Properties of Alkenes (OCR A-Level Chemistry A): Revision Notes

Properties of alkenes

Introduction

Alkenes belong to the family of unsaturated hydrocarbons, which means they contain fewer hydrogen atoms than the corresponding saturated alkanes. This characteristic arises from the presence of at least one carbon-carbon double bond (C=C) within their molecular structure. Understanding the properties of alkenes is essential because this functional group determines their chemical behavior and reactivity.

The double bond in alkenes makes them more reactive than alkanes and gives them distinctive geometric and bonding characteristics that we'll explore in detail.

Structure of alkenes

General formula and naming

Aliphatic alkenes that contain a single double bond follow the general formula $\text{C}_n\text{H}_{2n}$, where $n$ represents the number of carbon atoms in the molecule. This formula shows that alkenes have two fewer hydrogen atoms than the corresponding alkane ($\text{C}_n\text{H}_{2n+2}$), which is a direct consequence of the double bond.

The first three members of the alkene homologous series demonstrate this pattern clearly:

Each of these molecules contains exactly one C=C double bond:

• Ethene ($\text{C}_2\text{H}_4$): The simplest alkene with two carbon atoms
• Propene ($\text{C}_3\text{H}_6$): Contains three carbon atoms with the double bond between C1 and C2
• But-2-ene ($\text{C}_4\text{H}_8$): A four-carbon alkene with the double bond between the second and third carbon atoms

Structural variations in alkenes

While simple straight-chain alkenes follow the $\text{C}_n\text{H}_{2n}$ formula, alkenes can exist in several different structural forms that don't necessarily obey this pattern. Understanding these variations is important for recognizing the diversity of alkene chemistry.

Branched alkenes such as 2-methylpropene maintain the general formula $\text{C}_n\text{H}_{2n}$ because branching doesn't affect the hydrogen count—only the arrangement of carbon atoms changes.

Polyunsaturated alkenes contain more than one double bond in their structure. For example, buta-1,3-diene has two double bonds and therefore has the formula $\text{C}_4\text{H}_6$ rather than $\text{C}_4\text{H}_8$. Each additional double bond removes two more hydrogen atoms from the molecule.

Cyclic alkenes like cyclopentene have a ring structure combined with a double bond. These compounds also deviate from the simple $\text{C}_n\text{H}_{2n}$ formula because the ring structure itself removes two hydrogen atoms from what would be expected in a straight-chain molecule.

Nature of the carbon-carbon double bond

Bonding in carbon atoms

To understand why alkenes behave differently from alkanes, we need to examine the bonding around the carbon atoms in the double bond. Each carbon atom has four electrons in its outer shell that it can use to form covalent bonds with other atoms.

In the carbon-carbon double bond, each carbon atom uses three of its four outer electrons to form three sigma bonds (σ-bonds). One of these sigma bonds connects to the other carbon atom of the double bond, while the other two sigma bonds connect to additional atoms (which may be hydrogen, carbon, or other elements).

This bonding arrangement leaves one electron on each carbon atom of the double bond that is not involved in sigma bonding. This remaining electron occupies what we call a p-orbital, and it's this electron that participates in forming the second bond between the two carbon atoms.

Formation of the pi bond

The second bond in the C=C double bond forms through a completely different mechanism than the sigma bond. This bond, called a pi bond (π-bond), forms through the sideways or lateral overlap of the p-orbitals from each carbon atom.

When two p-orbitals overlap sideways, the electron density becomes concentrated in two regions: one above the plane of the carbon-carbon sigma bond and one below it. Each carbon atom contributes one electron to this shared electron pair, creating a region of high electron density that extends above and below the line joining the nuclei of the bonding atoms.

This arrangement is fundamentally different from a sigma bond, where electron density is concentrated directly between the two bonded nuclei along the bonding axis.

Consequences of pi bonding

This restricted rotation is responsible for the existence of E-Z isomerism (geometric isomerism) in alkenes, where different groups attached to the double bond can be arranged on the same side or opposite sides, creating distinct isomers that cannot interconvert at room temperature.

Molecular geometry around the double bond

Trigonal planar shape

The shape around each carbon atom in the double bond is trigonal planar, which means the three bonded groups around each carbon are arranged in a flat, triangular pattern. This geometry arises because there are three regions of electron density around each carbon atom in the double bond:

• One region from the sigma bond to the other carbon atom
• Two regions from the sigma bonds to the other attached atoms (hydrogen or carbon)

According to electron-pair repulsion theory, these three regions of electron density repel each other and position themselves as far apart as possible in three-dimensional space. This maximum separation occurs when the three regions are arranged at 120° angles to each other in the same plane.

The pi bond doesn't count as a separate region of electron density for determining shape because the pi electron density is simply an extension of the region already created by the sigma bond between the two carbons.

Planar structure

An important consequence of the trigonal planar arrangement around both carbon atoms of the double bond is that all the atoms directly attached to the double bond lie in the same plane. This planar structure, combined with the restricted rotation, makes the geometry around the double bond rigid and fixed.

Worked example: Predicting shapes in molecules

Alkenes in the natural world

Beta-carotene and pigmentation

Alkenes play fascinating roles in nature, often contributing to colors, scents, and important biological functions. One striking example is the bright pink-orange coloration of flamingo feathers.

Flamingos obtain their distinctive color from their diet. These birds consume algae and crustaceans that contain pigment molecules called carotenoids. In the flamingo's liver, enzymes convert these carotenoids into pink and orange pigments that are then deposited in the growing feathers.

The extended system of alternating single and double bonds (called conjugation) in beta-carotene is responsible for its ability to absorb certain wavelengths of light, producing its characteristic orange color.

Limonene in citrus fruits

Another naturally occurring alkene is limonene, a cyclic alkene with the molecular formula $\text{C}_{10}\text{H}_{16}$. This compound is found naturally in the rinds of citrus fruits such as oranges and lemons, where it serves as the primary contributor to their characteristic smell and flavor.

Limonene's pleasant fragrance has led to its widespread use in perfumes, household cleaning products, and air fresheners. This demonstrates how the chemical properties of alkenes—particularly their distinctive structures and resulting aromas—make them valuable in both nature and commercial applications.

Key takeaways