Acetylides from Alkynes, And Substitution Reactions of Acetylides
Last updated: February 23rd, 2020 |
Deprotonation of Alkynes And Substitution Reactions (SN2) Of Acetylides Are The Two Most Important Reactions Of Alkynes
With the series of posts on alkenes in the can, let’s move on and talk about a closely related functional group that shares many reactions in common with alkenes: acetylenes, or as they are more commonly referred to, “alkynes”.
Alkynes, as we shall see, do share many reactions in common with alkenes. However there are some reactions of alkenes which do *not* work for alkynes (dihydroxylation with OsO4 and epoxidation with peroxyacids to give two examples), and conversely several reactions which only apply to alkynes and not to alkenes.
Today we’ll introduce two reactions that are of key importance for alkynes – and don’t have a corollary in alkene chemistry (as far as we’re concerned, anyway).
Table of Contents
- Deprotonation of Alkynes To Give “Acetylides”
- Alkylation of Acetylides With Alkyl Halides
- Why Is This So Important? Because It Forms A New Carbon-Carbon Bond!
- Summary: Acetylide Formation and Carbon-Carbon Bond Formation
- (Advanced) References and Further Reading
Let’s start with an important fact. Alkynes are unusually acidic hydrocarbons. Recall that the acidity of a compound is related to the stability of the conjugate base. In an alkyne, where the carbon is sp hybridized, the lone pair resides in an orbital with 50% s character [the 2s orbital is closer to the positively-charged nucleus than the 2p orbital, increasing stability]. Compare that to alkenes (sp2, 33% s-character) and alkanes (sp3, 25% s character) and we have an explanation as to why alkynes have a pKa of 25, which is a factor of 1017 more acidic than your typical alkene (pKa about 42) and 1025 more acidic than alkanes (pKa 50).
This means that alkynes can be deprotonated without resorting to heavy artillery (i.e. organolithium bases, or Schlösser’s base – not that you probably need to know about that particular reagent) required to deprotonate alkenes. Instead, readily available sodium amide (NaNH2) the conjugate base of ammonia (pKa 38) can be used, which is a big plus for convenience.
Deprotonation of alkynes with a strong base yields the conjugate base, often referred to as an “acetylide” :
So that’s certainly one way in which the chemistry of alkynes is distinct from that of alkenes: alkynes can be readily deprotonated.
Knowing that, care to take a guess what the next extension of this is? You might recall that in all cases that the conjugate base is a stronger nucleophile. And almost every reaction in organic chemistry involves the attack of a nucleophile upon an electrophile. So one logical application of being able to deprotonate alkynes into acetylide ions is that they are excellent nucleophiles and we can combine them with various electrophiles.
One of the simplest and yet most powerful reactions in terms of generating a diverse array of functional groups is the SN2 reaction, in which a nucleophile attacks a carbon with a good leaving group (alkyl halides or sulfonates). The “big barrier” to the SN2 is steric hindrance so the reaction works particularly well for methyl and primary alkyl halides (secondary alkyl halides are a bit iffy in this context).
Here’s an example of how the SN2 can be applied to alkynes.
It’s a typical substitution reaction: we’re forming and breaking a bond on the same carbon.
Now comes the important part. This SN2 reaction is particularly useful.
Notice the type of bond that’s being formed here: we’re forming a carb0n-carbon bond. If you’ve read previous posts leading up to this one, scratch your head for a moment: can you think of any examples where we’re forming a carbon-carbon bond? Probably not! [one example is cyanide ion with an alkyl halide].
Why is this important? Consider that organic chemistry is the study of carbon containing molecules. The reaction of acetylides with alkyl halides, therefore, is a way of extending the carbon skeleton of the molecule. We haven’t yet learned any other reactions that perform this function nearly as well.
If you are studying alkynes, it is likely that in the near future you will be asked to solve “synthesis” problems, where you build more complex molecules out of simpler parts. The SN2 reaction of acetylides with alkyl halides often plays a pivotal role, since it allows for the extension of the carbon chain. So pay particular attention to it!
In the next post, we’ll talk about some other reactions that are unique to alkynes, and see – once again – how stereochemistry can rear its ugly head.
Next Post: Partial Reduction of Alkynes
Not much to say here. This is a pretty standard acid-base reaction, driven by the acidity of the sp-H atom. The utility lies in that this is still a robust method of C-C bond formation, and a useful way to introduce alkynyl groups if desired.
- PREPARATION AND USE OF LITHIUM ACETYLIDE: 1-METHYL-2-ETHYNYL-endo-3,3-DIMETHYL-2-NORBORNANOL
Mark Midland, Jim I. McLoughlin, and Ralph T. Werley Jr
Org. Synth. 1990, 68, 14
I was initially a little surprised that something like this was published so recently in Organic Syntheses, but reading the discussion gives some context. The selective formation of the monolithiated species from deprotonation of acetylene is tricky.
Lars Skattebøl, E. R. H. Jones, and Mark C. Whiting
Org. Synth. 1959, 39, 56
Alkynyl Grignards can also be formed by deprotonation of a terminal alkyne with a Grignard reagent, as this procedure demonstrates.
- Stable Carbonium Ions. XVIII.1a Alkynyl Carbonium Ions
Charles U. Pittman Jr. and George A. Olah
Journal of the American Chemical Society 1965, 87 (24), 5632-5637
The experimental procedure has many methods for forming nucleophilic alkynyl groups, which are then reacted with various ketones to form alkynyl alcohols, and then ionized in superacid media in order to characterize the resulting carbocations.
Kenneth N. Campbell and Barbara K. Campbell
Org. Synth. 1950 30, 15
An extremely simple example of this reaction. The deprotonation is done with Na metal in liquid ammonia, and care has to be taken to avoid the conditions of dissolving metal reduction (the procedure states that the reaction should not turn blue)
- Synthesis of Unsymmetrical Alkynes via the Alkylation of Sodium Acetylides. An Introduction to Synthetic Design for Organic Chemistry Students
Jennifer N. Shepherd and Jason R. Stenzel
Journal of Chemical Education 2006, 83 (3), 425
A nice paper that describes the adaptation of this reaction for undergraduate teaching labs.