A few organic reaction mechanisms that a chemistry major must understand (2)

A few organic reaction mechanisms that chemistry majors must understand
For intramolecular ester condensation of binary carboxylic acid esters see Dieckmann condensation reaction.
Reaction Mechanism
α-Hydrogen is very weakly acidic (pKa-24.5),and sodium is a relatively weak base (pKa~15.9), so the negative ions formed by interaction with sodium are rare in the equilibrium system. However, since the product acetyl is a relatively strong acid, it can interact with sodium to form stable negative ions, thus shifting the equilibrium towards the product. Therefore, despite the low concentration of negative ions in the reaction system, the reaction continues as soon as it is formed, and the resultant reaction can still be completed successfully.
Commonly used alkaline condensers besides sodium are potassium tert-butoxide, sodium tert-butoxide, potassium hydride, sodium hydride, sodium triphenylmethyl, lithium diisopropylammonium (LDA), and Grignard’s reagent.

Two different esters can also undergo ester condensation, theoretically giving four different products, called mixed ester condensation, which is not of much significance in preparation. If there is neither α-hydrogen atom in one of the ester molecules and the alkoxycarbonyl group is relatively reactive, only one condensation product is produced. Such as benzoate, formate, oxalate, carbonate, etc.. When reacting with other esters containing α-hydrogen atoms, only one condensation product is generated.
In fact, this reaction is not limited to the condensation of esters themselves, but esters and compounds containing active methylene groups can undergo such a condensation reaction, which can be expressed by the following general formula:
Claisen-Schmidt
Condensation of an aldehyde without an α-hydrogen atom with an aliphatic aldehyde or ketone with an α-hydrogen atom in the presence of a dilute aqueous sodium hydroxide or alcohol solution, and loss of water to give an α,β-unsaturated aldehyde or ketone.
Reaction mechanism

Reaction examples
Claisen rearrangement
Allyl aryl ethers can rearrange at high temperatures (200°C) to form allyl phenols.
When the two neighboring positions of the allyl aryl ether are not occupied by substituents, the rearrangement mainly yields the neighboring product, and when both neighboring positions are occupied by substituents, the rearrangement yields the para product. When both neighboring positions are occupied by substituents, the rearrangement gives the para-position product, and when both neighboring positions are occupied by substituents, the rearrangement does not occur.
Cross-reaction experiments demonstrate that the Claisen rearrangement is an intramolecular rearrangement. The rearrangement is carried out using a g-carbon 14C-labeled allylic ether, and after the rearrangement the g-carbon atom is attached to the benzene ring and the carbon-carbon double bond is displaced. Aryl allyl phenols with both neighboring positions substituted remain with the a-carbon atom attached to the benzene ring after rearrangement.
Reaction Mechanism

The Claisen rearrangement is a synergistic reaction with a cyclic transition state in the middle, so that the electronic effects of the substituents on the aryl ring have no effect on the rearrangement.
The rearrangement from allyl aryl ether to o-allyl phenol takes place through a [3,3]s migration and a ketone to allyl alcohol isomerism; the rearrangement of allyl aryl phenol, in which both neighboring sites are occupied by substituent groups, first takes place through a [3,3]s migration to the neighboring site (Claisen rearrangement), and since the neighboring site is occupied by a substituent group, the interconversion isomerism is not possible, and then a [3,3]s migration takes place (Cope rearrangement) to the neighboring site (Claisen rearrangement). Cope rearrangement) to the para position, followed by a [3,3]s migration ( Cope rearrangement) to the para position, and then the para allylphenol is obtained by mutual isomerization.
When the substituted allyl aryl ether is rearranged, the conformation of the new double bond after rearrangement is E-type (trans) regardless of whether the original allyl double bond is in the Z-configuration or the E-configuration, due to the stabilized chair conformation of the six-membered cyclic transition state through which the rearrangement reaction takes place.
The Claisen rearrangement is universal. In ethers, the Claisen rearrangement is possible if there is an allyloxy-carbon-carbon structure.
Favorskii rearrangement
Rearrangement of a-halogenated ketones by heating in aqueous sodium hydroxide to form carboxylic acids containing the same number of carbon atoms; in the case of cyclic a-halogenated ketones, this results in ring narrowing.