Transcript Document
Chapter 11 Alkynes 1 Alkynes Introduction—Structure and Bonding • general molecular formula CnH2n-2 • the triple bond introduces two degrees of unsaturation. 2 Introduction—Structure and Bonding Cf. p bond in CH2CH2 : 64 Kcal/mol • Both p bonds of a C-C triple bond are weaker than the CC double bond for homolytic 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. 3 Alkynes Nomenclature • 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 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 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 derived from acetylene is called an ethynyl group. 4 Alkynes Nomenclature 5 Alkynes Physical Properties • 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 soluble in organic solvents and insoluble in water. 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 O Natural female sex hormones CH3 CH3 OH CH3 C CH3 H H H H H H O HO Progesterone Estradiol Oral contraceptive drugs CH3 OH CH3 C H C CH H H CH H H H OH H O HO 8 Ethynylestradiol Norethindrone 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 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 hormone levels in a woman, thereby preventing pregnancy. 9 Alkynes Acetylene and Other Interesting Alkynes 10 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 11 Alkynes Alkynes from Nature 12 Alkynes Preparation of Alkynes 13 Alkynes Preparation of Alkynes 14 Introduction to Alkyne Reactions—Additions 15 Introduction Nucleophile R C C H Terminal alkyne pKa = 25 to Alkyne + :B _ Reactions—Acids R Strong base C _ C: + H & B An acetylide anion NH2- and H- are strong enough bases (pKa (NH3) = 38, pKa(H2) = 35) to favor left-right equilibrium bias, OH- and OR- (pKa (H2O) = 15.7, pKa(ROH) = 15.5-18) are not R C _ C: Nucleophile + E+ R C C E Electrophile 16 Hydrohalogenation—Electrophilic Addition of HX hydrohalogenation 17 Alkynes Hydrohalogenation—Electrophilic Addition of HX 18 Alkynes Introduction to Alkyne Reactions • Electrophilic addition of HX to alkynes electrophilic addition of HX to alkenes is slower than • Markovnikov addition 19 Alkynes Introduction to Alkyne Reactions • Carbocation A is stabilized by resonance, but B is not. 20 Alkynes Halogenation—Addition of Halogen 21 Alkynes Halogenation—Addition of Halogen 22 Alkynes Hydration—Electrophilic Addition of Water • In the presence of strong acid or Hg2+ catalyst, the elements of H2O add to the triple bond. 23 Alkynes Hydration—Electrophilic Addition of Water • Internal alkynes undergo hydration with concentrated acid • terminal alkynes require the presence of an additional Hg2+ catalyst—usually HgSO4—to yield methyl ketones by Markovnikov addition of water. 24 Hydration—Electrophilic Addition of Water • 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. • Equilibrium favors the keto form largely because the C=O is much stronger than a C=C. • Tautomerization (the process of converting one tautomer 25into another), is catalyzed by both acid and base. Alkynes Hydration—Electrophilic Addition of Water 26 Alkynes Hydration—Electrophilic Addition of Water 27 Alkynes Hydroboration—Oxidation Hydroboration—oxidation is a two step reaction sequence that converts an alkyne to a carbonyl compound. 28 Alkynes Hydroboration—Oxidation 29 Alkynes Introduction to Alkyne Reactions—Acetylide anions 30 Alkynes Reactions of Acetylide Anions • Acetylides are strong nucleophiles • the mechanism of substitution is SN2, and thus the reaction is fastest with CH3X and 10 alkyl halides. 31 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. 32 Alkynes Reactions of Acetylide Anions 33 Alkynes Reactions of Acetylide Anions 34 Alkynes Reactions of Acetylide Anions • Carbon—carbon 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 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 desired 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 another—that is, functional group interconversions. 38 Alkynes Synthesis 39 40 41 Homework 11.28, 11.37, 11.40, 11.41, 11.44, 11.46, 11.47, 11.50, 11.51, 11.55 42 Preview of Chapter 12 Oxidation and Reduction Reduction Reducing agents Reduction of alekene, alkyne Oxidation Oxidizing agents Oxidation of alcohol, alkene, and alkyne 43