Isomers

Alkene Stereoisomers

The geometry around the C=C double bond in an alkene plays an important role in the chemistry of these compounds. To understand why, let's return to the hypothetical intermediate from the previous section in which we have a C₂H₄ molecule with an unpaired electron on each of the carbon atoms.

The sigma bond skeleton in this molecule is formed by the overlap of sp² hybridized orbitals on each carbon atom with either a 1s orbital on a hydrogen atom or the sp² hybridized orbital on the other carbon atom. This leaves one unpaired electron in an empty 2p orbital on each carbon atom. The orbitals that hold these electrons interact to form a bond. Students sometimes ask, "Why are there three lines between the carbon atoms in this drawing if there are only two bonds?" The answer is simple: Both of the curved lines are needed to represent a single bond.

The geometry around a C=C double bond is therefore different from the geometry around a CC single bond. Because of the double bond, the six atoms in a C₂H₄ molecule all lie in the same plane, as shown in the figure below.

The presence of the bond restricts rotation around a C=C double bond. There is no way to rotate one end of this bond relative to the other without breaking the bond. Because the bond is relatively strong (270 kJ/mol), rotation around the C=C double bond cannot occur at room temperature.

Alkenes therefore form stereoisomers that differ in the way substituents are arranged around the C=C double bond. The isomer with similar substituents on the same side of the double bond is called cis, a Latin stem meaning "on this side." The isomer in which similar substituents are across from each other, is called trans, a Latin stem meaning "across." The cis isomer of 2-butene, for example, has both CH₃ groups on the same side of the double bond. In the trans isomer the CH₃ groups are on opposite sides of the double bond.

Stereogenic Atoms

Isomers are defined as compounds that have the same molecular formula, but different structures. There are two ways in which isomers can differ. When they differ in the way the atoms are connected, they are called constitutional isomers. Butane and isobutane, for example, are constitutional isomers.

Butane			Isobutane

Constitutional isomers have similar chemical properties but different physical properties. Butane melts at -138.4C and boils at -0.5C, whereas isobutane melts at -159.6C and boils at -11.7C .

Isomers in which the atoms are connected in the same way, but differ in the way the atoms are arranged in space are called stereoisomers. Thus, as noted in the previous section, cis/trans isomers such as cis- and trans-2-butene are stereoisomers.

Cis/trans isomers also have similar chemical properties but different physical properties. Cis-2-Butene, for example, freezes at -138.9C, whereas trans-2-butene freezes at -105.6C.

The carbon atoms that form the C=C double bond in 2-butene are called stereocenters or stereogenic atoms. A stereocenter is an atom for which the interchange of two groups converts one stereoisomer into another. The carbon atoms in the C=C double bond in 2-butene, for example, are stereocenters. Interchanging the CH₃ and H substituents on the carbon atom on either side of the C=C double bond would convert cis-2-butene into trans-2-butene, and vice versa.

The limitations of the cis/trans convention for describing stereoisomers of alkenes can be appreciated by considering the following compound.

Is this trans-3-methyl-2-pentene, because the two CH₃ substituents are across the double bond from each other? Or should it be called cis-3-methyl-2-pentene because the two "bulky" substituents (CH₃ and CH₃CH₂) are on the same side of the double bond?

An unambiguous system for describing stereoisomers of alkenes has been developed, in which strict rules are used to assign a priority to the substituents on each end of the double bond.

• The highest priority is assigned to the atom that has the largest atomic number. Carbon (Z = 6), for example, would have a higher priority than hydrogen (Z = 1), and bromine (Z = 35) would have a higher priority than chlorine (Z = 17).
• If the atoms directly attached to the double bond are the same, continue down the substituent chain until you find a difference. Consider the CH₃ and CH₂CH₃ substituents in 3-methyl-2-pentene, for example. The carbon atom on the CH₂CH₃ substituent would be assigned a higher priority because it is bound to two H atoms and a C atom, whereas the carbon atom in the CH₃ substituent is bound to three H atoms.
• Identify the substituent on each carbon atom that has the highest priority.
• If the substituents with the highest priority on both carbon atoms are on the same side of the double bond, the compound is described as the Z isomer. (From the German zusammen, which means "together.")
• If the substituents with the highest priority on the carbon atoms are on opposite sides of the double bond, the compound is the E isomer. (From the German entgegen, "opposite.")

Organic Chemistry: Structure and Nomenclature of Hydrocarbons

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