Question

Which of the following is correct order of reactivity of \(\mathrm{H}\) atoms of alkanes? (1) Primary II < secondary II < tertiary II (2) Sccondary II > primary II > tertiary II (3) Tertiary II > primary II > sccondary II (4) Tertiary II > secondary II > primary II

Short answer

Option (4) tertiary > secondary > primary is correct.

Step-by-step solution

- Understand the types of hydrogen atoms in alkanes

In alkanes, hydrogen atoms are classified based on the carbon atom they are attached to. A primary hydrogen (1°) is attached to a primary carbon (a carbon attached to only one other carbon). A secondary hydrogen (2°) is attached to a secondary carbon (a carbon attached to two other carbons). A tertiary hydrogen (3°) is attached to a tertiary carbon (a carbon attached to three other carbons).

- Review the reactivity order of hydrogen atoms in alkanes

The reactivity of hydrogen atoms follows the order: tertiary > secondary > primary. This is because tertiary carbons form the most stable free radicals during reactions, followed by secondary, and then primary carbons.

- Identify the correct option

Given the reactivity order (tertiary > secondary > primary), the correct option that matches this order should be identified.

- Match with given options

Option (4) Tertiary > secondary > primary matches the correct reactivity order. This means that option (4) is the correct answer.

Key concepts

primary hydrogen

In alkanes, a primary hydrogen atom (1°) is connected to a primary carbon. A primary carbon is one that is bonded to only one other carbon atom. This makes primary hydrogens the least substituted type of hydrogen in alkanes.

Primary hydrogens are found at the end of a carbon chain. They are generally the least reactive, due to the lower stability of the free radicals they form. Free radicals are atoms or molecules with an unpaired electron, making them highly reactive. When one of these primary hydrogens is removed, the resulting free radical (primary radical) is less stable than secondary or tertiary radicals. This reduced stability means primary hydrogens are less likely to participate in chemical reactions.

When studying alkanes, identifying primary hydrogens can help you predict their reactivity. Knowing that primary hydrogens are the least reactive in most cases, you can anticipate the behavior of alkanes in various chemical reactions.

secondary hydrogen

A secondary hydrogen atom (2°) in alkanes is attached to a secondary carbon. A secondary carbon, in turn, is a carbon that is bonded to two other carbons. This makes secondary hydrogens more reactive than primary hydrogens.

Secondary hydrogens are usually found in the middle of the carbon chain. Their increased reactivity compared to primary hydrogens is due to the relatively higher stability of the secondary free radicals they form. Secondary free radicals are more stable because the unpaired electron is better dispersed among neighboring carbon atoms, reducing the overall energy of the system.

In chemical reactions, secondary hydrogens participate more readily than primary hydrogens. This higher reactivity makes secondary hydrogens an important consideration when predicting the outcomes of alkane-related reactions.

tertiary hydrogen

A tertiary hydrogen atom (3°) is connected to a tertiary carbon. A tertiary carbon is one that is bonded to three other carbons. Tertiary hydrogens are the most reactive in alkanes.

These hydrogens are typically found in more branched alkanes. The high reactivity of tertiary hydrogens is due to the exceptional stability of tertiary free radicals. When a tertiary hydrogen is removed, the resulting free radical has its unpaired electron highly stabilized by three adjacent carbon atoms. The electron's energy is even more effectively dispersed, making the radical much more stable.

Because of this increased stability, tertiary hydrogens are highly reactive and participate in chemical reactions more readily than both primary and secondary hydrogens. Knowing about the position and reactivity of tertiary hydrogens helps predict how alkanes will behave under different conditions or during specific chemical processes.

reactivity order

The reactivity order of hydrogen atoms in alkanes is crucial for understanding their chemical behavior. The order is: tertiary (3°) > secondary (2°) > primary (1°).

This order is based on the stability of the free radicals formed when these hydrogens are removed. Tertiary carbons form the most stable free radicals owing to their three carbon neighbors, which effectively disperse the unpaired electron's energy. Secondary carbons create moderately stable free radicals because they have two carbon neighbors, while primary carbons yield the least stable free radicals with only one carbon neighbor.

This reactivity order explains why, in reactions involving alkanes, tertiary hydrogens react the fastest, followed by secondary, and then primary hydrogens. Understanding and remembering this reactivity order helps you predict and explain the outcomes of various chemical reactions involving alkanes.