Question

Which of the following is not polar? (a) tert-Butyl free radical (b) tert-Butyl carbocation (c) tert-Butyl carbanion (d) allyl cabanion

Short answer

Tert-Butyl Free Radical (a) is non-polar.

Step-by-step solution

Identify the Components

First, let's identify each component in the question. A tert-Butyl free radical is a carbon center with an unpaired electron. A tert-Butyl carbocation has a positively charged carbon. A tert-Butyl carbanion is a negatively charged carbon ion. An allyl carbanion involves a structure with resonance allowing delocalization of the negative charge.

Determine Molecular Polarity

Molecular polarity generally depends on the molecular geometry and the presence of polar bonds. The presence of opposite charges within the molecular structure usually makes a molecule polar unless the shape cancels them out.

Evaluate tert-Butyl Free Radical

The tert-Butyl free radical has a single unpaired electron. This does not contribute to a net dipole moment, as radicals themselves do not imply polarity due to lack of charge.

Evaluate tert-Butyl Carbocation

The tert-butyl carbocation has a central positively charged carbon. This charge makes the molecule polar because of the asymmetrical distribution of charge.

Evaluate tert-Butyl Carbanion

The tert-butyl carbanion has a central negatively charged carbon. The central negative charge makes this molecule polar, again due to asymmetrical charge distribution.

Evaluate Allyl Carbanion

The allyl carbanion, while possessing a lateral negative charge, exhibits resonance that can distribute the charge across different atoms. This resonance potentially reduces any overall dipole moment but does not necessarily make it non-polar.

Identify the Non-Polar Molecule

Tert-Butyl Free Radical is the only structure without a clear charge, making it the non-polar molecule among the choices.

Key concepts

Tert-Butyl Free Radical

The tert-butyl free radical is a fascinating molecule centered around a carbon atom with a peculiar unpaired electron. This unpaired electron makes it a radical, a type of species known for being highly reactive. However, when it comes to polarity, the absence of a formal charge on the radical centers means that it is not inherently polar. Polarity usually involves separation of charge across a molecule.
Since a tert-butyl free radical does not possess a positive or negative charge, it does not exhibit a net dipole moment. This lack of charge separation means the tert-butyl free radical remains non-polar, despite its reactivity. The structure's non-polarity can be attributed to its symmetry and lack of charged atoms.
When considering the molecular structure, the tert-butyl group, which is a bulky arrangement of carbon and hydrogen atoms, does not introduce any charges into the molecule to create an overall dipole moment. Therefore, in exercises concerning polarity, this radical is frequently the non-polar choice.

Tert-Butyl Carbocation

A tert-butyl carbocation features a positively charged carbon atom at its center, leading to significant molecular polarity. A carbocation is characterized by having a carbon atom with only three bonds, thus carrying a positive charge. This makes the molecule highly polar due to the presence of an unbalanced charge.
In a tert-butyl carbocation, the positive charge arises because the central carbon atom has lost an electron, giving it only six valence electrons compared to the usual eight. This creates a strong dipole moment, as the charge is not balanced by any negative counterpart within the structure.
Due to its positively charged nature, the tert-butyl carbocation shows strong electrophilic behavior, seeking out electrons from other molecules to stabilize itself. The polarity of this compound is a crucial consideration in chemical reactions, especially those involving nucleophiles looking to donate electron pairs.

Tert-Butyl Carbanion

The tert-butyl carbanion presents an interesting scenario where the central carbon atom carries a negative charge, making it polar. Unlike the carbocation, the carbanion has gained an extra electron, resulting in a carbon center with eight electrons, giving rise to a full negative charge.
This negative charge creates a significant dipole moment due to the imbalance of charge distribution, making the tert-butyl carbanion polar. The structure consists of the central negatively charged carbon atom and the surrounding alkyl groups, which do not offset this negative charge.
The presence of a negative charge makes tert-butyl carbanions strong nucleophiles, often involved in reactions where they donate their excess electron pair to form bonds with electrophiles. This molecular shape and charge distribution underline its role in many organic reaction mechanisms.

Allyl Carbanion

The allyl carbanion is unique due to its ability to stabilize a negative charge through resonance. Unlike a typical carbanion, the negative charge in an allyl carbanion can be delocalized across the molecule. This delocalization occurs because of the pi-bond network present in the allyl system.
The structure typically involves a central carbon atom bonded to two other carbon atoms, with available p orbitals allowing electrons to resonate across multiple positions. This means that the negative charge is not fixed on a single atom but spreads out over the framework, reducing the overall dipole moment.
However, despite the resonance, the allyl carbanion still exhibits polarity due to the presence of a net negative charge. The resonance only lessens the extremity of the dipole moment, rather than eliminating it. As such, allyl carbanions are considered polar molecules, though with nuances in their charge distribution owing to resonance.