OC Question...Alkyne Reduction

[TX Hopeful]

I'm a bit confused on the reduction of alkynes... I know that is you reduce an alkyne with Na and NH3, the result is a trans-alkene. BUT...

Kaplan flashcards say that the reduction of an alkyne on a metal surface (Pd) results in an alkane.

Schaum's states that the reduction of an alkyne with Lindlar's catalyst (H2/Pd) results in a cis-alkene.

An old DAT (1990) that I took says that an alkyne reacted with excess H2/Pd forms an alkane

Examkrackers states that syn addition of H2/Ni, Pd, or Pt creates a cis-alkene.

So, my question is... is it an alkane or a cis-alkene? Any help is appreciated. I take the DAT tomorrow morning. Thanks in advance.

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[aceking]

I'm a bit confused on the reduction of alkynes... I know that is you reduce an alkyne with Na and NH3, the result is a trans-alkene. BUT...

Kaplan flashcards say that the reduction of an alkyne on a metal surface (Pd) results in an alkane.

Schaum's states that the reduction of an alkyne with Lindlar's catalyst (H2/Pd) results in a cis-alkene.

An old DAT (1990) that I took says that an alkyne reacted with excess H2/Pd forms an alkane

Examkrackers states that syn addition of H2/Ni, Pd, or Pt creates a cis-alkene.

So, my question is... is it an alkane or a cis-alkene? Any help is appreciated. I take the DAT tomorrow morning. Thanks in advance.

it's been a long time, but i think i may remember it
Lindar Catalyst is basically a sheet of metal flat that has the H+ top and the the triple bond thing comes and attaches to the metal sheet; therefore, the only to get reduced is Cis because it is a flat sheet of menta (e.g. Pd)

the other one that does the Trans ene thing is because it aint no mental, it is revolving with liquid stuff. I believe it is like liq. nitrogen. Trans is usually more stable than cis.
you dont need to how they become what. just know that by using something you get something.

I AM NOT SO SURE IF I GOT IT RIGHT ABOUT THE CIS AND TRANS, BUT ABOUT 80% SURE it has been so long for me

 

[luder98]

I'm a bit confused on the reduction of alkynes... I know that is you reduce an alkyne with Na and NH3, the result is a trans-alkene. BUT...

Kaplan flashcards say that the reduction of an alkyne on a metal surface (Pd) results in an alkane.

Schaum's states that the reduction of an alkyne with Lindlar's catalyst (H2/Pd) results in a cis-alkene.

An old DAT (1990) that I took says that an alkyne reacted with excess H2/Pd forms an alkane

Examkrackers states that syn addition of H2/Ni, Pd, or Pt creates a cis-alkene.

So, my question is... is it an alkane or a cis-alkene? Any help is appreciated. I take the DAT tomorrow morning. Thanks in advance.

I don't have the book with me, I'm recalling from memory. You should check with Kaplan book. It's mentioned in there.

From what I remember, it's very difficult to stop at alkene. You must have either (Na, NH3) or Lin. catalyst to stop the rxn at alkene. Depending which one you use, you will produce cis or trans. I don't quite agree with Examkrackers' comment. Maybe they have mole ratio restriction. The others are correct including yours.

 

[TX Hopeful]

I don't have the book with me, I'm recalling from memory. You should check with Kaplan book. It's mentioned in there.

From what I remember, it's very difficult to stop at alkene. You must have either (Na, NH3) or Lin. catalyst to stop the rxn at alkene. Depending which one you use, you will produce cis or trans. I don't quite agree with Examkrackers' comment. Maybe they have mole ratio restriction. The others are correct including yours.

Thanks Lude. I think this is the answer. Let me know if I'm completely wrong.

You can't stop it at an alkene unless you have 1.) Na/NH3 in which the product is trans. or 2.) Lindlar's in which the product is cis. If it's a typical catalytic hydrogenation with H2 and metal (w/o Na/NH3 or Lindlar's), then it is reduced to an alkane.

 

[frogger33]

You can't stop it at an alkene unless you have 1.) Na/NH3 in which the product is trans. or 2.) Lindlar's in which the product is cis. If it's a typical catalytic hydrogenation with H2 and metal (w/o Na/NH3 or Lindlar's), then it is reduced to an alkane.[/QUOTE]

This is correct. I also have in my notes that instead of Na/NH3, it can also be LiNH3. Just that I would add that in there.

 

[MSZ]

Sorry if I'm repeating anything that's already been said, but I'm too lazy to go through everyone's response to see whether the correct answer has already been stated. I'm tutoring four people in organic chem right now, so it's fresh. This is how it is to my knowledge:

1. Catalytic hydrogenation is H2/Pd,Ni,Pt and they will reduce both an alkyne and an alkene to an alkane...so alkane is the answer to your question. However, it is important to recognize that H2 addition is a syn addition (it could affect the stereochemistry in your answer).

2. Dissoloving metal catalysts such as Li or Na metal with liquid NH3 will reduce an alkyne to a TRANS alkene (don't worry about the mechanism).

3. Lindlar's catalyst (H2/Pd and quinoline) will result in a CIS alkene which is why you may have seen a cis alkene as a possible answer in one of your answer guides. They aren't being specific and telling you that it's Lindlar's catalyst (they're forgetting about the quinoline). So, unless it says Lindlar, assume that H2 will reduce it to an alkane.

GOOD LUCK TOMORROW 🙂

 

[bouncy_ball]

I'm a bit confused on the reduction of alkynes... I know that is you reduce an alkyne with Na and NH3, the result is a trans-alkene. BUT...

Kaplan flashcards say that the reduction of an alkyne on a metal surface (Pd) results in an alkane.

Schaum's states that the reduction of an alkyne with Lindlar's catalyst (H2/Pd) results in a cis-alkene.

An old DAT (1990) that I took says that an alkyne reacted with excess H2/Pd forms an alkane

Examkrackers states that syn addition of H2/Ni, Pd, or Pt creates a cis-alkene.

So, my question is... is it an alkane or a cis-alkene? Any help is appreciated. I take the DAT tomorrow morning. Thanks in advance.

The difference between lindlar's catalyst from a plain old Platinum/Palladium catalyst is that lindlars has a 'poison' called Quinoline that limits the reaction from going all the way to an alkane. Thus, Lindlar's Catalyst (Pt/PD, H2, Quinoline) yields a cis alkene product.