Question

Which of the following are more stable than \(\mathrm{CH}_{3}-\mathrm{CH}_{2}\) here? (a) \(\mathrm{CH}_{3}-{\mathrm{C}} \mathrm{H}-\mathrm{CH}_{3}\) (b) \(\mathrm{Ph}-\stackrel{\vartheta}{\mathrm{C}} \mathrm{H}\) (c) \(\mathrm{HC}=\hat{\mathrm{C}}\) (d) \(\mathrm{CH}_{2} \mathrm{Cl}\)

Short answer

(a) and (b) are more stable than \( \mathrm{CH}_{3}-\mathrm{CH}_{2} \).

Step-by-step solution

Analyze Reference Stability

The reference structure given is the ethyl radical, \( \mathrm{CH}_{3}-\mathrm{CH}_{2} \). Ethyl radicals are primary carbon radicals, generally less stable than secondary or tertiary radicals and those that benefit from resonance stabilization.

Evaluate Structure (a)

For structure (a), \( \mathrm{CH}_{3}-\mathrm{C} \mathrm{H}-\mathrm{CH}_{3} \), the radical is attached to a secondary carbon, making it a secondary radical, which is more stable than the primary ethyl radical.

Evaluate Structure (b)

Structure (b) is \( \mathrm{Ph}-\stackrel{\vartheta}{\mathrm{C}} \mathrm{H} \), the phenyl radical. This radical benefits from resonance due to electron delocalization with the aromatic ring, making it more stable than the ethyl radical.

Evaluate Structure (c)

The structure \( \mathrm{HC}=\hat{\mathrm{C}} \) represents an ethynyl radical where the radical is "cumulatively pinned" between two sp hybridized carbons. This doesn't offer resonance stabilization, thus this radical is likely less stable than a secondary or resonance-stabilized radical.

Evaluate Structure (d)

For \( \mathrm{CH}_{2} \mathrm{Cl} \), which is a chloromethyl radical. The electronegative Cl atom can withdraw electron density but does not provide significant resonance stabilization like aromatic systems. It is generally less stable than secondary or resonance-stabilized radicals.

Conclude Stability Comparisons

Comparing the radicals, structures (a) and (b) are more stable than \( \mathrm{CH}_{3}-\mathrm{CH}_{2} \) due to being a secondary radical and an aromatically stabilized radical, respectively. Structures (c) and (d) are not more stable due to lack of sufficient resonance stabilization or lack of secondary radical stability.

Key concepts

Primary Carbon Radical

A primary carbon radical refers to a molecule where one of the carbon atoms bears an unpaired electron and is connected to only one other carbon atom. These radicals are characterized by:

• Having the free electron on a carbon atom that is only attached to one other alkyl group.
• Exhibiting lower stability compared to secondary and tertiary radicals.
• Being highly reactive due to the lack of stabilizing interactions from neighboring groups.

A classic example of a primary carbon radical is the ethyl radical (\(\mathrm{CH}_{3}-\mathrm{CH}_{2}\)). In this molecule, the radical is located on a carbon atom that is only connected outwardly to an ethyl group, making it a primary radical. Due to their high reactivity, primary radicals quickly participate in chemical reactions, striving to achieve a more stable electronic state.

Secondary Carbon Radical

Secondary carbon radicals are molecules with an unpaired electron on a carbon that is attached to two other carbon atoms. Here's what you need to know about them:

• The radical center is sandwiched between two alkyl groups, allowing for greater stability.
• This stabilizing effect results from hyperconjugation and the inductive effects of adjacent atoms.
• Secondary radicals are less reactive compared to primary radicals, offering more stable reaction intermediates.
• An example is the radical in structure (\(\mathrm{CH}_{3}-\mathrm{C} \mathrm{H}-\mathrm{CH}_{3}\)), where the radical center is attached to two alkyl groups making it secondary.

This position allows secondary radicals to participate in more select chemical transformations due to their enhanced stability.

Resonance Stabilization

Resonance stabilization is a critical concept in radical chemistry and impacts the stability of molecules:

• Resonance occurs when electrons in a molecule can be shared over multiple atoms.
• Aromatic rings, like benzene, offer excellent resonance stabilization by delocalizing the unpaired electron.
• This drastically lowers the molecule's energy, increasing its stability.
• For example, in a phenyl radical (\(\mathrm{Ph}-\stackrel{\vartheta}{\mathrm{C}} \mathrm{H}\)), the electron can move across the aromatic ring structure.

This resonance effect makes certain radicals far more stable than others that cannot disperse their unpaired electron in a similar manner.

Ethyl Radical

An ethyl radical is a specific type of primary carbon radical with significant reactivity due to its structure:

• The ethyl radical (\(\mathrm{CH}_{3}-\mathrm{CH}_{2}\)) has the unpaired electron on a carbon atom bonded to another carbon and two hydrogen atoms.
• It is less stable than secondary or tertiary radicals, making it more susceptible to participating in reactions.
• This instability arises because it lacks the electron-donating effects that can stabilize radicals attached to more carbon atoms.

As a primary radical, the ethyl radical is essential in understanding organic transformations but requires conditions that account for its reactivity.

Chloromethyl Radical

The chloromethyl radical is another kind of primary carbon radical, influenced by its substituent:

• Formed by (\(\mathrm{CH}_{2} \mathrm{Cl}\)), it has the radical on a carbon atom attached to a chlorine atom and two hydrogen atoms.
• The presence of the electronegative chlorine atom influences its reactivity by drawing electron density away from the radical center.
• Unlike resonance-stabilized radicals, the chloromethyl radical does not provide the same level of stability.
• Its reactivity can be harnessed in various synthetic processes, though with limited applications compared to more stable radicals.

While chlorine's electron-withdrawing nature affects its behavior, it cannot offer the stabilization seen in bound aromatic radicals.