What are the Properties of Alkynes

While answering the question ‘what are the properties of alkynes,’ this article also presents you adequate information about alkynes. Alkyne group is a member of the hydrocarbon family; contains a Carbon-Carbon triple bond in their structure. Alkynes possess similar physical properties as alkanes and alkenes. Alkynes are more reactive than alkanes and alkynes show reactions similar to alkanes. This article briefly explains about the similar properties which alkynes share with the other hydrocarbons and the differences which they possess. Similar to alkanes and alkenes, alkynes also have cyclic members apart from the acyclic (non-cyclic) members. However, this article focuses only on the acyclic alkynes, which have the general molecular formula of C_nH_2n-2.

What are Alkynes

Alkynes are a category of hydrocarbons containing a Carbon-Carbon triple (C=C) bond. Because of the triple bond, an alkyne has four fewer hydrogen atoms than its’ corresponding alkane. Similar to alkanes and alkenes, alkynes also form both cyclic and non-cyclic molecular structures. The general molecular formula for acyclic (non-cyclic) alkanes is C_nH_2n-2 and that of cyclic alkanes is C_nH_2n-4. Some properties of alkynes are different from other hydrocarbons.

General Molecular Formula of Alkynes

Acyclic alkanes have the general molecular formula of C_nH_2n-2. The smallest alkane is Methane (Acetylene C₂H₂). Alkynes that have triple bonds at the end of Carbon chain (RC=H), are called terminal alkynes or monosubstituted alkynes. As a group, terminal alkynes are the most acidic hydro carbon group. Disubstituted alkynes have internal triple bonds (RC=R’).

Chemical Properties of Alkynes

Molecular structure of alkynes

The Carbon-Carbon triple bond has the linear geometry. It characterizes the -C=C- and C-C=C-C units of terminal and internal triple bonds respectively.The triply bonded Carbon atoms are sp-hybridized.

Acidity of terminal alkynes

The =C-H bond of the alkynes shows little tendency to ionize. However, alkanes and alkenes are all weak acids.

Reactivity of alkynes

Unlike alkanes, alkynes show many reactions with chemical reagents. Most of them are addition reactions as in alkenes. However, terminal alkynes undergo substitution reactions too. 

In general, both of alkanes and alkynes are more reactive than alkanes and alkenes. Alkynes undergo similar types of reactions. For example, they both undergo electrophilic addition reactions with halogens, acids and oxidizing agents. One of the most significant differences in chemical reactivity between alkenes and alkynes is due to the differences in acidity. Terminal alkynes contain an acidic hydrogen that can be removed by a strong base to yield an acetylide anion. As acetylide anion’s negative charge is in a hybrid orbital with more of s character, which allow the charge to be closer to the nucleus, it is more stable than either alkyl anion or vinylic anion.

Alkanes and alkenes do not react as follows.

Physical Properties of Alkynes

Alkynes resemble the alkenes and alkanes in physical properties.The molecules that have a lower molecular weight are gases at room temperature and as the molecular weight increases their physical state changes from gas to liquid and then to solids.

Solubility of Alkynes

Both alkenes and alkynes are relatively polar molecules. Therefore, both of these groups are soluble in polar solvents or solvents in low polarity. Alkynes are slightly soluble in water; that of alkanes is lower than the alkynes.

Alkynes _{solubility in water} > Alkenes _{solubility in water}

Density of Alkynes

Alkynes have a low-density value as alkanes and alkenes. The densities of alkynes are usually less than that of water. (Ref: Table 2)

Boiling points of Alkynes

Generally, alkynes have slightly higher boiling points than the corresponding alkanes and alkenes.

Example: 

Alkane                                               Alkene                                                   Alkyne
Butane                                               Butene                                                  1-Butyne        2-butyne
0⁰C                                                     6⁰C                                                        8.1⁰C             27.0 ⁰C

Properties of Alkynes – Summary

Alkynes are a group in hydrocarbon family that have a Carbon-Carbon triple bond in the structure. Acyclic alkynes have the general chemical formula of C_nH_2n-2, and that of cyclic alkyne is C_nH_2n-4. Acetylene is the smallest member in alkyne group. Terminal alkynes are the most acidic group in hydrocarbon family. Alkynes undergo both addition and substitution reactions. Alkynes are more reactive than alkanes.

Alkynes resemble alkanes and alkynes in physical properties (boiling point, solubility in water, dipole moment). Solubility of alkynes in water is slightly higher than the other hydrocarbons.Their densities are lower than that of water as other hydrocarbons.