Lesson 7: Alkenes - Review Session on HBr, NMR, and Synthesis

Lecture 7: Alkenes: Review and Applications in HBr, NMR, and Synthesis

I. HBr Addition to Alkenes:

1. Mechanism:

   • Electrophilic Addition: HBr adds to alkenes in an electrophilic addition reaction.

   • Step 1: Formation of Bromonium Ion: The pi electrons of the double bond attack the hydrogen of HBr, forming a bromonium ion intermediate (a cyclic three-membered ring with a positive charge on the bromine).

   • Step 2: Nucleophilic Attack: A bromide ion (Br-) attacks the bromonium ion. This results in the formation of a dibromide product where the bromine adds to the more substituted carbon (following Markovnikov's rule).

2. Regioselectivity:

   • Markovnikov's Rule: The bromine atom typically adds to the more substituted carbon of the double bond.

3. Stereochemistry:

   • Anti Addition: H and Br add to opposite faces of the double bond due to the cyclic nature of the bromonium ion.

II. NMR Spectroscopy of Alkenes:

1. Principle:

   • Nuclear Magnetic Resonance (NMR): Alkenes exhibit distinct NMR spectra due to the different chemical environments of their protons (H atoms).

   • Chemical Shifts: Protons attached to sp2 hybridized carbons in alkenes typically show upshifted chemical shifts (around δ 5-6 ppm) compared to saturated hydrocarbons (δ 0-4 ppm).

2. Coupling Patterns:

   • Spin-Spin Coupling: Protons on adjacent carbons in the alkene (vicinal coupling) or protons on the same carbon (geminal coupling) show characteristic splitting patterns in the NMR spectrum.

III. Synthesis Applications:

1. Functional Group Transformations:

   • Hydration: Alkenes can be hydrated to form alcohols via acid-catalyzed or oxymercuration-demercuration methods.

   • Oxidation: Alkenes can undergo oxidation reactions to form carbonyl compounds (aldehydes or ketones) using oxidizing agents like potassium permanganate (KMnO4) or ozone (O3).

2. Importance in Organic Synthesis:

   • Diversity: Alkenes serve as versatile intermediates in organic synthesis, facilitating the creation of complex molecular structures through controlled addition reactions and subsequent functional group transformations.