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Electrophilic Addition Reactions of Alkenes

Curved Arrows

• Curved arrows with a two-barbed arrowhead signify the movement of two electrons.

• Arrows point from electron donor (nucleophile) to electron acceptor (electrophile).

Alkenes

• Alkenes are compounds containing a carbon-carbon double bond.

• They act as nucleophiles in electrophilic addition reactions.

• Electrophiles add to one sp² carbon, and nucleophiles add to the other sp² carbon.

Key Question

• Which is more harmful: synthetic pesticides or those made by plants?

Electrophilic Addition Overview

• Organic compounds can be divided into families based on their structural features. Alkenes undergo electrophilic addition reactions due to their weak double bonds.

Mechanism of Electrophilic Addition to Alkenes

1. First Step: Slow addition of a proton (electrophile) to one sp² carbon (nucleophile), forming a carbocation intermediate.

2. Second Step: Rapid reaction of the carbocation with a nucleophile (e.g., Br-) to form products such as alkyl halides.

Types of Addition Reactions

• Addition of Hydrogen Halide (HX): Results in alkyl halides. Reaction involves carbocation formation and bromide ion addition.

• Stereochemistry: Analysis shows features in common such as the reaction of π electrons with the electrophile.

Carbocation Stability

• Carbocation stability increases with the number of alkyl groups attached (tertiary > secondary > primary).

• The stability factor influences the overall rate of reaction.

Carbocation Rearrangements

• Rearrangements occur to form more stable carbocations via 1,2-hydride and 1,2-methyl shifts.

Stereochemistry of Addition Reactions

• If the product has an asymmetric center, two enantiomers may be formed (racemic mixture).

• When creating two centers, various stereoisomer configurations arise depending on the addition mechanism (syn or anti).

  • Anti addition results in different configured stereoisomers (threo and erythro).

Examples of Reactions

• Addition of Water: Acid-catalyzed addition converts alkenes to alcohols.

• Hydroboration-Oxidation: Yields alcohols with syn addition characteristics where bulkiness signifies favored outcomes for alkyl groups.

• Bromination: Reaction proceeds via three-membered ion intermediates leading to anti addition of Br2.

Ozonolysis

• Ozonolysis converts alkenes to carbonyls; each sp² bond results in fragmentation and the formation of ketones and aldehydes based on the substituents present.

Regioselectivity and Mechanisms

• Regioselectivity pertains to preferential product formation based on which sp² carbon is attacked. Products depend on electrophile strength and nucleophiles in excess.

Enzymatic Reactions

• Enzymes exhibit strict stereochemistry leading to the formation of single stereoisomers without the formation of undesirable isomers.

Conclusion

• Alkenes are key in organic reactions, forming various products through addition mechanisms that undergo different stereochemical transformations.