Key Organic Reactions: Cannizzaro, Aldol, Grignard, Friedel-Crafts, Diels-Alder, Lecture notes of Organic Chemistry

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NAME REACTIONS

Cannizzaro Reaction

The Cannizzaro reaction involves the treatment of an aldehyde (without alpha-hydrogens) with concentrated NaOH or KOH. Aldehydes when heated with NaOH, undergo a disproportion reaction. One half of aldehyde molecules are oxidized to a carboxylic acid and one half to an alcohol. The Cannizzaro reaction, named after its discoverer Stanislao Cannizzaro.

Step- 2 : Hydride-ion transfer

Mechanism :???

Step- 3 : Acid-base reaction

Crossed Cannizzaro reaction

When two different aldehydes, each lacking alpha-hydrogen are heated in an alkaline solution, the reaction is said to be crossed Cannizzaro reaction. For example, when benzaldehyde and formaldehyde is heated in the presence of NaOH, a mixture of benzyl alcohol and sodium formate is formed.

Aldol Condensation

Aldehydes containing alpha-hydrogens undergo self-addition in the presence of a base to form products called aldols. The reaction is called aldol condensation. The term aldol is derived from the combination of the words Aldehyde and Alcohol, the two functional groups present in the product. When two acetaldehyde molecules combine with each other in the presence of dilute NaOH, it form aldol ( 3 - hydroxybutanal).

Chemical Reaction

Mechanism

The reaction is reversible and involves the following steps

Step- 1 : Formation of enolate ion

Step- 3 : The negative oxygen in the product accepts a proton from water to give aldol

Aldols can easily be dehydrated either by heating or by treatment with dilute acid to form α,β-unsaturated aldehydes.

Dehydration product Ketols are also easily dehydrated by heating or by treatment with dilute acid to form α,β-unsaturated ketones.

Mixed Aldol Condensation

The reaction of two different carbonyl compounds (one of

which must contain α-H ) in the presence of a base is known

as mixed aldol condensation. For example, acetaldehyde

react with benzaldehyde (which has no α-H ) in the

presence of a base to form cinnamaldehyde

Key points in aldol condensation

1. The product is a larger molecule that contains newly formed C-C bond.
2. The products contains two functional groups; one carbonyl group and one hydroxyl group.
3. The hydroxyl group is always attached to the beta carbon, which is two carbons away from the carbonyl group.
4. An aldol condensation is a condensation reaction in organic chemistry in which an enol or an enolate ion reacts with a carbonyl compound to form a β-hydroxyaldehyde or β-hydroxyketone, followed by dehydration to give a conjugated enone.

Grignard Reactions

Of all the organometallic compounds known, organomagnesium halides (RMgX) or Grignard reagents are the most important. They are so named after Victor Grignard discovered them and developed their use as synthetic agents. Grignard reagents occupy pride of place in organic synthesis by virtue of their versatility and wide application. Almost all classes of organic compounds can be prepared from them. Grignard earned one-half of a Nobel Prize in 1912 for his remarkable contribution to synthetic organic chemistry. General formula: Where R = alkyl, alkenyl, alkynl or aryl group X = Cl, Br or I. Organomagnesium fluorides are not known