Give the structure of the addition product formed when...

Give the structure of the addition product formed when ethylene reacts with each of the following reagents: (a) $\mathrm{H}-\mathrm{I}$ (b) $\mathrm{Br}_{2}$ (c) $\mathrm{BH}_{3}$ (Hint: Each of the $\mathrm{B}-\mathrm{H}$ bonds undergoes an addition to one molecule of ethylene. That is, three moles of ethylene react with one mole of $\mathrm{BH}_{3}$.)

Give the structure of the addition product formed when ethylene reacts with each of the following reagents:
(a) $\mathrm{H}-\mathrm{I}$
(b) $\mathrm{Br}_{2}$
(c) $\mathrm{BH}_{3}$ (Hint: Each of the $\mathrm{B}-\mathrm{H}$ bonds undergoes an addition to one molecule of ethylene. That is, three moles of ethylene react with one mole of $\mathrm{BH}_{3}$.)

Key Concepts

Electrophilic Addition Reactions

Electrophilic addition reactions involve the attack of an alkene’s electron?rich double bond on an electrophile, leading to the formation of new sigma bonds. These reactions typically proceed through the formation of an intermediate (such as a carbocation or a cyclic halonium ion) and are fundamental to understanding how unsaturated compounds, like alkenes, react with various electrophilic species.

Regioselectivity and Markovnikov's Rule

In many electrophilic additions, the distribution of atoms in the final product is determined by regioselectivity, often explained by Markovnikov’s rule. This rule states that in the addition of a hydrogen halide to an unsymmetrical alkene, the hydrogen atom tends to attach to the carbon with more hydrogen substituents, thereby generating the more stable carbocation intermediate. This concept is critical for predicting where groups will add along a double bond.

Anti Addition Mechanism

The anti addition mechanism refers to a stereochemical pathway where substituents add to opposite faces of a double bond. In reactions where intermediates such as cyclic halonium ions are formed (as in halogen additions), the nucleophile attacks from the side opposite to the leaving group, resulting in an overall anti stereochemistry. Understanding this concept is crucial for predicting the three-dimensional arrangement of atoms in the product.

Syn Addition in Hydroboration

Syn addition is a process in which both atoms or groups add to the same side (face) of a double bond. In hydroboration reactions, borane (BH3) reacts with alkenes through a concerted mechanism where both the boron atom and hydrogen add simultaneously on the same face, leading to the characteristic syn stereochemistry. This concept is particularly important when analyzing subsequent reactions such as oxidation following hydroboration.

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