1、1Chapter 11Alkynes2Alkynes general molecular formula CnH2n-2 the triple bond introduces two degrees of unsaturation.IntroductionStructure and Bonding3IntroductionStructure 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 homolyt
2、ic cleavage.Alkynes are more polarizable than alkenes.Cyclooctyne is the smallest isolable cycloalkyne,though it decomposes upon standing at room temperature after a short time.4Alkynes Alkynes are named in the same general way that alkenes are named.In the IUPAC system,change the ane ending of the
3、parent alkane 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 d
4、ouble 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 acetylene i
5、s called an ethynyl group.Nomenclature5AlkynesNomenclature6Alkynes 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 increases.Al
6、kynes are soluble in organic solvents and insoluble in water.Physical Properties7AlkynesAcetylene 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.8Acetylene and Other Interesting Alky
7、nesOHCH3HHHHOEstradiolHHHCH3CCH3OOCH3HCH3OHHOHHCCHOHHCH3OCHHHCHProgesteroneNatural female sex hormonesOral contraceptive drugsEthynylestradiolNorethindrone9AlkynesAcetylene and Other Interesting Alkynes Ethynylestradiol and norethindrone are two components of oral contraceptives that contain a carbo
8、n-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 elevating ho
9、rmone levels in a woman,thereby preventing pregnancy.10AlkynesAcetylene and Other Interesting Alkynes11AlkynesAcetylene and Other Interesting Alkynesblocks the effects of progesterone,thus preventing pregnancyand inducing abortionsinterferes with ovulation,and so it prevents pregnancy12AlkynesAlkyne
10、s from Nature13AlkynesPreparation of Alkynes14AlkynesPreparation of Alkynes15Introduction to Alkyne ReactionsAdditions16Introduction to Alkyne ReactionsAcids&NucleophileStrong baseAn acetylide anionTerminal alkynepKa=25:HB_+:BRCCRCCHNH2-and H-are strong enough bases(pKa(NH3)=38,pKa(H2)=35)to favorle
11、ft-right equilibrium bias,OH-and OR-(pKa(H2O)=15.7,pKa(ROH)=15.5-18)are notRCC_:+E+RCCENucleophile Electrophile17HydrohalogenationElectrophilic Addition of HXhydrohalogenation18AlkynesHydrohalogenationElectrophilic Addition of HX19Alkynes Electrophilic addition of HX to alkynes is slower than electr
12、ophilic addition of HX to alkenesIntroduction to Alkyne Reactions Markovnikov addition20Alkynes Carbocation A is stabilized by resonance,but B is not.Introduction to Alkyne Reactions21AlkynesHalogenationAddition of Halogen22AlkynesHalogenationAddition of Halogen23Alkynes In the presence of strong ac
13、id or Hg2+catalyst,the elements of H2O add to the triple bond.HydrationElectrophilic Addition of Water24AlkynesHydrationElectrophilic Addition of Water Internal alkynes undergo hydration with concentrated acid terminal alkynes require the presence of an additional Hg2+catalystusually HgSO4to yield m
14、ethyl ketones by Markovnikov addition of water.25 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 favors the keto form largely because the C=
15、O is much stronger than a C=C.Tautomerization(the process of converting one tautomer into another),is catalyzed by both acid and base.26AlkynesHydrationElectrophilic Addition of Water27AlkynesHydrationElectrophilic Addition of Water28AlkynesHydroborationOxidationHydroborationoxidation is a two step
16、reaction sequence that converts an alkyne to a carbonyl compound.29AlkynesHydroborationOxidation30AlkynesIntroduction to Alkyne ReactionsAcetylide anions31AlkynesReactions of Acetylide Anions Acetylides are strong nucleophiles the mechanism of substitution is SN2,and thus the reaction is fastest wit
17、h CH3X and 10 alkyl halides.32AlkynesReactions of Acetylide Anions nucleophilic substitution with acetylide anions forms new carbon-carbon bonds in high yield only with unhindered CH3X and 1 alkyl halides.33AlkynesReactions of Acetylide Anions34AlkynesReactions of Acetylide Anions35AlkynesReactions
18、of Acetylide Anions Carboncarbon bond formation with acetylide anions is a valuable reaction used in the synthesis of numerous natural products.36AlkynesReactions of Acetylide Anions Acetylide anions are good nucleophiles that open epoxide rings by an SN2 mechanism.Backside attack occurs at the less
19、 substituted end of the epoxide.37AlkynesSynthesis 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,we use the process of retrosynthetic analysis:workin
20、g backwards from a desired product to determine the starting materials from which it is made.38AlkynesSynthesis In designing a synthesis,reactions are often divided into two categories:1.Those that form new carbon-carbon bonds.2.Those that convert one functional group into anotherthat is,functional group interconversions.39AlkynesSynthesis40414211.28,11.37,11.40,11.41,11.44,11.46,11.47,11.50,11.51,11.55Homework43Preview of Chapter 12Oxidation and ReductionReductionReducing agentsReduction of alekene,alkyneOxidationOxidizing agents Oxidation of alcohol,alkene,and alkyne
