3.11: Physical Properties of Alkynes

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The characteristic of the triple bond helps to explain the properties and bonding in the alkynes. Hybridization due to triple bonds allows the uniqueness of alkyne structure. This triple bond contributes to the nonpolar bonding strength, linear, and the acidity of alkynes.

Solubility

Physical Properties include nonpolar due to slight solubility in polar solvents and insoluble in water. This solubility in water and polar solvents is a characteristic feature to alkenes as well. Alkynes dissolve in organic solvents.

Che 118B Properties of Alkynes image 3.bmp

Boiling Points

Compared to alkanes and alkenes, alkynes have a slightly higher boiling pointa. Ethane has a boiling point of -88.6 ºC, while Ethene is -103.7 ºC and Ethyne has a higher boiling point of -84.0 ºC. That is because the triple binds concentrate more electron density, making alkynes more polarizable and increasing the intensity of the London Dispersion forces (more polarizable electrons means easier to form temporary dipoles). Increasing the number of carbon atoms results in higher boiling points.

Boiling points for selected compounds
Compound	Boiling point (ºC)
Ethane	-88.6
Ethene	-103.7
Ethyne	-84
1-Butyne	8.08

Outside links

References

Bloch, D.R. Organic chemistry demystified, New York : McGraw-Hill, 2006.
Vollhardt. Schore, Organic Chemistry Structure and Function Fifth Edition, New York: W.H. Freeman and Company, 2007.

Problems

What is the carbon-carbon, carbon-hydrogen bond length for alkyne? Is it shorter or longer than alkane and alkene?
Which is the most acidic and most stable, alkane, alkene, or alkyne? And depends on what?
How many pi bonds and sigma bonds are involved in the structure of ethyne?
Why is the carbon-hydrogen bond so short?
What is the alkyne triple bond characterizes by? How is this contribute to the weakness of the pi bonds?
How is heat of hydrogenation effects the stability of the alkyne?

Contributors

Bao Kha Nguyen, Garrett M. Chin