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 (c) has a complete octet around the carbon atom.

Step-by-step solution

Understanding the Octet Rule

The octet rule states that atoms tend to be more stable when they have eight electrons in their valence shell. We need to identify which intermediate from the options provided has a carbon atom with a complete octet.

Analyzing Free Radical

A free radical is a species with an unpaired electron. In a carbon-centered free radical, there are usually 7 electrons around the carbon, since one electron is unpaired. Thus, it does not have a complete octet.

Analyzing Carbene

A carbene is a neutral species with the general formula R2C: where carbon has only six electrons in its valence shell—two lone pairs and two bonded pairs. Therefore, carbenes do not have a complete octet.

Analyzing Carbanion

A carbanion is a species where carbon has a negative charge, which typically means an additional electron is present. It forms three bonds and has one lone pair, totaling eight electrons around the carbon, thus completing the octet.

Analyzing Carbonium Ion

A carbonium ion is a positively charged species where carbon has only six electrons, due to loss of one electron in a bond, thus not satisfying the octet rule.

Conclusion

Based on the analysis, the carbanion is the intermediate with a complete octet around the carbon atom, as it has eight electrons surrounding it.

Key concepts

Octet Rule

The octet rule is a fundamental concept in chemical bonding. It suggests that atoms are most stable when they have eight electrons in their outer shell, resembling the electron configuration of a noble gas. This rule guides the way atoms interact, form bonds, and create stable molecules.
Atoms strive to complete their octet through various means such as sharing, losing, or gaining electrons.

• In covalent bonds, atoms share electrons to fulfill the octet.
• In ionic bonds, atoms transfer electrons to achieve a full outer shell.

Although the octet rule is a good guideline, it's important to note that there are exceptions. Some molecules may have less or more than eight electrons and still be stable.

Carbon Intermediates

Carbon intermediates are transient species that form during chemical reactions. These are neither the reactants nor the final products but play a vital role in the pathway of the reaction. Understanding these intermediates helps in comprehending reaction mechanisms and predicting the outcome of reactions.
Some of the most common carbon intermediates include free radicals, carbenes, carbanions, and carbonium ions.

• Each has a unique structure and electron configuration, influencing its reactivity and stability.
• They are essential in organic chemistry for understanding how transformations occur.

Identifying the stability and behavior of these intermediates is crucial for developing new chemical reactions and synthesis pathways.

Free Radicals

Free radicals are chemical species with an unpaired electron, making them highly reactive. This unpaired electron gives them a total of seven electrons in their outer shell, falling short of the complete octet.
Free radicals can form through various processes such as homolytic bond fission, where a bond splits evenly, leaving each atom with one electron.

• Due to their reactivity, free radicals can initiate chain reactions, particularly in polymerizations and combustion.
• In biological systems, they can cause oxidative damage, leading to aging and diseases.

Though short-lived, their reactivity makes them crucial in both industrial and biological processes.

Carbenes

Carbenes are fascinating intermediates with the general formula R₂C:. These neutral species have two electrons in a lone pair and two additional electrons involved in bonding, summing up to six.
Because they lack a complete octet, carbenes are typically very reactive and short-lived.

• There are two types of carbenes, singlet and triplet, based on their electronic configuration and spin states.
• They are involved in various organic transformations, such as cyclopropanation, where they add to double bonds.

Their unique reactivity makes them useful in synthetic organic chemistry.

Carbanions

Carbanions are negatively charged carbon species with three covalent bonds and a lone pair of electrons. This configuration gives them eight electrons in their outer shell, satisfying the octet rule and making them relatively more stable than other intermediates, like carbenes or radicals.
Carbanions form under basic conditions where hydrocarbons can lose a proton, leaving behind the electron pair.

• They play a key role in various organic reactions, such as the nucleophilic attacks in aldol condensations.
• Their stability can be influenced by the presence of electron-withdrawing groups, which stabilize the negative charge.

The complete octet around the carbon in carbanions makes them crucial intermediates in many synthetic pathways.

Carbonium Ions

Carbonium ions, or carbocations, are positively charged carbon species that arise when a carbon atom loses an electron, leading to only six electrons in their outer shell. Due to this electron deficiency, they do not satisfy the octet rule, making them highly reactive.
These ions typically form in the presence of polar solvents where a leaving group departs from a molecule.

• They are key intermediates in reactions such as electrophilic addition and substitution.
• The stability of carbonium ions increases with greater substitution by alkyl groups, due to hyperconjugation, which helps in dispersing the charge.

Understanding their behavior helps chemists manipulate electrophilic reactions for desired synthetic outcomes.