Problem 43 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

The carbanion (c) has the complete octet around the carbon atom.

Step 1: Understanding the Complete Octet Rule

A complete octet for a carbon atom means that it has eight electrons in its valence shell, fulfilling the octet rule for stable electron configuration, similar to a noble gas.

Step 2: Analyzing the Free Radical

A free radical contains a carbon atom with an unpaired electron, which means it has only 7 electrons in its outer shell. Hence, it does not have a complete octet.

Step 3: Evaluating the Carbene

A carbene has two valence electrons on carbon that form a lone pair and two covalent bonds with other atoms, resulting in only 6 electrons around carbon, thus not completing the octet.

Step 4: Reviewing the Carbanion

A carbanion is formed when a carbon atom has three bonds and an additional lone pair of electrons, providing a total of 8 electrons around carbon, thereby completing its octet.

Step 5: Checking the Carbonium Ion

A carbonium ion has a positively charged carbon atom due to the loss of an electron, leaving it with only 6 electrons and therefore an incomplete octet.

Key Concepts

Free Radical

Free radicals are unique chemical species that contain an unpaired electron. This unpaired electron is usually in an outer orbit, making these molecules highly reactive. The presence of this unpaired electron results in an outer shell configuration of seven electrons for the carbon atom, making it unstable. As a consequence, free radicals do not satisfy the octet rule, which states that atoms tend to bond in such a way that each atom has eight electrons in its valence shell.

• High reactivity due to an unpaired electron.
• Typically formed during chemical reactions or by breaking covalent bonds.
• In search of an electron to achieve stability, leading to chain reactions.

Free radicals are often seen in atmospheric chemistry, biological systems, and in various chemical processes where they can act as intermediates for chemical transformations.

Carbene

A carbene is a molecule featuring a carbon atom with two electrons and two empty valence orbitals, generally represented as having a divalent carbon atom. Carbenes can exist in two electronic states: singlet and triplet, depending on how the non-bonding electrons are arranged:

• Singlet Carbenes: Both electrons are paired, and the carbon atom possesses an sp² hybridization.
• Triplet Carbenes: Electrons are unpaired and occupy different orbitals, resulting in sp hybridization.

Since carbenes have just six electrons around the carbon atom, they fall short of the octet rule. However, they are highly reactive, commonly serving as intermediates in various organic reactions such as cyclopropanation and insertions.

Carbanion

Carbanions are negatively charged species with a full octet of electrons around a carbon atom, making them quite stable in certain contexts. This extra electron is generally accommodated as a lone pair, resulting in three covalent bonds and one unshared pair of electrons. The key features of a carbanion include:

• Possession of an extra electron giving it a negative charge.
• Typically sp³ hybridization leading to a tetrahedral geometry.
• Stability greatly influenced by the surrounding electron-donating or withdrawing groups.

Carbanions play a critical role as strong nucleophiles in organic chemistry, participating in numerous reactions, including nucleophilic substitutions and eliminations.

Carbonium Ion

Carbonium ions, also known as carbocations, are positively charged ions with a significant deficit of electrons on the carbon atom. Unlike carbanions, these layers have only six electrons, which violates the octet rule. Characteristics of carbonium ions include:

• Typically sp² hybridized, resulting in a planar, trigonal geometry.
• High reactivity due to the electropositive charge seeking additional electrons.
• Stability of carbonium ions is greatly affected by the surrounding substituents; more substituted carbocations are generally more stable.

These ions are invaluable in organic reaction mechanisms, especially in SN1 reactions and catalytic processes, where they serve as key intermediates facilitating the transformation of molecules.