Which compound reacts with propene to form tripropyl borane?

Diborane

The correct answer is option 2. Diborane.

Hydroboration is the key reaction involving diborane and alkenes. This process is used to add boron across the double bond of an alkene, leading to the formation of organoboranes (trialkylboranes in this case).

Diborane (\( \text{B}_2\text{H}_6 \)) first reacts with the alkene (propene) to form a cyclic intermediate. In this intermediate, the boron atom bonds to the carbon atoms of the double bond, with one of the hydrogen atoms from the diborane attaching to each of the carbon atoms initially involved in the double bond. The reaction of diborane with propene results in the addition of the boron atom across the double bond of propene. The overall reaction is:

Here, tripropyl borane is formed, where each propyl group (derived from propene) is attached to the boron atom.

Diborane (\( \text{B}_2\text{H}_6 \)) is used because it can effectively react with alkenes to form trialkylboranes. The reaction typically occurs in a solution where diborane is either dissolved or generated in situ.

Reactivity: Diborane reacts with the alkene in a 1:1 molar ratio, where each mole of diborane reacts with three moles of propene to yield one mole of tripropyl borane. The process proceeds through the formation of a cyclic transition state in which boron and hydrogen atoms are added to the carbons of the double bond.

Borane (\( \text{BH}_3 \)): While borane itself can react with alkenes, it is often used in a solvent like tetrahydrofuran (THF) and is less commonly used directly compared to diborane in the hydroboration reaction.

Boric Acid (\( \text{H}_3\text{BO}_3 \)): Boric acid does not react in the same manner as diborane. It is typically used in other types of reactions or as a weak acid and does not directly form trialkylboranes.

Sodium Borohydride (\( \text{NaBH}_4 \)): Sodium borohydride is primarily a reducing agent for carbonyl compounds and does not participate in hydroboration reactions with alkenes.

Summary:

In the hydroboration reaction, diborane reacts with propene to form tripropyl borane. The reaction involves the addition of boron and hydrogen across the double bond of the alkene, resulting in the formation of the organoborane product.