The compound which reacts fastest with Lucas reagent at room temperature is: (a) Butan - 1 -ol (b) Butan-2-ol (c) 2 -methylpropan-2-ol (d) 2 -methylpropan-1-ol

When exploring the reactivity of alcohols, particularly in the context of the Lucas test, it is essential to understand the classification of alcohols based on the carbon atom to which the hydroxyl (OH) group is attached.

Primary alcohols have the OH group connected to a carbon atom that is attached to at most one other carbon. This structural feature causes them to react relatively slowly due to the less stable carbonium ion formed during the reaction. Secondary alcohols are bound to a carbon atom connected to two other carbons, and they exhibit moderate reactivity due to a more stable carbonium ion.

Tertiary alcohols, however, are connected to a carbon that is attached to three other carbons, leading to the most stable carbonium ion and thus the fastest reaction rate with the Lucas reagent. The test is often characterized by a change in the solution's appearance, usually becoming cloudy or turbid as the alcohol is converted to an alkyl chloride.

To apply this knowledge, let's consider an illustrative example where multiple alcohols are presented, and we need to predict their reactivity. In this context, it becomes quite straightforward: the more alkyl groups surrounding the carbon holding the OH group, the faster the reaction with the Lucas reagent at room temperature.

Understanding the structure of tertiary alcohols is crucial for many chemical analyses, including the Lucas test. A tertiary alcohol is one where the hydroxyl group is connected to a carbon atom (tertiary carbon) that itself is directly bonded to three other carbon atoms.

Due to this arrangement, tertiary alcohols are particularly notable for their reactivity. The presence of three alkyl groups imparts steric hindrance and electronic effects that facilitate the formation of a carbocation—a positively charged carbon ion—during reactions such as the one with the Lucas reagent.

For example, 2-methylpropan-2-ol, a typical tertiary alcohol, has a central carbon connected to three other carbons and the hydroxyl group. This structure is key to its reactivity and the resulting speedy conversion to an alkyl chloride in the Lucas test.

Qualitative analysis in chemistry involves determining the presence or absence of particular compounds or functional groups within a substance. It doesn't measure the quantity but rather the characteristics or constituents of a sample. The Lucas test is a prime example of qualitative analysis used to distinguish between different alcohols based on their classification as primary, secondary, or tertiary.

The test's simplicity and reliability make it a staple in qualitative organic chemistry. Once a substance is treated with the Lucas reagent, observations are made regarding the reaction's rapidity and the physical changes in the solution, such as cloudiness caused by the insoluble alkyl chloride.

Qualitative analysis, such as the Lucas test, provides valuable initial data that can inform further quantitative analysis or be part of compound characterizations in syntheses, research, and educational settings.