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chapter8 - 8. Alkynes: An Introduction to Organic Synthesis...

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8. Alkynes: An Introduction to Organic Synthesis Based on McMurry’s Organic Chemistry , 7 th edition
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2 Alkynes Hydrocarbons that contain carbon-carbon triple bonds Acetylene, the simplest alkyne is produced industrially from methane and steam at high temperature Our study of alkynes provides an introduction to organic synthesis, the preparation of organic molecules from simpler organic molecules
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3 Why this chapter? We will use alkyne chemistry to begin looking at general strategies used in organic synthesis
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4 8.1 Naming Alkynes General hydrocarbon rules apply with “- yne as a suffix indicating an alkyne Numbering of chain with triple bond is set so that the smallest number possible for the first carbon of the triple bond
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5 8.2 Preparation of Alkynes: Elimination Reactions of Dihalides Treatment of a 1,2-dihalidoalkane with KOH or NaOH produces a two-fold elimination of HX Vicinal dihalides are available from addition of bromine or chlorine to an alkene Intermediate is a vinyl halide
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6 8.3 Reactions of Alkynes: Addition of HX and X 2 Addition reactions of alkynes are similar to those of alkenes Intermediate alkene reacts further with excess reagent Regiospecificity according to Markovnikov ANTI-addition
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7 Electronic Structure of Alkynes Carbon-carbon triple bond results from sp orbital on each C forming a sigma bond and unhybridized p X and p y orbitals forming π bonds. The remaining sp orbitals form bonds to other atoms at 180º to C-C triple bond. The bond is shorter and stronger than single or double Breaking a π bond in acetylene (HCCH) requires 318 kJ/mole (in ethylene it is 268 kJ/mole)
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8 Addition of Bromine and Chlorine Initial addition gives trans intermediate Product with excess reagent is tetrahalide
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Addition of HX to Alkynes Involves Vinylic Carbocations Addition of H-X to alkyne should produce a vinylic carbocation intermediate Secondary vinyl Secondary vinyl carbocations form carbocations form less readily than less readily than primary alkyl primary alkyl carbocations carbocations Primary vinyl Primary vinyl carbocations carbocations probably do not probably do not form at all form at all Nonethelss, H-Br can add to an alkyne to give a vinyl bromide if the Br is not on a primary carbon For the purposes of this course we will assume the same For the purposes of this course we will assume the same mechanism as for the alkenes, and a vinylic carbocation mechanism as for the alkenes, and a vinylic carbocation intermediate is acceptable. intermediate is acceptable.
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This note was uploaded on 04/07/2008 for the course CHGN 222 taught by Professor Cowley during the Spring '08 term at Mines.

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chapter8 - 8. Alkynes: An Introduction to Organic Synthesis...

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