Problem 81 - A Complete Resource Book in Chemistry for JEE Main

Question

The three products obtained in the reaction between glycerol and excess hydroiodic acid would include in that order (a) allyl iodide, propene and isopropyl iodide (b) allyl iodide, isopropyl iodide and propene (c) isopropyl iodide, allyl iodide and propene (d) propene, allyl iodide and isopropyl iodide

Step-by-Step Solution

Answer

The correct answer is (c) isopropyl iodide, allyl iodide, and propene.

Step 1: Understand the Reaction

The reaction in question involves glycerol (1,2,3-propanetriol) reacting with excess hydroiodic acid (HI). HI acts as a reducing agent that can convert alcohol groups in glycerol into iodides.

Step 2: Initial Formation of Iodoform

Initially, one of the steps involves converting the primary and secondary alcohols of glycerol to iodoforms. This typically results in the removal of the hydroxyl groups as water, and iodine from HI replacing them.

Step 3: Formation of Iso-iodopropane

The first main product is isopropyl iodide. This is formed when two iodine atoms substitute the hydroxyl groups on the glycerol, leading to secondary iodination at the central carbon, typically observed with 2-iodopropane (isopropyl iodide).

Step 4: Subsequent Formation of Allyl Iodide

After isopropyl iodide forms, further dehydroiodination can yield allyl iodide, which is formed by the removal of another iodine, resulting in the reformation of a double bond. This structure is characterized by I attached to a carbon that is adjacent to a double bond.

Step 5: Final Product as Propene

The last product is propene. Further elimination of iodine and water could convert any remaining intermediary compounds into the unsaturated hydrocarbon propene, resulting from complete elimination reactions.

Key Concepts

Glycerol and Hydroiodic Acid Reaction

When glycerol, a three-carbon molecule with three hydroxyl (OH) groups, reacts with excess hydroiodic acid (HI), several transformations occur. This reaction provides a fascinating example of how alcohols can be converted to iodides through a process known as halogenation. Glycerol can react with HI, which serves as a source of iodine that replaces the hydroxyl groups, leading to a significant structural change in the molecule.
The process involves more than just a simple substitution. Instead, it includes multiple steps where various intermediates and products form. Understanding this reaction is crucial in organic chemistry because it illustrates how alcohols, like those in glycerol, are transformed into more reactive iodide forms, setting the stage for further reactions.

Alcohol to Iodide Conversion

The conversion of alcohol groups to iodides is key in many organic reactions. In the case of glycerol reacting with hydroiodic acid, each hydroxyl group on the glycerol is eventually replaced by an iodine atom.
This alcohol to iodide conversion is facilitated by the HI, where it donates an iodine atom to replace the OH group. This is a type of nucleophilic substitution reaction. It's particularly useful because the iodide group is a good leaving group and can easily be displaced in subsequent reactions.
By understanding the replacement of OH groups with I, one can appreciate how initial products like isopropyl iodide form when glycerol undergoes this transformation with HI.

Dehydroiodination Process

Following the initial conversion of alcohol to iodide forms, such as isopropyl iodide, the reaction can proceed to dehydroiodination. This step involves the removal of iodine (I) and hydrogen, leading to the formation of a double bond.
This process is crucial because it transforms saturated carbon compounds into unsaturated ones, thereby forming alkenes. For example, after isopropyl iodide formation in this reaction, dehydroiodination leads to allyl iodide. This is a pivotal step, as it highlights the diversity of products that can stem from a single starting molecule through different mechanisms. Dehydroiodination is a fundamental process that illustrates the reactivity of haloalkanes towards elimination reactions.

Iodoform Formation

Iodoform formation is an important initial step in the reaction process. When glycerol interacts with excess HI, iodoform formation occurs as a side reaction. This process involves the complete substitution of hydrogen atoms in the alcohol with iodine atoms.
Iodoform, a yellow crystalline solid known for its distinct antiseptic smell, can sometimes form when iodinated intermediates like from glycerol are further processed. While not the primary focus of this reaction, understanding how iodoform might form provides insights into side reactions that might take place and how excess reagents like HI can lead to multiple, sometimes unexpected, pathways in reaction sequences.

Product Formation Sequence in Organic Reactions

The order in which products form during this reaction is an excellent example of reaction pathways in organic chemistry. Initially, primary iodination leads to the formation of isopropyl iodide. As the reaction proceeds and conditions allow, further chemical changes can lead to allyl iodide through dehydroiodination.
Finally, continued reaction under certain conditions can result in propene, completing a sequence where alcohol groups are first converted to iodides, then altered through elimination processes to yield alkenes. Understanding this product formation sequence helps highlight the dynamic nature of organic reactions and how different conditions can steer the direction of these transformations.