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Lesson Video: Alkynes Chemistry

In this video, we will learn how to name and describe the reactions of alkynes.

16:49

Video Transcript

In this video, we will learn about the alkynes. We will learn how to name them and explain their physical properties. We will also look at how they are prepared and some common addition reactions that they undergo.

Let’s start by asking what is an alkyne. An alkyne is an unsaturated hydrocarbon that contains at least one carbon-carbon triple bond. Hydrocarbon means a molecule composed only of carbon and hydrogen, and unsaturated refers to a multiple bond between carbon atoms. In the alkene family of organic chemicals, there is a least one carbon-carbon double bond. And in the alkyne family, there’s a least one carbon-carbon triple bond.

We can represent a general alkyne like this, where R and R dash are alkyl groups, either the same or different or hydrogen atoms. The triple bond consists of a 𝜎 bond and two 𝜋 bonds. This section of an alkyne molecule is always linear because of two 180-degree bond angles. The 𝜎 bond is very strong, but the two 𝜋 bonds are weaker. As a result, a 𝜋 bond can break and open up, and other substituents can be added on here and here. If the second 𝜋 bond also breaks, further substituents can be added on here and here. These two or four substituents may or may not be the same. We will investigate this later. What we need to remember for now is that the triple bond acts as a functional group. This part of the molecule can undergo reactions.

Alkyne molecules can be straight chains or branched chains. The general formula for an alkyne is C𝑛H2𝑛−2, where 𝑛 is equal to the number of carbon atoms. Note that this general formula is only for alkynes that contain one carbon-carbon triple bond. All alkynes belong to the same family or homologous series. A homologous series is a family of compounds that have the same functional group and thus similar chemical properties, the same general formula with each compound in the series differing from the next by a simple structural unit. For example, a chain of three carbon atoms with a triple bond and a chain of four carbon atoms with a triple bond are both members of a homologous series known as alkynes.

Two other homologous series that you may be familiar with are the alkanes and the alkenes. The general formula for the alkanes is C𝑛H2𝑛+2, and alkenes C𝑛H2𝑛.

Now let’s learn how to name alkynes. Taking the general formula for an alkyne with one triple bond and substituting the number two for 𝑛, we get C2H2. Let’s put that into a table. The displayed formula of this molecule is HC triple bond CH. This is the simplest alkyne, and we call it ethyne: eth- meaning there are two carbons and -yne indicating that there is a triple bond between the carbons.

The name ethyne is an IUPAC name. IUPAC or the International Union of Pure and Applied Chemistry is a global body of chemists and scientists that have made a systematic set of naming rules for compounds. The common name for ethyne, which you may have heard, is acetylene. Now ethyne is a colorless gas at room temperature. And when mixed with oxygen and ignited, ethyne releases a large amount of heat energy. This combustion process produces an extremely hot flame. This gas mixture is used in oxyacetylene welding torches, which are highly effective at both melting and cutting through metals.

When 𝑛 in the general formula of an alkyne is three, we get the molecular formula C3H4. And here is its displayed formula. And its IUPAC name is propyne: prop- telling us there are three carbons, and -yne that there is a carbon-carbon triple bond.

When in 𝑛 is four, the molecular formula is C4H6, with two possible displayed formulas. The first displayed formula is but-1-yne and the second but-2-yne: but- indicating a four-carbon chain, -yne indicating a carbon-carbon triple bond, and the numbers one and two indicating where in the chain the triple bond starts. In but-1-yne, the triple bond is between carbons number one and two, but we choose the lower of the two numbers to indicate where the carbon triple bond starts. And so this molecule is but-1-yne. In the second displayed formula, the triple bond is between carbons two and three. The lower of the two numbers is two, and so this molecule is but-2-yne.

Now we could have chosen to number but-2-yne from right to left instead like this: one, two, three, four. We would still get but-2-yne. However, for the first displayed structure but-1-yne, if we were to number the carbons from right to left instead, one, two, three, four, notice that the triple bond this time is between carbons number three and four, which would give us a different name. So the rule is we must always number the longest carbon chain from the side that is closest to the carbon-carbon triple bond. So how we had it before was correct: one, two, three, four.

Notice that the more carbons there are in the chain, the more possible positions the triple bond could be in. When there are five carbons in the chain with one triple bond, we get the molecular formula C5H8 with two possible displayed formulas: pent-1-yne and pent-2-yne. Pent- indicates the longest chain consists of five carbons, -yne tells us we’re dealing with an alkyne, and the numbers one and two indicating the carbon on which the triple bond starts.

Let’s look at one last example. When 𝑛 is six, we get the molecular formula C6H10 with three possible structures. Here, I’ve drawn skeletal formulas instead of displayed formulas, where the end of each line indicates a carbon and the hydrogens have not been shown. Because there are six carbons in each structure, the names begin with hex-. Again, -yne is the suffix because we are dealing with alkynes, and the numbers in the name indicating the smallest carbon number on which the triple bond starts. In hex-1-yne, the triple bond starts on carbon number one. In hex-2-yne, it starts on carbon number two, and in hex-3-yne, on carbon number three.

Now let’s compare the first five alkynes in terms of boiling point. For these five compounds, the triple bond starts on carbon number one. Notice that ethyne does not boil under normal conditions but instead sublimes straight from a solid to a gas. However, for the subsequent alkynes, the boiling points increase with increasing carbon chain length. In other words, shorter chains have lower boiling points, and longer chains have higher boiling points.

Why is this? The intermolecular forces of attraction between shorter carbon chains are relatively weak. Little energy is required to separate these molecules during boiling and therefore the lower the boiling point. However, the attractive forces between longer chains is generally stronger. More energy is required to separate the molecules during boiling, and therefore the higher the boiling point. These intermolecular forces are Van der Waals forces. And because these molecules are largely nonpolar, we can specify them as London dispersion forces.

Now we know how to name alkynes, and we know the trend in the boiling point for those alkynes whose carbon-carbon triple bond starts on carbon number one. Now we’re going to turn our attention to some reactions.

We’ll start by looking at two reactions by which ethyne can be prepared. Ethyne can be made in various ways in the lab and in industry, but let’s have a look at the more common methods used. In the lab, ethyne can be made by reacting calcium carbide with water. Water in a fissile funnel is added dropwise to calcium carbide, CaC2. Calcium hydroxide, CaOH2, and gaseous ethyne, C2H2, are produced. Ethyne travels up the glass tubing and is collected in a gas jar. In industry, the partial combustion of methane, CH4, produces ethyne plus other compounds. This mixture is then separated, and ethyne is collected.

Now that we know how ethyne is prepared, let’s have a look at some of the reactions of alkynes. And we’ll use ethyne as an example. We’ll have a look at four reactions, which are all specific types of addition reactions.

Earlier we said, that this triple bond can open up to give an alkene with two added substituents or an alkane with four added substituents. Because the substituents add on, these are called addition reactions. When an alkyne such as ethyne reacts with hydrogen gas in the presence of a metal catalyst, for example, nickel, a hydrogenation addition reaction occurs. First, one of the 𝜋 bonds opens up to produce an alkene, ethene. Then the other 𝜋 bond opens up to produce an alkane, ethane. And this is fully saturated with hydrogen.

Reaction with bromine in the addition reaction called bromination gives a haloalkane end product called 1,1,2,2-tetrabromoethane. If the bromine is dispersed in water as bromine water, the red-brown mixture will slowly lose its color over time as it reacts. This addition reaction can therefore be used as a test for unsaturation. The presence of an unsaturated compound can be confirmed by the slow disappearance of the red-brown color of the bromine in bromine water.

Reaction of an alkyne with hydrogen bromide in a hydrobromination addition reaction gives an interesting product. This major product has both bromine atoms attached to the same carbon atom, which is 1,1-dibromoethane instead of the expected 1,2-dibromoethane. Why do both bromines add to the same carbon atom? This is in accordance with Markownikoff’s rule, which says when an acid HA, in our case, HBr, is added to an unsaturated bond, the major product has hydrogen added to the carbon atom with the most hydrogen substituents.

Remember, this hydrobromination is a two-step process. First, a 𝜋 bond opens up to add on one hydrogen atom and one bromine atom to different carbons. Now we can apply Markownikoff’s rule to understand what the major product will be. Further hydrobromination with another HBr molecule will open up the second 𝜋 bond. And the hydrogen from the second HBr molecule will add onto the carbon atom that has the most number of hydrogen. This carbon has one, two hydrogen atoms. And the other carbon only has one hydrogen atom. And so the hydrogen adds on here. Therefore, the bromine must add on here, which is what we have over here.

The last addition reaction we’ll look at is the reaction of an alkyne with water in the presence of an acid catalyst. This is called hydration. First, the alkyne is converted to a molecule that has a double bond as well as an alcohol group, in this case ethenol. Ethenol then reacts further to form ethanol, which is an aldehyde. If the starting material is not ethyne but another alkyne, we won’t get an aldehyde as the product but rather a ketone.

Did you notice in all these addition reactions there is only one final product? We could define an addition reaction as a type of reaction where two or more molecules combine to form one larger molecule without any by-products forming. We now know that alkynes can undergo addition reactions because of the carbon-carbon triple bond, which is a functional group. Because alkynes are hydrocarbons, they can also undergo combustion reactions like other hydrocarbons.

For example, ethyne can react with oxygen. When there is sufficient or excess oxygen, the products are carbon dioxide and water. This is complete combustion. However, if ethyne burns in air or insufficient oxygen, carbon in the form of soot from a smoky flame or carbon monoxide is produced. This is called incomplete combustion.

We’ve learned a lot about alkynes. Let’s now summarize what we’ve learned. Alkynes are unsaturated hydrocarbons containing at least one carbon-carbon triple bond. We looked at ethyne, C2H2, which is the simplest alkyne. And it is also called acetylene, which is mixed with oxygen in oxyacetylene torches. We saw that the longer the carbon chain length is in an alkyne, the higher the boiling point and that this is due to increased intermolecular forces of attraction.

We learned that alkynes undergo a number of addition reactions at the triple bond, for example, hydrogenation, bromination — which is a type of halogenation — hydrobromination, and hydration. We learned that hydrobromination follows Markownikoff’s rule. And lastly, we learned that alkynes undergo combustion reactions just like other hydrocarbons.

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