Problem 16 - A Complete Resource Book in Chemistry for JEE Main

Question

The number of dibromo derivatives of propane are (a) 5 (b) 4 (c) 3 (d) 2

Step-by-Step Solution

Answer

There are 4 dibromo derivatives of propane.

Step 1: Understand Propane Structure

Propane (\[\text{C}_3\text{H}_8\]) is a saturated hydrocarbon with a linear chain of three carbon atoms.

Step 2: Consider Possible Positions for Bromine

Examine the possible positions on the propane molecule where bromine atoms can be substituted; these are onto different carbon atoms.

Step 3: Generate Substitution Variants

Consider bromination at each carbon atom. For propane, the first carbon (\[\text{C}_1\]) and the last carbon (\[\text{C}_3\]) are equivalent due to symmetry, allowing substitution at equivalent positions.

Step 4: Count Isomers with Replacement

Replace two hydrogens with two bromines: - 1,1-dibromopropane - 2,2-dibromopropane - 1,2-dibromopropane - 1,3-dibromopropane This gives four different dibromo isomers.

Key Concepts

Propane Structure

Propane, with the chemical formula \(\text{C}_3\text{H}_8\), is a simple, yet fundamental, saturated hydrocarbon. It consists of three carbon atoms linked in a straight chain, each one forming single bonds with hydrogen atoms to complete their valence shells. The structure is straightforward:

• The first carbon atom is bonded to three hydrogens and one of the intermediate carbons.
• The middle carbon connects to two other carbons and two hydrogens.
• The last carbon mirrors the first with three hydrogens and a connection to the adjacent carbon.

This setup, known scientifically as a linear structure, provides propane with a stable form as all carbons are in a continuous chain. Understanding this structure is key when discussing chemical reactions and substitutions, especially when considering modifications with other elements like bromine.

Bromine Substitution

Bromine substitution in organic chemistry refers to the replacement of hydrogen atoms with bromine atoms in a molecule. Propane can undergo a substitution reaction where bromine atoms replace one or more hydrogen atoms attached to the carbon backbone. In the case of propane:

• Substitution typically occurs at carbon atoms which can host more bromine atoms, thereby creating different molecules.
• The outer carbon atoms (\(\text{C}_1\) and \(\text{C}_3\)) are equivalent due to the symmetry of the propane molecule. This symmetry simplifies potential substitution patterns, as replacing hydrogen on these carbons can yield equivalent results.

Bromine's electronegativity and larger atomic size compared to hydrogen make substituted propane derivatives relatively stable while altering the molecule's physical and chemical properties.

Isomer Counting

Isomers are molecules with the same molecular formula but different arrangements of atoms in space. Counting isomers involves identifying all the different possible molecular configurations that a compound like a dibromo derivative of propane can form. For dibromo derivatives of propane:

• Four distinct isomers arise from different bromine substitution patterns.
• These isomers include 1,1-dibromopropane, 2,2-dibromopropane, 1,2-dibromopropane, and 1,3-dibromopropane.
• Each arrangement is unique based on the position of the bromine atoms relative to the carbon chain.

Determining the number of isomers requires understanding of chemical symmetry and how different substitutions modify the original molecule's structure.

Organic Chemistry Reactions

Organic chemistry often examines the transformation of molecules through various reactions. Substitution reactions, like those involving bromine and propane, are key examples of how organic compounds can be transformed. These reactions typically involve the breaking of a bond in an organic molecule and formation of a new bond with a different atom or group. Key aspects in the context of propane include:

• Substitution reaction mechanisms might proceed through radical processes, especially with bromine, which can form under ultraviolet light or heat.
• These reactions are harnessed to synthesize different molecules selectively, like the dibromo derivatives studied here.

Understanding these processes not only illustrates how organic compounds can be modified but also provides insight into the reactivity and functional diversity of hydrocarbons in chemical synthesis.