Problem 190 - A Complete Resource Book in Chemistry for JEE Main

Question

On mixing a certain alkane with chlorine and irradiating it with ultraviolet light, it forms only one monochloroalkane. This alkane could be [2003] (a) propane (b) pentane (c) isopentane (d) neopentane

Step-by-Step Solution

Answer

(d) Neopentane is the alkane that forms only one monochloroalkane.

Step 1: Understanding the Reaction

The exercise involves an alkane reacting with chlorine under ultraviolet light. This is a typical free-radical halogenation reaction where hydrogen atoms in the alkane can be replaced by chlorine atoms, forming monochloro products.

Step 2: Identifying Monochloroalkane Formation

We are given that this process only forms one type of monochloroalkane. This means that there is only one type of hydrogen atom in the alkane molecule. For there to be only one monochloro product, the equivalent positions must all be the same, leading to a single product.

Step 3: Analyzing Each Option

(a) Propane (CH₃-CH₂-CH₃) has 2 types of hydrogen positions (primary and secondary) leading to 2 monochloro products. (b) Pentane (CH₃-(CH₂)₃-CH₃) has 3 types of hydrogen positions, leading to more than one monochloro product. (c) Isopentane (CH₃)₂CH-CH₂-CH₃ has multiple types of hydrogen positions, leading to multiple monochloro products. (d) Neopentane ((CH₃)₄C) has only one type of hydrogen position making it possible to form only one type of monochloro product.

Step 4: Selecting the Correct Alkane

Since neopentane has carbon atoms with all equivalent hydrogens, chlorination will result in only one type of monochloroalkane, meeting the requirement of the exercise.

Key Concepts

Free-radical Halogenation

Free-radical halogenation is a reaction where alkanes react with halogens like chlorine or bromine under specific conditions like heat or ultraviolet light.
This process involves three main steps: initiation, propagation, and termination.

• Initiation: This step is the starting point where chlorine molecules (**Cl₂**) are split into two chlorine radicals (**Cl•**) by ultraviolet light or heat. Radicals are highly reactive atoms with unpaired electrons.
• Propagation: In this phase, the chlorine radical reacts with an alkane, first removing a hydrogen atom to form hydrogen chloride (**HCl**) and an alkyl radical. The alkyl radical then reacts with another chlorine molecule, continuing the cycle by forming a chlorinated product and another chlorine radical.
• Termination: During this final stage, two radicals come together to form a stable molecule, ending the reaction cycle.

This reaction is very useful for introducing chlorine into hydrocarbons and is an example of how selectively reactive radicals can be in organic chemistry.

Hydrogen Atom Position

The position of hydrogen atoms in an alkane molecule is crucial in determining the outcome of free-radical halogenation. Not all hydrogen atoms are equivalent in a molecule; they may vary depending on their bonding and location.

• Primary Hydrogens: These are the hydrogen atoms attached to a carbon with two other hydrogen atoms and one carbon, i.e., **R-CH₃.**
• Secondary Hydrogens: Found on a carbon atom attached to two other carbons and one other hydrogen, i.e., **R₂-CH₂.**
• Tertiary Hydrogens: These attach to a carbon bonded to three other carbon atoms, i.e., **R₃-CH.**

Each type of hydrogen can result in a different product during chlorination. For the reaction to create only one monochloroalkane, as in the case of neopentane, all hydrogen positions in the molecule must be equivalent.

Chlorination Reaction

The chlorination reaction is a specific type of halogenation where chlorine is used to replace hydrogen atoms in a hydrocarbon. In the presence of ultraviolet light, the energy helps chlorine molecules split into radicals, setting off a chain reaction that substitutes hydrogen with chlorine. Unlike bromination, chlorination is less selective, meaning it can react with many different types of carbon-hydrogen bonds with varying ease. Because of this low selectivity, controlling the specific hydrogen atom that chlorine will replace is complex in molecules with different hydrogen types. In the unique case of neopentane, all hydrogen atoms are equivalent, simplifying the reaction and ensuring that only a single type of monochloroalkane forms. This is a valuable reaction in organic synthesis due to the uniformity and predictability of the product.

Neopentane Chemistry

Neopentane, also known as 2,2-dimethylpropane, has the molecular formula **C₅H₁₂** and a distinctive structure where all carbon atoms are tetrahedrally arranged around a central carbon atom.

• Structural Simplification: This unique arrangement results in every hydrogen being in the same environment, meaning all hydrogen positions are equal.
• Monochloro Product Formation: When neopentane undergoes chlorination, it can only form one type of monochloro product since there is no variation in the hydrogen positions.

The chemistry of neopentane makes it an interesting study for reactions requiring specificity, as it simplifies analysis and prediction of reaction products. This particular characteristic of neopentane allows it to produce a single monochlorinated derivative under chlorination, highly beneficial for obtaining uniform products in chemical synthesis.