Alkynes

Alkynes are unsaturated hydrocarbons that contain a carbon–carbon triple bond. Alkynes are unsaturated because they have fewer hydrogen atoms than the corresponding alkenes and therefore the general formula is C_nH_2n-2. Remember, the general formula for alkanes is C_nH_2n+2. That is a four-hydrogen difference, which corresponds to two degrees of unsaturation.

 

 

In the nomenclature of alkynes, the ending changes from “ane” to “yne”:

 

 

Alkynes are classified into two main types – Internal and Terminal Alkynes.

If the triple bond is at the periphery, the alkyne is classified as terminal. These are also known as monosubstituted acetylenes.

In disubstituted acetylenes, the triple bond is located elsewhere along the chain:

 

 

Structure and Hybridization of Alkynes

The carbon atoms in the triple bond are sp-hybridized. Remember, in the sp hybridization, the s orbital of the excited state carbon is mixed with only one out of the three 2p orbitals. It is called sp hybridization because two orbitals (one s and one p) are mixed:

 

 

The resulting two sp hybrid orbitals are then arranged in a linear geometry (180^o), and the two unhybridized 2p orbitals are placed at 90^o:

 

 

Let’s see how this happens in acetylene- C₂H₂. The two carbon atoms make a sigma bond by overlapping the sp orbitals.

 

 

One hydrogen bonds to each carbon atom by overlapping its s orbital with the other sp orbital.

The two p orbitals of each carbon overlap to make two π bonds.

The key parameters of the sp hybridization and triple bond:

* In a triple bond, there is one σ (sigma) and two π (pi) bonds.

* All the atoms have linear geometry.

* The angle between atoms is 180^o which corresponds to the linear geometry:

 

 

This angle occasionally may vary since alkynes can exist as cyclic compounds. The smallest cycloalkyne is a 9-membered ring cyclononyne.

 

 

However, due to the ring strain, it decomposes at room temperature.

 

The Acidity of Terminal Alkynes

Alkynes are a lot more acidic than alkanes and alkenes:

 

 

This trend cannot be explained by any of the factors of “Atom”, “Resonance”, and “Induction” discussed in ARIO. We have the same atom (carbon), no resonance stabilization, and no inductive effect.

The last factor to consider is the orbital. Let’s put a negative charge on each carbon and take a closer look:

 

 

The only difference in these carbons is their hybridization state. Remember, alkanes are sp³, alkenes are sp^2, and alkynes are sp-hybridized.

What you need to know is that s orbitals are more electronegative than p orbitals, and the more s character the hybrid orbital has, the better it stabilizes the negative charge. These are the percentages of the s orbital (s-character) in each hybrid orbital:

 

 

This is an important feature that makes it possible to deprotonate terminal alkynes with strong yet safe-to-handle bases such as the sodium amide or sodium hydride:

 

 

The resulting acetylide ion is both a strong base and an excellent nucleophile, which is why it is a very important reagent in organic synthesis.

When reacted with sterically unhindered 1^o substrates, they give nucleophilic substitution by the S_N2 mechanism:

 

 

More details and practice problems can be found at Alkylation of Terminal Alkynes in Organic Synthesis

 

Check Also 

• Naming Alkynes by IUPAC Nomenclature Rules – Practice Problems
• Preparation of Alkynes by Elimination Reactions
• Hydrohalogenation of Alkynes
• Addition of Water to Alkynes
• Acid-Catalyzed Hydration of Alkynes with Practice Problems
• Reduction of Alkynes
• Halogenation of Alkynes
• Hydroboration-Oxidation of Alkynes with Practice Problems
• Ozonolysis of Alkynes with Practice Problems
• Alkylation of Terminal Alkynes in Organic Synthesis with Practice Problems
• Reactions of Acetylide Ions
• Alkyne reactions summary practice problems
• Alkyne Synthesis Reactions Practice Problems
• Alkyne Naming and Reactions Practice Quiz
• Reactions Map of Alkynes