2.5.1 – Alkene Structure

Chapter 2.3 introduced the various functional groups commonly seen in organic molecules. Each functional group has distinct properties that affects the molecule’s reactivity. This chapter will focus on the structure, properties, stereochemistry, and nomenclature of alkenes. Chapter 3.2 will cover the reactivity of alkenes.

Alkene Structure

Molecules are in constant motion, including translation through space, bond vibration, and molecular rotation. Rotation about a carbon-carbon single bond changes the 3-dimensional arrangement of atoms of a molecule. These different arrangement of atoms are called conformations. In Figure 2.5.1.a, two different conformations of butane (an alkane) are shown on the left. These conformations represent rotation about the central carbon-carbon single bond. They are the same molecule and are named the same way: butane.

In contrast, two different structures are shown for the alkene but-2-ene (Figure 2.5.1.a, right). They are different configurations of the molecule. A configuration is a permanent geometry of a molecule resulting from spatial arrangements of its bonds. The two configurations of but-2-ene are different molecules and are named differently: trans–but–2–ene vs cis–but–2–ene.

Although similar, configurations (shown for but-2-ene on the right) and conformations (shown for butane on the left) have different meanings when applied to molecules. This is because molecules with single bonds can rotate freely, but molecules with double bonds (or triple bonds) exhibit restricted rotation.

The reason for restricted rotation in alkenes relates to the orbitals discussed in Chapter 2.2. Carbon atoms in alkenes are sp² hybridized, meaning they contain three hybridized sp² orbitals and one unhybridized p-orbital. This unhybridized p-orbital can make a π-bond by overlapping with another p-orbital. The two unhybridized p-orbitals overlap above and below the plane of the molecule (Figure 2.5.1.b). If one of the carbon atoms in the π-bond were to rotate, the p-orbitals would no longer overlap above and below the plane of the molecule, which would break the π-bond. Therefore, alkenes have a restricted rotation.

Note the different terminology used for the structural arrangement of alkenes compared to alkanes. Alkenes have different configurations, which are permanent geometries due to restricted rotation about the carbon-carbon double bond. This means that two configurations of an alkene cannot be interconverted, and they are different molecules with different properties. Alkanes have different conformations due to the free rotation about carbon-carbon single bonds. Two conformations of an alkane can be interconverted, which happens regularly at room temperature as molecules are in constant motion.

Impact of Geometry on Melting Point

The cis and trans configurations of alkenes are seen in fatty acids such as oleic and elaidic acid pictured below in Figure 2.5.1.c.

The cis configuration of this molecule (left) creates kinks in the chain, while the trans configuration (right) has a similar zig-zag structure as a saturated hydrocarbon. The kinks in oleic acid prevent molecules from packing tightly together. The lower packing efficiency prevents intermolecular forces of attraction between chains, and thus requires less energy to melt. In contrast, the trans configuration will pack together more efficiently. Greater possibilities for intermolecular attractive forces in elaidic acid means that more energy is required to melt it, raising the melting point compared to the cis configuration. This can be seen in the melting points, where oleic acid has a melting point of 17^oC and elaidic acid has one of 52^oC.

(The full solution to this problem can be found in Chapter 5.1).