more stable carbocation as an intermediate • Regioselective Reaction: When a reaction that can potentially yield two or more constitutional isomers actually produces only one or a predominance of one isomer Chapter3

25. Addition of Sulfuric Acid to Alkenes • Addition of concentrated sulfuric acid to alkenes leads to alkyl hydrogen sulfates which are soluble in the acid • The addition follows Markovnikov’s rule • The sulfate can be hydrolyzed by heating with water • The net result is Markovnikov addition of water to an alkene Chapter3

26. Addition of Water to Alkenes: Acid-Catalyzed Hydration • The reaction of alkenes with dilute aqueous acid leads to Markovnikov addition of water Chapter3

27. Addition of Bromine and Chlorine to Alkenes • Addition produces vicinal dihalides • This reaction is used as a test for alkenes because the red color of the bromine reagent disappears when an alkene (or alkyne) is present • The hydration of alkenes and the dehydration of alcohols are simply reverse reactions of one other Chapter3

28. Mechanism of Halogen Addition • A bromonium ion intermediate results instead of the carbocation seen in other addition reactions Chapter3

29. Halohydrin Formation • If halogenation is carried out in aqueous solvent, the water molecule can act as a nucleophile to open the halonium ion • The product is a halohydrin • In unsymmetrical alkenes, the bromonium ion will have some of its d+ charge density on the most substituted of the two carbons • The most substituted carbon can best accommodate d+ charge • The water nucleophile will tend to react at the carbon with the most d+ charge Chapter3

30. Oxidations of Alkenes: Syn 1,2-Dihydroxylation • Either OsO4 or KMnO4 will give 1,2 diols (glycols) • Mechanism for Syn Hydroxylation of Alkenes • Cyclic intermediates result from reaction of the oxidized metals • The initial syn addition of the oxygens is preserved when the oxygen-metal bonds are cleaved and the products are syndiols Chapter3

31. Oxidative Cleavage of Alkenes • Reaction of an alkene with hot KMnO4 results in cleavage of the double bond and formation of highly oxidized carbons • Unsubstituted carbons become CO2, monosubstituted carbons become carboxylates and disubstituted carbons become ketones • This be used as a chemical test for alkenes in which the purple color of the KMnO4 disappears and forms brown MnO2 residue if alkene (or alkyne) is present Chapter3

32. Ozonolysis of Alkenes • Cleavage of alkenes with ozone and workup with zinc in acetic acid leads to less highly oxidized carbons than products from cleavage with hot KMnO4 • Unsubstituted carbons are oxidized to formaldehyde, monosubstituted carbons are oxidized to aldehydes and disubstituted carbons are oxidized to ketones Chapter3

33. Addition of Bromine and Chlorine to Alkynes • Addition of halogen to alkynes can occur once or twice depending on how many equivalents of the halogen are added • Addition of one equivalent usually proceeds to give the trans dihalide Chapter3

34. Addition of Hydrogen Halides to Alkynes • Addition of hydrogen halides occurs once or twice depending on how many molar equivalent of hydrogen halide are added • Both additions are Markovnikov and give gem-halides • HBr can be generated by reaction of acetyl bromide and alumina • Anti-Markovnikov addition of HBr occurs in the presence of peroxide (See Chapter 10) Chapter3