Difference Between Alkenes and Alkynes

The key difference between alkenes and alkynes is that the alkenes have carbon-carbon double bonds whereas the alkynes have carbon-carbon triple bonds.

Both alkenes and alkynes are hydrocarbons having carbon and hydrogen atoms. There can be other substituents attached to these molecules instead of hydrogens. Therefore, large numbers of molecules are possible. Due to multiple bonds, they can polymerize and make larger chains. Thus, they are especially valuable in useful polymer synthesis. For example, PVC, rubber, various plastic types, etc.

CONTENTS

1. Overview and Key Difference
2. What are Alkenes
3. What are Alkynes
4. Side by Side Comparison – Alkenes vs Alkynes in Tabular Form
5. Summary

What are Alkenes?

Alkenes are hydrocarbons with carbon-carbon double bonds. We call them as olefins as well. Ethene is the simplest alkene molecule, having two carbons and four hydrogens. It has one carbon-carbon double bond, and the molecular formula is C₂H₄. The chemical structure of this molecule is as follows:

H₂C ₌ CH₂

When naming the alkenes, we use the suffix “ene” instead of “ane” at the end of the name of an alkane. We should take the longest carbon chain containing the double bond and should number it in a way, to give the minimum number to the double bond. The physical properties of alkenes are similar to the corresponding alkanes.

Nature

Usually, the alkenes having low molecular weights are in the gaseous form at room temperature. For instance, ethane and propene are gases. Alkenes are relatively non-polar molecules; therefore, they dissolve in nonpolar solvents or solvents with very low polarity. Hence, alkenes are slightly soluble in water. Moreover, the density of alkenes is less than water.

These compounds undergo addition reactions due to the double bonds. For example, in the hydrogenation reaction, two hydrogens attached to the double bond and making the alkene to the corresponding alkane. This reaction speeds up in the presence of a metal catalyst. In an addition reaction like this, if the reagent that is going to attach to the double bond attaches to the same side of the molecule, we call it a syn addition. If the addition is on opposite sides, then we call it an anti addition.

Likewise, alkenes undergo various types of additions with molecules like halogens, HCl, water etc. The additions can take place as Markonikov or anti- Markonikov type. Moreover, we can make these molecules via elimination reactions. When considering the stability of alkenes, the more highly substituted the carbon atoms of the double bond, the greater is the stability. Further, alkenes may have diastereoisomers; therefore, can show stereoisomerism.

What are Alkynes?

Hydrocarbon molecules with carbon-carbon triple bond are alkynes. The common name for this family is acetylenes. Ethylene is the simplest molecule in this family with two carbons and two hydrogens. It has the molecular formula of C₂H₂ and following is its structure.

 H — C _≡ C — H

We can name these compounds in much the same way as alkenes. That is, we can name them by replacing the “ane” with “yne” at the end of the name of the corresponding alkane. There, we should number the chain of carbons to give the carbon atoms of the triple bond the lowest possible number.

Furthermore, the physical properties of alkynes are similar to the corresponding alkanes. Usually, the alkynes having low molecular weights are in the gaseous form at room temperature. For example, ethyne is a gas. Moreover, these compounds are relatively non-polar molecules; therefore, they dissolve in nonpolar solvents or solvents with very low polarity. Therefore they are slightly soluble in water. The density of alkynes is less than water. Alkynes undergo addition reactions, due to its triple bond. Also, we can synthesize them by elimination reactions.

What is the Difference Between Alkenes and Alkynes?

Alkenes and alkynes are unsaturated hydrocarbons. The key difference between alkenes and alkynes is that the alkenes have carbon-carbon double bonds whereas alkynes have carbon-carbon triple bonds. Moreover, Double bond carbons are sp² hybridized in alkenes, and triple bond carbons are sp hybridized in alkynes. A further difference between alkenes and alkynes is that the alkenes have no acidic hydrogen while alkynes have acidic hydrogen atoms.

The below infographic is a tabular representation of the difference between alkenes and alkynes.

Summary – Alkenes vs Alkynes

Alkenes and alkynes are hydrocarbon compound containing carbon atoms hydrogen atoms. Moreover, they are unsaturated compounds (have either double or triple bonds). The key difference between alkenes and alkynes is that the alkenes have carbon-carbon double bonds whereas the alkynes have carbon-carbon triple bonds.

Reference:

1. Libretexts. “Nomenclature of Alkenes.” Chemistry LibreTexts, National Science Foundation, 26 Nov. 2018. Available here  
2. “Alkene.” Wikipedia, Wikimedia Foundation, 28 Nov. 2018. Available here

Image Courtesy:

1.”Angles of Alkane Alkene Alkyne”By B Levin13 – Own work, (CC BY-SA 3.0) via Commons Wikimedia  
2.”IUPAC-alkyne”By Fvasconcellos (CC BY-SA 3.0) via Commons Wikimedia