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Unformatted text preview: Organic Lecture Series CH 310/318 M
Textbook Assignment: Chapter 7 LECTURE 16 Today's Topics: Alkene- Hydrogenations; Alkynes-intro Notice & Announcements:
HW 6 posted-Due 3/19 1 Organic Lecture Series Alkynes Chapter 7
2 Organic Lecture Series Preparation from Alkenes Treatment of a vicinal dibromoalkane with two moles of base, most commonly sodium amide, results in two successive dehydrohalogenation reactions (removal of H and X from adjacent carbons) and formation of an alkyne. 3 Organic Lecture Series Preparation from Alkenes For a terminal alkene to a terminal alkyne, 3 moles of base are required: 4 Organic Lecture Series Addition of X2 Alkynes add one mole of bromine to give a dibromoalkene. Addition shows anti stereoselectivity. 5 Organic Lecture Series Addition of X2 The intermediate in bromination of an alkyne is a bridged bromonium ion. 6 Organic Lecture Series Addition of HX Alkynes undergo regioselective addition of either 1 or 2 moles of HX, depending on the ratios in which the alkyne and halogen acid are mixed. Follows Markovnikov's rule, but why?
7 Organic Lecture Series Addition of HX The intermediate in addition of HX is a 2 vinylic carbocation in preference to a 1vinylic carbocation. Reaction of the vinylic cation (an electrophile) with halide ion (a nucleophile) gives the product. 8 Organic Lecture Series Addition of HX In the addition of the second mole of HX, Step 1 is reaction of the electron pair of the remaining pi bond with HBr to form a carbocation. Of the two possible carbocations, the favored one is the resonance-stabilized 2carbocation. 9 Organic Lecture Series Reduction Treatment of an alkyne with hydrogen in the presence of a transition metal catalyst, most commonly Pd, Pt, or Ni, converts the alkyne to an alkane. No selectivity-complete reduction!!
10 Organic Lecture Series Reduction With the Lindlar catalyst, reduction stops at addition of one mole of H2.
This reduction shows syn stereoselectivity. Pt on CaCO3 11 Organic Lecture Series Hydroboration Addition of borane to an internal alkyne gives a trialkenylborane. Addition is syn stereoselective. 12 Organic Lecture Series Hydroboration To prevent dihydroboration with terminal alkynes, it is necessary to use a sterically hindered dialkylborane, such as (sia)2BH. Treatment of a terminal alkyne with (sia)2BH results in stereoselective and regioselective hydroboration. 13 Organic Lecture Series Hydroboration Treating an alkenylborane with H2O2 in aqueous NaOH gives an enol. Enol: A compound containing an OH group on one carbon of a carbon-carbon double bond. Combination of alkene and alcohol
14 Organic Lecture Series Hydroboration An enol is in equilibrium with a keto form by migration of a hydrogen from oxygen to carbon and migratrion of the double bond from C=C to C=O. Keto forms generally predominate at equilibrium. Keto and enol forms are tautomers and their interconversion is called tautomerism. 15 Hydroboration Organic Lecture Series Hydroboration/oxidation of an internal alkyne gives a ketone: Hydroboration/oxidation of a terminal alkyne gives an aldehyde: 16 Organic Lecture Series Addition of H2O: hydration In the presence of sulfuric acid and Hg(II) salts, alkynes undergo addition of water. (Omit mechanism)
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