Ethane-1, 2- diol when heated with \(\mathrm{PI}_{3}\) (or excess HI) gives (a) Iodoethane (b) \(1,2-\) di iodoethane (c) 2 - Iodoethanol (d) ethene

In organic chemistry, nucleophilic substitution reactions are fundamental processes where an electron-rich nucleophile selectively bonds with or attacks the positive charge of an atom to replace a leaving group. This can often be seen in the reactions of alcohols with phosphorus trihalides or halogen acids, like in our example of ethane-1,2-diol and PI3 or excess HI.

When the nucleophile, in this case iodine from PI3 or HI, approaches the electrophilic carbon (the carbon bonded to the leaving group -OH), the hydroxyl groups are displaced. This displacement, due to the carbon's partial positive charge, allows the nucleophile to bond, forming a new product. Students should note that the nucleophilicity of a molecule is influenced by several factors, including the molecule's charge, the solvent, the electronegativity of the atom, and the steric hindrance.

Understanding the mechanism of nucleophilic substitution helps in predicting the outcome of reactions involving halogens and alcohols. A classic example of such a reaction is the conversion of ethane-1,2-diol to 1,2-diiodoethane.

Ethane-1,2-diol, commonly known as ethylene glycol, is an organic compound widely used as an antifreeze in cooling and heating systems due to its low freezing point. Structurally, it is composed of a two-carbon chain with hydroxyl groups (-OH) attached to each carbon, denoted chemically as CH2(OH)-CH2(OH).

It's pivotal for students to visualize the molecule and understand how the hydroxyl groups' positioning can affect the molecule’s chemical reactivity. In the context of nucleophilic substitution reactions, the hydroxyl groups in ethane-1,2-diol are prone to substitution, making it a perfect candidate for demonstrating how alcohols react with halogenating agents such as PI3 or excess HI to form halogenated products.

Halogenation refers to the process where one or more halogen atoms are introduced into a molecule, often by replacing certain groups such as hydroxyl (-OH) in alcohols. This chemical transformation is important in creating various compounds in organic synthesis and industrial applications. With alcohols like ethane-1,2-diol, halogenation typically occurs via a nucleophilic substitution reaction.

The alcohol's hydroxyl group makes it susceptible to reaction with halogenating agents. When reacting with PI3 or excess HI, a clear example of this reactivity is observed. The halogenation of alcohols can lead to the formation of alkyl halides, which have a wide range of uses, including serving as intermediates in the manufacture of other organic compounds. In the provided exercise, ethane-1,2-diol is converted into 1,2-diiodoethane, showcasing how halogenation can result in significant changes to an organic molecule's structure and properties.