9: Alkynes - An Introduction to Organic Synthesis
After you have completed Chapter 9, you should be able to
- fulfillall of the detailed objectives listed under each individual section.
- solve road-map problems involving any of the reactions introduced to this point.
- design multistep syntheses using any of the reactions introduced to this point, and determine the viability of a given synthesis.
- define, and use in context, the key terms introduced.
Addition reactions not only dominate the chemistry of alkenes, they are also the major class of reaction you will encounter. This chapter discusses an important difference between (terminal) alkynes and alkenes, that is, the acidity of the former; it also addresses the problem of devising organic syntheses. Once you have completed this chapter you will have increased the number of organic reactions in your repertoire, and should be able to design much more elaborate multistep syntheses. As you work through Chapter 9, you should notice the many similarities among the reactions described here and those in Chapters 7 and 8.
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- 9.1: Naming Alkynes
- Alkynes are organic molecules made of the functional group carbon-carbon triple bonds and are written in the empirical formula of CnH2n−2 . They are unsaturated hydrocarbons. Like alkenes have the suffix –ene, alkynes use the ending –yne; this suffix is used when there is only one alkyne in the molecule.
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- 9.3: Reactions of Alkynes - Addition of HX and X₂
- Reactions of alkynes, a class of hydrocarbons containing carbon-carbon triple bonds, exhibit various chemical transformations due to the presence of the highly reactive π-bonds. These reactions can be broadly categorized into addition reactions and substitution reactions.
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- 9.4: Hydration of Alkynes
- As with alkenes, hydration (addition of water) to alkynes requires a strong acid, usually sulfuric acid, and is facilitated by mercuric sulfate. However, unlike the additions to double bonds which give alcohol products, addition of water to alkynes gives ketone products ( except for acetylene which yields acetaldehyde ). The explanation for this deviation lies in enol-keto tautomerization.
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- 9.5: Reduction of Alkynes
- Reactions between alkynes and catalysts are a common source of alkene formation. Because alkynes differ from alkenes on account of their two procurable π bonds, alkynes are more susceptible to additions. Aside from turning them into alkenes, these catalysts affect the arrangement of substituents on the newly formed alkene molecule. Depending on which catalyst is used, the catalysts cause anti- or syn-addition of hydrogens. Alkynes can readily undergo additions because of their availability of tw
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- 9.7: Alkyne Acidity - Formation of Acetylide Anions
- Terminal alkynes are much more acidic than most other hydrocarbons. Removal of the proton leads to the formation of an acetylide anion, RC=C:-. The origin of the enhanced acidity can be attributed to the stability of the acetylide anion, which has the unpaired electrons in an sp hybridized orbital. The stability results from occupying an orbital with a high degree of s-orbital character.