Carbanion Reactions and Mechanisms PDF

**[WHAT IS A CARBANION ?]** A carbanion can be defined as a negatively charged ion in which a carbon atom exhibits trivalence (implying it forms a total of three bonds) and holds a formal negative charge whose magnitude is at least -1. When pi delocalization does not occur in the organic molecule (...

**[WHAT IS A CARBANION ?]** A carbanion can be defined as a negatively charged ion in which a carbon atom exhibits trivalence (implying it forms a total of three bonds) and holds a formal negative charge whose magnitude is at least -1. When pi delocalization does not occur in the organic molecule (as it does in the case of aromatic compounds), carbanions typically assume a bent, linear, or a trigonal pyramidal molecular geometry. It is important to note that all carbanions are conjugate bases of some carbon acids. IMG\_256 An illustration detailing the possible [resonance structures](https://byjus.com/chemistry/resonance-structures/) of a carbanion in which the carbon holding the negative charge is bound to three different R groups is provided above. It can be noted that each of the R-groups in this illustration can either denote an alkyl group, an aryl group, or a hydrogen atom. **CONDENSATION REACTION OF CARBANIONS** The condensation reaction of a carbanion involves several key steps What is Aldol Condensation? --------------------------- Aldol condensation occurs in aldehydes having **α-hydrogen** with a dilute base to give **β-hydroxy aldehydes** called aldols. This reaction is most commonly known as aldol condensation. If the condensation reaction occurs between two different carbonyl compounds it is called **[crossed aldol condensation]**. Aldol Condensation Reaction --------------------------- Aldol Condensation can be defined as an organic reaction in which enolate ion reacts with a [carbonyl compound](https://byjus.com/chemistry/carbonyl-compounds/) to form β-hydroxy ketone or β-hydroxy aldehyde, followed by dehydration to give a conjugated enone. Aldol **Condensation plays a vital role in organic synthesis**, creating a path to form carbon-carbon bonds. The general reaction of aldol condensation is  General Aldol Condensation Reaction **MECHANISM** **Step-I: **In this step, an alkali hydroxide ion gives a carbanion (i.e. enolate ion) by removing a proton from the α -- carbon of one molecule of ethanal. Since hydrogen acts as a base, it moves the acidic a-hydrogen and creates the reactive enolate ion. You can think of this reaction as an acid-base reaction. IMG\_256 **Step II: **In this step, an alkoxide ion is created by the nucleophilic addition of an enolate ion to the carbonyl carbon of the second molecule of ethanal. This attack produces an alkoxide intermediate and is a nucleophilic addition reaction.  **Step III: **The alkoxide ion accepts a proton from water in this step to form β -- hydroxy aldehyde (aldol). IMG\_258 **Step IV: **Dehydration of aldol products- When heated with diluted acids, the aldol condensation product undergoes dehydration to produce α, β- unsaturated aldehydes or ketones.   IMG\_256 Used in the synthesis of various drugs and active pharmaceutical ingredients (APIs).Fundamental in forming carbon-carbon bonds, which is crucial for constructing complex organic molecules. **Favorskii rearrangement** The Favorskii rearrangement is a fascinating organic reaction where **cyclopropanones** or **α-halo ketones** are rearranged to form carboxylic acid derivatives. This reaction is typically base-catalyzed and can lead to the formation of acids, esters, or amides, depending on the base used **Mechanism of the Favorskii Reaction** ---------------------------------------  Esters are obtained if alkoxide bases are used: IMG\_257 A direct conversion from α-halo ketones is possible:  Ring-contraction: IMG\_259 This reaction is used to synthesize complex bicyclic ester structures It\'s employed in the preparation of various steroids and nonsteroid compounds The photo-Favorskii reaction is used in the photochemical deprotection of certain phosphates, such as those protected by p-hydroxyphenacyl groups **Wittig reaction** **Wittig reaction** or Wittig olefination is a chemical reaction of an aldehyde or ketone with a triphenyl phosphonium ylide (often called a **Wittig reagent**) to give an alkene and triphenylphosphine oxide.The Wittig reaction was discovered in 1954 by Georg Wittig, for which he was awarded the Nobel Prize in Chemistry in 1979. It is widely used in organic synthesis for the preparation of alkenes. The reaction works for a wide variety of R groups, and with both aldehydes and ketones, and with simple alkyl or aryl groups it generally gives mainly the Z- alkene product, though if the R groups are similar it may give E/Z mixtures.  The driving force is the formation of a the highly stable triphenylphosphine oxide (Ph3P=O). ### **Preparation of Wittig reagents** Wittig reagents are usually prepared from a phosphonium salt, which is in turn prepared by the reaction of triphenylphosphine with an alkyl halide via an SN2 reaction. The alkylphosphonium salt is deprotonated with a strong base such as *n*-butyllithium: 1)  SN2 reaction IMG\_256 2\) Deprotonation (for simplicity the butyllithium has been written as if it were ionic, which it is not):  The \[1,2\]-Wittig rearrangement is used to convert ethers into secondary or tertiary alcohols. The \[2,3\]-Wittig rearrangement is employed to synthesize homoallylic It allows for the control of stereochemistry in the formation of carbon-carbon bonds, which is crucial in the synthesis of complex molecules **Stability of carbanion** -------------------------- Several factors affect the stability of the carbanion molecule. **1. Induction** If the substituent group is highly electronegative, it draws the shared electrons towards itself and stabilizes the molecule. On the other hand, if the substituent group is electropositive, it will "push" the shared electrons toward the carbanion, making the molecule less stable. There more the substitution, the more stable is the molecule. **2. Hybridization** The hybridization of the negatively-charged carbon is also another factor determining stability. The greater the s character, the more stable the anion. **3. Resonance** Conjugation is another factor that affects stability. Delocalization distributes the negative charge all over the molecule, resulting in resonance. Aromatic systems offer tremendous stability through this resonance effect when they are present as a substituent. IMG\_256 **[SIGNIFICANCE OF CARBANIONS]** Carbanions are fascinating and highly significant in the realm of organic chemistry due to their unique properties and reactivity. Here's why they matter: **Reaction Intermediates**: Carbanions act as intermediates in many important organic reactions, such as nucleophilic substitutions, eliminations, and certain rearrangement reactions. **Base Catalysts**: They are strong bases and play a crucial role in catalyzing reactions, especially in the formation of carbon-carbon bonds. **Synthetic Applications**: Carbanions are used in the synthesis of various complex organic compounds, which are essential in pharmaceuticals, agrochemicals, and materials science. **Stabilization Insights**: The study of carbanions provides insight into the stability of compounds, especially through inductive and resonance effects.