Mason Delacroix
The E-Z system is used to name alkenes, which are organic molecules with distinct chemical structures. The system involves analyzing the two groups at each end of the double bond in an alkene molecule and ranking them based on their priority according to the CIP priority rules. The E or Z designation is then assigned based on whether the higher-priority groups are on the same side (Z, from German zusammen) or opposite sides (E, from German entgegen) of the double bond. This system is advantageous because it is comprehensive and always works, providing an absolute configuration of the molecule, unlike the cis-trans system which only gives relative configurations and has limited applicability.
| Characteristics | Values |
|---|---|
| E isomer | "entgegen" – the geometric isomer where the #1 ranked substituents are on the opposite side of the double bond |
| Z isomer | "zusammen" – the geometric isomer where the #1 ranked substituents are on the same side of the double bond |
| E/Z system | Comprehensive and describes the absolute configuration of the molecule |
| Cis/trans | Only works in certain cases and gives relative configurations |
| Trans | Generally preferred over cis |
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What You'll Learn
E and Z notation for alkenes
The E-Z system is a rigorous IUPAC system for naming alkene isomers. It is based on a set of priority rules, often called the Cahn-Ingold-Prelog (CIP) rules, which allow for the ranking of any groups. The general strategy of the E-Z system is to analyze the two groups at each end of the double bond.
The E and Z notation for alkenes is used to distinguish between geometric isomers that lack two identical substituents. The Z isomer ("zusammen", same) is the geometric isomer where the #1 ranked substituents are on the same side of the double bond. A handy trick to remember this is "zee zame zide". The E isomer ("entgegen") is the geometric isomer where the #1 ranked substituents are on the opposite sides of the double bond. Similarly, you can remember this with the mnemonic "epposite sides".
E/Z is the preferred nomenclature since it describes the absolute configuration of the molecule, whereas the cis-trans system merely describes the relative configuration. However, in simple cases where there are two identical substituents on each carbon of the alkene, the cis-trans system can be used to designate the isomers where those substituents are on the same and opposite sides of the double bond, respectively.
For example, let's consider the following alkene: On carbon 1, the chlorine is the first priority since it has a higher atomic number than carbon. There are two carbon atoms connected to carbon 2, and the priority cannot be determined solely based on their atomic number. The carbon on the top (isopropyl group) is connected to two carbons and one hydrogen, while the carbon on the bottom (ethyl group) is connected to a carbon and two hydrogens. Therefore, isopropyl gets higher priority. So, we have the two higher priority groups (Cl and isopropyl) on opposite sides of the double bond, as one is pointing down and the other one is pointing up. This alkene would be designated as E.
In another example, consider the two isomers of 2-butene. The left-hand structure (the cis isomer) has C and H on both carbons (C2 and C3) attached to the double bond. By the CIP priority rules, C is higher priority than H (higher atomic number). We see that the higher priority group is "down" at C2 and "down" at C3. Since the two priority groups are both on the same side of the double bond ("down", in this case), they are zusammen = together. Therefore, this is (Z)-2-butene. Now, looking at the right-hand structure (the trans isomer), the priority group is "down" on the left end of the double bond and "up" on the right end of the double bond. This is the E isomer.
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E isomer
The E isomer, or "entgegen", is a geometric isomer where the #1 ranked substituents are on the opposite sides of the double bond in a molecule. The E/Z system is comprehensive and describes the absolute configuration of a molecule. The E isomer is different from the Z isomer, where the #1 ranked substituents are on the same side of the double bond.
The E/Z notation is used for alkenes, and the two molecules can be described as diastereomers, or stereoisomers that are not mirror images of each other. These two molecules have different physical properties, such as different boiling points, melting points, reactivities, and spectral characteristics.
The E isomer is also different from the cis- and trans- configurations, which are used to denote the relative configuration of two groups to each other in situations with restricted rotation. While cis- and trans- can be used to describe simple alkene stereoisomers, they only give relative configurations and only work in certain cases.
When determining whether a molecule has an E or Z configuration, one must first determine if there is a difference between the two sides of the double bond. If there is no difference, then the groups are identical, and E/Z does not apply.
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Z isomer
The E/Z system is used to name alkene isomers, with Z corresponding to the German word "zusammen", meaning "together". The Z isomer is the geometric isomer where the #1 ranked substituents are on the same side of the double bond.
The E/Z system is especially useful when the cis-trans system is ineffective in describing the substituents' orientation in geometric isomers. For example, in the case of 1-bromo-2-chloro-2-fluoro-1-iodoethene, there are two distinct geometric isomers that cannot be named using the cis-trans system. Using the E/Z system, we can determine that the left-hand structure is the Z isomer because the higher priority groups are on the same side of the double bond.
It's important to note that the E/Z system is based on priority rules, known as the Cahn-Ingold-Prelog (CIP) rules, which allow for the ranking of any groups. These rules are used to determine the configuration of alkene isomers.
In simple cases, such as 2-butene, Z corresponds to cis and E to trans. However, this is not always the case, and the E-Z system is advantageous because it always works, whereas the cis-trans system breaks down in many ambiguous cases. For example, in cycloalkenes with ring sizes less than 8, the E/Z nomenclature is not required since the trans-double bond isomers are extremely unstable, and Z is assumed.
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Cis and trans isomers
Cis–trans isomerism, also known as geometric isomerism, describes certain arrangements of atoms within molecules. The terms "cis" and "trans" are derived from Latin, meaning "this side of" and "the other side of", respectively. Cis and trans isomers are stereoisomers, which are pairs of molecules with the same formula but different functional group orientations in three-dimensional space.
The difference in properties between cis and trans isomers arises from the arrangement of atoms in a double bond. In a cis isomer, similar atoms or groups of atoms lie on the same side of the double bond, resulting in a higher energy state and instability. In contrast, in a trans isomer, the similar atoms or groups are on opposite sides of the double bond, making it more stable and existing at a lower energy state. Trans isomers also tend to have lower densities and higher melting points than cis isomers.
Cis-trans isomerism occurs when there is restricted rotation in the molecule, and there are two non-identical groups on each doubly bonded carbon atom. This restricted rotation is a key distinction between cis-trans isomerism and conformational isomerism, where the terms syn and anti are used instead of "cis" and "trans."
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E-Z system for naming alkenes
The E-Z system is a rigorous IUPAC system for naming alkene isomers. It is based on priority rules, often called the Cahn-Ingold-Prelog (CIP) rules, which are used to rank groups at each end of the double bond. The general strategy is to analyze the two groups at each end of the double bond and rank them using the CIP priority rules. Then, determine if the higher-priority groups at each end are on the same side (Z, from the German "zusammen" meaning "together") or opposite sides (E, from the German "entgegen" meaning "opposite").
The E-Z system is particularly useful when the cis-trans system breaks down due to ambiguous cases. For example, consider the alkene 1-bromo-2-chloro-2-fluoro-1-iodoethene, which has four different substituents attached to the double bond. It is impossible to name these isomers as cis or trans, but the E-Z system works without any issues. In this case, the left end of the double bond has Br and I, with I being the higher priority according to the CIP rules. Moving to the right end of the double bond, we compare the groups attached to the first carbon atom, CCC, and OHH, with O being the higher priority. Therefore, the isomer is determined to be E, as the high-priority groups are on opposite sides.
It is important to note that while in simple cases, Z may correspond to cis and E to trans, this is not a rule. The E-Z system is more reliable and is particularly suited to tri- or tetra-substituted alkenes, especially when the substituents are not alkyl groups. The E-Z system is the IUPAC-approved way to name alkene stereoisomers and is based on absolute stereochemistry, while the cis-trans system is based on relative stereochemistry.
When naming alkenes using the E-Z system, the letters E and Z are printed in italic type within parentheses and are capitalized. For example, the chemical name of an alkene with multiple double bonds is (2E,4E,6Z,8E)-3,7-dimethyl-9-(2,6,6-trimethyl-1-cyclohexenyl)nona-2,4,6,8-tetraenoic acid, indicating the positions and configurations of the alkenes.
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