Entgegen

Definition

Entgegen is a German prefix that denotes a sense of opposition or against. In the context of organic chemistry, it is particularly relevant to the discussion of alkene stereochemistry and the E,Z designation.

5 Must Know Facts For Your Next Test

1. The term 'entgegen' is used to describe the relative orientation of substituents around a carbon-carbon double bond, where the substituents are on opposite sides of the double bond.
2. The E,Z designation is used to differentiate between the two possible stereoisomers of an alkene, with 'E' denoting the entgegen (opposite) configuration and 'Z' denoting the zusammen (same side) configuration.
3. Alkenes with the entgegen (E) configuration have higher stability compared to their zusammen (Z) counterparts due to decreased steric hindrance.
4. The E,Z designation is determined by comparing the priorities of the substituents attached to the double bond, with the higher priority substituents being used to assign the configuration.
5. Understanding the entgegen (E) configuration is crucial for predicting the stereochemical outcomes of various organic reactions involving alkenes, such as elimination reactions and addition reactions.

Review Questions

• Explain the relationship between the term 'entgegen' and the E,Z designation used to describe alkene stereochemistry.
  • The term 'entgegen' is a German word that means 'opposite' or 'against'. In the context of alkene stereochemistry, the E,Z designation uses the 'entgegen' (E) configuration to describe the relative orientation of substituents around a carbon-carbon double bond, where the higher priority substituents are on opposite sides of the double bond. This is in contrast to the 'zusammen' (Z) configuration, where the higher priority substituents are on the same side of the double bond. Understanding the entgegen (E) configuration is crucial for predicting the stereochemical outcomes of organic reactions involving alkenes.
• Discuss how the entgegen (E) configuration of alkenes affects their stability compared to the zusammen (Z) configuration.
  • Alkenes with the entgegen (E) configuration are generally more stable than their zusammen (Z) counterparts. This is due to the decreased steric hindrance between the substituents in the E configuration, as the higher priority substituents are positioned on opposite sides of the double bond. The reduced steric interactions in the E configuration result in a more favorable arrangement of the atoms, leading to a lower overall energy state for the molecule. This increased stability of the E configuration is an important factor to consider when predicting the outcomes of organic reactions involving alkenes.
• Analyze the significance of the entgegen (E) configuration in the context of organic reactions, such as elimination reactions and addition reactions.
  • The entgegen (E) configuration of alkenes plays a crucial role in determining the stereochemical outcomes of various organic reactions. In elimination reactions, the E configuration is often favored, as it allows for the anti-periplanar arrangement of the leaving group and the hydrogen atom, which is required for the successful removal of these groups. Similarly, in addition reactions to alkenes, the E configuration can influence the stereoselectivity of the reaction, as the approach of the incoming nucleophile or electrophile may be guided by the relative positions of the substituents. Understanding the entgegen (E) configuration is therefore essential for predicting and rationalizing the stereochemical outcomes of these important organic transformations.

Related terms

Stereochemistry: The study of the three-dimensional arrangement of atoms in molecules and how this arrangement affects the molecule's properties and reactivity.

E,Z Designation: A system used to describe the relative orientation of substituents around a carbon-carbon double bond, where E stands for entgegen (German for 'opposite') and Z stands for zusammen (German for 'together').

Cis-Trans Isomerism: A type of stereoisomerism in which two identical substituents are either on the same side (cis) or on opposite sides (trans) of a carbon-carbon double bond.