Olefin Metathesis: The Net Reaction

Introduction

Most reactions of alkenes add something across the double bond. Olefin metathesis is different: it cuts C=C bonds and stitches the fragments back together in new combinations. Formally, the two halves of each alkene swap partners. Because the number and type of bonds hardly change, metathesis reactions are usually close to thermoneutral, and chemists must find other ways to push them in the desired direction. This page focuses on what the reaction does and why it goes; the mechanism follows on the next page.

Core explanation

The fragment exchange. Think of each alkene as two alkylidene halves, R¹CH= and =CHR². Metathesis of R¹CH=CHR¹ with R²CH=CHR² gives two molecules of R¹CH=CHR²:

R¹CH=CHR¹ + R²CH=CHR² ⇌ 2 R¹CH=CHR²

No atoms are lost or gained; only the connectivity changes. The reaction was discovered in industry in the 1950s–60s with poorly defined molybdenum and tungsten catalysts; well-defined Schrock (Mo, W) and Grubbs (Ru) catalysts made it routine in synthesis. Chauvin, Grubbs and Schrock shared the 2005 Nobel Prize in Chemistry.

Near-zero enthalpy. In a simple exchange, one C=C bond is broken and one of similar strength is made for each pair of partners. ΔH is therefore small, and ΔS for exchange of similar molecules is also small. Without an additional driving force, a mixture of all possible alkenes forms, approaching a statistical equilibrium.

Main types.

- Cross metathesis (CM) exchanges fragments between two different alkenes. Selectivity problems arise because homodimers also form. Using one partner in excess, or one that dimerises only slowly, improves the yield of the cross product. - Ring-closing metathesis (RCM) joins two alkene ends within the same molecule to form a ring, releasing a small alkene, usually ethene, from terminal alkenes. - Ring-opening metathesis (ROM) and ring-opening metathesis polymerisation (ROMP) open strained cyclic alkenes such as norbornene or cyclooctene, forming linear chains or polymers. - Acyclic diene metathesis (ADMET) polymerises α,ω-dienes with release of ethene.

Driving forces.

1. Loss of a volatile product. Terminal alkenes release ethene in RCM, CM and ADMET. Ethene is a gas at room temperature and escapes, so by Le Chatelier's principle the equilibrium shifts towards products. Converting one molecule into two also increases entropy. 2. Relief of ring strain. ROMP of norbornene (strain energy about 80 kJ mol⁻¹) or cyclobutene releases strain, making ΔH strongly negative. Unstrained cyclohexene barely polymerises by ROMP. 3. Concentration effects. RCM is favoured by high dilution, which reduces competing intermolecular reactions, while ROMP and ADMET need high concentration.

Step-by-step reasoning

To predict a metathesis product:

1. Split each C=C bond into its two alkylidene halves. 2. Recombine halves to form new C=C bonds between the fragments you want joined. 3. Identify the small by-product formed by the leftover halves (often CH₂=CH₂). 4. Ask what drives the equilibrium: gas loss, strain relief or excess reagent. 5. Consider E/Z geometry: many catalysts favour the more stable E isomer.

Visual explanation

Draw two alkenes side by side as rectangles split vertically down the middle of each C=C. Colour the halves red–red and blue–blue. After metathesis, draw two rectangles coloured red–blue. For RCM, draw a chain with a CH₂= at each end; join the two inner CH= carbons into a ring, and release the two CH₂ ends as CH₂=CH₂.

Real-world analogy

Imagine two couples at a dance who swap partners. Nobody leaves the room and the number of pairs stays the same. To make the new pairings stick, something must be different afterwards — for example, one of the pairs leaving through the door (ethene escaping).

Real-world example

The Shell Higher Olefin Process uses metathesis to redistribute alkene chain lengths for detergent manufacture. ROMP of dicyclopentadiene produces tough thermoset plastics used for car panels and industrial housings. RCM is used to make large rings in drug molecules, including the macrocyclic protease inhibitors used to treat hepatitis C.

Why?

Why is removing ethene so effective? Every metathesis step is reversible. If a product escapes as a gas, the reverse reaction cannot occur, and the system is continually pulled towards the side that produces ethene.

Common misconception

"Metathesis breaks and forms σ bonds to substituents." It does not. The substituents stay attached to their original carbons; only the C=C double bonds are cut and reconnected.

Worked example

Question: Give the products of the ring-closing metathesis of hepta-1,6-diene, CH₂=CH–CH₂CH₂CH₂–CH=CH₂.

Reasoning: The two inner CH= carbons join to form a new C=C inside a ring containing those two carbons plus the three CH₂ groups. The two terminal CH₂= halves combine as ethene.

Answer: Cyclopentene and ethene.

Quick check

1. What is the main driving force for ring-opening metathesis polymerisation of norbornene? Answer: Relief of ring strain in the bicyclic alkene makes the reaction strongly exothermic.

Exam focus

Be able to predict products of CM, RCM and ROMP by cutting and rejoining C=C bonds. Explain why simple metathesis is nearly thermoneutral and identify the driving force in each case: ethene loss and entropy for RCM, ring strain for ROMP, excess reagent for CM.

Advanced insight

Because metathesis is reversible, product mixtures can reach thermodynamic control, giving mainly E alkenes. Specialised Mo, W and Ru catalysts now give kinetically controlled Z -selective metathesis by making the pathway to the E product sterically disfavoured. Ring-closing reactions to large rings compete with oligomerisation, and ring–chain equilibria govern which forms dominate at a given concentration.

Summary

Olefin metathesis exchanges alkylidene fragments between alkenes. The reaction is nearly thermoneutral, so it needs a driving force: loss of volatile ethene, relief of ring strain or use of excess partner. Its main forms are cross metathesis, ring-closing metathesis and ring-opening metathesis polymerisation, all of which have important industrial and pharmaceutical uses.

Practice questions

1. Give the products of the cross metathesis of propene with itself. Answer: But-2-ene and ethene (CH₃CH=CHCH₃ + CH₂=CH₂). 2. Why is RCM usually carried out at low concentration? Answer: Low concentration favours intramolecular ring closure over intermolecular reaction that forms oligomers. 3. Why does cyclohexene not readily undergo ROMP? Answer: Cyclohexene has very little ring strain, so opening the ring releases almost no energy to drive polymerisation. 4. State two driving forces for olefin metathesis. Answer: Removal of a volatile alkene such as ethene, and release of ring strain in cyclic alkenes.