Single, Double and Triple Carbon Bonds

Introduction

Two carbon atoms can share one, two or three pairs of electrons. Ethane, ethene and ethyne each contain two carbon atoms, yet they behave very differently: ethane is fairly unreactive, ethene is used to make plastics, and ethyne burns in the extremely hot oxy-acetylene flame used for cutting steel. The difference lies in the carbon–carbon bond. This page compares single, double and triple bonds by how many electrons they share, how long they are and how strong they are.

Core explanation

Counting shared pairs. A covalent bond is a shared pair of electrons. Carbon has four bonds to use, and it can use them in different combinations:

- Single bond (C–C): one shared pair. Ethane, H₃C–CH₃: each carbon also bonds to three hydrogens. - Double bond (C=C): two shared pairs. Ethene, H₂C=CH₂: each carbon bonds to only two hydrogens. - Triple bond (C≡C): three shared pairs. Ethyne, HC≡CH: each carbon bonds to one hydrogen.

In every case each carbon still forms four bonds in total. A double bond counts as two of carbon's four, and a triple bond as three.

Length and strength. More shared electrons between the nuclei pull the two atoms closer and hold them more tightly. Approximate values:

Bond Shared pairs Length (pm) Bond energy (kJ/mol) --- --- --- --- C–C 1 154 347 C=C 2 134 612 C≡C 3 120 838

As the number of shared pairs increases, bonds become shorter and stronger . (1 pm = 10⁻¹² m.)

Stronger, but not twice as strong. A C=C bond is less than twice as strong as a C–C bond (612 compared with 2 × 347 = 694). The second pair of electrons in a double bond sits above and below the line joining the nuclei and is held less tightly. That extra pair is exposed and can be attacked by other molecules. So although the double bond as a whole is stronger, it is the more reactive bond: its weaker second part breaks easily, allowing addition reactions .

Saturated and unsaturated. Compounds with only C–C single bonds are saturated : every carbon holds the maximum number of other atoms. Compounds with C=C or C≡C bonds are unsaturated , because the multiple bond can open up to accept more atoms.

Carbon's multiple bonds to other elements. Carbon also forms C=O double bonds (in carbon dioxide and many organic compounds) and C≡N triple bonds (in nitriles). Carbon, nitrogen and oxygen form multiple bonds readily because their atoms are small enough for the extra electron pairs to overlap well.

Step-by-step reasoning

To work out the bonding in a two-carbon hydrocarbon:

1. Count the hydrogens on each carbon. 2. Subtract from 4 to find how many bonds remain for the C–C link. 3. One remaining bond → single; two → double; three → triple. 4. For example, C₂H₄ has two H per carbon, leaving two bonds: C=C.

Visual explanation

Draw ethane, ethene and ethyne side by side. The carbons get closer from left to right, joined by one, two and then three lines. Around each, count the lines to each carbon: always four.

Real-world analogy

Imagine two people holding hands. With one hand each they are fairly far apart and easy to separate; with two hands they stand closer; with three "hands" they are pressed together and very hard to pull apart. But the extra grips are looser than the first, so they are the easiest ones to let go.

Real-world example

Ethyne (acetylene) burns in oxygen at temperatures above 3000 °C, which is why it is used for welding and cutting metals. Ethene, with its reactive double bond, is the starting material for poly(ethene), the most widely produced plastic in the world.

Why?

Why are multiple bonds shorter? More shared electrons between the two nuclei create a greater attraction for both nuclei, pulling them closer together until repulsion between the nuclei balances this attraction.

Common misconception

"A double bond is stronger, so ethene must be less reactive than ethane." The overall C=C bond is stronger, but its second shared pair is weaker and exposed, so ethene reacts much more readily than ethane.

Worked example

Question: Propene has the formula C₃H₆, with its structure CH₂=CH–CH₃. Check that every carbon has four bonds.

Reasoning: First carbon: 2 H + double bond (2) = 4. Middle carbon: 1 H + double bond (2) + single bond to CH₃ (1) = 4. End carbon: 3 H + single bond (1) = 4.

Answer: Every carbon has four bonds, so the structure is valid.

Quick check

1. How many pairs of electrons are shared in a C≡C bond? Answer: Three pairs.

Exam focus

Learn the trend: single → double → triple bonds get shorter and stronger. Be ready to draw ethane, ethene and ethyne and to explain why alkenes are more reactive than alkanes. Always check that each carbon in any structure you draw has exactly four bonds.

Advanced insight

A single bond is a σ (sigma) bond, formed by head-on overlap of orbitals. A double bond is one σ bond plus one π (pi) bond, formed by sideways overlap of p orbitals above and below the bond axis; a triple bond is one σ plus two π bonds. The π bonds prevent rotation about a C=C bond, which gives rise to cis–trans isomerism.

Summary

Carbon atoms can share one, two or three electron pairs, forming single, double or triple bonds, while each carbon keeps four bonds in total. Bond length falls (154, 134, 120 pm) and bond energy rises (347, 612, 838 kJ/mol) from single to triple. Multiple bonds are not proportionally stronger, and their extra electron pairs make unsaturated compounds more reactive.

Practice questions

1. Draw the structure of ethene and state how many hydrogens each carbon holds. Answer: H₂C=CH₂; each carbon holds two hydrogens. 2. Put C=C, C–C and C≡C in order of increasing bond length. Answer: C≡C (120 pm) < C=C (134 pm) < C–C (154 pm). 3. Why is ethene described as unsaturated? Answer: It contains a C=C double bond, which can open up so that the carbons bond to more atoms. 4. A C=C bond has energy 612 kJ/mol and a C–C bond 347 kJ/mol. What does this suggest about the second shared pair in the double bond? Answer: It contributes only about 265 kJ/mol, less than the first pair, so it is weaker and easier to break in reactions.