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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: webassignbuddy on September 11, 2013, 07:42:58 PM

Title: Can someone explain to me step by step how this NaNH2/NH3 reaction proceeds
Post by: webassignbuddy on September 11, 2013, 07:42:58 PM
I've looked EVERYWHERE on the internet for terminal alkynes reacting with NaNH2/NH3 and there is NO site that shows the FULL pushing mechanism. Every single diagram is either confusing or a "shortcut" to the answer, which is NOT what I need right now for O-Chem.

(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fi11.photobucket.com%2Falbums%2Fa179%2Fslourg%2FHELPPPPPPPP_zpse4848032.png&hash=287d00b9e83c48e17301d611cccf6847)

I already got the answer to that question. It was pretty obvious (visually speaking). However, what I don't understand Is how the terminal alkyne reacts with the reagents used.
The furthest I've gotten is the NH2- deprotonates the terminal alkyne, and THAT'S IT according to this site: http://www.masterorganicchemistry.com/2011/07/29/reagent-friday-sodium-amide-nanh2-2/

On the other hand, this site says something COMPLETELY different and, to make matters worse, completely confusing: http://www.assignmenthelp.net/assignment_help/partial-reduction-of-alkynes.php

Can someone please show or tell me how the electrons flow and how you get to the product after reacting the terminal alkyne in the pic above with NaNH2/NH3?
Title: Re: Can someone explain to me step by step how this NaNH2/NH3 reaction proceeds
Post by: TwistedConf on September 11, 2013, 08:07:15 PM
You know it can't be reduction of the alkyne, as the triple bond is still there in the product, and the link you gave is describing different reactions from what you posted anyway.

The furthest I've gotten is the NH2- deprotonates the terminal alkyne, and THAT'S IT

Exactly... that's it.  It's an acid-base reaction, which you have undoubtedly already studied and already know the simple mechanism for, right?
Title: Re: Can someone explain to me step by step how this NaNH2/NH3 reaction proceeds
Post by: Babcock_Hall on September 11, 2013, 08:28:42 PM
Once you deprotonate the terminal alkyne, you have a nucleophile.  Is a carbonyl-containing compound a good electrophile?
Title: Re: Can someone explain to me step by step how this NaNH2/NH3 reaction proceeds
Post by: webassignbuddy on September 11, 2013, 08:35:51 PM
Once you deprotonate the terminal alkyne, you have a nucleophile.  Is a carbonyl-containing compound a good electrophile?

Ohh. yes...
Ok, for some reason I thought it was more complicated. Thanks to you and TwistedConf!
Title: Re: Can someone explain to me step by step how this NaNH2/NH3 reaction proceeds
Post by: azmanam on September 12, 2013, 08:29:00 AM
Your initial confusion is a common one. NaNH2 with NH3 as solvent is a strong base, good for deprotonating things like alkynes. Na° with NH3 is something completely different - it is a reductant, good for reducing alkynes to trans alkenes.

NaNH2 is an anionic nitrogen atom with two hydrogens, two lone pairs, and a negative charge. This is what is used in this question, and on the Master Organic Chemistry site.
Na(+)    H-N(-)-H

Na° is elemental sodium metal with its one 3s valence shell electron. This is not what is being used here, but is what is being described on the assignmenthelp website.
Na·

They sure look like the same reagent, but they are not, and when they're written in line with the rest of the reagents, it can be hard to quickly see the difference. And sometimes the "°" isn't written next to the Na! Make sure to re-read these types of reagent to make sure you're using the correct reagent.