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Alkyne Reactions

By James Ashenhurst

Synthesis (5) – Reactions of Alkynes

Last updated: November 15th, 2022 |

A Reaction Map For Alkynes (PDF)

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.

  1. Summary of Alkyne Reactions: Addition, Deprotonation (+ SN2), And Oxidative Cleavage
  2. Key Reactions of Alkynes
  3. Visualizing The Addition and Oxidative Cleavage Reactions of Alkynes With A Reaction Map
  4. A Full Reaction Map PDF For First-Semester Organic Chemistry: Alkanes, Alkyl Halides, Alkenes, and Alkynes

1. Summary of Alkyne Reactions: Addition, Deprotonation (+ SN2), And Oxidative Cleavage

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 (see post: E and Z  Notation for Alkenes).

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”. (See post: Hydroboration and Oxymercuration of Alkynes)

And enols, as you’ll learn more about in Org 2, tend to be fairly unstable species. Through a process called tautomerism, (See post: Keto-Enol 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 (See post: Acetylides from Alkynes)

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].

2. Key Reactions of Alkynes

Here’s a list of the key reaction types:

image with key reactions of alkynes addition reactions such as hydrogenation bromination partial hydrogenation

3. Visualizing The Addition and Oxidative Cleavage Reactions of Alkynes With A Reaction Map

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.

transformations of alkynes gto give aldehydes ketones alkanes alkenes cis trans acetylides tetrahalides dihalides carbonyls

The previous 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).

4. A Full Reaction Map PDF For First-Semester Organic Chemistry: Alkanes, Alkyl Halides, Alkenes, and Alkynes

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-C≡C-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.

Test Yourself!

See if you can use the map to find ways to do these transformations (in any number of steps):

full-reaction-map-roadmap-alkynes-all-reactions-alkenes-all-reactions-alkyl-halides-over-50-reactions-on-one-sheet-scaled

Notes

Related Articles

00 General Chemistry Review
01 Bonding, Structure, and Resonance
02 Acid Base Reactions
03 Alkanes and Nomenclature
04 Conformations and Cycloalkanes
05 A Primer On Organic Reactions
06 Free Radical Reactions
07 Stereochemistry and Chirality
08 Substitution Reactions
09 Elimination Reactions
10 Rearrangements
11 SN1/SN2/E1/E2 Decision
12 Alkene Reactions
13 Alkyne Reactions
14 Alcohols, Epoxides and Ethers
15 Organometallics
16 Spectroscopy
17 Dienes and MO Theory
18 Aromaticity
19 Reactions of Aromatic Molecules
20 Aldehydes and Ketones
21 Carboxylic Acid Derivatives
22 Enols and Enolates
23 Amines
24 Carbohydrates
25 Fun and Miscellaneous
26 Organic Chemistry Tips and Tricks
27 Case Studies of Successful O-Chem Students

Comments

Comment section

21 thoughts on “Synthesis (5) – Reactions of Alkynes”

  1. Pingback: Reactions of Alkanes, Alkenes, and Alkynes

  2. Hey james, there’s a slight error in the key rxns of alkykes. In the last reaction(oxidative ozonolysis) the alkyne doesn’t break to give carboxylic acids, only the terminal alkynes do. But thanks for a nice worksheet anyway. It sure helos in JEE.

    Reply

  3. I haven been taught that whenever we use alcoholic KOH for the formation of unsaturated compounds ,rearrangement of double or triple bond will take place leading to more substituted unsaturated compounds.In case of Alkynes ,I thought the added heat will favour rearrangement not the KOH itself but this is not the case.So plz can you tell how KOH favours rearrangement in detail?

    Reply

  4. Please could you describe the mechanism for the reaction between Alkynes and benzene to form PAH’s? Thank you.

    Reply

  7. Just curious, when transforming a terminal alkyne specifically to an alkene, either with H2, Lindlar’s catalyst or Na, NH3 (l), does the reagent matter. If you the desired product is a terminal alkene with a double bond on one end of the compound, cis- and trans- specificity does not really matter. So can either of the above reagents be used?

    Reply

    1. Lindlar is milder and is probably the best choice. Na/NH3 would involve formation of a terminal acetylide ion and trying to pump electrons into a negatively charged species is not the easiest thing to do.

      Reply

      1. Hey James, does NH3 always have to be the solvent to create an acetylide anion from an alkyne? Can one just use water as a solvent and add NaNH2 to the alkyne to create the acetylide anion?

        Reply

        1. No, it doesn’t. There’s lots of other options. Alkyllithiums will do the job nicely, as will LDA in ethereal solvents and many other strong bases. Water is a no-go, though – it’s too acidic to use as a solvent for the deprotonation of alkynes.

          Reply

  8. Actually, in oxidation clevage, ozone makes either a ketone and a aldehyde,
    in your problem, it is a non-terminal alkyne, therefor it has to be ketone when reacted with O3

    Reply

  9. Hi, James,
    Your website is wonderful!
    Interestingly, I did a postdoc at McGill in 2012-2014. Now I’m working at a University in China with a bit of a struggle. Recently I’m working on a total synthesis project. Your summary of alkyne transformations is very helpful.
    Thank you!

    Yong

    Reply

  10. Sir, my professor told me that you can dehydrogenate an alkane to an alkene with the use of a metal catalyst like Pt. Also, I read in a book that you can reduce an alkyl halide to an alkane with LiAlH4. Why are these not in your map?

    (Please disregard the first comment I gave :) )

    Reply

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