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Rearrangement of Carbocations, Saytzeff’s Orientation and Evidences

Inorganic chemistry reactions, carbocation rearrangements are extremely common and are described as the process of moving a carbocation.

Rearrangement of carbocations

Inorganic chemistry reactions, carbocation rearrangements are extremely common and are described as the process of moving a carbocation from an unstable to a more stable state by using different structural redirections within the molecule. When the carbon atoms on the carbocation change to another carbon, we have a structural isomer. The process is complex.

The carbocation rearrangement process occurs whenever alcohols transform into various carbocations. In a nutshell, a carbocation is a positive charge on a molecule attached to three other groups and contains six electrons instead of eight. Despite this, carbocation rearrangements are also observed in reactions that lack alcohol. They, on the other hand, need more complex explanations than those listed below. The two types of rearrangements are hydride shifts and alkyl shifts. These rearrangements usually occur in many types of carbohydrate molecules. After the molecules have been rearranged, they can also undergo more unimolecular substitutions (SN1) or unimolecular eliminations (E1). The most common combination we see is a simple one or a complicated one. Before carbocation rearrangement, we can expect two products. However, once we undergo this process, we see the major product.



Hydride shift

We typically observe two products whenever a nucleophile attacks a molecule. There are typically both major and minor products, however. In general, the major product has more substitutes (i.e., more stability). A contract usually specifies that there will be a lesser amount of substituted (less stable) products. As a result of the reaction, carbocations can undergo modifications called hydride shifts. In the unimolecular substitution, the two-electron hydrogen is transferred from the hydrogen to the adjacent carbon. Hydride shift typically occurs when an alcohol reacts with HBr, HCl, or HI, which are hydrogen halides. The instability and rapid reactivity of HF makes it unsuitable for many applications. The following example shows how alcohol reacts with hydrogen chloride:



The nucleophilic Cl atom has been replaced for the alcohol component (-OH). However, unlike in SN2 reactions, there is no direct replacement of the OH atom. After obtaining a proton from the nucleophile, the leaving group, -OH, creates a carbocation on Carbon #3 to produce an alkyloxonium ion in this SN1 reaction. Carbon #2, the hydrogen atom connected to the Carbon atom right adjacent to the original Carbon (ideally the more stable Carbon), can undergo a hydride shift before the Cl atom hits. Formally, the hydrogen and the carbocation swap place. Because of hyperconjugation, the Cl atom may now attack the carbocation, forming a more stable structure. The carbocation is the most stable because it is attached to three different carbons (being attached to three different carbons). However, there are still trace quantities of the minor, unstable product. Hydride shift happens through a series of stages that comprise several intermediates and transition states. The mechanism for the above-mentioned response is as follows:



Hydration of alkenes: hydride shift

In a more complicated situation, we see a hydride shift when alkenes are hydrated. The interaction of 3-methyl-1-butene with H3O+ to produce 2-methyl-2-butanol is shown below:

Once again, we observe a variety of items. However, in this scenario, we observe two small items and one large one. Because the -OH substituent is bonded to the highly substituted carbon, we see the main product. The proton connects to carbon 2 during the hydration of the reactant. As a result, the carbocation is based on carbon 2. Hydride shifts occur when hydrogen from the neighboring carbon swaps places with the adjacent carbon. The carbocation, which is stable and hyper conjugated, is now ready to attack by H2O to form an alkyloxonium ion. As the process continues, a water molecule will attack the proton on the alkyloxonium ion, forming alcohol. This mechanism is represented in the diagram below:

Alkyl shift

A carbocation does not have to have a hydrogen atom (either secondary or tertiary) that is on the adjacent carbon atom available for rearrangement. Rearrangement by alkyl shift (or alkyl group migration), on the other hand, is an alternative option. The shift of the alkyl atom is very similar to the shift of the hydride atom. The proton (H) that would normally shift with the nucleophile now shifts with an alkyl group. Shifting groups transfer their electron pair with them to form bonds with neighboring or adjacent carbohydrates. The shifted alkyl group and positive charge of tertiary carbocation reactions result in a faster reaction than secondary carbocations. When SN1 reactions occur, secondary carbocation changes to tertiary carbocation:


There are slight differences and variations between the two reactions. We observe secondary substrate in reaction #1. The reason for the alkyl shift is the lack of hydrogen on the adjacent carbon. A similar reaction is observed during Hydride Shift as well. Only the alkyl group is shifted as opposed to the proton, but various intermediary steps still have to be taken to produce the final product. We can describe reaction #2, however, as undergoing a concerted mechanism. Essentially, everything is accomplished in one step. It is not possible to use primary carbocations as intermediates and these processes take longer to complete since higher temperatures and longer reaction times are necessary. So that the alkyloxonium ion can be formed without the formation of the unstable primary carbohydrate, water must leave the alkyl group from the adjacent carbon when it is protonated.

Saytzeff’s orientation and evidence

Saytzeff's Rule is also known as Zaitsev's Rule, Saytzev's Rule, and Z-rule. Alexander Zaitsev, a Russian scientist, studied many elimination processes and discovered a common pattern in the resultant alkenes. Based on this study, Zaitsev concluded that stable alkenes are generated when hydrogen is removed from -carbon with a low number of hydrogen substituents.

Saytzeff's Rule enters the scene during elimination reactions. The most substituted product would be the one that is the most stable and liked. This criterion solely applies to the radiochemistry of the elimination process and does not generalize about the resultant stereochemistry.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of pharmaguideline.com, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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