What are Alkynes?

Alkynes are organic compounds that contain at least one triple bond between carbon-carbon atoms. They are a series of unsaturated compounds with a general molecular formula, C_nH_2n-2.

Bonds

The molecules exist as three-dimensional figures in real life, just like every other substance. Therefore, there is an angle formed between the bonds and in alkynes, the bond angle is about 180 degrees. 

Also, it has sp hybridisation. The triple bond consists of one sigma bond and two pi bonds. Due to the presence of pi bonds, alkynes are a little unstable and the electrons are loosely held making it more reactive. 

The molecules with triple bond at the terminal end are terminal alkynes whereas the molecules with triple bond in between the carbon chain are the internal alkynes.

Nomenclature 

Alkynes use a common suffix while naming the molecules so that it is easily identified to be a part of this homologous series. The suffix is -yne. The prefix is decided depending on the number of carbon atoms, for example, meth, eth, prop and so on.

Many molecules have common names that are different from their IUPAC names. For example, ethyne is commonly known as acetylenes. 

Formulae 

The same molecule has various forms of expressing itself while highlighting different key features. Each method is known as a formula.  

(Using butyne as an example) 

Molecular formula 

It shows the exact number of each type of atom present. Written in the format

C_nH_2n-2.

Eg: C₄H₆

Empirical formula 

It shows the simplest whole-number ratio of the atoms present in the molecule.  

Eg: C₂H₃

 Condensed formula 

The atoms (symbols) are listed separately in the order it appears in the molecule’s arrangement. 

Displayed formula 

It is a graphical representation of the molecule showing the atoms and bonds present in the molecule. Bonds are represented as lines and atoms by their symbols. 

Skeletal formula 

Mostly displays only the functional group. Hydrogen atoms are omitted and the carbon atoms are represented by lines. It is the most common way and easiest way of representation.

Isomerism  

Even though the number of items is the same, there are various ways in which we can arrange them. Following this theory, the atoms in a molecule can be rearranged to form different structures with the same molecular formula; they are termed isomers.  

Alkynes show structural isomerism (compounds have the same molecular formula but different structural formula). Specifically,  

• Chain isomerism: The structures differ in the shape of their carbon skeleton. 
• Position isomerism: The position of the functional group (triple bond) varies. 
• Ring chain isomerism: These are cyclic structures of the compound with the same molecular formula.  

Preparation 

They can be produced in various reactions. 

• Double elimination of a geminal dihalide from alkyl halides. 
• Double elimination of vicinal dihalides: On treatment with alcoholic potassium hydroxide, it undergoes dehydrohalogenation.  
• By the reaction between an alkyl halide and an acetylide ion. 
• Ethyne can be prepared by reacting calcium carbide with water. 

 Physical Properties 

They have properties similar to alkanes and alkenes. 

Solubility 

The solubility of a substance is its ability to be dissolved in a solvent, especially water. 

Alkynes are soluble in organic solvents but not in water. This is because it is considered a non-polar molecule (very little difference in the electronegativity of the atoms).

Alkyne’s Van Der Waals force doesn’t release enough energy to break the hydrogen bonds in water. But it is enough to break the forces in organic solvents. 

 Boiling Point 

The very common term boiling point is the temperature at which the liquid substance starts to boil.

The boiling point increases as the molecular weight of the compound increase. This is because the Van Der Waals forces also increase with the increase in the molecular size. And a greater force between molecules indicates the need for more energy to overcome it.  

Straight chain alkanes are generally observed to be having a greater boiling point compared to their isomers.

 Melting point 

The melting point is the temperature at which a solid substance starts to melt and form a liquid version of it. 

Follows the same pattern as its boiling points; the melting point increases with the increase in weight. 

Combustion 

Just like any other fuel even hydrocarbons (alkynes) can be heated in the presence of oxygen to form carbon dioxide and water as products and most importantly energy (heat). However, at times when there is insufficient oxygen present during heating, the compound undergoes incomplete combustion. During this process, a harmful compound carbon monoxide is released which affects the human respiratory system. 

Addition Reaction 

As the name suggests, addition in this context means combining. When an alkyne reacts with a compound there are chances the atoms add across the triple bond to form a single molecule. This is very similar to the addition reaction of alkenes. There are several reagents possible. Some are listed below with conditions.  

• Hydrogen is used to first form an alkene then an alkane (140°C and nickel catalyst). 
• Water/ steam to terminal alkynes leads to the formation of aldehydes otherwise ketones (330°C, 60 atmospheres, and phosphoric acid). 
• Hydrogen halides to form di-halogenoalkanes. 
• Halogen to form the respective alkene and then a tetra haloalkane. 

Acidic character 

When sodium reacts with ethyne it forms sodium acetylide and dihydrogen gas. This has not been observed with ethene and ethane. Due to the high proportion of s character, alkynes have the highest electronegativity. Therefore, they attract the C – H bond to a greater extent than alkanes and alkenes. So, hydrogen atoms can liberate as protons more easily.  

These reactions are not shown by alkanes or alkenes. This means this can be a distinguishing factor between them. 

 Polymerization  

If we keep adding molecules one after the other, we can produce a long chain. This is polymerization. 

Polymer is an extremely long-chain molecule that is made up of many repeating units called a monomer. 

Linear polymerization: under suitable conditions, ethyne can be polymerized to polyethyne. 

Cyclic polymerization: Three molecules can be polymerized to form benzene. 

Common Mistakes 

• Students often forget to balance the equations. 
• Remember that bending a chain does not make it an isomer. It has to be branched. 
• Remember the conditions for each reaction correctly. 
• While drawing repeat units for polymerization, students often forget to bend the molecule. 

Context and Applications

This topic is useful for courses like Bachelors and Masters in Chemistry.