By James Ashenhurst
Synthesis (5) – Reactions of Alkynes
Last updated: March 26th, 2019
Today, we’re going to add the reactions of alkynes to our reaction map, which will bring to a close all the major reactions we’ve discussed so far in a typical first semester course.
Like alkenes, the main pathway found in the reactions of alkynes is “addition” – that is, breaking the C-C π bond and forming two new single bonds to carbon. The product of an addition reaction to an alkyne is an alkene – and, as we just mentioned, alkene reactions undergo addition reactions too. The upshot of this is that since alkynes possess two π bonds, one must always be alert to the possibility of two addition reactions occurring. Furthermore, should only one addition occur, the stereochemistry of the addition should be well noted, as it can lead to the formation of geometric isomers (i.e. E/Z isomers). Finally, there is an additional complexity in certain alkyne reactions that is not found in the reactions of alkenes. When water is added across an alkyne, the resulting product is an “enol”. And enols, as you’ll learn more about in Org 2, tend to be fairly unstable species. Through a process called tautomerism, they convert to their constitutional isomers containing carbonyl (C=O) groups such as aldehydes and ketones.
Another reaction not present in the reactions of alkenes is deprotonation. Alkynes are unusually acidic hydrocarbons, with a pKa of about 25 (compare that to alkenes (pka = 43) and alkanes (pKa = 50). The deprotonation of alkynes leads to its conjugate base, an “acetylide”, which is an excellent nucleophile. The reaction of acetylides with alkyl halides (in SN2 reactions) is one of the few carbon-carbon bond forming reactions learned in Org 1, which makes it arguably the most important reaction to learn for synthesis this semester.
Finally, alkynes also undergo oxidative cleavage reactions. Treatment of alkynes with either ozone or KMnO4 leads to carboxylic acids [terminal alkynes give carbonic acid, which decomposes to CO2 and water].
Here’s a list of the key reaction types:
One useful way to help visualize these reactions is to make a “spiderweb” diagram, showing how alkynes are transformed into a variety of functional groups. This is what it looks like for alkynes.
The last post in the series on alkynes was entitled, “Alkynes Are A Blank Canvas“. Alkynes are a blank canvas because on top of their own transformations, through partial reduction (Na/NH3 or Lindlar) alkynes can also be transformed to alkenes, (which themselves have a host of reactions) or even alkanes (which can then be transformed to alkyl halides, which also have a host of reactions).
This updated reaction map shows all the key reactions of alkanes, alkyl halides, alkenes, and alkynes covered in this and previous blog posts. One note – in a large map such as this, compromises had to be made: it is impossible to maintain complete self-consistency between all the structures drawn for each functional group and the resulting reactions. For example, on the sheet, alkynes are depicted as R-CΞC-R, (internal alkyne) while the products of certain reactions clearly come from a terminal alkyne. Each functional group should be interpreted figuratively (i.e. including its common variations) and not literally.
See if you can use the map to find ways to do these transformations (in any number of steps):
- alkane to alkyne
- alkyne to an alcohol (4 ways)
- alkyne to an alkyl halide
- (challenging) go from a cis-alkene to a trans-alkene