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fy name reactions sem ii poc i
- 1. REACTION SEM- II NAME Compiled By- Dr. Atul
- 2. 06 React to convert an aromatic aldehyde and an anhydride to an α,ÎČ- unsaturated carboxylic acid PERKIN CONDENSATION 05BENZOIN CONDENSATION carbon-carbon bond is formed when two molecules of aldehydes (particularly benzaldehy -de) reacts with each other to form a condensed product called Benzoin. INTRODUCTIO N 04 CROSSED CANNIZZARO REACTION If two different aldehyde having no âHydrogen reacts with each other, it is called Crossed Cannizzaro Reaction.. INTRODUCTIO N 03CANNIZZARO REACTION The disproportionation (self-redox) of aldehydes lacking α-hydrogen atoms in presence of strong base to form salt of an acid and a primary alcohol is known as Cannizzaro reaction. INTRODUCTIO N 02CROSSED ALDOL CONDENSATION An aldol condensation between two different aldehydes produces a cross â aldol condensation. If both aldehydes possess α hydrogens, a series of products will form. INTRODUCTIO N 01ALDOL CONDENSATION Aldehydes that have α hydrogens react with themselves when mixed with a dilute aqueous acid or base. The resulting compounds, ÎČâhydroxy aldehydes, are referred to as aldol compounds because they possess both an aldehyde and alcohol functional group. INTRODUCTION
- 3. ALDOL CONDENSATION 1 © Dr. Atul R. Bendale
- 4. Mechanism Reaction © Dr. Atul R. Bendale Aldol Condensation, in organic chemistry, is a condensation reaction of enolate and carbonyl compound. When enol ions react with carbonyl compound it forms a ÎČ-hydroxyketone or ÎČ-hydroxyaldehyde. This process is followed by dehydration which results in conjugated enone. Step 1: Removal of α-hydrogen from the base- enolate ion Step 2: The first step results in the formation of carbanion which undergoes a nucleophilic addition reaction with the carbonyl group present in the second molecule of the aldehyde (in this case ethanal). The second step results in the formation of the condensation product Step 3: Protonation of alkoxide ion will occur due to reaction with water. Animation Step 4: Heating of the aldol compound in the basic solution will help in dehydrating the product to form α ÎČâunsaturated aldehyde compound.
- 5. CROSSED ALDOL CONDENSATION 2 © Dr. Atul R. Bendale
- 6. Reaction © Dr. Atul R. Bendale Cross aldol condensation refers to the aldol condensation reaction that occurs in between two different organic compounds, in this case, two different aldehydes and ketones. Therefore, the reaction leads to the formation of a series of product. If the α-hydrogen atoms are present in both aldehyde and ketone, the reaction will produce a mixture of four compounds. The optimum result of cross aldol condensation reaction is possible if an aldehyde compound contains α-hydrogen and another aldehyde compound does not contain α-hydrogen. Refer to the example below to see the reaction with the mixture of ethanal and propanal. The success of these mixed aldol reactions is due to two factors. First, aldehydes are more reactive acceptor electrophiles than ketones, and formaldehyde is more reactive than other aldehydes. Second, aldehydes lacking alpha-hydrogens can only function as acceptor reactants, and this reduces the number of possible products by half. Mixed aldols in which both reactants can serve as donors and acceptors generally give complex mixtures of both dimeric (homo) aldols and crossed aldols. Because of this most mixed aldol reactions are usually not performed unless one reactant has no alpha hydrogens. Step 1: The hydroxide ion deprotonates the enolizable aldehyde reversibly & it is preferentially adds to the non-enolizable aldehyde, which has the sterically less hindered and, therefore, more accessible carbonyl carbon. Step 2: Alkoxide ion is protonated by water, Aldol 3 is an enolizable aldehyde. A small amount of it is converted to the corresponding enolate ion by the hydroxide ion. Step 3: Enolate ion loses a hydroxide ion. Mechanism
- 7. CANNIZZARO REACTION 3 © Dr. Atul R. Bendale
- 8. Mechanism Reaction The disproportionation (self-redox) of aldehydes lacking αα-hydrogen atoms in presence of strong base to form salt of an acid and a primary alcohol is known as Cannizzaro reaction. - Thus, aromatic aldehydes, formaldehydes, trialkyl acetaldehyde, heterocyclic aldehydes undergo Cannizaro reaction. Step 1 A nucleophile such as a hydroxide ion is used to attack the carbonyl group of the given aldehyde, causing a disproportionation reaction and giving rise to an anion carrying 2 negative charges Step 2: This resulting intermediate can now function as a hydride reducing agent. Due to its unstable nature, the intermediate releases a hydride anion. This hydride anion proceeds to attack another aldehyde molecule. Now, the doubly charged anion is converted into a carboxylate anion and the aldehyde is converted into an alkoxide anion. Step 3: In this final step, water offers a proton to the alkoxide anion which alcohol product. The reaction can proceed since the alkoxide is more basic than water. Now, the carboxylate ion gives carboxylic acid product when acid workup is used (the acid workup is required since carboxylate is less basic than water and therefore cannot obtain a proton from water). Cannizzaro Reaction Mechanism details the method to get one molecule of alcohol and one molecule of carboxylic acid from two molecules of a given aldehyde. The reaction is executed by a nucleophilic acyl substitution on an aldehyde where the leaving group attacks another aldehyde. A tetrahedral intermediate results from the attack of hydroxide on a carbonyl. This tetrahedral intermediate collapses, thereby reforming the carbonyl and transferring a hydride which attacks another colony. Now, a proton is exchanged by acid and alkoxide ions. When a base of high concentration is introduced, the aldehyde forms an anion which has a charge of 2. From this, a hydride ion is transferred to a second molecule of the aldehyde, forming carboxylate and alkoxide ions. The alkoxide ion also obtains a proton from the solvent for the reaction. © Dr. Atul R. Bendale
- 9. CROSSED CANNIZZARO REACTION 4 © Dr. Atul R. Bendale
- 10. 0 Mechanism Reaction © Dr. Atul R. Bendale Aldehydes with alpha hydrogen atoms undergo deprotonation due to the strongly alkaline conditions of the reaction, leading to enolates and/or aldol reactions of these enolates where beta hydroxy aldehydes or ketones are obtained. Therefore, it is not surprising that the reaction produces only 50% of the required alcohol and carboxylic acid at ideal conditions. This is why the crossed Cannizzaro reaction is more commonly used. A sacrificial aldehyde is combined with a more valuable chemical and formaldehyde is used as a reductant, oxidizing it to sodium formate. The required alcohol is obtained from the reduction of the other aldehyde chemical. Since 2 different aldehydes can be completely converted into the required product, the yield of the valuable chemical is increased. Step 1 A nucleophile such as a hydroxide ion is used to attack the carbonyl group of the given aldehyde, causing a disproportionation reaction and giving rise to an anion carrying 2 negative charges Step 2: This resulting intermediate can now function as a hydride reducing agent. Due to its unstable nature, the intermediate releases a hydride anion. This hydride anion proceeds to attack another aldehyde molecule. Now, the doubly charged anion is converted into a carboxylate anion and the aldehyde is converted into an alkoxide anion. Step 3: In this final step, water offers a proton to the alkoxide anion which alcohol product. The reaction can proceed since the alkoxide is more basic than water. Now, the carboxylate ion gives carboxylic acid product when acid workup is used (the acid workup is required since carboxylate is less basic than water and therefore cannot obtain a proton from water). In general, a mixture of aldehydes undergoes a Cannizzaro reaction to yield all possible products. If one of the aldehydes is formaldehyde, the reaction yields almost exclusively salt of formic acid and the alcohol. corresponding to the other aldehyde. Such a reaction is called a Crossed Cannizzaro Reaction. If two different aldehyde having no -Hydrogen, it is called Crossed Cannizzaro Reaction
- 11. BENZOIN CONDENSATION 5 © Dr. Atul R. Bendale
- 12. 0 Reaction © Dr. Atul R. Bendale Benzoin Condensation is an important reaction in which carbon-carbon bond is formed when two molecules of aldehydes (particularly benzaldehyde) reacts with each other to form a condensed product called Benzoin. - This reaction is catalyzed by cyanide ion. Benzoin condensation reaction is nothing but the coupling reaction that occurs between aldehydes for the formation of parent benzoin. In this case, the benzaldehyde is involved in this homocoupling process. In the third phase, rearrangement occurs and also the removal of the cyanide ions occurs resulting in the formation of benzoin. The Rearrangement process results in the reversal of polarity of the carbonyl group, later adding to the second group of carbonyl atom in the second nucleophilic addition The first phase of the process includes the reaction of cyanide ions with the benzaldehyde to form the product called cyanohydrin. In this step, the cyanide ion or sodium cyanide takes part in the nucleophilic addition reaction, and it is a reversible reaction. The cyanide ion helps the reaction to occur by acting as a nucleophile and facilitating the abstraction of protons, thus forming cyanohydrin. The cyanide ions serve as a catalyst in the reaction. The second step is the condensation reaction that occurs between the cyanohydrin and the benzaldehyde. Mechanism In the reaction, a strong base deprotonates the former carbonyl C-atom and the second kind of an equivalent aldehyde reacts with the carbanion, thus eliminating the catalyst and regenerating the carbonyl compound at the end of the condensation reaction. In this Benzoin Condensation reaction, the two different aldehydes serve two purposes. One aldehyde in the reaction donates the protons and whereas the other aldehyde accepts the protons.
- 13. PERKIN CONDENSATION 6 © Dr. Atul R. Bendale
- 14. Mechanism Reaction © Dr. Atul R. Bendale The Perkin reaction is an organic reaction developed by English chemist William Henry Perkin that is used to make cinnamic acids. It gives an α,ÎČ-unsaturated aromatic acid by the aldol condensation of an aromatic aldehyde and an acid anhydride, in the presence of an alkali salt of the acid. The alkali salt acts as a base catalyst, and other bases can be used instead This product is now hydrolyzed to finally give alpha, beta- unsaturated acid Under the influence of the base, the anhydride gives the carbanion. This carbanion now attacks the carbonyl carbon of the aldehyde. This attack yields an intermediate. The abstraction of a proton from the active methyl group of the intermediate by the given base and the subsequent elimination of the hydroxyl group gives unsaturated anhydride.