Is tautomerism a rearrangement reaction
88 5 REACTION MECHANISMS 5.3 ELIMINATION Elimination is the reverse of addition. By splitting off atoms on neighboring carbon atoms (b-elimination), double bonds are created. Often halogenated hydrocarbons or alcohols serve as starting materials; alkenes are formed as products. The mechanism of elimination is similar to nucleophilic substitution (Fig. 88.1; competition of reactions). It also often occurs as a competitive reaction to S N reactions. Since higher activation energies are usually required for elimination reactions, the following rule of thumb applies: the higher the temperature, the more favored the elimination over the substitution; Example: Condensation reactions of alcohols (page 40). At low temperatures, ethers are formed (S N reaction), at high temperatures, alkenes are formed (elimination). Tertiary haloalkanes in particular have a very strong tendency to elimination instead of forming an ether in an S N reaction. As with S N reactions, the elimination can also proceed according to two different mechanisms. The elimination - E1 - takes place like the S N 1 reaction via a carbonium ion as an intermediate product. The first step - splitting off the leaving group - is completely the same. In the second step, however, the carbenium ion does not stabilize through the uptake of a nucleophile. Instead, the nucleophile splits off a proton from the neighboring carbon, creating a double bond. In the case of the elimination - E2 - the attack of a base on the neighboring C atom takes place first to form the leaving group. A proton is split off from there. At the same time, the lea- ving group leaves the molecule. This also creates a double bond. The stronger the base, the more likely the E2 mechanism is. This is noticeable in the reaction of haloalkanes with the strongly basic alcoholates (Williamson's ether synthesis). 5.4 REORGANIZATION During a rearrangement, the molecular structure changes under certain reaction conditions. The simplest and most common case is the migration of a proton within the molecule. This is the case, for example, with enols in which the OH group is located directly next to the C = C double bond. In such cases a proton migrates from the OH group to the carbon atom and the enol becomes a carbonyl compound. An equilibrium is created, but in almost every case it is on the side of the carbonyl compound. Therefore, one usually writes the carbonyl and not the enol structure. The equilibrium is called keto-enol tautomerism. In other cases the rearrangement reaction is irreversible. It is not a proton that migrates, but an alkyl group. A technically important example is the Beckmann rearrangement: In this process, an oxime becomes an amide. As in an S N 1 reaction, the OH group is protonated first. It then splits off as H 2 O. The resulting positive molecular ion then rearranges itself with migration of an alkyl residue to nitrogen and subsequently takes up an OH - ion (from the water). The structure finally changes into the amide with proton rearrangement (as in keto-enol tautomerism). This rearrangement is used in the manufacture of polyamides. The starting material for Perlon (nylon-6) is ε-caprolactam. This can be made from cyclohexanone and hydroxylamine. The oxime is transformed into the cyclic amide (= lactam) (Fig. 88.2). Rearrangement reactions are particularly important in petroleum processing. In catalytic reforming, unbranched hydrocarbons are rearranged into highly branched products at high temperatures via a radical mechanism. These then have better combustion properties. This enables carburetor fuels with a high octane rating. CCH Br CCHOHCC Br OOHCCHOHHHCCHOCCOH hydroxylamine COH 2 NOHCNOHCNHOH 2 OCH 2 OHNNCCNOHOH Fig. 88.1: The competition between SN 1 reaction and E1 reaction Fig. 88.2: Beckmann rearrangement to e-caprolactam Cyclohexanone carbon e -caprolactam E1 SN 1 ene For testing purposes only - property of the publisher öbv
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