7.4.1.1. From alkenes

Alkenes react with water in the presence of acid as catalyst to form alcohols. In case of unsymmetrical alkenes, the addition reaction takes place in accordance with Markovnikov’s rule.

In the preparation of alcohols from alkenes, the reaction is primarily achieved through acid-catalyzed hydration. This means that when we have an alkene (a molecule with a carbon-carbon double bond) and we add water (H2O) in the presence of an acid (like sulfuric acid), an alcohol is formed. If the alkene is unsymmetrical (for example, propene), the water molecule adds to the carbon atom of the double bond that has more hydrogen atoms connected to it. This behavior is explained by Markovnikov's rule, which states that in the addition of HX (where X can be a halogen or OH), the hydrogen atom will attach to the carbon with more hydrogen atoms already, making the reaction predictable.

Imagine you have a crowd of people standing in a line. The larger group stands closer to the front while the smaller group stays behind. If someone comes along to hand out balloons to the larger group first, they are following the principle of giving preferential treatment to the bigger crowd, just like how the hydrogen atom prefers to attach to the more 'populated' carbon in Markovnikov's rule.

The mechanism of the reaction involves the following three steps: Step 1: Protonation of alkene to form carbocation by electrophilic attack of H3O+. Step 2: Nucleophilic attack of water on carbocation. Step 3: Deprotonation to form an alcohol.

The preparation of alcohols through acid-catalyzed hydration involves a series of steps. In the first step, the alkene is protonated by the acid, leading to the formation of a carbocation (a positively charged carbon atom). This step occurs because the alkene acts as a nucleophile (it attracts positively charged species). In the second step, water (acting as a nucleophile) attacks this carbocation, resulting in a molecule that has both an -OH and a hydrogen atom attached to the previously double-bonded carbon. The final step involves removing a proton (H+) to yield the alcohol.

Consider a simple scenario where you have a ball (the alkene) that is thrown at a net (the acid). When the ball hits the net, it gets caught (protonation forms the carbocation). Then, someone (water) reaches in to grab the ball (nucleophilic attack). Finally, the ball is adjusted by removing a small additional piece (deprotonation), resulting in an ideally placed device (the alcohol).

Diborane (BH3) reacts with alkenes to give trialkyl boranes as addition product. This is oxidised to alcohol by hydrogen peroxide in the presence of aqueous sodium hydroxide.

Another method to prepare alcohols from alkenes is through hydroboration-oxidation. This process involves reacting diborane (BH3) with the alkene, which results in the formation of trialkyl borane. Then, this compound undergoes oxidation, often using hydrogen peroxide in the presence of sodium hydroxide, converting the boron atom into a hydroxyl group, forming an alcohol. Interestingly, this method leads to the formation of alcohols in a way that is contrary to Markovnikov's rule — the hydroxyl group connects to the less substituted carbon.

Imagine you have a cake (the alkene) where you want to add frosting (the alcohol). Instead of simply spreading the frosting on top of the cake, you first dip the cake in a cream (diborane) that evenly coats it. You then set the cake to cool (oxidation with hydrogen peroxide), allowing it to absorb the frosting uniformly. This process gives you a cake that, instead of just having frosting added on, has a perfectly flavored frosting throughout, showing that the process works differently than expected.

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