Dr. Pandit Khakre Asst. Prof Mrs. K.S.K. College, Beed

Preparation of Phenols Phenol is also called Carbolic acid. It was first isolated from coal Tar. But nowadays it can be manufactured synthetically (1) From Benzene Sulphonic Acid (2) From Diazonium Salts

(3) From Grignard Reagent (4) By decarboxylation of Sodium salt of Salicylic acid

(6) From Benzene (Rachig’s Method) (5) From Cumene (6) From Benzene (Rachig’s Method) (7) From ChloroBenzene (Dow’s Process)  

Fries Rearrangement The Fries Rearrangement enables the preparation of acyl phenols

Mechanism of the Fries Rearrangement The reaction is catalyzed by Brønsted or Lewis acids such as HF, AlCl3, BF3, TiCl4 or SnCl4. The acids are used in excess of the stoichiometric amount, especially the Lewis acids, since they form complexes with both the starting materials and products. The complex can dissociate to form an acylium ion. Depending on the solvent, an ion pair can form, and the ionic species can react with each other within the solvent cage. However, reaction with a more distant molecule is also possible:

After hydrolysis, the product is liberated. The reaction is ortho,para-selective so that, for example, the site of acylation can be regulated by the choice of temperature. Only sterically unhindered arenes are suitable substrates, since substituents will interfere with this reaction. The requirement for equimolar quantities of the catalyst, the corrosive and toxic conditions (HF), and the violent reaction of the catalyst with water have prompted the development of newer protocols. Zeolites have proven to be unsuitable, since they are deactivated, but strong acids, such as sulfonic acids, provide a reasonable alternative. An additional option for inducing a Fries Rearrangement is photochemical excitation, but this method is only feasible in the laboratory:

Claisen Rearrangement The aliphatic Claisen Rearrangement is a [3,3]-sigmatropic rearrangement in which an allyl vinyl ether is converted thermally to an unsaturated carbonyl compound. The aromatic Claisen Rearrangement is accompanied by a rearomatization: The etherification of alcohols or phenols and their subsequent Claisen Rearrangement under thermal conditions makes possible an extension of the carbon chain of the molecule.

Mechanism of the Claisen Rearrangement The Claisen Rearrangement may be viewed as the oxa-variant of the Cope Rearrangement:  Mechanism of the Cope Rearrangement Mechanism of the Claisen Rearrangement The reaction proceeds preferably via a chair transition state. Chiral, enantiomerically enriched starting materials give products of high optical purity.

A boat transition state is also possible, and can lead to side products: The aromatic Claisen Rearrangement is followed by a rearomatization: When the ortho-position is substituted, rearomatization cannot take place. The allyl group must first undergo a Cope Rearrangement to the para-position before tautomerization is possible.

Ireland-Claisen Rearrangement All Claisen Rearrangement reactions described to date require temperatures of > 100 °C if uncatalyzed. The observation that electron withdrawing groups at C-1 of the vinyl moiety exert a positive influence on the reaction rate and the yield has led to the development of the following variations: Ireland-Claisen Rearrangement Eschenmoser-Claisen Rearrangement Johnson-Claisen Rearrangement

The Riemer Tiemann reaction is a chemical reaction used for the ortho-formylation of phenols;[1][2][3][4][5] with the simplest example being the conversion of phenol to salicylaldehyde. The reaction was discovered by Karl Reimer[6] and Ferdinand Tiemann. The Reimer in question was Karl Reimer (1845-1883) not the less known Carl Ludwig Reimer (1856-1921).[7] Reaction mechanism The mechanism of the Reimer-Tiemann reaction Chloroform (1) is deprotonated by a strong base (normally hydroxide) to form the chloroform carbanion (2) which will quickly alpha-eliminate to give dichlorocarbene (3); this is the principal reactive species. The hydroxide will also deprotonate the phenol (4) to give a negatively charged phenoxide (5). The negative charge is delocalised into the aromatic ring, making it far more nucleophilic and increases its ortho selectivity. Nucleophilic attack of the dichlorocarbene from the ortho position gives an intermediate dichloromethyl substituted phenol (7). After basic hydrolysis, the desired product (9) is formed.