Carbocation rearrangements are extremely common in organic chemistry reactions and are defined as the movement of a carbocation from an unstable state to a more stable state through the use of various structural rearrangement "shifts" in the molecule. When the carbocation moves to another carbon, we can say that there is a structural isomer of the original molecule. However, this phenomenon is not as simple as it seems.
When alcohols undergo conversion to different carbocations, the carbocations undergo a phenomenon known as carbocation rearrangement. Simply put, a carbocation maintains a positive charge on a molecule that is attached to three other groups and has a sextet, not an octet. But we also see carbocation rearrangements in reactions that do not contain alcohol. These, in turn, require more difficult explanations than the two listed below. There are two types of rearrangements: hydride shift and alkyl shift. These rearrangements typically occur in many types of carbohydrates. Molecules can also be transferred after rearrangementunimolekyle substitution (SN1)Lubsingle-molecule elimination (E1). But most of the time we see either a simple or a complex mix of products. We can expect two products before the carbocation rearrangement, but after this phenomenon has gone through, we see the main product.
When a nucleophile attacks some molecules, we usually seetoProducts. But in most cases we normally see both a primary product and a secondary product. The main product is normally the rearranged productmore replaced(ie more stable). In the contract, the secondary product is normally the normal productless substituted(otherwise less stable).
Reaction: We see that the resulting carbocations can undergo so-called rearrangementshydride shift. That is, the two-electron hydrogen from the unimolecular substitution goes towith themcoal. The hydride shift phenomenon is usually observed in the reaction between alcohol and hydrogen halides, which include HBr, HCl, and HI. HF is not normally used because of its instability and high reactivity. Below is an example of a reaction between an alcohol and hydrogen chloride:
GREEN (Cl)= nucleophile BLUE (OH)= outgoing group ORANGE (H)= proton with hydride shift RED(H)= remaining proton
The alcohol moiety (-OH) has been substituted with a nucleophilic Cl atom. However, this is not a direct replacement of the OH atom as seen in SN2 reactions. inclN1, we see that the -OH leaving group forms a carbocation at carbon #3 after receiving a proton from a nucleophile to produce an alkyloxonium ion. Before attack by the Cl atom, the hydrogen atom attached to the carbon directly adjacent to the original carbon (preferably the more stable carbon), carbon #2, can undergo a hydride shift. The hydrogen and the carbocation formally change places. The Cl atom can now attack the carbocation, where it forms a more stable structure due to hyperconjugation. In this case, the carbocation is the most stable because it bonds to a tertiary carbon (it is bonded to 3 different carbon atoms). However, we can still see small amounts of a minor, unstable product. The hydride shift mechanism occurs inmany stepswhich include various intermediate and transition states. Below is the mechanism for the reaction given above:
Hydrogenation of alkenes: hydride shift
In the more complex case, when alkenes are hydrogenated, we also observe a hydride shift. The reaction of 3-methyl-1-butene with H is shown below3O+which allows for the production of 2-methyl-2-butanol:
Once again we see a lot of products. In this case, however, we are dealing with two smaller products and one larger product. We observe the major product because the -OH substituent is attached to the more substituted carbon. When the reactant hydrates, a proton binds to carbon no. 2. The carbocation is therefore at carbon no. 2. The hydride shift now occurs when the hydrogen on the adjacent carbon formally exchanges places with the carbon. Carbocation is now ready to be attacked by H2O to provide the alkyloxonium ion due to stability and hyperconjugation. The final step can be seen when another water molecule attacks the proton of the alkyloxonium ion and gives the alcohol. We see this mechanism below:
Not all carbon atoms have corresponding hydrogen atoms (secondary or tertiary) that are on adjacent carbon atoms available for rearrangement. In this case, the reaction may undergo another rearrangement state known asalkylskift(or migration of alkyl groups). The alkyl shift works in much the same way as the hydride shift. Instead of the proton (H) moving with the nucleophile, we see an alkyl group moving with the nucleophile. The alternating group carries its electron pair with it to ensure bonding to an adjacent or adjacent carbocation. Switched alkyl group and positive charge of the carbocation switch position in the molecule Tertiary carbocations react much faster than secondary carbocations. We see the transition of alkyl from the secondary carbocation to the tertiary carbocation at SN1 reactions:
We observe small changes and differences between the two responses. In reaction #1 we see that we have a secondary substrate. This undergoes an alkyl shift because there is no corresponding hydrogen on the adjacent carbon. Once again, the reaction resembles a hydride shift. The only difference is that instead of moving the proton, we move the alkyl group as we go through various intermediate steps to deliver the final product.
In the case of reaction no. 2, however, we can say that it occurscompatiblemechanism. In short, this means that everything happens in one step. This is due to primary carbohydratesI can notbe an intermediate product and are relatively difficult processes as they require higher temperatures and longer reaction times. After the alcohol substrate is protonated to form the alkyloxonium ion, the water must leaveat the same timewhen the alkyl group moves from the adjacent carbon to skip the formation of the unstable primary carbocation.
Carbocation rearrangements for the E1 reaction
El reactions are also affected by the alkyl shift. Again, we can see both small and large products. However, we see that the more substituted carbons undergo the effects of the E1 reaction and form a double bond. See the example worksheet #4 below because the properties and effects of carbocation rearrangements in E1 reactions are similar to alkyl shifts.
Shifting 1,3-hydride and more
Typically, hydride shift can occur at low temperatures. However, by heating the cation solution, the rearrangement process can be easily and easily accelerated. One way to account for a small barrier is to propose a 1,3-hydride shift that switches the functionality of two different types of methyl groups. Another possibility is a 1,2-hydride shift where a secondary carbocation intermediate can be obtained. Then a further 1,2 hydride shift would give a more stable rearranged tertiary cation.
More distant hydride shifts such as 1,4 and 1,5 hydride shifts have been observed, but these systems are too fast to undergo secondary cation intermediates.
Carbocation rearrangements occur very easily and are common in many organic chemical reactions. However, we usually neglect this step. Dr. Sarah Lievens, professor of chemistry at the University of California, Davis, once said that carbocation rearrangements can be observed using various analogies to help her students remember the phenomenon. For hydride shift: "A new friend (nucleophile) just joined the group (organic molecule). Since he is new, he has only made two new friends. However, the popular kid (Hydrogen) willingly left his friends for a new friend, so he could make even more friends. That's why everyone won't be so lonely and we can all be friends." This analogy also works for alkyl shifts combined with hydride shifts.
- Vogel, Pierre.Carbocation kemi.Amsterdam: Elsevier Science Publishers BV, 1985.
- Olah, George A. and Prakash, G.K. Sun.Carbocation kemi.New Jersey: John Wiley & Sons, Inc., 2004.
- Vollhardt, K. Peter C. and Schore, Neil E.Organic Chemistry: Structure and Function. New York: Bleyer, Brennan, 2007.
Answers to practical problems
- Jeffrey Ma
Carbocation rearrangements are extremely common in organic chemistry reactions are are defined as the movement of a carbocation from an unstable state to a more stable state through the use of various structural reorganizational "shifts" within the molecule.What is an example of a carbocation rearrangement? ›
Below is a carbocation rearrangement example, which is a reaction between an alcohol and hydrogen chloride. Protonation and Loss of Water Molecule: In this SN1 reaction, -OH which leaves the organic alcohol forms a carbocation on carbon (C-3) after receiving a proton from the nucleophile to produce an alkyloxonium ion.What are the types of carbocation rearrangements? ›
There are two types of carbocation rearrangements: a hydride shift and an alkyl shift. Once rearranged, the resultant carbocation will react further to form a final product which has a different alkyl skeleton than the starting material.How do you know which carbocation will rearrange? ›
A quick way to tell whether a substrate will produce a carbocation prone to rearrangement is to look at the carbon that bears the leaving group. If this carbon is next to a higher order carbon (meaning secondary, tertiary, allylic, etc.)Does carbocation rearrangement occur in sn1? ›
An adjacent bonding pair of electrons (i.e. a C-H bond) interacts with the empty p-orbital, and before you know it, the C-H bond has moved and a new, more stable carbocation has formed! The carbocation is then attacked by the nucleophile, giving a substitution reaction (SN1) with rearrangement!What makes carbocations rearrange? ›
The answer lies in the observation that formation of carbocations is sometimes accompanied by a structural rearrangement. Such rearrangements take place by a shift of a neighboring alkyl group or hydrogen, and are favored when the rearranged carbocation is more stable than the initial carbocation.What is a simple example of rearrangement reaction? ›
In a rearrangement reaction, a molecule undergoes a reoraganization of its constituent parts. For example, alkene on heating with strong acid from another isomeric alkene.What are two examples of carbocation? ›
A carbocation is an ion with a positively-charged carbon atom. Among the simplest examples are methenium CH3+, methanium CH5+, and ethanium C2H7+.What are the three types of rearrangement reaction? ›
Three key rearrangement reactions are 1,2-rearrangements, pericyclic reactions and olefin metathesis.Does sn2 have carbocation rearrangement? ›
Carbocations are only formed in unimolecular (SN1 and E1) reactions since the SN2 and E2 reactions are concerted mechanisms where everything happens at the same time.
Solution: By methyl or hydride shift, they become unstable carbocation, hence not involved in rearrangement.How do you arrange the carbocation in stability order? ›
Thus the observed order of stability for carbocations is as follows: tertiary > secondary > primary > methyl.Which is the most stable rearranged form of given carbocations? ›
The formed six membered carbocation can undergo further 1, 2 H-shift to give six membered tertiary carbocation (c) which is the most stable one.Is carbocation formed in SN1 or SN2? ›
In SN1, there is a stage where carbocation forms. The anion or the negatively charged atoms or compounds then gets attracted to the carbocation. In SN2 , there is only a transition stage and no formation of intermediates.Does rearrangement occur in SN1 and E1? ›
3. Elimination (E1) With Rearrangement: Alkyl Shift. You might remember that these types of rearrangements can occur in SN1 reactions too. And if you read that post, you might recall that in addition to shifts of hydrogen (“hydride”, because there's a pair of electrons attached) we can also have alkyl shifts.How can you tell the difference between SN1 and SN2? ›
|Difference between SN1 and SN2|
|The rate of reaction is unimolecular.||The rate of reaction is bimolecular|
|It is a two-step mechanism||It is only a one-step mechanism|
|Carbocation is formed as an intermediate part of the reaction.||No carbocation is formed during the reaction.|
Rearrangements occur to create more stable carbocations.What determines carbocation stability? ›
Some of the major factors that affect carbocation stability are resonance, electronegativity, hyperconjugation, and inductive effect. Resonance: The more resonance structures a carbocation has, the more stable it is.What is the purpose of rearrangement reaction? ›
A rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule. Often a substituent moves from one atom to another atom in the same molecule.What happens in rearrangement reaction? ›
A reaction in which an atom or bond moves or migrates, having been initially located at one site in a reactant molecule and ultimately located at a different site in a product molecule.
Rearrangement: A mechanism step or reaction in which an atom or group migrates from one carbon atom to another. The reaction often includes the breaking and/or making of carbon-carbon sigma bonds.What are the three types of carbocation? ›
The primary carbocation, secondary carbocation, and tertiary carbocation, respectively, are formed when one, two, or three carbons are connected to the carbon with the positive charge.What reactions form a carbocation? ›
Adding strong acid to an alkene gets us to a carbocation (A–>C).What are the three reactions of carbocation? ›
- Nucleophile CaptureEdit.
- Elimination to form a pi bondEdit.
The term “rearrangement” is used to describe two different types of organic chemical reactions. A rearrangement may involve the one -step migration of an H atom or of a larger molecular fragment within a relatively short lived intermediate.In which of the following reactions can rearrangements occur? ›
Addition of HBr to alkene proceeds through carbocation as an intermediate. This carbocation may undergo rearrangement to form more stable carbocation. So rearrangement can occur in addition of HBr to an alkene.What are the 5 main reaction types? ›
This becomes much easier for students to do when they learn the pattern of 5 basic categories of chemical reactions: synthesis, decomposition, single replacement, double replacement, and combustion.Is rearrangement possible in SN1 or SN2? ›
So out of these following reactions SN1,E1,SN2(AR),EAS(Electrophilic armatic substitution), electrophilic addition on alkenes involve the formation of carbocation and hence rearrangement of carbon skeleton is possible.Which carbocation is formed in SN1 reaction? ›
In an SN1 there is loss of the leaving group generates an intermediate carbocation which is then undergoes a rapid reaction with the nucleophile.. In an SN1 reaction, the rate determining step is the loss of the leaving group to form the intermediate carbocation.Is a methyl shift a form of carbocation rearrangement? ›
Notice that in the observed product, the carbon framework has been rearranged: the methyl carbon indicated by a red dot has shifted from carbon #3 to carbon #2. This is an example of another type of carbocation rearrangement, called a methyl shift.
Carbocation rearrangements occur most frequently on secondary carbocations. Simple alkyl primary carbocations are too high in energy to form so you don't tend to see a primary carbocation. There are some exceptions to this general rule for primary carbocations.How many carbocation can undergo rearrangement? ›
How many carbocations undergo rearrangements? Correct answer is '8'.Do carbocation rearrangements have to be adjacent? ›
Yes these rearrangements typically move a hydride or alkyl group to an adjacent carbon.Why tertiary carbocation is more stable? ›
A tertiary carbocation is more stable than secondary carbocation because: Tertiary carbocations include three alkyl groups, whereas secondary carbocations contain just two alkyl groups. Thus, tertiary carbocation is more stable than secondary carbocation, according to the inductive effect.Why tertiary carbanion is least stable? ›
Because of the +I effect, the methyl groups increase the intensity of the negative charge on central carbon in tertiary carbanion which makes tertiary carbanion more unstable.What is the correct order of decreasing stability of carbocations? ›
Thus, the stability of carbocations decreases in the order: II > I > III.What type of carbocation is most stable? ›
A tertiary carbocation is the most stable carbocation due to the electron releasing effect of three methyl groups. An increased + I effect by three methyl groups stabilizes the positive charge on the carbocation.Why carbocation is not formed in SN2? ›
SN2 reactions proceed via a concerted mechanism, meaning that there is just one step. The nucleophile attacks the carbon attached to the leaving group, from the other side as the leaving group, and the C-LG bond cleaves simultaneously. Being unstable carbocation is not formed in sn2 and direct product is formed.Which will form the most stable carbocation in an SN1 reaction? ›
Since the tertiary carbocation formed by the dissociation of iodide from will the be most stable, this substrate will react the fastest.Does SN1 prefer primary or tertiary? ›
Tertiary carbons have the largest number of adjacent C-C bonds, the largest inductive effect, the most stable carbocation intermediate, and are thus favored in SN1.
1,2-Hydride shifts and 1,2-methyl shifts will occur in E1 reactions if the rearrangement leads to a more stable carbocation. These rearrangements do not occur for obvious reasons in the E2 reaction.Is there carbocation rearrangement in E1? ›
Since carbocation intermediates are formed during an E1, there is always the possibility of rearrangements (e.g. 1,2-hydride or 1,2-alkyl shifts) to generate a more stable carbocation.Can rearrangements happen in E2? ›
The carbocation intermediate formed can undergo rearrangements to form a more stable carbocation. E2 reactions are elimination reactions occurring in a single step. Hence there is no likely occurrence of rearrangements during an E2 reaction.Does SN2 prefer primary or tertiary? ›
SN2 indicates a substitution reaction that takes place in one step. A primary alcohol is preferred to prevent steric congestion caused by the simultaneous binding of the nucleophile and release of the leaving group.Does SN2 favor primary or tertiary? ›
5. For SN2, The Rate Of Reaction Increases Going From Tertiary To Secondary To Primary Alkyl Halides. For SN1 The Trend Is The Opposite. For the SN2, since steric hindrance increases as we go from primary to secondary to tertiary, the rate of reaction proceeds from primary (fastest) > secondary >> tertiary (slowest).Can a reaction be both SN1 and SN2? ›
Obviously, SN1 can occur only in Polar protic solvents, and in reactions where the alkyl halide is a conflicting one (reaction can occur both by SN1 and SN2), the mentioning of the said solvent is essential.What are some examples of a carbocation? ›
A carbocation is an ion with a positively-charged carbon atom. Among the simplest examples are methenium CH3+, methanium CH5+, and ethanium C2H7+. Some carbocations may have two or more positive charges, on the same carbon atom or on different atoms; such as the ethylene dication C2H42+.Which is most likely carbocation rearrangement? ›
Carbocation rearrangements occur most frequently on secondary carbocations. Simple alkyl primary carbocations are too high in energy to form so you don't tend to see a primary carbocation. There are some exceptions to this general rule for primary carbocations.How many carbocations undergo rearrangements? ›
How many carbocations undergo rearrangements? Correct answer is '8'.Which carbocation is most stabilize? ›
Compound D is a most stable carbocation as it has a +ve charge on tertiary carbocation and also the O lone pair is in conjugation with a double bond and in turns, it is in conjugation with +ve charge so most stable carbocation.
Benzyl carbocations are more stable.What is an example of rearrangement? ›
In a rearrangement reaction, a molecule undergoes a reoraganization of its constituent parts. For example, alkene on heating with strong acid from another isomeric alkene.Does E1 have carbocation rearrangement? ›
Since carbocation intermediates are formed during an E1, there is always the possibility of rearrangements (e.g. 1,2-hydride or 1,2-alkyl shifts) to generate a more stable carbocation.