Unformatted text preview: Organic Lecture Series CH 310/318 M
Textbook Assignment: Chapter 7 LECTURE 23 Today's Topics: Alkynes: reactions & preparations Notice & Announcements:
SPRING BREAK!!! 3-12 TO 3-16 1 Organic Lecture Series Alkynes Chapter 7
2 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?
3 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. 4 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. 5 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!!
6 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 7 Organic Lecture Series Hydroboration Addition of borane to an internal alkyne gives a trialkenylborane. Addition is syn stereoselective. 8 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. 9 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
10 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. 11 Hydroboration Organic Lecture Series Hydroboration/oxidation of an internal alkyne gives a ketone: Hydroboration/oxidation of a terminal alkyne gives an aldehyde: 12 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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