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MAKING ALKYNES

This guide explores the synthesis of alkynes through double elimination reactions involving compounds with two halogens. If both halogens are on the same carbon, two E2 eliminations can produce an alkyne, although the second elimination is more challenging and necessitates a stronger base such as sodium amide (NaNH2) or sodium ethoxide (NaOEt). We will discuss the typical reagents and conditions used, including the preference for the trans isomer formation and various examples of suitable bases and solvents.

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MAKING ALKYNES

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  1. MAKING ALKYNES “DOUBLE ELIMINATION”

  2. COMPOUNDS WITH TWO HALOGENS If you have a compound with two halogens it can react twice (two E2 eliminations). If both halogens are on the same carbon, an alkyne is produced. The second elimination is more difficult than the first one - it requires a stronger base. more difficult, requires a stronger base like NH2- most E2 bases will work

  3. AMIDE VS. ETHOXIDE .. .. : : N O CH2CH3 H .. H more basic (N is less electro- negative then O) less basic (O accommodates the charge better) The usual reagent is sodium amide in liquid ammonia (-33o C): The usual reagent is sodium ethoxide in ethanol : NaNH2 / NH3 (liq) NaOEt / EtOH Both are made by adding sodium metal to the solvent.

  4. EXAMPLES KOH EtOH D NaNH2 NH3 (liq) NaNH2 NH3 (liq) trans (the reaction is more difficult for cis )

  5. THE REACTION CAN BE DONE IN TWO STEPS stops here with sodium ethoxide NaOEt EtOH D NaNH2 predominantly the trans isomer NH3 (liq) (lower energy product) stronger base brings about the second step

  6. COMPOUNDS WITH TWO HALOGENS If you have a compound with two halogens it can react twice (two E2 eliminations). If the halogens are on different carbons, a diene is usually produced. KOH EtOH D NaOEt Br2 EtOH D CCl4

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