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Chapter: Organic Chemistry: Stereochemistry

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Configurational isomers – alkenes and cycloalkanes

Configurational isomers have the same molecular formula and the same bonds. However, some of the atoms are arranged differently in space with respect to each other, and the isomers cannot be interconverted without breaking a covalent bond. Substituted alkenes and cycloalkanes can exist as configurational isomers.

CONFIGURATIONAL ISOMERS – ALKENES AND CYCLOALKANES

Key Notes

Definition

Configurational isomers have the same molecular formula and the same bonds. However, some of the atoms are arranged differently in space with respect to each other, and the isomers cannot be interconverted without breaking a covalent bond. Substituted alkenes and cycloalkanes can exist as configurational isomers.

Alkenes – cis and trans isomerism

Alkenes having two different substituents at each end of the double bond can exist as two configurational isomers. Simple alkenes can be defined as cis ortransdepending on whether substituents at different ends of thealkene are on the same side of the alkene (i.e. cis) or on opposite sides (i.e. trans).

Alkenes – Z and E nomenclature

Alkenes can be assigned as Z or E depending on the relative positions of priority groups. If the priority groups at each end of the alkene are on the same side of the double bond, the alkene is the Z isomer. If they are on opposite sides, the alkene is defined as the E isomer. Priority groups are determined by the atomic numbers of the atoms directly attached to the alkene. If there is no distinction between these atoms, the next atom of each substituent is compared.

Cycloalkanes

Substituted cycloalkanes can exist as configurational isomers where the substituents are cis or trans with respect to each other.

 

Definition

Configurational isomers are isomers which have the same molecular formula and the same molecular structure. In other words, they have the same atoms and the same bonds. However, the isomers are different because some of the atoms are arranged differently in space, and the isomers cannot be interconverted without breaking and remaking covalent bonds. As a result, configurational isomers are different compounds having different properties.

Common examples of configurational  isomers  are  substituted  alkenes  and  substituted  cycloalkanes where the substituents are arranged differently with respect to each other.

Alkenes – cis and transisomerism

Alkenes having identical substituents at either end of the double bond can only exist as one molecule. However, alkenes having different substituents at both ends of the double bond can exist as two possible isomers. For example, 1-butene (Fig. 1a) has two hydrogens at one end of the double bond and there is only one way of constructing it. On the other hand, 2-butene has different substituents at both ends of the double bond (H and CH3) and can be constructed in two ways. The methyl groups can be on the same side of the double bond (the cis isomer; Fig. 1b), or on opposite sides (the trans isomer; Fig. 1c). The cis and trans isomers of an alkene are configurational isomers (also called geometric isomers) because they have different shapes and cannot interconvert since the double bond of an alkene cannot rotate. Therefore, the substituents are ‘fixed’ in space relative to each other. The  structures  are  different  compounds  with  different  chemical  and  physical properties.


Alkenes – Z and E nomenclature

The cis and trans nomenclature for alkenes is an old method of classifying the configurational isomers of alkenes and is still commonly used. However, it is only suitable for simple 1,2-disubstituted alkenes where one can compare the relative position of the two substituents with respect to each other. When it comes to trisubstituted and tetrasubstituted alkenes, a different nomenclature is required.

The Z/E nomenclature allows a clear, unambiguous definition of the configu-ration of alkenes. The method by which alkenes are classified as Z or E is illus-trated in Fig. 2. First of all, the atoms directly attached to the double bond are identified and given their atomic number (Fig. 2b). The next stage is to compare the two atoms at each end of the alkene. The one with the highest atomic number takes priority over the other (Fig. 2c). At the left hand side, oxygen has a higher atomic number than hydrogen and takes priority. At the right hand side, both atoms are the same (carbon) and we cannot choose between them.


Therefore, we now need to identify the atom of highest atomic number attached to each of these identical carbons. These correspond to a hydrogen for the methyl substituent and a carbon for the ethyl substituent. These are now different and so a priority can be made (Fig. 3a). Having identified which groups have priority, we can now see whether the priority groups are on the same side of the double bond or on opposite sides. If the two priority groups are on the same side of the double bond, the alkene is designated as Z (from the German word ‘zusammen’ meaning together). If the two priority groups are on opposite sides of the double bond, the alkene is designated as E (from the German word ‘entgegen’ meaning across). In this example, the alkene is E (Fig. 3b).


Cycloalkanes

Substituted cycloalkanes can also exist as configurational isomers. For example, there are two configurational isomers of 1,2-dimethylcyclopropane depending on whether the methyl groups are on the same side of the ring or on opposite sides (Fig. 4). The relative positions of the methyl groups can be defined by the bonds. A solid wedge indicates that the methyl group is coming out the page towards you, whereas a hatched wedge indicates that the methyl group is pointing into the page away from you. If the substituents are on the same side of the ring, the structure is defined as cis. If they are on opposite sides, the structure is defined as trans.

 

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