Polar Reactions. Indian Institute of Technology Madras CH 3 OH 2 -H 2 O CH 2. HCl. H 3 C CH 3 neopentyl alcohol CH 3 CH 3. 1,2-shift -H + H 3 C

Transcription

1 Polar Reactions Important points to be discussed in this module: 1. Chemistry of Carbocations with respect to: a. 1,2-shifts of carbocations b. Pinacol-pinacolone rearrangement c. Semipinacolone rearrangement 2. Chemistry of Carbanions: a. Alkylation and acylation of enolates b. Aldol condensation c. Michael reaction d. Claisen and Dieckman condensation reactions Chemistry of Carbocations During the Friedel-Crafts alkylation, we saw that n-propyl cation ( C 2 C 2 ) undergoes rearrangement to give rise to a more stable secondary carbocation (C 3 C 2 ). The driving force for this reaction is generation of a more stable carbocation it is estimated that approximately 16 kcal/mol (67 kj/mol) is gained by conversion of a primary to a secondary or a secondary to tertiary carbocation. These types of rearrangements are broadly classified as Wagner-erwein rearrangements and involve 1,2-shift of an adjacent group. These types of reactions are common to all the reactions which involve carbocation intermediate and the fate of the carbocation intermediate is dependent of the reaction conditions. The reaction of neopentylic alcohol with a mineral acid such as Cl can nicely illustrate this point. In this reaction, depending on the exact reaction conditions, varying amounts 2-methyl-2-butene and 2- chloro-2-methylbutane are formed and neopentyl chloride is conspicuous by its absence! neopentyl alcohol Cl 2-2 C 2 1,2-shift 2-methyl-2-butene - CCl - 3C 3 Cl 2-chloro-2-methylbutane 1

2 What happens in this transformation then? The answer to this question could be obtained by writing the mechanism for this transformation. In the first step, the Cl protonates the hydroxyl group of the neopentyl alcohol which leads to dehydration and subsequent formation of neopentylic carboaction. This intermediate is extremely short lived and rearranges via the migration of a methyl group along with its pair of bonding electrons to generate a more stable tertiary carbocation. This process of intramolecular migration is considerably faster than the intermolecular trapping of the carbocation with solvent or chloride nucleophile. The migration of a group from one carbon to an adjacent carbocation is commonly encountered when the reactive intermediate involved is carbocations and is commonly referred to as 1,2-shift. Pinacol Rearrangement: 1,2-diols, known as pinacols, also undergo a similar type of dehydration reaction when treated with acid to furnish a ketone or aldehyde which have rearranged structure compared to the initial diol. This transformation is commonly referred to as Pinacol Rearrangement. Let s take an example when pinacol reacts with an acid, it leads to the formation of pincolone. It is believed that this reaction also involves the formation of a tertiary carbocation in the first step which is followed by 1,2-shift of the methyl group. Now, the question will be if we already have a stable carbocation, why should it undergo rearrangement? The answer is - the oxygen lone pair - which can stabilize the newly formed carbocation by donating its lone pair to carbocation, thereby completing octet for all the atoms involved. In fact, one could view this reaction as a push-pull reaction! The carbocation pulls the alkyl group along with its bonding electrons towards itself while the oxygen lone pair pushes the alkyl group away. pinacol pinacolone 2

3 It is clear now that there are two important steps involved in pinacol rearrangement: (1) loss of water from the protonated diol to form the carbocation; and (2) rearrangement of the carbocation by a 1,2-shift to give rise to the protonated ketone. The next question that arises is what happens when the pinacol is unsymmetrical? In such situations, the product obtained is determined (a) by which group is lost in step (1) and then (b) by which group migrates in step (2) to the electron deficient carbon thus formed. The group is usually lost from the carbon atom that will form the more stable carbocation. The rate of migration of the groups in the second step is referred to as the migratory aptitude of the group. Since the rearrangement involves movement of the migrating group with its bonding electrons to an electron deficient centre, migratory aptitudes are greatest for groups in which the migrating atom is most electron rich. It should be kept in mind though that the direction of the rearrangement is largely determined by the relative ease of removal of a group (first step) and rearrangement may not necessarily involve the group of highest migratory aptitude. Et Et C A general order of migratory aptitudes for the pinacol rearrangements is: p-anisyl > p-tolyl > phenyl > tert.-alkyl > primary alkyl > It should be noted however, that even though the migratory aptitudes generally follow the order given above, the absolute comparison depends on the actual reaction and condition. Semipinacol rearrangement: The pinacol type of rearrangement is not limited only to vicinal diols. Deamination of a- amino alcohols also leads to a reaction which closely resembles the pinacol 3

4 rearrangement and is often called the semipinacol rearrangement. In this reaction, nitrous acid is used to form an alkanediazonium ion which readily looses nitrogen to give the intermediate carbocation. N 2 N 2 N 2 -N 2 - This reaction can be used for expansion as well as the contraction of medium sized rings. N 2 N 2 C + N 2 N 2 N 2 + N 2 4

5 Assignment A. Suggest a suitable mechanism for the following reactions. Write each step and show the rearrangement clearly by means of arrow. 1. Cl Cl C Br Br 5

6 B. Identify the products of the following reactions and suggest a mechanism for the 1. reaction N 2 C 2 N 2 N 2 6. N 2 N 2 N 2 6