The Alkynes: A First Look

Introduction

Alkanes have only single bonds and alkenes have a double bond. The next step is a carbon–carbon triple bond , C≡C, the functional group of the alkynes . The simplest alkyne, ethyne (often called acetylene), burns with one of the hottest flames of any common fuel gas and is used to cut and weld steel. Alkynes complete the set of simple hydrocarbon families and show clearly how bond type controls formula, shape and reactivity.

Core explanation

The triple bond. In an alkyne, two carbon atoms share three pairs of electrons. Each of these carbons then has only one bond left to form, so it bonds to one other atom.

General formula. Alkynes with one triple bond fit CₙH₂ₙ₋₂ . Each extra carbon–carbon bond removes two hydrogens, so compare the families with two carbons:

Family Example Formula C–C bond Bond length (pm) --- --- --- --- --- alkane ethane C₂H₆ single 154 alkene ethene C₂H₄ double 134 alkyne ethyne C₂H₂ triple 120

More shared electrons pull the carbon atoms closer together, so the bond gets shorter and stronger: about 347, 612 and 838 kJ/mol for single, double and triple bonds.

Naming. Alkynes use the familiar stems with the ending -yne : ethyne (C₂H₂), propyne (C₃H₄), but-1-yne and but-2-yne (C₄H₆). Position numbers work exactly as they do for alkenes.

Shape. Around each triple-bonded carbon there are only two groups of bonding electrons, which get as far apart as possible, at 180° . Ethyne, H–C≡C–H, is a straight, linear molecule. Compare this with tetrahedral carbon in alkanes (109.5°) and flat carbon in alkenes (120°).

Unsaturated and reactive. Alkynes are unsaturated. They decolourise bromine water and undergo addition reactions, and because there are two "spare" bonds, they can add two molecules in turn. For example, ethyne can add hydrogen to form ethene and then ethane:

C₂H₂ + H₂ → C₂H₄, then C₂H₄ + H₂ → C₂H₆

Combustion. Ethyne burns in oxygen with a very hot flame, above 3000 °C, which is why oxy-acetylene torches can cut and weld steel. In air it burns with a very smoky, sooty flame, because it has a high proportion of carbon.

Hazards. Ethyne is extremely flammable, forms explosive mixtures with air over a wide range of concentrations, and can decompose violently under pressure. For this reason it is supplied dissolved in a solvent inside specially packed cylinders and handled only by trained people.

Step-by-step reasoning

To work out the formula of an alkyne from its name:

1. Use the stem to find the number of carbons, n (prop- = 3). 2. Apply CₙH₂ₙ₋₂: 2 × 3 − 2 = 4 hydrogens. 3. Write the formula: C₃H₄. 4. Check that the triple-bonded carbons have four bonds each.

Visual explanation

In SIM-ORG-001, compare the three two-carbon models. Ethane is a pair of tetrahedra, ethene is flat like a letter H, and ethyne is a straight rod: H, C, C and H all in one line, with three sticks joining the carbons. The carbons sit visibly closer together as the number of bonds increases.

Real-world analogy

Think of two magnets joined by one, two or three springs. The more springs you add, the tighter and shorter the connection becomes, and the more effort it takes to pull the magnets fully apart. The triple bond is the three-spring connection.

Real-world example

Oxy-acetylene torches mix ethyne with pure oxygen to produce a flame hot enough to melt steel. They are used for cutting metal in scrapyards, for welding pipes and in some repair work. Before electric lighting, ethyne was also burned in miners' and bicycle lamps for its bright, white flame.

Why?

Why is ethyne linear? Each carbon has just two regions of electrons around it: the triple bond to the other carbon and the single bond to hydrogen. Electron regions repel each other, and two regions are furthest apart on opposite sides, at 180°.

Common misconception

"A triple bond is very strong, so alkynes must be unreactive." The triple bond is stronger overall than a double bond, but its second and third parts are relatively easy to break. Alkynes are unsaturated and react readily by addition, just like alkenes.

Worked example

Question: Give the molecular formula of the alkyne with five carbons and compare it with pentane and pentene.

Reasoning: Alkyne: 2 × 5 − 2 = 8, giving C₅H₈. Pentane: 2 × 5 + 2 = 12, giving C₅H₁₂. Pentene: 2 × 5 = 10, giving C₅H₁₀.

Answer: Pentyne is C₅H₈ — two fewer hydrogens than pentene and four fewer than pentane.

Quick check

1. What is the H–C–C bond angle in ethyne? Answer: 180°, because the molecule is linear.

Exam focus

Recognise the three general formulae: CₙH₂ₙ₊₂ (alkanes), CₙH₂ₙ (alkenes) and CₙH₂ₙ₋₂ (alkynes). Be able to link bond type to shape: 109.5°, 120° and 180°. State that alkynes are unsaturated and decolourise bromine water.

Advanced insight

A triple bond is made of one sigma bond and two pi bonds at right angles to each other, forming a cylinder of electron density around the C–C axis. Because hydrogen on a triple-bonded carbon is slightly acidic, terminal alkynes such as ethyne can react with very strong bases, a property that alkanes and alkenes do not share.

Summary

Alkynes are unsaturated hydrocarbons containing a C≡C triple bond, with general formula CₙH₂ₙ₋₂. The simplest is ethyne, C₂H₂, a linear molecule with 180° bond angles. The triple bond is shorter and stronger than double or single bonds. Alkynes undergo addition reactions and decolourise bromine water. Ethyne burns with a very hot flame used for cutting and welding.

Practice questions

1. State the general formula of the alkynes. Answer: CₙH₂ₙ₋₂. 2. Give the molecular formula of propyne. Answer: C₃H₄. 3. Explain why ethyne is described as unsaturated. Answer: It contains a carbon–carbon triple bond, so further atoms can be added across it. 4. Arrange the C–C bonds in ethane, ethene and ethyne in order of increasing length. Answer: Ethyne (triple) is shortest, then ethene (double), then ethane (single) is longest. 5. Give one use of ethyne and explain what property makes it suitable. Answer: Cutting and welding metal; it burns in oxygen with an extremely hot flame, above 3000 °C.