What is a carbanion? How is it formed. Briefly discuss its properties and reactions.

A carbanion is a chemical species that has a negative charge on a carbon atom. It is formed when the carbon atom has three covalent bonds to other atoms and one lone pair of electrons. Carbanions are considered to be the base of the specific organic molecule and can act as nucleophiles in reactions.

Carbanions are formed when a molecule loses a proton (H+) from a carbon atom, leaving it with a lone pair of electrons and a negative charge. This frequently occurs in the presence of a strong base. For example, when an alkyl halide reacts with a strong base like sodium amide (NaNH2), it will result in the formation of a carbanion.

Some common properties of carbanions are: 1. Carbanions are negatively charged species, which makes them strong bases and nucleophiles. 2. Due to their negative charge, carbanions are more reactive than their neutral counterparts. 3. Carbanions are typically unstable and are often stabilized by the presence of electron-withdrawing groups (EWGs) such as halogens or carbonyls that can help to distribute the negative charge. 4. The stability of carbanions is also affected by their hybridization. The more s-character there is in the orbital containing the lone pair of electrons, the more stable the carbanion. For example, an sp-hybridized carbanion is more stable than an sp2-hybridized one, which is more stable than an sp3-hybridized one. 5. Carbanions act as both Lewis bases and Bronsted bases since they can donate electron pairs and accept protons, respectively.

Carbanions are involved in various types of organic reactions due to their high reactivity as nucleophiles and bases. Some common reactions are: 1. Nucleophilic Substitution: Carbanions can replace a leaving group in a molecule via an S_N2 or S_N1 mechanism. 2. Deprotonation: Carbanions can be formed when a strong base removes a proton from a weak acid. 3. Nucleophilic Addition: Carbanions can add to carbonyl groups like ketones, aldehydes, and esters, forming new carbon-carbon bonds. 4. Michael Addition: In this reaction, carbanions attack α, β-unsaturated carbonyl compounds, resulting in a 1,4-addition product. 5. Wittig Reaction: Carbanions derived from phosphonium salts (Wittig reagents) react with carbonyl compounds to form alkenes and a phosphine oxide.