When do methyl shifts occur

The rearranged carbocation is the major product of a synthesis reaction because it is the most stable form. If a secondary carbocation is vicinal to a tertiary carbon atom bearing a hydrogen atom, then a hydride shift occurs.

We call this a 1,2-hydride shift. This shift is possible when there is a positive charge on the carbon atom where its adjacent carbon atom has a removable hydrogen atom.

Methyl shift is the movement of a methyl group from one carbon atom to a charged, adjacent carbon atom of the same compound. We call this a methyl shift if the moving chemical species is a methyl group, and it can be any other possible alkyl group as well.

Here, the smaller substituent alkyl group tends to be the moving chemical species that attach to the charged carbon atom. So, lets try a hydride shift with that one. So, this hydrogen and these two electrons are gonna move over here to the carbon in magenta. We have our ring, and the methyl group stays there. The hydrogen in blue just moved over to here, so actually, let me go ahead and make that blue, so we can distinguish it, so here is the hydrogen, and those two electrons that moved.

There was already a hydrogen on that carbon, so I will draw in the original hydrogen here, and we took a bond away from the carbon in blue. So, here's the carbon in blue, which means that is where our carbocation is. That is a plus one formal charge. Now, let's look at this resulting carbocation. The carbon that's in blue is directly bonded to one, two, three other carbons So, this is a tertiary carbocation.

And we know from the previous video that a tertiary carbocation is more stable than a secondary carbocation. So, this is the rearrangement that we would see. We're going from a secondary carbocation to a tertiary carbocation via a hydride shift. And just like the previous examples, we don't need to draw in the hydrogens. We could just show our tertiary carbocation leaving out the hydrogens.

So, put a methyl group in here, plus one formal charge on this carbon. So, this tertiary carbocation is more stable than the secondary ones. Let's do another carbocation rearrangement problem.

So, this one's actually a little bit easier than the previous one. So, here's our carbocation, and the carbon with the plus one formal charge is directly bonded to two other carbons, which makes this a secondary carbocation. So, let's think about what kind of shifts that we could possibly have.

So first, we know that this carbon has two hydrogens on it, so we could try doing a hydride shift with one of those hydrogens. So, one of these hydrogens and these two electrons could do a hydride shift and move over here to this carbon.

So, let's draw what we would make. Let's draw our ring in here, and let's put in these two methyl groups coming off of that carbon. So, the hydrogen in red is now this hydrogen. Let me go ahead and make this red, and let me highlight these two electrons. So, that's what moved in our hydride shift.

Typically, hydride shifts can occur at low temperatures. However, by heating the solutionf of a cation, it can easily and readily speed the process of rearrangement. One way to account for a slight barrier is to propose a 1,3-hydride shift interchanging the functionality of two different kinds of methyls. Another possibility is 1,2 hydride shift in which you could yield a secondary carbocation intermediate.

Then, a further 1,2 hydride shift would give the more stable rearranged tertiary cation. More distant hydride shifts have been observed, such as 1,4 and 1,5 hydride shifts, but these arrangements are too fast to undergo secondary cation intermediates.

Carbocation rearrangements happen very readily and often occur in many organic chemistry reactions. Yet, we typically neglect this step. Sarah Lievens, a Chemistry professor at the University of California, Davis once said carbocation rearrangements can be observed with various analogies to help her students remember this phenomenon.

For hydride shifts: "The new friend nucleophile just joined a group the organic molecule. Because he is new, he only made two new friends. However, the popular kid the hydrogen glady gave up his friends to the new friend so that he could have even more friends. Therefore, everyone won't be as lonely and we can all be friends. Introduction Whenever alcohols are subject to transformation into various carbocations, the carbocations are subject to a phenomenon known as carbocation rearrangement.

Hydride Shift Whenever a nucleophile attacks some molecules, we typically see two products. Hydration of Alkenes: Hydride Shift In a more complex case, when alkenes undergo hydration, we also observe hydride shift. Alkyl Shift Not all carbocations have suitable hydrogen atoms either secondary or tertiary that are on adjacent carbon atoms available for rearrangement. We see alkyl shift from a secondary carbocation to tertiary carbocation in S N 1 reactions: We observe slight variations and differences between the two reactions.

Carbocation Rearrangements for E1 Reactions E1 reactions are also affected by alkyl shift. Analogy Carbocation rearrangements happen very readily and often occur in many organic chemistry reactions. References Vogel, Pierre. Carbocation Chemistry. Click for additional MCAT tutorials. Click for additional orgo tutorial videos. Click to see the entire series Ready to test your skills? Leave a Reply Cancel reply Your email address will not be published. Organic Chemistry Reference Material and Cheat Sheets The true key to successful mastery of alkene reactions lies in practice practice practice.