Properties of Alkynes

Terminal alkynes have dipole moments that are larger than the dipole moments of terminal alkenes. Disubstituted alkynes with identical or even similar alkyl groups have no dipole moment. 

We have learned that the 71 bond of alkenes reacts in characteristic ways with electrophiles, oxidizing agents, and reducing agents. Alkynes, with two 7t bonds, undergo similar regiospecific reactions. However, both Ji bonds of alkynes can react, and this means that we have to pay attention to the experimental conditions (Sections 7.6 and 7.7). 

Do the conditions for adding a reagent to one Ji bond to give an alkene allow addition to the second 7i bond  

We have to consider the regiospecilicity of the addition reactions to the first and second 7i bonds. 

We have to learn how to control the stepwise progression of such reactions. 

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We have already discussed one important chemical property of alkynes the acidity of acetylene and terminal alkynes In the remaining sections of this chapter several other reactions of alkynes will be explored Most of them will be similar to reactions of alkenes Like alkenes alkynes undergo addition reactions We 11 begin with a reaction familiar to us from our study of alkenes namely catalytic hydrogenation... 

Examples of physical properties of alkynes are given in Appendix 1. 

Physical properties of alkynes [49, p. 251] are essentially similar to those of alkanes and alkenes. These compounds are weakly polar and are insoluble in water, but they are quite soluble in organic solvents of low polarity (e.g., ether, benzene, CCl ). Chemically, alkynes are more reactive than alkanes but behave like alkenes. The triple bond appears to be less reactive than the double bond in some reagents while more reactive in others. In a chemical reaction, the triple bond is usually broken into a double bond, which may eventually split into single bonds. 

Nomenclature of Alkynes—Physical and Chemical Properties of Alkynes... 

As might be expected, the physical properties of alkynes are very similar to those of alkenes and alkanes with the same number of carbons. For example, the boiling points of hexane. 1-hexene, and 1-hexyne are 69°C, 63°C, and 71°C. respectively. 

The excellent coordination properties of alkynes with transition metals led to their use as partners for the coupling with a large variety of unsaturated molecules. Two partners such as alkynes and alkenes can produce various modes of C-C bond formation. Linear or cyclic couplings can occur via different pathways, similar to those reported for two C=C bonds couplings (Scheme 1). 

Introduction 392 9-2 Nomenclature of Alkynes 393 9-3 Physical Properties of Alkynes 394 9-4 Commercial Importance of Alkynes 395 9-5 Electronic Structure of Alkynes 396 9-6 Acidity of Alkynes Formation of Acetylide Ions 397 9-7 Synthesis of Alkynes from Acetylides 399 9-8 Synthesis of Alkynes by Elimination Reactions 403 Summary Syntheses of Alkynes 404 9-9 Addition Reactions of Alkynes 405... 

The physical properties of alkynes (Table 9-2) are similar to those of alkanes and alkenes of similar molecular weights. Alkynes are relatively nonpolar and nearly insoluble in water. They are quite soluble in most organic solvents, including acetone, ether, methylene chloride, chloroform, and alcohols. Many alkynes have characteristic, mildly offensive odors. Ethyne, propyne, and the butynes are gases at room temperature, just... 

The physical properties of alkynes are similar to those of alkanes and alkenes. They are insoluble in water but quite soluble in organic solvents. Densities of liquid alkynes are less than that of water. 

Alkynylgold compounds are readily available from reactions of the parent alkyne and gold(I) halides in the presence of even a very weak base, owing to the enhanced CH acidic properties of alkynes. The donor-free products are oligomeric, probably through a combination of a- and... 

The physical properties of alkynes resemble those of hydrocarbons having a similar shape and molecular weight. 

At infrequent intervals, different chemists have juxtaposed properties of alkynes and alkenes. The purpose, of course, was to enrich their understanding of both families In so doing, a number of misconceptions of alkyne vs. alkene reactivity were clarified. The major conclusion that evolved was simple nucleophiles react faster with alkynes than with alkenes electrophiles (including radicals ) react slower with alkynes than with alkenes. Since new kinds of data are available, we believe that the comparisons are especially illuminating now. 

Properties of alkynes Alkynes have physical and chemical properties similar to those of the alkenes. Many of the reactions alkenes undergo, alkynes undergo as well. However, alkynes generally are more reactive than alkenes because the triple bond of alkynes has even greater electron density than the double bond of alkenes. This cluster of electrons is effective at inducing dipoles in nearby molecules, causing them to become unevenly charged and thus reactive. 

We have completed the first total syntheses of three types of natural products secosyrins, ichthyothereol, and SP-hydroxystreptazolone. Application of the unique properties of alkyne-Co2(CO)6 complexes as a crucial step made these total syntheses possible. The facile preparation of alkyne-Co2(CO)6 complexes as well as their intriguing properties will make them useful for other total synthesis of other natural products. 

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