The Alkyne Homologous Series of Hydrocarbons

An Alkyne is a molecule made solely of the element Carbon and the element Hydrogen that contains one triple bond between two of the Carbon atoms. The series is very similar to the alkene and alkane series of molecules as they are all made from Carbon and Hydrogen.

Making the Molecules

When constructing these molecules from the dot diagrams, the Carbon atoms are always linked together first. Then any spare unbonded electrons left over on the Carbon atoms are matched with Hydrogen atoms to complete the molecule. Identification of the location of the double bond is important in molecules with more than three Carbon atoms. See further down this page for a summary of the naming process.

Take note of the construction process and how it always results in the molecules following the formula C n H (2n-2) where n is the number of Carbons in the molecule.

Ethyne, which is also known as acetylene, is the smallest molecule in the Alkyne series and has two Carbon atoms which are triple bonded together. The diagram to the right shows the construction of this molecule from the electron dot diagrams of Carbon and Hydrogen. There is only one possible structure for this molecule so it does not require and numbering of the Carbon atoms.

Propyne, C3H4

By adding one more Carbon atom we have Propyne. The triple bond is between two of the Carbon atoms with the third singly bonded. This leaves four free electrons overall, meaning that four Hydrogens will be needed to complete the molecule.

It is possible to have the Carbon - Carbon triple bond on either the left or right of the Carbon chain. If one of these versions was rotated or flipped horizontally it would be identical to the other. This means that they are both the same structure, so there is only one isomer of Propyne. There is no need to put a "1" in the name (Prop-1-yne) so it is left out for simplcity's sake.

This is the first alkyne where there are two possible locations for the triple bond. Both are shown in the image. In but-1-yne, the triple bond is between the endmost carbon, C1 and its neighbour C2. The other possibility is but-2-yne where the triple bond is between the second and third Carbons in the skeleton of the molecule.

These two structural isomers have the same chemical formula C4H6, but have slightly different chemical properties. For example, But-1-yne boils at 8.1 degrees celsius whereas but-2-yne boils at 27 degrees celsius. But-1-yne is also far more flammable. This difference in properties allow these variations of Butyne to be identified and separated if required.

Using five Carbon atoms to build the backbone of Pentyne also yields two possible structural isomers. These are, of course, pent-1-yne and pent-2-yne. These are shown and labeled in the diagram below.

It is an interesting exercise to continue building the series of these hydrocarbon molecules either with a model kit or with pencil and paper. It becomes clear that with each additional two Carbon atoms added to the Carbon chain, another one structural isomer is added to the list of possible structures. This is shown to be the case in the image below of the three isomers of Hexyne, the 6-carbon alkyne molecule.

The standard international naming system for alkynes gives the same names as for the alkane series, but with "-ane" at the end of the name being replaced with "-yne". Thus the name for the simplest member of the series is ethyne. This is the same as the name for the 2-Carbon alkane, but with the ending changed to indicate the presence of the triple bond somewhere in the molecule.

Identifying the Location of the Triple Bond

Because longer molecules have more possible variations, called isomers, in the location of the triple bond the position of that bond needs to be identified. As with the alkene homologous series, the Carbon atoms are numbered from the end that is closest to the triple bond. The number that the bond starts at is then inserted into the middle of the name. For example, hex-1-yne and hex-2-yne are two isomers of hexyne. Hex-1-yne has the triple bond between the first and second Carbon atoms, while hex-2-yne has the bond between the second and third Carbon atoms.

Chemical Properties of Alkynes

The triple bond is unstable and so alkynes are generally quite reactive. They are also able to act as acids, are highly volatile and combust readily. As such Propyne is being considered as a rocket fuel.