Organic Chemistry

1. Organic Chemistry Jin Hongwei College of Chemical Engineering and Materials Science jhwei828@zjut.edu.cn

2. Chapter Three Unsaturated Hydrocarbons: Alkenes and Alkynes • Alkenes and Alkynes. • Structures of Alkenes and Alkynes. • Isomerism of Alkenes and Alkynes. • Nomenclature of Alkenes and Alkynes. • Physical Properties of Alkenes and Alkynes. • Chemical Properties of Alkenes and Alkynes. • Synthesis of Alkenes and Alkynes.

3. Alkenes and Alkynes • Alkenes are hydrocarbons that contain a carbon-carbon double bond. • Alkynes are hydrocarbons that contain a carbon-carbon triple bond. • Alkenes have the general formula CnH2n, where n is an integer. • Alkynes have the general formula CnH2n-2. • BACK

4. Structures of Alkenes and Alkynes(1) • It was recognized well over 100 years ago that ethylene carbons can be tetravalent only if the two carbon atoms share four electrons and are linked by a doublebond. • Furthermore, ethylene is planar(flat) and has bond angles of approximately 1200.

5. Structures of Alkenes and Alkynes(2) • Ansp2-hybridized carbon. Three equivalent sp2 hybrid orbitals (green) lie in a plane at angles of 1200 to one other, and a single unhybridized p orbital (red) is perpendicular to the sp2 plane.

6. Structures of Alkenes and Alkynes(3) • When two sp2-hybridized carbons approach each other, they form asigma(σ)bond by sp2-sp2 overlap according to valence bond theory. At the same time, the unhybridized p orbitals approach with the correct geometry for side-ways overlap, leading to the formation of what is calleda pi(П)bond.

7. Structures of Alkenes and Alkynes(4) • To complete the structure of ethylene, four hydrogen atoms form bonds with the remaining four sp2 orbitals. The combination of an sp2-sp2σbond and a 2p-2pПbond results in the sharing of four electrons and the formation of a carbon-carbon double bond.

8. Structures of Alkenes and Alkynes(5) • Acetylene, the simplest alkyne, was once widely used in industry as the starting material for the preparation of several chemicals. The structure of acetylene, the angles of the hydrogen –carbon bond and carbon-carbon triple bond are 180o.

9. Structures of Alkenes and Alkynes(6) • A carbon-carbon triple bond results from the interaction of two sp-hybridized carbon atoms. Recall that the two sp hybrid orbitals of carbon lie at an angle of 180 0to each other along an axis perpendicular to the axes of the two unhybridized 2py and 2pz orbitals. When two sp-hybridized carbons approach each other, one sp-sp bond and two p-pbonds are formed. • BACK

10. Isomerism of Alkenes and Alkynes(1) • Cis-Trans Isomerism in Alkenes. • The requirement for cis-trans isomerism in alkenes: Compounds that have one of their carbons bonded to two identical groups can’t exist as cis-trans isomers. • Only when both carbons are bonded to two different groups are cis-trans isomers possible.

11. Isomerism of Alkenes and Alkynes(2) • For example: • Which of the following compound has cis or trans geometry? • The question can’t be answered because the prefixes cis and trans describe only the geometry of disubstituted double bonds.

12. Isomerism of Alkenes and Alkynes(3) • A more general method for describing double-bond geometry is provided by the E,Z system of nomenclature, which uses a series of sequence rules to assign priorities to the substituent groups on the double-bond carbons. • Called the Cahn-Ingold-Prelog rules after the chemists who proposed them, the sequence rules are as follow: • Rule 1: Considering each of the double-bond carbons separately, identify the two atoms directly attached and rank them according to atomic number. An atom with higher atomic number receives higher priority than an atom with lower atomic number. （将与两个双键碳原子直接相连的原子按原子序数分别比较大小，大者为“较优”基团。）

13. Isomerism of Alkenes and Alkynes(4) • Rule 2:If a decision can’t be reached by ranking the first atoms in the substituent, look at the second, third, or fourth atoms away from the double-bond carbons until the first difference is found. （如果与双键碳原子直接相连的原子序数相同，则需要比较由该原子外推至相邻的第二、第三个、第四等原子直至比较出较优基团为止。） • Rule 3:Multiple-bonded atoms are equivalent to the same number of single bonded atoms.（重键原子相当于相同数目的单原子。） • If the higher-priority groups attached to each carbon of double-bond are on the same side of the double bond, the alkene is designatedZ; If the higher-priority groups attached to each carbon of double-bond are on the opposite sides of the double bond, the alkene is designated E. • BACK

14. Nomenclature of Alkenes and Alkynes(1) • Alkenes are named using a series of rules similar to those for alkanes, with the suffix –ene used instead of –ane to identify the family. There are three steps: • Step 1:Name the parent hydrocarbon. Find the longest carbon chain containing the double bond, and name the compound accordingly, using the suffix –ene . • Step 2:Number the carbon atoms in the chain. Begin at the end nearer the double bond or, if the double bond is equidistant from the two ends, begin at the nearer the first branch point. • Step 3:Write the full name. Number the substituents according to their positions in the chain, and list them alphabetically. Indicate the position of the double bond by giving the number of the first alkene carbon and placing that number immediately before the parent name.

15. Nomenclature of Alkenes and Alkynes(2) • Alkynes are named using a series of rules similar to those for alkanes, with the suffix –yne used instead of –ane to identify the family. There are three steps: • Step 1:Name the parent hydrocarbon. Find the longest carbon chain containing the triple bond, and name the compound accordingly, using the suffix –yne . • Step 2:Number the carbon atoms in the chain. Begin at the end nearer the triple bond or, if the triple bond is equidistant from the two ends, begin at the nearer the first branch point. • Step 3:Write the full name. Number the substituents according to their positions in the chain, and list them alphabetically. Indicate the position of the triple bond by giving the number of the first alkyne carbon and placing that number immediately before the parent name. • BACK

16. Chemical Properties of Alkenes and Alkynes(1) • A. Reduction of Alkenes • Alkenes react with H2 in the presence of a catalyst to yield the corresponding saturated alkane addition products. We describe the result by saying that the double bond has beenhydrogenated, orreduced. • For example:

17. Chemical Properties of Alkenes and Alkynes(2) • B. Reduction of Alkyne • Alkynes are easily reduced to alkanes by addition of H2 over a metal catalyst. • Hydrogenation of alkynes in the presence of Lindlar’s catalyst or P-2 catalyst causessyn addition of hydrogento take place, and the alkene that is formed from an alkyne with an internal triple bond has the (Z) or cis configuration. 3-Hexyne (Z)-3-Hexene(cis-3-hexene)

18. Chemical Properties of Alkenes and Alkynes(3) • An anti addition of hydrogen atoms to the triple bond occurs when alkynes are reduced with lithium or sodium in ammonia at low temperatures. For example: 3-Hexyne (E)-3-Hexene (trans-3-hexene)

19. Chemical Properties of Alkenes and Alkynes(4) • Heats of hydrogenation and stability of alkenes and alkynes • Heat of hydrogenation: The heat that is released in the hydrogenation of unsaturated hydrocarbon per mole. • (一摩尔不饱和烃氢化时所放出的热量－－－氢化热）

20. Chemical Properties of Alkenes and Alkynes(5) • The data show that alkenes become more stable with increasing substitution. As a general rule, alkenes follow the stability order: • Tetrasubstitued>Trisubstituted>Disubstituted>Monosubstituted （烯烃的稳定性：四取代烯烃>三取代烯烃>二取代烯烃>单取代烯烃） • The stability of alkynes as follow: • Disubstituted>Monosubstituted>Acetylene （炔烃的稳定性：二取代炔烃>单取代炔烃>乙炔）

21. Chemical Properties of Alkenes and Alkynes(6) • C.Electrophilic Additions of Alkenes • (a). Addition of Halogens to Alkenes （烯烃与卤素的加成） • (b). Hydrohalogenation of Alkenes（烯烃与卤化氢的加成） • (c). Hydration of Alkenes（烯烃与酸生成醇的加成） • (d). Halohydrin Formation（烯烃与氢卤酸的加成） • (e). Hydroboration of Alkenes（烯烃的硼氢化反应） • (f). Oxymercuration of Alkenes（烯烃的羟汞化反应） • D.Electrophilic Additions of Alkynes • (a). Addition of Halogens to Alkynes (炔烃与卤素的加成） • (b). Hydrohalogenation of Alkynes（炔烃与卤化氢的加成） • (c). Hydration of Alkynes（炔烃与水的加成） • (d). Hydroboration of Alkynes（炔烃的硼氢化反应）

22. Chemical Properties of Alkenes and Alkynes(7) • Addition of Halogens to Alkenes • Bromine and chlorine both add readily to alkenes to yield 1,2-dihaloalkanes. For example: • An explanation for the observed anti stereochemistry of addition was suggested in 1937 by George Kimball and Irving Roberts, who proposed that the true reaction intermediated is not a carbocation but is instead a bromonium ion (溴鎓离子）. NOT formed BACK

23. Chemical Properties of Alkenes and Alkynes(8) • Hydrohalogenation of Alkenes • The hydrogen halides HCl, HBr, and HI can react with alkenes to yield alkyl halides. For example: • In this case, an unsymmetrically substituted alkene has given a single addition product, rather than the mixture that might have been expected. We say that such reactions are regiospecific. • After looking at the results of many such reactions, the Russian chemist Vladimir Markovnikov proposed in 1869 what has become known as Markovnikov’s rule.

24. Chemical Properties of Alkenes and Alkynes(9) • Markovnikov’s rule: • In the addition of HX to an alkene, the H attaches to the carbon with fewer alkyl substituents and the X attaches to the carbon with more alkyl substituents. • 一个烯烃与卤化氢发生加成反应时，氢总是加到取代基较少的双键碳原子上，而卤素原子则加到取代基较多的双键碳原子相连。Why? • Mechanism of electrophilic addition of HX to alkenes. • The reaction occurs in two steps and involves a carboncation intermediate.

25. Chemical Properties of Alkenes and Alkynes(10) Some special examples: • What happened? BACK

26. Chemical Properties of Alkenes and Alkynes(11) • Hydration of Alkenes • Water adds to alkenes to yield alcohols, a process called hydration. • The reaction take place on treatment of the alkene with water and a strong acid catalyst by a mechanism similar to that of HX addition. For example: BACK

27. Chemical Properties of Alkenes and Alkynes(12) • Halohydrin Formation • Many different kinds of electrophilic additions to alkenes take place. For example, alkenes add HOCl or HOBr under suitable conditions to yield 1,2-halo alcohols, called halohydrins (卤代醇）. • The addition is usually done indirectly by reaction of the alkene with either Br2 or Cl2 in the presence of water. • For example: BACK

28. Chemical Properties of Alkenes and Alkynes(13) • Hydroboration of Alkenes • One of the most useful methods for preparing alcohols from alkenes is the hydroboration reaction reported in 1959 by H. C. Brown (1979 Nobel Prize winner). • Hydroboration involves addition of a B-H bond of borane, BH3, to an alkene to yield an organoborane intermediate, RBH2. For example: • Syn-addition product BACK

29. Chemical Properties of Alkenes and Alkynes(14) • Oxymercuration of Alkenes • In the laboratory, alkenes are often hydrated by the oxymercuration procedure. • When an alkene is treated with mercury acetate[Hg(OAc)2] in aqueous THF solvent, electrophilic addition to the double bond rapidly occurs. The intermediate organomercury compound is then treated with sodium borohydride, NaBH4 , and an alcohol is produced. For example: BACK

30. Chemical Properties of Alkenes and Alkynes(15) • Addition of Halogens to Alkynes • Bromine and chlorine also add to alkynes to give addition products, and trans stereochemistry again results: BACK

31. Chemical Properties of Alkenes and Alkynes(16) • Hydrohalogenation of Alkynes • The reaction of alkynes with HX, often can be stopped after addition of 1 equivalent of HX, but reaction with an excess of HX often leads to a dihalide product. The regiochemistry of the addition of alkynes with HX follows Markovnikov’s rule: Halogen adds to the more highly substituted side of the alkyne bond, and hydrogen adds to the less highly substituted side.Trans stereochemistry of H and X is normally found in the product. For example: BACK

32. Chemical Properties of Alkenes and Alkynes(17) • Mercury(Ⅱ)- Catalyzed Hydration of Alkynes • Alkynes don’t react directly with aqueous acid but will undergo hydration readily in the presence of mercury(Ⅱ) sulfate catalyst. The reaction occurs with Markovnikov regiochemistry: The –OH group adds to the more highly substituted carbon, and the –H attaches to the less highly substituted one. For example: BACK

33. Chemical Properties of Alkenes and Alkynes(18) • Hydroboration of Alkynes • Hydroboration/oxidation of an internal alkyne such as 3-hexyne gives a ketone, and hydroboration/oxidation of a terminal alkyne gives an aldehyde. For example:

34. Chemical Properties of Alkenes and Alkynes(19) • E.Nucleophilic Additions of Alkynes • Alkynes can occur nucleophilic addition with alcohols in the presence of base. For example:

35. Chemical Properties of Alkenes and Alkynes(20) • F. Oxidation of Alkenes :Hydroxylation and Cleavage • Hydroxylation of an alkene • The addition of an –OH group to each of the two alkene carbons---can be carried out by reaction of the alkene with osmium tetraoxide (OsO4) or potassium permanganate(KMnO4) in basic cold solution. The reaction occurs with syn stereochemistry and yield a 1,2-dialcohol, or diol product (also called a glycol).

36. Chemical Properties of Alkenes and Alkynes(21) • Alkene Cleavage • Powerful oxidizing reagents such as ozone(O3), hot basic potassium permanganate(KMnO4). For example:

37. Chemical Properties of Alkenes and Alkynes(22) • Treatment with hot basic potassium permanganate oxidatively cleaves the double bond of an alkene. Alkenes with monosubstituted carbon atoms are oxidatively cleaved to salts of carboxylic acids. Disubstituted alkene carbons are oxidatively cleaved to ketones. Unsubstituted alkene carbons are oxidized to carbon dioxide. For example:

38. Chemical Properties of Alkenes and Alkynes(23) • G. Epoxidation of Alkenes • The most widely used method for synthesizing epoxides(环氧化合物) is the reaction of an alkene with an organic peroxy acid(过氧酸). For example:

39. Chemical Properties of Alkenes and Alkynes(24) • H. Oxidative Cleavage of Alkynes • Treating alkynes with ozone or with basic potassium permanganate leads to cleavage at the carbon-carbon triple bond. The products are carboxylic acids:

40. Chemical Properties of Alkenes and Alkynes(25) • I. Free Radical Substitution of Alkenes • α-Hydrogen of alkene can occur free radical substitution with halogen in high temperature. For example:

41. Chemical Properties of Alkenes and Alkynes(26) • J. The Acidity of Terminal Alkynes • The hydrogen bonded to the carbon of a terminal alkyne is considerably more acidic than those bonded to carbons of an alkene or alkane. The pKa values for ethyne, ethene, and ethane illustrate this point: pKa=25 pKa=44 pKa=50 • Relative Basicity:

42. Chemical Properties of Alkenes and Alkynes(27) • K. Alkylation of Terminal Alkynes • We can replace a hydrogen attached to a triply bonded carbon of a terminal alkyn( called an acetylenic hydrogen) by an alkyl group. This type of reaction, called an alkylation of alkynes. For example, an acetylide anoin can react with an alkyl halide such as bromomethane to substitute for the halogen and yield a new alkyne product:

43. Chemical Properties of Alkenes and Alkynes(27) • Organic Synthesis examples: BACK

44. Synthesis of Alkenes and Alkynes(1) • The two most common alkene-forming elimination reactions are dehydrohalogenation---the loss of HX from an alkyl halide, and dehydration---the loss of water from an alcohol. For example:

45. Synthesis of Alkenes and Alkynes(2) • Alkynes can be prepared by elimination of HX from a alkyl halides in much the same manner as alkenes. Treatment of a 1,2-dihalide with excess strong base such as KOH or NaNH2 results in a twofold elimination of HX and formation of an alkyne. For example: BACK

46. Additional problems of this chapter(1) • 3.1 Give IUPAC names for the following compounds: • (a) (b) • (c) (d) • 3.2 Draw structures corresponding to the following names: • (a) (4E)-2,4-Dimethyl-1,4-hexadiene (b) 3-Butyl-2-heptene • (c) 3,3-Dimethyl-4-octyne (d) 3,5-Heptadien-1-yne • 3.3 Addition of HCl to 1-isopropenyl-1-methylcyclopentane yields 1-chloro-1,2,2-trimethylcyclohexane. Propose a mechanism, showing the structures of the intermediates and using curved arrows to indicate electron flow in each step.

47. Additional problems of this chapter(2) • 3.4 Predict the major product(s) of the following reactions: • (a) (b) 3.5 Synthesize the following compounds using 1-butyne as the only source of carbon, along with any inorganic reagents you need. More than one step may be needed. (a) (b) (c) (d)