Problem 18 - A Complete Resource Book in Chemistry for JEE Main

Question

1,1-dibromoethane when heated with zinc dust produces (a) ethyl bromide (b) ethene (c) vinyl bromide (d) 2 -butene

Step-by-Step Solution

Answer

1,1-dibromoethane reacts with zinc dust to form ethene.

Step 1: Understand the Reaction Type

The reaction involved here is the dehalogenation of a vicinal dibromide (1,1-dibromoethane) using zinc dust as a reducing agent. A vicinal dibromide has two bromine atoms attached to adjacent carbon atoms.

Step 2: Identify the Reaction Outcome

Heating 1,1-dibromoethane with zinc will remove both bromine atoms as zinc bromide (ZnBr₂), leading to the formation of a carbon-carbon double bond, resulting in an alkene.

Step 3: Determine the Alkene Product

When the two bromine atoms are removed from 1,1-dibromoethane, the remaining molecule is ethene (C₂H₄), a simple alkene with a double bond between the two carbon atoms.

Key Concepts

Vicinal Dibromide

In organic chemistry, a vicinal dibromide is a chemical compound featuring two bromine atoms bonded to adjacent carbon atoms within a molecule. It is important to note that the positioning of these bromine atoms is key in determining the reactivity and outcome of chemical reactions, particularly in dehalogenation processes. The "vicinal" aspect indicates that the bromine atoms are located on neighboring carbons, which is different from a geminal dibromide, where both bromine atoms would be on the same carbon atom.
This distinction plays a critical role when predicting products because the hollow space created once the bromine atoms are removed allows for a straightforward double bond formation, as seen in many reactions involving alkenes.

• Vicinal means adjacent or neighboring.
• Dibromides have two bromine atoms connected to the carbon backbone.
• This setting is an excellent precursor for the removal of halogens, leading to unsaturated compounds like alkenes.

Alkene Formation

The transformation of a vicinal dibromide into an alkene is a significant reaction in organic synthesis. This process is highlighted by the removal of the halogen atoms and the formation of a carbon-carbon double bond. The carbon backbone restructures itself once the bromine atoms are expelled, which typically results in an unsaturated compound known as an alkene. Alkenes are characterized by the presence of one or more carbon-carbon double bonds, enhancing their reactivity in polymer production and chemical industries.
During the dehalogenation reaction, the elimination of bromine atoms transforms the formerly saturated molecule into an unsaturated one, drastically altering its chemical properties:

• Alkenes are unsaturated hydrocarbons with double bonds.
• Double bonds open up avenues for reactions, such as polymerization.
• The reaction often requires a catalyst or specific conditions to initiate and sustain the dehalogenation.

Zinc Dust Reduction

The role of zinc dust in the dehalogenation of vicinal dibromides is fundamental. Zinc works as a reducing agent, meaning it donates electrons to other molecules during a chemical reaction. In the case of vicinal dibromides, zinc facilitates the removal of bromine atoms from the organic compound in the form of zinc bromide (ZnBr₂), an inorganic salt. This step is crucial for the formation of the carbon-carbon double bond in the newly formed alkene.
Simplifying this concept, consider how zinc dust is used:

• Zinc acts as a reducing agent, aiding the removal of bromate ions.
• It forms zinc bromide as a byproduct.
• The metallic nature of zinc allows it to effectively "pull" bromine out of the compound, making it essential for the reaction to proceed.

This dehalogenation technique highlights the utility of metal dusts like zinc in organic reactions, showcasing its effectiveness in enabling desired transformations.