ADDITION REACTIONS OF ALKYNES

Transcription

1 ADDITIN REATINS F ALKYNES

2 ADDITIN REATINS F ALKYNES two pi bonds A carbon-carbon triple bond is like two carbon-carbon double bonds. Most double bond reagents will react twice.

3 ALKYNES ARE LESS REATIVE TAN ALKENES (sp 2 ) bonds are longer weaker less s character (sp) bonds are shorter stronger more s character SSS RULE Shorter - Stronger - more S character

4 SIMPLE ADDITINS D IT TWIE!

5 ADDITIN F l T AN ALKYNE 3 l first mole 3 l l second mole Follows the Markovnikoff Rule in both additions. 3 l l Also Br and I..

6 ADDITIN F BRMINE T AN ALKYNE 3 Br 2 first mole 3 Br Br anti addition Br - Br 2 second mole 3 + Br 3 Br Br Br Br Both additions proceed through a bridged intermediate and both additions are stereospecific anti. Also l 2 and I 2

7 ENL-KET TAUTMERISM YDRATIN F ALKYNES SMETING DIFFERENT When you add water to a triple bond, it doesn t add twice - rather only once, usually giving a ketone as the product.

8 ADDITIN F WATER ALKYNES ARE LESS REATIVE TAN ALKENES 3 + dilute 2 S reaction is slow Water doesn t add under the usual conditions (dil. 2 S 4 ). A stronger electrophile than 3 + is required.. gs 4 is added.

9 MERURY IS LIKE A BIG PRTN g 2+ is a stronger electrophile than slower + g 2+ faster g 2+ is not only a stronger Lewis acid, it is large and it forms a bridged ion. So we use a mixture of 2 S 4, 2 and gs 4 as our reagents. ( gs 4 = mercuric sulfate )

10 ADDITIN F WATER T AN ALKYNE 3 2 S 4 / gs g +2 stronger electrophile g 2 + bridged ion, anti opening 3 unstable enol 3 + g + exchanges for a proton 3 3 ketone Follows the Markovnikoff Rule

11 ENLS ARE UNSTABLE (USUALLY( ol USUALLY) discussed later ENLS : ( have - attached to a double bond) Think of this combination as unstable. ene NTE : Phenols are not enols and they are very stable (benzene resonance).

12 ENL - KET TAUTMERISM

13 ENL 3 2 unstable enol TAUTMERISM KET 3 2 TAUTMERS : species in equilibrium that differ in the position of a proton or other group. Most enols are not favored, keto enol they rapidly change to keto. E N E R G Y To interconvert tautomers, a proton is transferred from oxygen to carbon. Mechanism, next slide.. Bond Energies: = 179 =

14 NVERSIN F AN ENL T A KETNE ( catalyzed by small amounts of acid ) ENL KETNE

15 VERALL RESULT (NT( A STEPWISE MEANISM) ENL : KET

16 YDRGENATIN F ALKYNES

17 YDRGENATIN F ALKYNES Alkynes react twice with hydrogen when a metal catalyst is present. The alkene is formed first, and then the alkane Pd 3 3 syn addition cis 2 /Pd Since alkenes are more reactive than alkynes, it is difficult to stop the reaction after the first addition.

18 MEANISM F ATALYTI YDRGENATIN REVIEW R R R R R R.. R R ALKENE PIKS UP TW YDRGENS syn addition metal catalyst with adsorbed hydrogen atoms

19 LINDLAR onverting an alkyne to a cis alkene.

20 LINDLAR ATALYST There is a special deactivated catalyst, Lindlar, that allows the hydrogenation of an alkyne to stop at the alkene, after the addition of only one mole of hydrogen. Lindlar is a palladium catalyst, where the palladium is precipitated onto particles of barium sulfate from a solution which has quinoline present. Lindlar = Pd / BaS 4 / quinoline N.. quinoline The quinoline is thought to poison, or reduce the activity, of the catalyst. Many sulfur and nitrogencontaining compounds act as poisons.

21 With Lindlar catalyst, only one mole of hydrogen will add to an alkyne. As with other catalysts, hydrogenations with Lindlar are syn stereospecific additions Lindlar catalyst 3 3 cis syn addition Pd (without Lindlar)

22 PISNING MEANISM IS UNKNWN A PSSIBLE YPTESIS IS GIVEN ERE Quinoline binds to sites on the catalyst. Linear alkynes can approach and pick up hydrogens. Planar alkenes are blocked. bound quinoline QUINLINE QUINLINE surface of Lindlar catalyst R- -R QUINLINE QUINLINE alkyne fits R R QUINLINE QUINLINE = R QUINLINE R QUINLINE QUINLINE alkene blocked ( probably over-simplified )

23 MAKING A trans -ALKENE an you also convert an alkyne to a trans -alkene? YES...

24 SDIUM METAL IN LIQUID AMMNIA This is a different type of reaction (not a catalytic hydrogenation) and it converts an alkyne to a trans alkene. 3 3 Na N 3 (liq) - 60 o 3 3 trans anti addition Just keep in mind that it is possible to make trans alkenes as well as cis alkenes, however, a different reaction is used.

25 NTRLLING TE UTME hemists are always trying to either discover or design pairs of reactions that allow one to control the outcome of a change. especially regioselectivity or stereochemistry. onsider: Br (peroxide free) Markovnikoff alkyne to cis alkene (catalytic hydrogenation) versus and now versus Br (peroxides Anti-Markovnikoff alkyne to trans alkene (sodium/ liquid ammonia)

26 SYNTESIS If the organic chemist didn t have methods giving different results, it would be difficult to do synthesis. The task of a synthetic chemist is to make molecules where each atom is arranged in the correct place and with the correct stereochemistry. There are 256 stereoisomers of cholesterol! To synthesize (construct) a molecule of cholesterol in the lab, the chemist would have to arrange the atoms in the correct order with the correct types of bonds and control the outcome at all eight stereocenters or the molecule would not be cholesterol.