Last updated: February 8th, 2020 |
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
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].
2. Key Reactions of Alkynes
Here’s a list of the key reaction types:
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.
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).
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-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.
Test Yourself!
See if you can use the map to find ways to do these transformations (in any number of steps):
In order to continue enjoying our site, we ask that you confirm your identity as a human. Thank you very much for your cooperation.
Learning Objective
- predict the products and specify the reagents for the full or partial reduction of alkynes
Alkynes can undergo reduction reactions similar to alkenes. These reactions are also called hydrogenation reactions. With the presence of two pi bonds within the carbon-carbon triple bonds, the reduction reactions can be partial or complete depending on the reagents. Since partial reduction of an alkyne produces an alkene, the stereochemistry of the addition mechanism determines whether the cis- or trans- alkene is formed. The three most significant alkyne reduction reactions are summarized below.
Like alkenes, alkynes readily undergo catalytic hydrogenation partially to cis- or trans- alkenes or fully to alkanes depending on the reaction employed.
The catalytic addition of hydrogen to 2-butyne provides heat of reaction data that reflect the relative thermodynamic stabilities of these hydrocarbons, as shown above. From the heats of hydrogenation, shown in blue in units of kcal/mole, it would appear that alkynes are thermodynamically less stable than alkenes to a greater degree than alkenes are less stable than alkanes. The standard bond energies for carbon-carbon bonds confirm this conclusion. Thus, a double bond is stronger than a single bond, but not twice as strong. The difference ( 63 kcal/mole ) may be regarded as the strength of the π-bond component. Similarly, a triple bond is stronger than a double bond, but not 50% stronger. Here the difference ( 54 kcal/mole ) may be taken as the strength of the second π-bond. The 9 kcal/mole weakening of this second π-bond is reflected in the heat of hydrogenation numbers ( 36.7 - 28.3 = 8.4 ).
Alkynes can be fully hydrogenated into alkanes with the help of a platinum, paladium, or nickel catalyst. Because the reaction is catalyzed on the surface of the metal, it is common for these catalysts to dispersed on carbon (Pd/C) or finely dispersed as nickel (Raney-Ni). The full reduction of 2-butyne is shown below as an example.
Since alkynes are thermodynamically less stable than alkenes, we expect addition reactions of alkynes to be more exothermic and relatively faster than equivalent reactions of alkenes. For catalytic hydrogenation, the Pt, Pd, or Ni catalysts are so effective in promoting addition of hydrogen to both double and triple carbon-carbon bonds that the alkene intermediate formed by hydrogen addition to an alkyne cannot be isolated. A less efficient catalyst, Lindlar's catalyst permits alkynes to be converted to alkenes without further reduction to an alkane. Lindlar’s Catalyst transforms an alkyne to a cis-alkene because the hydrogenation reaction is occurring on the surface of the metal. Both hydrogen atoms are added to the same side of the alkyne as shown in the syn-addition mechanism for hydrogenation of alkenes in the previous chapter.
Lindlar's catalyst is prepared by deactivating (or poisoning) a conventional palladium catalyst. Lindlar’s catalyst has three components: palladium-calcium carbonate, lead acetate and quinoline. The quinoline serves to prevent complete hydrogenation of the alkyne to an alkane. This approach is similar to the one used for hydration of alkynes using a dialkyl borane for hydroboration. A strong reagent is modified into a less reactive form.
Alkynes can be reduced to trans-alkenes with the use of sodium dissolved in an ammonia solvent. A sodium radical donates an electron to one of the p-orbitals in the carbon-carbon triple bond. This reaction forms an anion that can be protonated by a hydrogen atom in the ammonia solvent which prompts another sodium radical to donate an electron to the second p-orbital. The resulting anion is also protonated by a hydrogen from the ammonia solvent to produce a trans-alkene according to the mechanism shown below.
Mechanism for Hydrogenation of Alkynes to trans-Alkenes
Exercise
1. Using any alkyne how would you prepare the following compounds: pentane, trans-4-methyl-2-pentene, cis-4-methyl-2-pentene.
Answer1.
Contributors
- Was this article helpful?