Trisubstituted Alkenes

Definition

Trisubstituted alkenes are organic compounds containing a carbon-carbon double bond with three substituents attached to one of the carbon atoms. These types of alkenes are important in the context of understanding alkene stereochemistry, the stability of alkenes, and the hydration of alkenes through hydroboration reactions.

5 Must Know Facts For Your Next Test

1. Trisubstituted alkenes can exist as either E or Z stereoisomers, which is determined by the relative positions of the three substituents.
2. Trisubstituted alkenes are generally more stable than disubstituted alkenes due to increased substitution and greater electron delocalization.
3. In the hydroboration-oxidation reaction, trisubstituted alkenes undergo anti-Markovnikov addition, with the borane reagent adding to the less substituted carbon.
4. The stability of trisubstituted alkenes is influenced by the nature and size of the substituents, with more bulky and electron-donating groups increasing stability.
5. Trisubstituted alkenes are important intermediates in many organic synthesis reactions, such as the Wittig reaction and the Grignard reaction.

Review Questions

• Explain how the E/Z designation is determined for trisubstituted alkenes and discuss the factors that influence their stability.
  • The E/Z designation for trisubstituted alkenes is determined by the relative positions of the three substituents around the carbon-carbon double bond. If the two largest substituents are on the same side (cis), the alkene is designated as Z. If the two largest substituents are on opposite sides (trans), the alkene is designated as E. The stability of trisubstituted alkenes is influenced by the size and electronic properties of the substituents, with more bulky and electron-donating groups generally increasing the stability of the alkene due to greater electron delocalization and steric effects.
• Describe the hydroboration-oxidation reaction of trisubstituted alkenes and explain how it relates to Markovnikov's rule.
  • In the hydroboration-oxidation reaction of trisubstituted alkenes, the borane reagent adds to the less substituted carbon, resulting in an anti-Markovnikov addition product. This is because the trisubstituted alkene is more stable than the disubstituted intermediate that would be formed if the borane added to the more substituted carbon, as predicted by Markovnikov's rule. The anti-Markovnikov addition is favored due to the increased stability of the trisubstituted alkylborane intermediate, which is then oxidized to produce the corresponding alcohol.
• Analyze the role of trisubstituted alkenes as important intermediates in organic synthesis reactions, such as the Wittig reaction and the Grignard reaction.
  • Trisubstituted alkenes are versatile intermediates in many organic synthesis reactions due to their stability and the ability to control their stereochemistry. In the Wittig reaction, a trisubstituted alkene is formed as the product, which can then be used to introduce new carbon-carbon double bonds with specific E/Z geometry. Similarly, in the Grignard reaction, trisubstituted alkenes can be used as electrophilic partners to form new carbon-carbon bonds, with the stereochemistry of the alkene being preserved. The ability to selectively form and manipulate trisubstituted alkenes makes them valuable building blocks in the construction of more complex organic molecules.

Related terms

Stereoisomers:

Molecules with the same molecular formula and connectivity but different spatial arrangements of atoms.

Markovnikov's Rule:

A rule that predicts the major product of an electrophilic addition reaction to an unsymmetrical alkene.

Anti-Markovnikov Addition:

The addition of a hydrogen and a heteroatom-containing group to an unsymmetrical alkene in a way that is the opposite of what Markovnikov's rule predicts.