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1、1,Chapter 11 Alkynes,2,Alkynes,general molecular formula CnH2n-2 the triple bond introduces two degrees of unsaturation.,IntroductionStructure and Bonding,3,IntroductionStructure and Bonding,Cf. p bond in CH2CH2 : 64 Kcal/mol Both bonds of a C-C triple bond are weaker than the C-C double bond for ho
2、molytic 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.,4,Alkynes,Alkynes are named in the same general way that alkenes are named. In the IUPAC system, change the ane en
3、ding of the 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. Co
4、mpounds both a double and triple bond 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 d
5、erived from acetylene is called an ethynyl group.,Nomenclature,5,Alkynes,Nomenclature,6,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
6、 number of carbons increases. Alkynes are soluble in organic solvents and insoluble in water.,Physical Properties,7,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、.,8,Acetylene and Other Interesting Alkynes,9,Alkynes,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 e
8、stradiol 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 hormone levels in a woman, thereby preventing pregnancy.,10,Alkynes,Acetylene and Other Interesting Alkynes
9、,11,Alkynes,Acetylene and Other Interesting Alkynes,blocks the effects of progesterone, thus preventing pregnancy and inducing abortions,interferes with ovulation, and so it prevents pregnancy,12,Alkynes,Alkynes from Nature,13,Alkynes,Preparation of Alkynes,14,Alkynes,Preparation of Alkynes,15,Intro
10、duction to Alkyne ReactionsAdditions,16,Introduction to Alkyne ReactionsAcids & Nucleophile,17,HydrohalogenationElectrophilic Addition of HX hydrohalogenation,18,Alkynes,HydrohalogenationElectrophilic Addition of HX,19,Alkynes,Electrophilic addition of HX to alkynes is slower than electrophilic addi
11、tion of HX to alkenes,Introduction to Alkyne Reactions,Markovnikov addition,20,Alkynes,Carbocation A is stabilized by resonance, but B is not.,Introduction to Alkyne Reactions,21,Alkynes,HalogenationAddition of Halogen,22,Alkynes,HalogenationAddition of Halogen,23,Alkynes,In the presence of strong a
12、cid or Hg2+ catalyst, the elements of H2O add to the triple bond.,HydrationElectrophilic Addition of Water,24,Alkynes,HydrationElectrophilic Addition of Water,Internal alkynes undergo hydration with concentrated acid terminal alkynes require the presence of an additional Hg2+ catalystusually HgSO4to
13、 yield methyl 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
14、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.,26,Alkynes,HydrationElectrophilic Addition of Water,27,Alkynes,HydrationElectrophilic Addition of Water,28,Alkynes,HydroborationOxidation,Hydroborat
15、ionoxidation is a two step reaction sequence that converts an alkyne to a carbonyl compound.,29,Alkynes,HydroborationOxidation,30,Alkynes,Introduction to Alkyne ReactionsAcetylide anions,31,Alkynes,Reactions of Acetylide Anions,Acetylides are strong nucleophiles the mechanism of substitution is SN2,
16、 and thus the reaction is fastest with CH3X and 10 alkyl halides.,32,Alkynes,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.,33,Alkynes,Reactions of Acetylide Anions,34,Alkynes,Re
17、actions of Acetylide Anions,35,Alkynes,Reactions of Acetylide Anions,Carboncarbon bond formation with acetylide anions is a valuable reaction used in the synthesis of numerous natural products.,36,Alkynes,Reactions of Acetylide Anions,Acetylide anions are good nucleophiles that open epoxide rings by
18、 an SN2 mechanism. Backside attack occurs at the less substituted end of the epoxide.,37,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 on
19、e step, we use the process of retrosynthetic analysis : working backwards from a desired product to determine the starting materials from which it is made.,38,Alkynes,Synthesis,In designing a synthesis, reactions are often divided into two categories: 1. Those that form new carbon-carbon bonds. 2. T
20、hose that convert one functional group into anotherthat is, functional group interconversions.,39,Alkynes,Synthesis,40,41,42,11.28, 11.37, 11.40, 11.41, 11.44, 11.46, 11.47, 11.50, 11.51, 11.55,Homework,43,Preview of Chapter 12 Oxidation and Reduction,Reduction Reducing agents Reduction of alekene, alkyne Oxidation Oxidizing agents Oxidation of alcohol, alkene, and alkyne,