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Organic Reagents

By James Ashenhurst

Reaction Friday – Reduction of Alkynes to Alkenes with Na/NH3

Last updated: March 27th, 2019

My apologies for the interruption. I seem to have wrested control of my blog back from the cats for now, but I’m not sure how long this will last. Clever little bugger. One thing I can say though – the cat picked a lot of questions to answer that I’d been ignoring because they seemed “dumb”. A lot of students are reluctant to ask those types of questions in public, in front of their class, or to their intimidating professor. So if he hacks in again, I hope he keeps answering those types of questions. Although he could sure use some spelling and grammar tips.

Let’s get back to talking about reactions again. Reaction Friday  – this instalment – involves the reduction of alkynes to give trans-alkenes using sodium in ammonia (Na/NH3). Not to be confused with NaNH2/NH3!

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9 thoughts on “Reaction Friday – Reduction of Alkynes to Alkenes with Na/NH3

  1. Hi, I read some where ( cant remember sorry) that free radical reactions dont involve any charges. I have told by my teacher many times when use the one headed arrow do not show any charges during the mechanism and obvioulsy you showed some !! . I am confused.


    1. The reason is that for the vast majority of free radical reactions you will encounter, you will form/break covalent bonds. Here we have sodium (Na) which is forming an ionic bond (electronegativity of Na is about 1.0). So in this case we end up drawing the charges. Hope that helps

  2. I am wondering whether this is really done, like in practice. One just use solid sodium and ammonia, and really get trans alkene?

  3. We were told not to confuse NaNH3 with NaNH2. Is the only difference that one is a stronger base than the other?

    1. Na in NH3 is a reducing agent. Sodium can easily donate an electron, and that’s what leads to reduction of alkynes to alkenes. Sodium amide (NaNH2) is a strong base.

  4. Hey,
    I was wondering about the significance of -78°C in the reaction of an alkyne plus Na & NH3 to form a trans alkene. Is it because the solution is put in an ice bath or something?

    1. Great question!!! If the reaction was run at -74 or -72 it wouldn’t make much of a difference, it’s just that -78 happens to be the temperature of the very cheap and convenient dry ice/acetone bath.

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