The Alkenes: Unsaturated Hydrocarbons

Introduction

Alkanes are full of hydrogen and fairly unreactive. Take two hydrogen atoms away from neighbouring carbons, however, and the carbons form a double bond with each other. The result is an alkene . That one change transforms the chemistry: alkenes react readily, and they are the starting point for most plastics and many industrial chemicals. Ethene, the simplest alkene, is produced in larger quantities than any other organic chemical.

Core explanation

The functional group. An alkene contains a carbon–carbon double bond , C=C. This is the functional group of the series, and it is where alkene reactions happen. In a double bond the two carbon atoms share two pairs of electrons instead of one.

General formula. Alkenes with one double bond fit CₙH₂ₙ . Compared with the alkane with the same number of carbons, an alkene has two fewer hydrogen atoms, because two bonds are now used between the doubly bonded carbons.

The first members:

n Name Molecular formula Structural formula Boiling point (°C) --- --- --- --- --- 2 ethene C₂H₄ CH₂=CH₂ −104 3 propene C₃H₆ CH₂=CHCH₃ −48 4 but-1-ene C₄H₈ CH₂=CHCH₂CH₃ −6

There is no alkene with one carbon, because a double bond needs two carbon atoms.

Shape. Around each carbon of the C=C bond, the three attached atoms lie in a flat triangle with angles of about 120°. In ethene all six atoms lie in one plane. Unlike a single bond, the double bond does not allow free rotation.

Unsaturated. Alkenes are unsaturated : they do not contain the maximum possible number of hydrogen atoms. The double bond can open up so that each carbon forms a new single bond to another atom. This is called an addition reaction , and it is the characteristic reaction of alkenes.

Reactivity. The double bond is a region of high electron density that attracts reagents such as bromine. As a result, alkenes react with many substances that leave alkanes unchanged.

Physical properties. Alkenes are similar to alkanes of the same size: non-polar, insoluble in water, with boiling points rising along the series. They burn in air, but with a smokier flame than alkanes because of their higher proportion of carbon.

Sources and uses. Alkenes are rare in crude oil. They are made by cracking larger alkanes. Ethene is used to make poly(ethene) and ethanol; propene is used to make poly(propene).

Step-by-step reasoning

To draw the displayed formula of propene:

1. Draw three carbon atoms in a row. 2. Put a double bond between the first two carbons. 3. Give each carbon four bonds in total: two hydrogens on the first carbon, one on the second, three on the third. 4. Count: C₃H₆, which fits CₙH₂ₙ.

Visual explanation

In SIM-ORG-001, build ethane and then ethene side by side. Ethane has two tetrahedral carbons that spin freely about the single bond. Ethene is flat, with the two carbons joined by a double stick and each carrying two hydrogens at 120° — like an open letter H lying on a table.

Real-world analogy

A double bond is like two people holding both hands. They are very close together, but each could let go with one hand to hold on to someone new. That free hand is exactly what an addition reaction uses.

Real-world example

Ripening fruit gives off ethene, which acts as a plant hormone. A ripe banana placed next to unripe fruit in a bag speeds up its ripening. Fruit warehouses control ethene levels to delay or trigger ripening before sale.

Why?

Why are alkenes more reactive than alkanes? The C=C bond concentrates electrons between two carbons, attracting reagents that seek electrons. Part of the double bond is also easier to break than a C–C single bond, so addition across it happens readily.

Common misconception

"A double bond is twice as strong as a single bond, so alkenes are less reactive." A C=C bond (about 612 kJ/mol) is stronger than a C–C bond (about 347 kJ/mol) but less than twice as strong. Its second part breaks relatively easily, which makes alkenes more reactive, not less.

Worked example

Question: An alkene has 5 carbon atoms. Give its molecular formula and the formula of the alkane with the same number of carbons.

Reasoning: Alkene: CₙH₂ₙ with n = 5 gives C₅H₁₀. Alkane: CₙH₂ₙ₊₂ with n = 5 gives C₅H₁₂.

Answer: C₅H₁₀ (pentene) and C₅H₁₂ (pentane) — the alkene has two fewer hydrogens.

Quick check

1. What is the functional group of the alkenes? Answer: The carbon–carbon double bond, C=C.

Exam focus

Learn the general formula CₙH₂ₙ and be able to draw ethene and propene with every bond shown, including the double bond. When defining unsaturated, say "contains a carbon–carbon double bond". Be careful: cycloalkanes also fit CₙH₂ₙ, so a formula alone does not prove a compound is an alkene.

Advanced insight

A C=C double bond consists of a sigma bond, formed by head-on overlap of orbitals, and a pi bond, formed by sideways overlap of p orbitals above and below the plane. The pi bond is weaker and exposed, which explains both the reactivity of alkenes and the lack of rotation about the double bond.

Summary

Alkenes are unsaturated hydrocarbons containing a C=C double bond, with general formula CₙH₂ₙ. The first members are ethene, propene and butene. The atoms around the double bond are flat with 120° angles. Alkenes undergo addition reactions and are far more reactive than alkanes. They are made by cracking and used to make plastics and ethanol.

Practice questions

1. Give the general formula of the alkenes and the molecular formula of the alkene with 6 carbons. Answer: CₙH₂ₙ; C₆H₁₂. 2. Explain why there is no alkene with only one carbon atom. Answer: A double bond is formed between two carbon atoms, so at least two carbons are needed. 3. State the meaning of unsaturated. Answer: Containing at least one carbon–carbon double or triple bond, so more atoms can be added to the molecule. 4. Give one industrial source and one use of ethene. Answer: Ethene is made by cracking larger alkanes; it is used to make poly(ethene) or ethanol.