Ch_322a_7.012

Ch_322a_7.012 - Example CH 3 CH 2 CH=CH 2 1-butene Br 2 CCl...

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Alkynes by Elimination Reactions Alkynes may be prepared from alkenes in a two-step synthetic sequence of bromination-debromination. General Scheme RCH=CHR Br 2 RCH-CHR Br Br Bromination vicinal dibromide alkene 2 B: - -2HBr RC CR alkyne Debromination The base must be sufficiently strong to affect two successive dehydrobromination reactions. The details of the bromination reaction of alkenes will be discussed later.
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Alkynes from Vicinal Dibromides RCH-CHR Br Br vicinal dibromide 2 B: - -2HBr RC CR alkyne Debromination A base sufficiently strong for the two successive dehydrobromination reactions is sodium amide (Na + NH 2 - ) in liquid ammonia (-33 o C) or in a hydrocarbon solvent at elevated temperature. Two Successive Dehydrobrominations H-N: H : - amido anion a strong base C + C H Br R Br R H C=C Br R R H bromoalkene (vinyl bromide) + NH 3 + Br - R-C C-R + NH 3 + Br - C=C Br R R H + :N-H H -
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Synthesis of Terminal Alkynes Three equivalents of base must be used in the synthesis of terminal alkynes because of the acidity of the alkynyl-H (pK a ~ 25).
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Unformatted text preview: Example CH 3 CH 2 CH=CH 2 1-butene Br 2 CCl 4 (solvent) CH 3 CH 2 CH Br CH 2 Br CH 3 CH 2 CH Br CH 2 Br NaNH 2 hydrocarbon solvent >100 o C CH 3 CH 2 CH=CH Br + CH 3 CH 2 C Br =CH 2 1 mixture NaNH 2 2 CH 3 CH 2 C C-H pK a ~ 25 NaNH 2 3 very fast CH 3 CH 2 C C:-+ NH 3 pK a ~ 38 NH 4 Cl/H 2 O (workup) CH 3 CH 2 C C-H 1-butyne The Acidity of Terminal Alkynes The order of base strength of the conjugate bases is: CH 3 CH 2 :-> strongest base H 2 C=CH :--> H C C:-weakest base Among a broader range of compounds, the relative acidity of terminal alkynes is: H-O-H RO-H > > pK a 15.7 16-17 RC C-H > 25 H 2-N-H 38 > RC H =C H-H > 44 RC H 2-H 50 > ~ and the relative base strengths of the conjugate bases are: RCH 2 : H 2 N: RCH=CH: RC C: RO: > H-O: > > >------> C-C H H H H H H pK a > 50 least acidic Acidities of Hydrocarbons C=C H H H H H 44 43 H C C-H 25 most acidic...
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Ch_322a_7.012 - Example CH 3 CH 2 CH=CH 2 1-butene Br 2 CCl...

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