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1、1,Alkynes,general molecular formula CnH2n-2the triple bond introduces two degrees of unsaturation.,IntroductionStructure and Bonding,2,IntroductionStructure and Bonding,Cf.p bond in CH2CH2:64 Kcal/molBoth bonds of a C-C triple bond are weaker than the C-C double bond for homolytic cleavage.Alkynes a
2、re more polarizable than alkenes.Cyclooctyne is the smallest isolable cycloalkyne,though it decomposes upon standing at room temperature after a short time.,3,Alkynes,Alkynes are named in the same general way that alkenes are named.In the IUPAC system,change the ane ending of the parent alkane name
3、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 and triple bo
4、nd are named as enynes.The chain is numbered to give the first site of unsaturation(either C=C or CC)the lower number.The simplest alkyne,H-CC-H,named in the IUPAC system as ethyne,is more often called acetylene,its common name.The two-carbon alkyl group derived from acetylene is called an ethynyl g
5、roup.,Nomenclature,4,Alkynes,Nomenclature,5,Alkynes,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.Alkynes are solub
6、le in organic solvents and insoluble in water.,Physical Properties,6,Alkynes,Acetylene and Other Interesting Alkynes,Acetylene(H-CC-H)is a colorless gas that burns in oxygen to form CO2 and H2O.Closest hydrocarbon to charcoal and is an excellent fuel.,7,Acetylene and Other Interesting Alkynes,8,Alky
7、nes,Acetylene and Other Interesting Alkynes,Ethynylestradiol and norethindrone are two components of oral contraceptives that contain 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 th
8、ey can be administered in lower doses.Most oral contraceptives contain both of these synthetic hormones.They act by artificially elevating hormone levels in a woman,thereby preventing pregnancy.,9,Alkynes,Acetylene and Other Interesting Alkynes,10,Alkynes,Acetylene and Other Interesting Alkynes,bloc
9、ks the effects of progesterone,thus preventing pregnancyand inducing abortions,interferes with ovulation,and so it prevents pregnancy,11,Alkynes,Alkynes from Nature,12,Alkynes,Preparation of Alkynes,13,Alkynes,Preparation of Alkynes,14,Introduction to Alkyne ReactionsAdditions,15,Introduction to Alk
10、yne ReactionsAcids&Nucleophile,16,HydrohalogenationElectrophilic Addition of HXhydrohalogenation,17,Alkynes,HydrohalogenationElectrophilic Addition of HX,18,Alkynes,Electrophilic addition of HX to alkynes is slower than electrophilic addition of HX to alkenes,Introduction to Alkyne Reactions,Markovn
11、ikov addition,19,Alkynes,Carbocation A is stabilized by resonance,but B is not.,Introduction to Alkyne Reactions,20,Alkynes,HalogenationAddition of Halogen,21,Alkynes,HalogenationAddition of Halogen,22,Alkynes,In the presence of strong acid or Hg2+catalyst,the elements of H2O add to the triple bond.
12、,HydrationElectrophilic Addition of Water,23,Alkynes,HydrationElectrophilic Addition of Water,Internal alkynes undergo hydration with concentrated acidterminal alkynes require the presence of an additional Hg2+catalystusually HgSO4to yield methyl ketones by Markovnikov addition of water.,24,Tautomer
13、ism: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=O is much stronger than a C=C.Tautomerization(the process
14、 of converting one tautomer into another),is catalyzed by both acid and base.,25,Alkynes,HydrationElectrophilic Addition of Water,26,Alkynes,HydrationElectrophilic Addition of Water,27,Alkynes,HydroborationOxidation,Hydroborationoxidation is a two step reaction sequence that converts an alkyne to a
15、carbonyl compound.,28,Alkynes,HydroborationOxidation,29,Alkynes,Introduction to Alkyne ReactionsAcetylide anions,30,Alkynes,Reactions of Acetylide Anions,Acetylides are strong nucleophilesthe mechanism of substitution is SN2,and thus the reaction is fastest with CH3X and 10 alkyl halides.,31,Alkynes
16、,Reactions of Acetylide Anions,nucleophilic substitution with acetylide anions forms new carbon-carbon bonds in high yield only with unhindered CH3X and 1 alkyl halides.,32,Alkynes,Reactions of Acetylide Anions,33,Alkynes,Reactions of Acetylide Anions,34,Alkynes,Reactions of Acetylide Anions,Carbonc
17、arbon bond formation with acetylide anions is a valuable reaction used in the synthesis of numerous natural products.,35,Alkynes,Reactions of Acetylide Anions,Acetylide anions are good nucleophiles that open epoxide rings by an SN2 mechanism.Backside attack occurs at the less substituted end of the
18、epoxide.,36,Alkynes,Synthesis,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:working backwards from a d
19、esired product to determine the starting materials from which it is made.,37,Alkynes,Synthesis,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.,38,Alkynes,Synthesis,39,40,41,11.28,11.37,11.40,11.41,11.44,11.46,11.47,11.50,11.51,11.55,Homework,42,Preview of Chapter 12Oxidation and Reduction,ReductionReducing agentsReduction of alekene,alkyneOxidationOxidizing agents Oxidation of alcohol,alkene,and alkyne,
