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

(a) allyl iodide, propene and isopropyl iodide

Step 1: Understanding the Reactants

The reaction involves glycerol, a triol (having three hydroxyl groups), and excess hydroiodic acid (HI). Excess HI implies that there's more than enough HI to fully react with all hydroxyl groups of glycerol.

Step 2: Reaction of Glycerol with HI

Excess HI would initially convert glycerol into primary iodides by replacing hydroxyl groups. Glycerol will lose its hydroxyl groups and step down to form allyl iodide by removal of one water molecule (dehydration).

Step 3: Dehydration to Form Propene

Further interaction and possible dehydration in the presence of HI can lead to the formation of propene from intermediates such as allyl iodide. HI provides conditions that facilitate this dehydration.

Step 4: Forming Isopropyl Iodide

Under conditions with excess HI and after dehydration to form propene, a Markovnikov addition of HI to propene will lead to the formation of isopropyl iodide. The addition of HI follows Markovnikov's rule.

Step 5: Analyzing Product Order

Given the steps, the initial major products are allyl iodide, then propene from further dehydration. Finally, isopropyl iodide is formed from propene.

Key Concepts

Dehydration Reaction

Dehydration reactions are key processes in organic chemistry where water molecules are removed from the reactants, leading to the formation of double bonds or unsaturated substances. In the case of glycerol reacting with excess hydroiodic acid (HI), dehydration plays a pivotal role. Initially, the HI facilitates the substitution of the hydroxyl groups in glycerol, transforming it into iodide intermediates. As the reaction progresses, one water molecule is removed from these intermediates, resulting in the formation of allyl iodide.
The dehydration process doesn't stop there. Further dehydration under the influence of HI can lead to the formation of propene. Here, the removal of water is crucial for converting the single-bonded saturated compounds into ones with double bonds, thus transforming into alkenes like propene. This progressive removal of water reveals the importance of dehydration reactions not only in changing the structural properties of molecules but also in transforming functional groups, ultimately paving the way for the creation of various organic compounds.

Markovnikov's Rule

Markovnikov's rule is an essential concept in chemistry, especially when predicting the outcome of the addition of hydrogen halides to alkenes. This rule states that in the addition reaction of HX (like hydroiodic acid, HI) to an alkene, the hydrogen atom attaches itself to the carbon with more hydrogen atoms (less substituted carbon), while the halide component attaches to the carbon with fewer hydrogen atoms (more substituted carbon).
Applying this rule in the context of our original exercise, after propene is formed through dehydration, HI returns to react with the propene. According to Markovnikov’s rule, the hydrogen from HI bonds to the terminal carbon of propene. This leaves the iodine to attach to the central carbon. This addition effectively transforms propene into isopropyl iodide. Markovnikov's rule is pivotal in predicting the structure of the major product from alkene reactions and exemplifies how molecule configuration can be controlled through specific reaction conditions.

Formation of Haloalkanes

The formation of haloalkanes, such as allyl iodide and isopropyl iodide, are notable outcomes during the reaction between glycerol and excess hydroiodic acid. These compounds are characterized by the substitution of a halogen atom for an atom or group in a hydrocarbon chain. In our exercise, glycerol undergoes a series of transformations facilitated by hydroiodic acid. Initially, one of the hydroxyl groups of glycerol is replaced with an iodine atom to form allyl iodide. This conversion primes the structure for further transformations.
Through subsequent dehydration reactions, an alkene (propene) can form, which is then transformed into isopropyl iodide through Markovnikov's addition of HI. The resultant isopropyl iodide is a haloalkane, demonstrating how functional groups can be modified using halogen elements. Understanding the formation of haloalkanes is fundamental in organic synthesis, as these compounds serve as critical intermediates and reactants in the creation of numerous other organic molecules.