Question

On boiling 2,2 -dichloropropane with aqueous alkali, the product obtained is (a) 2,2-propanediol (b) acetone (c) propanal (d) 2 -chloro-2-propanol

Short answer

Answer: Acetone

Step-by-step solution

Draw the structure of 2,2-dichloropropane

Start by drawing the structure of 2,2-dichloropropane, where there are two chlorine atoms attached to the second carbon of the propane chain. The structure would be: H-C-C(Cl)-C(H)(Cl)-H

Identify the E2 reaction mechanism

The reaction involves aqueous alkali, which indicates an elimination reaction. The E2 (bimolecular elimination) mechanism involves the use of a strong alkali as the base, such as OH-, to remove the two halide groups from the molecule.

Perform the E2 reaction

In the E2 mechanism, the base (OH-) will remove a proton from a neighboring carbon atom while the carbon-chlorine bond is broken, and the Cl- leaves the molecule. In our case, the OH- removes a proton from the first carbon atom, and the chloride ion leaves from the second carbon atom. The same process will happen for the other chlorine atom: H-C(-)-C(Cl)=C(H)(Cl)-H -> H-C=C(Cl)=C(-)-H As a result, we have removed both chloride ions and formed a double bond between the first and second carbons, giving us the final product: H-C=C-C(-)-H

Identify the final product

The final product, H-C=C-C(-)-H, is identical to the structure of acetone. Thus, the correct answer is: (b) Acetone

Key concepts

2,2-dichloropropane

2,2-Dichloropropane is an organic compound that belongs to the class of chlorinated hydrocarbons. It's a derivative of propane where two hydrogen atoms are replaced with chlorine atoms. These chlorine atoms are attached to the second carbon in the carbon chain, making it a 2,2-position isomer. The molecular structure can be visualized as a central carbon atom (C2) bonding with two chlorine atoms and four additional hydrogen atoms distributed among the three carbon atoms in the propane chain.

This specific arrangement of chlorine atoms plays a crucial role in how the molecule reacts in chemical reactions such as the E2 mechanism. Understanding its structure is essential as it directly influences the reaction pathways and the nature of the products formed during chemical processes. A clear grasp of this could help you predict and manipulate outcomes in synthesis reactions.

aqueous alkali

When we talk about an aqueous alkali in chemistry, we usually refer to a solution of water ( H_2O ) and a strong base. One of the most commonly used aqueous alkalis is sodium hydroxide ( NaOH ). Such solutions are known for their high hydroxide ion ( OH^- ) concentration.

In reactions involving 2,2-dichloropropane, the base, such as NaOH in water, plays a critical role in initiating the elimination reaction. The OH^- ions from the alkali solution are reactive species that can abstract protons from organic molecules, as seen in the E2 mechanism. These ions help in removing specific atoms or groups from the molecule, leading to the formation of the desired product – in this case, acetone through the elimination of chloride ions.

elimination reaction

The elimination reaction is a type of chemical reaction where certain atoms or functional groups are removed from a molecule, forming new double bonds in the process. One of the efficient types of elimination reactions is the E2 mechanism. In E2 reactions, the 'E' stands for elimination, and '2' denotes a bimolecular process, meaning two reactant species participate in the transition state.

During the E2 reaction with 2,2-dichloropropane and an aqueous alkali like NaOH , an OH^- ion abstracts a proton from one carbon while simultaneously a chlorine atom leaves from the adjacent carbon. This leads to the formation of a double bond between the two carbon atoms formally occupied by these groups. The reaction mechanism is concerted, occurring in a single step.

Ultimately, in this context, the elimination reaction transforms 2,2-dichloropropane into a compound that is reminiscent of acetone, following the removal of both chloride ions and the formation of new double bonds.