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Alkynes • Alkynes Are Molecules That Incorporate a C≡C Triple Bond

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Alkynes • Alkynes Are Molecules That Incorporate a C≡C Triple Bond 10.1 Alkynes • Alkynes are molecules that incorporate a C≡C triple bond. Copyright 2012 John Wiley & Sons, Inc. 10 -1 Klein, Organic Chemistry 1e 10.1 Alkynes • Given the presence of two pi bonds and their associated electron density, alkynes are similar to alkenes in their ability to act as a nucleophile. • Converting pi bonds to sigma bonds generally makes a molecule more stable. WHY? Copyright 2012 John Wiley & Sons, Inc. 10 -2 Klein, Organic Chemistry 1e 10.1 Alkyne Uses • Acetylene is the simplest alkyne. – It is used in blow torches and as a precursor for the synthesis of more complex alkynes. • More than 1000 different alkyne natural products have been isolated. – One example is histrionicotoxin, which can be isolated from South American frogs, and is used on poison-tipped arrows by South American tribes. Copyright 2012 John Wiley & Sons, Inc. 10 -3 Klein, Organic Chemistry 1e 10.1 Alkyne Uses • An example of a synthetic alkyne is ethynylestradiol. • Ethynylestradiol is the active ingredient in many birth control pills. • The presence of the triple bond increases the potency of the drug compared to the natural analog. • How do you think a C≡C triple bond affects the molecule’s geometry? Its rigidity? Its intermolecular attractions? Copyright 2012 John Wiley & Sons, Inc. 10 -4 Klein, Organic Chemistry 1e 10.2 Alkyne Nomenclature • Alkynes are named using the same procedure we used in Chapter 4 to name alkanes with minor modifications: 1. Identify the parent chain, which should include the C≡C triple bond. 2. Identify and name the substituents. 3. Assign a locant (and prefix if necessary) to each substituent, giving the C≡C triple bond the lowest number possible. 4. List the numbered substituents before the parent name in alphabetical order. Ignore prefixes (except iso) when ordering alphabetically. 5. The C≡C triple bond locant is placed either just before the parent name or just before the -yne suffix. Copyright 2012 John Wiley & Sons, Inc. 10 -5 Klein, Organic Chemistry 1e 10.2 Alkyne Nomenclature • Alkynes are named using the same procedure we used in Chapter 4 to name alkanes with minor modifications: 1. Identify the parent chain, which should include the C≡C triple bond. 2. Identify and name the substituents. Copyright 2012 John Wiley & Sons, Inc. 10 -6 Klein, Organic Chemistry 1e 10.2 Alkyne Nomenclature • Alkynes are named using the same procedure we used in Chapter 4 to name alkanes with minor modifications: 3. Assign a locant (and prefix if necessary) to each substituent. giving the C≡C triple bond the lowest number possible. – The locant is ONE number, NOT two. Although the triple bond bridges carbons 2 and 3, the locant is the lower of those two numbers. Copyright 2012 John Wiley & Sons, Inc. 10 -7 Klein, Organic Chemistry 1e 10.2 Alkyne Nomenclature • Alkynes are named using the same procedure we used in Chapter 4 to name alkanes with minor modifications: 4. List the numbered substituents before the parent name in alphabetical order. Ignore prefixes (except iso) when ordering alphabetically. 5. The C≡C triple bond locant is placed either just before the parent name or just before the -yne suffix. Copyright 2012 John Wiley & Sons, Inc. 10 -8 Klein, Organic Chemistry 1e 10.2 Alkyne Nomenclature • In addition to the IUCAP naming system, chemists often use common names that are derived from the common parent name acetylene. • You should also be aware of the terminology below. • Practice with SKILLBUILDER 10.1. Copyright 2012 John Wiley & Sons, Inc. 10 -9 Klein, Organic Chemistry 1e 10.2 Alkyne Nomenclature • Name the molecule below. • Recall that when triple bonds are drawn, their angles are 180°. Copyright 2012 John Wiley & Sons, Inc. 10 -10 Klein, Organic Chemistry 1e 10.3 Alkyne Acidity • Recall that terminal alkynes have a lower pKa than other hydrocarbons. • Acetylene is 19 pKa units more acidic than ethylene, which is 1019 times stronger. – Does that mean that terminal alkynes are strong acids? Copyright 2012 John Wiley & Sons, Inc. 10 -11 Klein, Organic Chemistry 1e 10.3 Alkyne Acidity • Because acetylene (pKa=25) is still much weaker than water (pKa=15.7), a strong base is needed to make it react. • Recall from Chapter 3 that we used the acronym ARIO to rationalize differences in acidity strengths. – Use ARIO to explain why acetylene is a stronger acid than ethylene which is stronger than ethane. Copyright 2012 John Wiley & Sons, Inc. 10 -12 Klein, Organic Chemistry 1e 10.3 Alkyne Acidity • Use ARIO to rationalize the equilibria below. • A base’s conjugate acid pKa must be greater than 25 for it to be able to deprotonate a terminal alkyne. Copyright 2012 John Wiley & Sons, Inc. 10 -13 Klein, Organic Chemistry 1e 10.4 Preparation of Alkynes • Like alkenes, alkynes can also be prepared by elimination. Copyright 2012 John Wiley & Sons, Inc. 10 -14 Klein, Organic Chemistry 1e 10.4 Preparation of Alkynes • Such eliminations usually occur via an E2 mechanism: – GEMINAL dihalides can be used. – VICINAL dihalides can also be used. – E2 requires anti-periplanar geometry. Copyright 2012 John Wiley & Sons, Inc. 10 -15 Klein, Organic Chemistry 1e 10.4 Preparation of Alkynes • Often, excess equivalents of NaNH2 are used to shift the equilibrium toward the elimination products. 1- • NH2 is quite strong, so if a terminal alkyne is produced, it will be deprotonated. • That equilibrium will greatly favor products. Copyright 2012 John Wiley & Sons, Inc. 10 -16 Klein, Organic Chemistry 1e 10.4 Preparation of Alkynes • A proton source is needed to produce the alkyne. • Predict the products in the example below. • Practice with CONCEPTUAL CHECKPOINT 10.7. Copyright 2012 John Wiley & Sons, Inc. 10 -17 Klein, Organic Chemistry 1e 10.5 Reduction of Alkynes • Like alkenes, alkynes can readily undergo hydrogenation. • Two equivalents of H2 are consumed for each alkynealkane conversion. • The cis alkene is produced as an intermediate. WHY cis? Copyright 2012 John Wiley & Sons, Inc. 10 -18 Klein, Organic Chemistry 1e 10.5 Reduction of Alkynes – Poisoned Catalyst • A deactivated or poisoned catalyst can be used to selectively react with the alkyne. • Lindlar’s catalyst and P-2 (Ni2B complex) are common examples of a poisoned catalysts. Copyright 2012 John Wiley & Sons, Inc. 10 -19 Klein, Organic Chemistry 1e 10.5 Reduction of Alkynes – Poisoned Catalyst • Is this a syn or anti addition? • Practice with CONCEPTUAL CHECKPOINT 10.9. Copyright 2012 John Wiley & Sons, Inc. 10 -20 Klein, Organic Chemistry 1e 10.5 Reduction of Alkynes – Dissolving Metal Reductions • Reduction with H2 gives syn addition. • Dissolving metal conditions can give anti addition producing the trans alkene. • Ammonia has a boiling point of –33°C, so the temperature for these reactions must remain very low. • Why can’t water be used as the solvent? Copyright 2012 John Wiley & Sons, Inc. 10 -21 Klein, Organic Chemistry 1e 10.5 Reduction of Alkynes – Dissolving Metal Reductions • Mechanism—step 1: – Note the single-barbed and double-barbed (fishhook) arrows. – Why does Na metal so readily give up an electron? Copyright 2012 John Wiley & Sons, Inc. 10 -22 Klein, Organic Chemistry 1e 10.5 Reduction of Alkynes – Dissolving Metal Reductions • Mechanism—step 1: • Why is the first intermediate called a RADICAL ANION? • The radical anion adopts a trans configuration to reduce repulsion. Copyright 2012 John Wiley & Sons, Inc. 10 -23 Klein, Organic Chemistry 1e 10.5 Reduction of Alkynes – Dissolving Metal Reductions • Mechanism—step 2 and 3: • Draw the product for step 3 of the mechanism. Copyright 2012 John Wiley & Sons, Inc. 10 -24 Klein, Organic Chemistry 1e 10.5 Reduction of Alkynes – Dissolving Metal Reductions • Mechanism—step 4: • Do the pKa values for NH3 and the alkene favor the proton transfer? Copyright 2012 John Wiley & Sons, Inc. 10 -25 Klein, Organic Chemistry 1e 10.5 Reduction of Alkynes – Dissolving Metal Reductions • Predict the product(s) for the following reactions. • Practice with CONCEPTUAL CHECKPOINT 10.10. Copyright 2012 John Wiley & Sons, Inc. 10 -26 Klein, Organic Chemistry 1e 10.5 Reduction of Alkynes – Summary • Familiarize yourself with the reagents necessary to manipulate alkynes. • Practice with CONCEPTUAL CHECKPOINT 10.11. Copyright 2012 John Wiley & Sons, Inc. 10 -27 Klein, Organic Chemistry 1e 10.6 Hydrohalogenation of Alkynes • Like alkenes, alkynes also undergo hydrohalogenation. • Draw the final product for the reaction above. • Do the reactions above exhibit Markovnikov regioselectivity? Copyright 2012 John Wiley & Sons, Inc. 10 -28 Klein, Organic Chemistry 1e 10.6 Hydrohalogenation of Alkynes • Modeled after the hydrohalogenation of alkenes, you might expect alkynes to react by the same mechanism. • Yet, the mechanism above does not explain all observed phenomena: – A slow reaction rate – 3rd order overall rate law – Vinylic carbocations are especially unstable Copyright 2012 John Wiley & Sons, Inc. 10 -29 Klein, Organic Chemistry 1e 10.6 Hydrohalogenation of Alkynes • Kinetic studies on the hydrohalogenation of an alkyne suggest that the rate law is 1st order with respect to the alkyne, and 2nd order with respect to HX. • What type of collision would result in such a rate law? – Unimolecular, bimolecular, or termolecular? Copyright 2012 John Wiley & Sons, Inc. 10 -30 Klein, Organic Chemistry 1e 10.6 Hydrohalogenation of Alkynes • Reaction rate is generally slow for termolecular collisions. WHY? • Considering the polarizability of the alkyne, does the mechanism explain the regioselectivity? • May involve multiple competing mechanisms. Copyright 2012 John Wiley & Sons, Inc. 10 -31 Klein, Organic Chemistry 1e 10.6 Hydrohalogenation of Alkynes • Peroxides can be used in the hydrohalogenation of alkynes to promote anti-Markovnikov addition just like with alkenes. • Which product is E and which is Z? • The process proceeds through a free radical mechanism that we will discuss in detail in Chapter 11.
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