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

(a) Tert-Butyl free radical is not polar.

Step-by-step solution

Identify the Characteristics of Each Species

We need to identify the nature of each species in the given options. Polar molecules have a dipole moment due to an uneven distribution of electrons. The polarity often arises from differences in electronegativities of the atoms involved.

Analyze Tert-Butyl Free Radical

A free radical has an unpaired electron. The tert-butyl free radical, with a central carbon atom bonded to three methyl groups (CH₃), is symmetrical and lacks a definite charge.

Analyze Tert-Butyl Carbocation

A carbocation is a species with a positively charged carbon atom. The tert-butyl carbocation has a positively charged central carbon bonded to three other carbon atoms, creating a distribution of positive charge and thus, a polar character.

Analyze Tert-Butyl Carbanion

A carbanion is a negatively charged species with an extra pair of electrons on the carbon atom. The tert-butyl carbanion, although symmetrical in structure, carries this negative charge, making it polar.

Analyze Allyl Carbanion

The allyl carbanion is formed by a negative charge on the carbon within an allyl group, which is resonance-stabilized. This resonating negative charge distribution results in polarity.

Determine the Non-Polar Species

Based on the analysis, the tert-butyl free radical is the only species without a fixed charge or significant polarity as it is symmetrically structured and has no partial positive or negative poles.

Key concepts

Polarity in Molecules

Understanding polarity in molecules involves looking at how electrons are shared between atoms. When electrons are distributed unevenly, resulting in regions of partial positive and negative charges, a molecule becomes polar. Polarity often arises due to differences in electronegativity, which is the tendency of an atom to attract electrons towards itself. If the atoms in a molecule have different electronegativities, the electrons will be drawn more towards one atom, creating a dipole. When electronegativities are similar, electrons are shared more equally, resulting in a non-polar molecule. It's essential to understand these concepts when determining the physical properties and reactions of the molecules.

Tert-Butyl Radicals

Tert-butyl radicals are highly reactive species with an unpaired electron on the central carbon atom. The structure of a tert-butyl radical can be thought of as a central carbon atom bonded to three methyl groups (CH₃). This creates a symmetrical, stable distribution around the central carbon. Because there is no significant difference in electronegativity between the atoms, and the molecule lacks any charged ends or distinct polarity, it is classified as non-polar. In this symmetrical arrangement, the unpaired electron is not enough to create an imbalance of charge that would generate a dipole moment, thus rendering the tert-butyl radical non-polar in nature.

Carbocations and Carbanions

Carbocations and carbanions are ions where the carbon atom either loses or gains electrons to attain a charge. A carbocation, such as the tert-butyl carbocation, has a positively charged central carbon atom because it has one less electron. This positivity attracts electron clouds from neighboring atoms, giving it a polar character due to uneven charge distribution.
Conversely, a carbanion, like the tert-butyl carbanion, carries a negative charge due to an extra pair of electrons. These excess electrons create regions of negative charge, causing the molecule to be polar as well. These differences in charge define the molecule's reactivity and its interactions within chemical reactions. Understanding the nature and behavior of these ions is crucial in predicting the outcomes of organic reactions.

Dipole Moment

A dipole moment is a quantitative measure of the polarity of a molecule. It arises when there is a separation of charge within a molecule, leading to a partial positive and a partial negative pole. Dipole moments are often represented by arrows pointing from the positive to the negative pole, indicating the direction of electron displacement. A higher dipole moment corresponds to a more polar molecule. For instance, in the analysis of the molecules mentioned, the allyl carbanion exhibits polarity due to the resonance stabilization, affecting its electron distribution and resulting in a dipole moment. Recognizing molecules with significant dipole moments is vital in predicting their solubility, boiling points, and reactivity in chemical processes.