Question

Why is the hydrolysis, during the preparation of alkane diols from 1,2 dihaloalkanes, carried out with mild alkali like \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) and not with \(\mathrm{NaOH} ?\)

Short answer

Na₂CO₃ is used to minimize side reactions and ensure controlled formation of alkane diols.

Step-by-step solution

Understand the Chemical Reaction

The reaction is an alkaline hydrolysis of 1,2-dihaloalkanes to form alkane diols. Typically, halogen atoms are replaced with hydroxyl groups. A base is used to facilitate this hydrolysis.

Consider the Reactivity of NaOH

NaOH is a strong base and can lead to highly reactive conditions. Its strong alkalinity may cause unwanted side-reactions, such as elimination reactions forming alkenes instead of substitution reactions forming diols.

Analyze the Advantage of Using Na₂CO₃

Na₂CO₃ is a mild alkali which provides a more controlled reaction environment. Its moderate basicity reduces the chance of elimination reactions, aiding the desired formation of the diol.

Key concepts

1,2-Dihaloalkanes

1,2-Dihaloalkanes are a type of organic compound where two halogen atoms are bonded to two adjacent carbon atoms in an alkane chain. These compounds are versatile building blocks in organic chemistry. They are especially valuable because they can undergo various reactions, including hydrolysis, leading to significant products like alkane diols. The halogens in these compounds, often chlorine or bromine, are much more electronegative than carbon. This means the carbon-halogen bond is polar, making the carbon atoms electrophilic. As a result, they are more susceptible to nucleophilic attack, which is a crucial characteristic during the alkaline hydrolysis reactions they undergo. In summary, 1,2-dihaloalkanes are important intermediates in synthetic pathways because of their reactivity and ability to transform into a range of useful products through reactions like alkaline hydrolysis, which we will discuss next.

Alkaline Hydrolysis

Alkaline hydrolysis is a chemical reaction where water, in the presence of a base, reacts with a compound to cause breaking of bonds and formation of new products. In the context of 1,2-dihaloalkanes, it's when halogen atoms are replaced by hydroxyl groups, transforming the compound into a diol, specifically an alkane diol. This process involves a nucleophilic substitution mechanism. The base facilitates the attack by a hydroxide ion on the electrophilic carbon bearing the halogen. As a result, the halogen is displaced, and a hydroxyl group takes its place. The key to successful alkaline hydrolysis with dihaloalkanes is the choice of base. A balanced approach is needed to suppress side-reactions that might yield unwanted products. Using the appropriate base ensures a smooth reaction towards the desired diol.

Mild Alkali vs Strong Base

The choice between a mild alkali and a strong base for reactions like alkaline hydrolysis significantly affects the outcome, especially when synthesizing alkane diols from 1,2-dihaloalkanes. **Mild Alkali (e.g., Na₂CO₃):** - Provides a gentle reaction environment. - Its moderate basicity limits the formation of side-products. - Reduces the risk of elimination reactions, hence favoring the substitution pathway. **Strong Base (e.g., NaOH):** - Leads to highly reactive conditions. - Can promote elimination reactions, where halogen atoms are removed without forming desired diols. - Might cause the formation of alkenes rather than completing the substitution to form diols. In synthesis, selecting the right base is crucial. While strong bases like NaOH can force the reaction to occur faster, they may induce unwanted complications. Mild alkalis like Na₂CO₃ serve better when precision and specificity are needed.