Lesson Explainer: Alkynes Chemistry

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

We will look at the homologous series of alkynes and how some of them are synthesized (in the lab and in industry) and study their hydrogenation, bromination, hydrobromination, and hydration reactions.

An alkyne is a type of organic chemical. Alkynes are unsaturated hydrocarbons with the general formula CH22.

Alkynes can have backbones of carbon atoms that are single chains, or they can have branches.

Alkynes contain a carbon–carbon triple bond functional group:

Definition: Functional Group

A part of a molecule made of particular atoms (or ions) in a particular bonding arrangement; functional groups usually behave in specific, predictable ways. For example, a carboxylic acid group (COOH) can make a substance more water soluble and more reactive to metal carbonates.

A carbon–carbon triple bond consists of a 𝜎 bond (2sp–2sp) and two 𝜋 bonds (2p–2p).

We can see a progression when we compare the general formulas and structures of alkanes, alkenes, and alkynes:

Alkynes contain only one carbon–carbon triple bond, with all other carbon–carbon bonds being saturated. They can form a homologous series.

Definition: Homologous Series

A sequence of organic compounds with the same key functional group; typically, a member of a homologous series will have one more carbon atom than the one before it.

Members of a homologous series have similar chemical properties and can be linear or branched.

The most well-known alkyne is ethyne. The common name of ethyne (and the preferred name according to IUPAC) is acetylene. However, ethyne is the systematic IUPAC name of CH22 that fits with the names of the other alkynes we will be looking at in this explainer.

We will explore the synthesis and uses of ethyne in a moment. In the meantime, here are some examples of the family of linear alkynes.

We could also include branched variations such as 3-methylbut-1-yne (CH58):

CCHCHCCHHHHHH

Example 1: Describing the Bond Types of the Three Bonds in a Triple Bond in an Alkyne Group

Which of the following combinations describes the triple bond in an alkyne group?

  1. The triple bond consists of two 𝜎 bonds and one dative bond.
  2. The triple bond consists of one 𝜎 bond and two polar bonds.
  3. The triple bond consists of two 𝜋 bonds and one polar bond.
  4. The triple bond consists of one 𝜎 bond and two 𝜋 bonds.
  5. The triple bond consists of two 𝜎 bonds and one 𝜋 bond.

Answer

An alkyne group consists of two carbon atoms joined by a triple bond:

CC

A carbon atom with a single bond and a triple bond is said to be “sp hybridized,” with electrons occupying 2sp orbitals and 2p orbitals.

An sp orbital is involved in the single bond (the one we are not interested in here). The other sp orbital forms part of the first bond of the carbon–carbon triple bond—this is the 𝜎 bond.

The other two bonds in the triple bond are formed from the overlap of the 2p orbitals of one carbon atom and the two p orbitals of the other carbon atom—these are 𝜋 bonds.

Therefore, the triple bond in an alkyne group consists of two 𝜋 bonds and one 𝜎 bond.

The answer is D.

Let’s take a look at the homologous series of linear alk-1-ynes (alkynes where the carbon–carbon triple bond begins on the carbon with index 1).

We can appreciate the meaning of this information if we look at a graph.

The melting points of members of this homologous series are erratic, and we would have to look further to demonstrate a trend. However, the boiling points show a clear trend: as the alkyne molecules get bigger, their boiling points increase. This demonstrates that the intermolecular forces increase as the chains get longer (in the same manner as the equivalent alkanes and alkenes). Since alkynes have little to no permanent dipole, London dispersion forces are the dominant type of intermolecular force at work.

Alkynes consist entirely of nonpolar carbon–carbon bonds and very weakly polar carbon–hydrogen bonds. Generally, we consider alkynes to be nonpolar.

When naming an alkyne, the index for the “yne” group is the index of the first carbon in the carbon–carbon triple bond. The name with the lowest index for the “yne” group takes priority:

For branched alkynes, we select the name with the lowest total for the indices:

We apply the same rules for haloalkynes (substances containing halogen functional groups and a carbon–carbon triple bond):

Ethyne can be made easily in the lab using calcium carbide and water. The carbide ion in calcium carbide has the form C22, so the formula for calcium carbide is CaC2.

Calcium carbide is made in large quantities by heating carbon (in the form of coke) with calcium-rich ores.

When calcium carbide is mixed with water, a spontaneous reaction occurs that releases ethyne:

This method produces lots of by-product (for each gram of ethyne generated this way, about 2.8 grams of calcium hydroxide is produced). Even though industrial quantities of ethyne can be produced using this method, there is an alternative that is much more popular.

In industry, ethyne is often produced by partially combusting methane. At high temperatures, methane can react to produce a variety of other compounds, including ethyne.

Ethyne is separated from this mixture before it is used.

We are going to look at a number of reactions of alkynes. Some of these reactions are more complex with larger alkynes. However, we do not need to examine those details, so we will be using ethyne (acetylene) in all the examples.

Besides combustion, all the reactions we will look at are addition reactions.

Definition: Addition Reaction

An addition reaction is a type of reaction where 2 or more molecules combine to form a larger molecule, without any by-products forming.

Like other hydrocarbons, alkynes burn in oxygen to produce carbon, carbon monoxide, carbon dioxide, and water, depending on the temperature and the relative amount of oxygen.

The complete combustion of alkynes follows the same principles as the complete combustion of alkanes or alkenes. The products are carbon dioxide and water:

Ethyne burns extremely well, generating an extremely hot flame if mixed with pure oxygen first (~3150C). This is one of the reasons that oxyacetylene torches (torches that use oxygen and ethyne together) are so effective at melting and “cutting” metals.

The incomplete combustion of ethyne happens readily if it is allowed to burn freely in air. Ethyne burns with a sooty flame, indicating that carbon is produced.

Here are a couple of examples of how ethyne might combust incompletely, producing either pure carbon (soot) or carbon monoxide:

By definition, alkynes are unsaturated. We should, therefore, be able to add hydrogen atoms, forming the equivalent alkene and alkane. In practice, it is difficult to stop hydrogenation at the alkene, so it is more common to completely hydrogenate alkynes, using a nickel, palladium, or platinum catalyst:

Example 2: Naming a Reaction That Converts Alkynes to Alkanes

What is the name of the reaction that converts alkynes into alkanes?

  1. Halogenation
  2. Hydrogenation
  3. Hydrohalogenation
  4. Alkylation
  5. Hydration

Answer

Alkanes are saturated hydrocarbons (substances composed solely of carbon and hydrogen, with only single bonds); alkanes have a general formula CH2+2.

Alkynes are hydrocarbons whose molecules contain a carbon–carbon triple bond; they have the general formula CH22.

Without even knowing which alkynes we start with, we can see that the general formula for the equivalent alkane has 4 more hydrogens: CHCHCHH2+2222+2(22)4==

This means that the simplest method of converting an alkyne to the equivalent alkane is to add 4 hydrogen atoms (or 2 hydrogen molecules (H2)). The addition of hydrogen to an unsaturated bond is called hydrogenation. This can often be accomplished using a catalyst, like finely divided nickel:

The answer is B.

Of course, alkynes can undergo addition reactions with chemicals other than hydrogen. Bromine (in the form of bromine water (Br()2aq) is used to indicate the presence of unsaturated hydrocarbons. If bromine water is mixed with a chemical with carbon–carbon double or triple bonds, the bromine will react; if all the bromine is consumed, the bromine water will lose its color.

Alkynes react with bromine to produce haloalkanes:

Hydrobromination is, as it were, in between hydrogenation and bromination. Adding HBr across the triple bond does have a seemingly strange outcome:

It might seem more elegant that each carbon gets one bromine, but that is not the case in the major product. Instead, we form 1,1-dibromoethane, which follows Markownikoff’s rule.

Definition: Markownikoff’s Rule

In the addition of an acid HA to an unsaturated carbon–carbon bond, the major product is the one where the hydrogen is added to the carbon with the greatest number of hydrogen substituents (and the A group is added to the other carbon).

In the addition of the first HBr, since ethyne is symmetrical, we do not see a difference whichever way we add the HBr:

In the addition of the second HBr, we have an asymmetrical alkene, bromoethene. The hydrogen in hydrogen bromide is added onto the carbon with the most hydrogens, and then the bromine is added onto the other carbon:

The reasons for this are beyond the scope of this explainer. We just need to remember Markownikoff’s rule.

Example 3: Understanding Intermediates and Products in the Hydrobromination of Ethyne

When ethyne reacts with hydrogen bromide gas, an asymmetric alkene intermediate (1-bromoethene) is formed:

  1. Which carbon atom has a partial positive charge, 𝛿+?
  2. What is the final product of this addition reaction?
    1. 1,2-Dibromoethane
    2. 1,2-Dibromoethene
    3. 1,1,3-Tribromoethane
    4. 1,1-Dibromoethane
    5. Bromoethane

Answer

Hydrogen bromide, HBr, can be added to unsaturated carbon–carbon bonds in a process called hydrobromination. Ethene (HCCH) has a carbon–carbon triple bond and can be hydrobrominated twice. If we hydrobrominate ethene just once, we get 1-bromoethene.

Part 1

To identify the carbon that is 𝛿+, we need to know the electronegativity of carbon and that of the substituents (H and Br). Carbon has a higher electronegativity than hydrogen (C: 2.6, H: 2.2), so we expect carbon–hydrogen bonds to be polarized, with the carbon 𝛿 and the hydrogen 𝛿+.

Carbon has a lower electronegativity than bromine (C: 2.6, Br: 3.0), so we expect carbon–bromine bonds to be polarized the other way around, with the carbon 𝛿+ and the bromine 𝛿.

As carbon B in the diagram of 1-bromoethene is bonded to a bromine, we expect it to be the more 𝛿+ carbon.

The answer is B.

Part 2

1-Bromoethene is described as an intermediate in the question. This means that 1-bromoethene is reacting with hydrogen bromide to form the product.

Hydrobromination generally follows Markownikoff’s rule: “In the addition of an acid HA to an unsaturated carbon–carbon bond, the major product is the one where the hydrogen is added to the carbon with the greatest number of hydrogen substituents (and the A group is added to the other carbon).”

Carbon A has more hydrogens than carbon B (2 versus 1), so Markownikoff’s rule predicts that the major product will be 1,1-dibromoethane:

The answer is D.

Water (HO2) can be treated as an acid of the form H(OH). However, water is not reactive enough on its own to react with the carbon–carbon triple bond. Instead, we need to add an acid catalyst. A mixture of mercury sulfate and sulfuric acid is commonly used.

If we hydrate ethyne, rather than forming ethane-1,1-diol, something else happens before the second hydration can occur. This is the acid-catalyzed hydration of ethyne to ethenol (pronounced “ethene-ol”):

In the presence of the same acid catalyst, ethenol can undergo a further reaction that produces a chemical without a carbon–carbon double bond (an aldehyde: ethanal). This stops a second hydration from occurring. This is the acid-catalyzed conversion of ethenol to ethanal:

Overall, the acid-catalyzed hydration of ethyne is as follows:

If the carbon–carbon triple bond of an alkyne is not on the end of a carbon chain (if we are not dealing with an alk-1-yne), we will produce a ketone instead of an aldehyde.

The aldehydes and ketones formed from the hydration of alkynes can be useful precursors in the production of other chemicals. For example, ethanal can be reduced to form ethanol or oxidized to form ethanoic acid.

However, the question remains, why does the acid catalyst not react with alkyne instead? This is beyond the scope of this explainer; however, it is worth keeping in mind.

Key Points

  • Alkynes are unsaturated hydrocarbons with the general formula CH22; apart from the 1 carbon–carbon triple bond, alkynes consist of carbons saturated with hydrogens.
  • The most well-known alkyne is ethyne (acetylene) (CH22), which is used in oxyacetylene torches and in chemical synthesis.
  • In small quantities, ethyne is often synthesized by adding water to calcium carbide.
  • In large quantities, ethyne is commonly synthesized by partially combusting methane.
  • As the length of the carbon chain of an alkyne increases, the boiling point increases; this is due to increased London dispersion forces between the molecules.
  • Alkynes, like other hydrocarbons, undergo combustion, producing carbon dioxide and water (with complete combustion) or mixtures of carbon dioxide, carbon monoxide, carbon, and water (with incomplete combustion).
  • Alkynes undergo a number of addition reactions, as shown in the following table.

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