Chemistry » Organic Molecules » Organic Molecular Structures

# The Alkynes

## The alkynes

In the alkynes there must be at least one triple bond between two of the carbon atoms. They are unsaturated compounds and are therefore more reactive than alkanes. Their general formula is $$\color{red}{\textbf{C}_{\textbf{n}}\textbf{H}_{\textbf{2n-2}}}$$. For example but-1-yne has the molecular formula $${\textbf{C}_{4}{\textbf{H}_{6}}}$$. The simplest alkyne is ethyne (see figure below), also known as acetylene. Many of the alkynes are used to synthesise other chemical products.

#### Fact:

Acetylene is the industrial name for the organic compound ethyne. The raw materials that are needed to make acetylene are calcium carbonate and coal. An important use of acetylene is in oxyacetylene gas welding. The fuel gas burns with oxygen in a torch. Because the combustion of alkenes and alkynes is exothermic an incredibly high heat is produced, which is hot enough to melt metal.

The (a) structural, (b) condensed structural and (c) molecular representations of ethyne (acetylene). (d) An atomic model of ethyne.

Remember that organic molecules do not need to be straight chains. They can have branched groups as well, as shown in the figure below.

A methyl branched group on carbon 2 of butane (2-methylbutane).

A summary of the relative reactivity and the homologous series that occur in the hydrocarbons is given in the table below.

 Functional group Homologous series Reactivity alkane $$\color{red}{\textbf{C}_{\textbf{n}}\textbf{H}_{\textbf{2n+2}}}$$ low reactivity alkene $$\color{red}{\textbf{C}_{\textbf{n}}\textbf{H}_{\textbf{2n}}}$$ high reactivity alkyne $$\color{red}{\textbf{C}_{\textbf{n}}\textbf{H}_{\textbf{2n-2}}}$$ high reactivity

Table: A summary of the homologous series of the hydrocarbons.

#### Fact:

Liquid bromine is highly corrosive and toxic. Handle with extreme care!

For the experiment on saturated and unsaturated compounds, if cyclohexane and cyclohexene are not available any alkanes/alkenes that are liquids at room temperature are acceptable. These should be relatively easy to source.

Liquid bromine is corrosive and toxic, including the fumes. If you have to prepare your own bromine water from liquid bromine do not let the learners handle the liquid bromine. Wear full safety gear and do not breath in the fumes. Work in a well ventilated area (preferably a fume hood) and be careful.

To make $$\text{50}$$ $$\text{ml}$$ bromine water:

• Take a bottle with a capacity of at least $$\text{100}$$ $$\text{ml}$$ and a lid that closes securely (screw top).

• Label the bottle and place $$\text{50}$$ $$\text{ml}$$ of water into the bottle.

• Decant the vapours from a bottle of liquid bromine (with appropriate precautions) into the labelled bottle until the airspace above the water is completely full of the reddish-brown gas.

• Cap both bottles and gently swirl the bromine water bottle so that the bromine gas dissolves in the water.

• Repeat the process at least one more time, until the water retains an orange colour.

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