1、1Alkynes general molecular formula CnH2n-2 the triple bond introduces two degrees of unsaturation.IntroductionStructure and Bonding2IntroductionStructure and BondingCf. p p bond in CH2CH2 : 64 Kcal/mol Both p p bonds of a C-C triple bond are weaker than the C-C double bond for homolytic cleavage. Al
2、kynes are more polarizable than alkenes. Cyclooctyne is the smallest isolable cycloalkyne, though it decomposes upon standing at room temperature after a short time.3Alkynes Alkynes are named in the same general way that alkenes are named. In the IUPAC system, change the ane ending of the parent alk
3、ane name to the suffix yne. Choose the longest continuous chain that contains both atoms of the triple bond and number the chain to give the triple bond the lower number. Compounds with two triple bonds are named as diynes, those with three are named as triynes and so forth. Compounds both a double
4、and triple bond are named as enynes. The chain is numbered to give the first site of unsaturation (either C=C or C C) the lower number. The simplest alkyne, H-C C-H, named in the IUPAC system as ethyne, is more often called acetylene, its common name. The two-carbon alkyl group derived from acetylen
5、e is called an ethynyl group.Nomenclature4AlkynesNomenclature5Alkynes The physical properties of alkynes resemble those of hydrocarbons of similar shape and molecular weight. Alkynes have low melting points and boiling points. Melting point and boiling point increase as the number of carbons increas
6、es. Alkynes are soluble in organic solvents and insoluble in water.Physical Properties6AlkynesAcetylene and Other Interesting Alkynes Acetylene (H-C C-H) is a colorless gas that burns in oxygen to form CO2 and H2O. Closest hydrocarbon to charcoal and is an excellent fuel.7Acetylene and Other Interes
7、ting AlkynesOHCH3HHHHOEstradiolHHHCH3CCH3OOCH3HCH3OHHOHHCCHOHHCH3OCHHHCHProgesteroneNatural female sex hormonesOral contraceptive drugsEthynylestradiolNorethindrone8AlkynesAcetylene and Other Interesting Alkynes Ethynylestradiol and norethindrone are two components of oral contraceptives that contai
8、n a carbon-carbon triple bond. Both molecules are synthetic analogues of the naturally occurring female sex hormones estradiol and progesterone, but are more potent so they can be administered in lower doses.Most oral contraceptives contain both of these synthetic hormones.They act by artificially e
9、levating hormone levels in a woman, thereby preventing pregnancy.9AlkynesAcetylene and Other Interesting Alkynes10AlkynesAcetylene and Other Interesting Alkynesblocks the effects of progesterone,thus preventing pregnancyand inducing abortionsinterferes with ovulation, and so it prevents pregnancy11A
10、lkynesAlkynes from Nature12AlkynesPreparation of Alkynes13AlkynesPreparation of Alkynes14Introduction to Alkyne ReactionsAdditions15Introduction to Alkyne ReactionsAcids & NucleophileStrong baseAn acetylide anionTerminal alkynepKa = 25:HB_+:BRCCRCCHNH2- and H- are strong enough bases (pKa (NH3) = 38
11、, pKa(H2) = 35) to favorleft-right equilibrium bias, OH- and OR- (pKa (H2O) = 15.7, pKa(ROH) = 15.5-18)are notRCC_:+ E+RCCENucleophile Electrophile16HydrohalogenationElectrophilic Addition of HXhydrohalogenation17AlkynesHydrohalogenationElectrophilic Addition of HX18Alkynes Electrophilic addition of
12、 HX to alkynes is slower than electrophilic addition of HX to alkenesIntroduction to Alkyne Reactions Markovnikov addition19Alkynes Carbocation A is stabilized by resonance, but B is not.Introduction to Alkyne Reactions20AlkynesHalogenationAddition of Halogen21AlkynesHalogenationAddition of Halogen2
13、2Alkynes In the presence of strong acid or Hg2+ catalyst, the elements of H2O add to the triple bond.HydrationElectrophilic Addition of Water23AlkynesHydrationElectrophilic Addition of Water Internal alkynes undergo hydration with concentrated acid terminal alkynes require the presence of an additio
14、nal Hg2+ catalystusually HgSO4to yield methyl ketones by Markovnikov addition of water.24 Tautomerism : the conversion of a general enol A to the carbonyl compound B. A and B are tautomers: A is the enol form and B is the keto form of the tautomer.HydrationElectrophilic Addition of Water Equilibrium
15、 favors the keto form largely because the C=O is much stronger than a C=C. Tautomerization (the process of converting one tautomer into another), is catalyzed by both acid and base.25AlkynesHydrationElectrophilic Addition of Water26AlkynesHydrationElectrophilic Addition of Water27AlkynesHydroboratio
16、nOxidationHydroborationoxidation is a two step reaction sequence that converts an alkyne to a carbonyl compound.28AlkynesHydroborationOxidation29AlkynesIntroduction to Alkyne ReactionsAcetylide anions30AlkynesReactions of Acetylide Anions Acetylides are strong nucleophiles the mechanism of substitut
17、ion is SN2, and thus the reaction is fastest with CH3X and 10 alkyl halides.31AlkynesReactions of Acetylide Anions nucleophilic substitution with acetylide anions forms new carbon-carbon bonds in high yield only with unhindered CH3X and 1 alkyl halides. 32AlkynesReactions of Acetylide Anions33Alkyne
18、sReactions of Acetylide Anions34AlkynesReactions of Acetylide Anions Carboncarbon bond formation with acetylide anions is a valuable reaction used in the synthesis of numerous natural products.35AlkynesReactions of Acetylide Anions Acetylide anions are good nucleophiles that open epoxide rings by an
19、 SN2 mechanism. Backside attack occurs at the less substituted end of the epoxide.36AlkynesSynthesis You can now begin to consider (for example) how to prepare a five-carbon product from three smaller precursor molecules using the reactions you have learned. To plan a synthesis of more than one step
20、, we use the process of retrosynthetic analysis : working backwards from a desired product to determine the starting materials from which it is made.37AlkynesSynthesis In designing a synthesis, reactions are often divided into two categories:1. Those that form new carbon-carbon bonds.2. Those that c
21、onvert one functional group into anotherthat is, functional group interconversions.38AlkynesSynthesis39404111.28, 11.37, 11.40, 11.41, 11.44, 11.46, 11.47, 11.50, 11.51, 11.55Homework42Preview of Chapter 12Oxidation and ReductionReductionReducing agentsReduction of alekene, alkyneOxidationOxidizing agents Oxidation of alcohol, alkene, and alkyne
