Question

Which of the following intermediate has the complete octet around the carbon atom? (a) free radical (b) carbene (c) carbanion (d) carbonium ion

Short answer

The carbanion has the complete octet around the carbon atom.

Step-by-step solution

Understanding the Concept of an Octet

An octet refers to the presence of eight electrons in the valence shell of an atom, which is usually associated with stability. Carbon, having four valence electrons, achieves an octet by sharing or gaining electrons to have a total of eight valence electrons in its outer shell.

Evaluating the Free Radical

A free radical contains a carbon atom with seven electrons in its outer shell (as it has an unpaired electron), which means it does not complete an octet around the carbon. This is because radicals are neutral species with odd numbers of electrons.

Analyzing the Carbene

A carbene has carbon with only six electrons in its outer shell because it contains a lone pair and two bonds (either sigma or pi). This makes carbene an electron-deficient species and thus, it does not possess a complete octet around the carbon atom.

Examining the Carbanion

A carbanion has carbon with a lone pair and three bond pairs, totaling eight electrons. This results in a full octet around the carbon atom. Carbanions are negatively charged because of the excess electron pair.

Investigating the Carbonium Ion

A carbonium ion is positively charged, with a carbon atom having only six electrons in its valence shell due to three bonding pairs and no lone pairs. Therefore, it does not have a complete octet.

Key concepts

Carbon Octet Rule

Carbon wants to be like its more stable noble gas neighbors by having eight electrons in its outer shell, known as a full octet. Typically, this is achieved through sharing or transferring electrons in bonds with other atoms. When a carbon atom possesses eight electrons in its valence shell, it is considered stable—because all its electron orbitals are filled. This stability is a driving force behind many organic reactions as carbon strives to achieve a full octet.

In organic chemistry, understanding the octet rule is crucial because it helps predict the behavior and stability of compounds. Most stable organic molecules conform to the octet rule. However, there are exceptions, such as free radicals and some ions, which do not meet this criterion but are still significant in chemical reactions.

• The octet rule is fulfilled by having eight electrons in the valence shell.
• Carbon achieves this by forming four covalent bonds.
• Deviations from the octet often signify reactive or unstable species.

Understanding when carbon does or does not follow the octet rule allows scientists to predict how it will react under different conditions, making it easier to design and analyze chemical reactions.

Chemical Intermediates

Chemical intermediates are the short-lived, high-energy species that appear temporarily during the conversion of reactants to products. These intermediates help us understand the step-by-step journey of a chemical reaction. In organic chemistry, mastering intermediate structures is important for predicting the pathways and speeds of reactions.

Intermediates include entities like carbanions, carboanions, free radicals, and carbocations. These intermediates differ in terms of charge, electron configuration, and geometry, influencing their reactivity and role in a reaction.

• They typically have high energy and are less stable than reactants or products.
• Recognizing intermediates helps chemists deduce reaction mechanisms.
• Each intermediate type reacts differently based on its structure and electron arrangement.

By understanding the nature of intermediates, chemists can manipulate reactions to achieve desired products efficiently, saving time and resources.

Carbanion Stability

Carbanions are negatively charged carbon species that have an extra pair of electrons, making them rich in electrons compared to neutral species like carbon radicals. The stability of a carbanion depends on several factors, including the inductive effect, resonance, and hybridization of the atom that bears the negative charge.

Resonance plays a vital role in carbanion stability, as the delocalization of the extra electron pair across a molecule can increase stability significantly. For example, when the negative charge is spread over a larger area or shared between multiple atoms, the carbanion becomes more stable.

• Carbanions are most stable when the negative charge is on a less electronegative atom, such as carbon.
• They are vital intermediates in many organic mechanisms, including eliminations and nucleophilic substitutions.
• Polar solvents can stabilize carbanions by solvation, further increasing their usefulness in reactions.

Understanding the factors influencing carbanion stability enables chemists to predict where a reaction might proceed and harness carbanions effectively to synthesize complex molecules.