Physical properties of alkynes

Alkynes resemble alkanes and alkenes in their physical properties. They share with these Examples of physical proper- 

Alkynes resemble alkanes and alkenes in their physical properties. They share with these other hydrocarbons the properties of low density and low water-solubility. Their boiling points are similar to those of alkanes. 

Acetylene is linear, with a carbon-carbon bond distance of 120 pm and carbon-hydrogen bond distances of 106 pm. 

The geometry at carbon changes from tetrahedral trigonal planar linear. 

All of these trends can be accommodated by the orbital hybridization model. The bond angles are characteristic for the sp, sp, and sp hybridization states of carbon and don t require additional comment. The bond distances, bond strengths, and acidities are related to the s character in the orbitals used for bonding, s Character is the fraction of the hybrid orbital contributed by an s orbital. Thus, an sp orbital has one quarter s character and three quarters p, an sp orbital has one third s and two thirds p, and an sp orbital one half s and one half p. We then use this information to analyze how various qualities of the hybrid orbital reflect those of its s and p contributors. 

Take C—H bond distance and bond strength, for example. Recalling that an electron in a 2s orbital is, on average, closer to the nucleus and more strongly held than an electron in a 2p orbital, it follows that an electron in an orbital with more s character will be more 

The general molecular formula for alkynes is because an alkyne has four fewer hydrogen 

The members of each series containing fewer than five carbon atoms are gases at room temperature. The boihng points of the alkynes, like those of alkanes and alkenes, increase as the number of carbon atoms increases because the London forces increase (Table 7.2). Branching tends to decrease the boiling point. 

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. 

At room temperature, the first three members of the series (ethyne, propyne and butyne) are gases, the others are liquids. Branching, as in the other hydrocarbons, decreases the boiling point. 

Name Number of Carbon Atoms Structural Formula Melting Point (°C) Boiling Point (°C) Density (water = 1) 

Alkynes are unsaturated compounds like alkenes and therefore their chemical properties are similar to those of alkenes. 

Alkynes undergo combustion reactions and addition reactions, as alkenes do. In addition, alkynes undergo substitution reactions with metals. 

The physical properties of alkynes (Table 9-1) 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. Acetylene, propyne, and the butynes are gases at room temperature, just like the coitesponding alkanes and alkenes. In fact, the boiling points of alkynes are nearly the same as those of alkanes and alkenes with similar carbon skeletons. 

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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. 

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. 

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 resemble those of hydrocarbons having a similar shape and molecular weight. 

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