Why does ionization of a tertiary substrate

The figure below shows the mechanism of an S N 1 reaction of an alkyl halide with water. Since water is also the solvent, this is an example of a solvolysis reaction. Examples of polar protic solvents are: acetic acid, isopropanol, ethanol, methanol, formic acid, water, etc. An S N 1 reaction speeds up with a good leaving group. This is because the leaving group is involved in the rate-determining step. A good leaving group wants to leave so it breaks the C-Leaving Group bond faster.

Once the bond breaks, the carbocation is formed and the faster the carbocation is formed, the faster the nucleophile can come in and the faster the reaction will be completed.

A good leaving group is a weak base because weak bases can hold the charge. They're happy to leave with both electrons and in order for the leaving group to leave, it needs to be able to accept electrons. Strong bases, on the other hand, donate electrons which is why they can't be good leaving groups.

As you go from left to right on the periodic table, electron donating ability decreases and thus ability to be a good leaving group increases. Halides are an example of a good leaving group whos leaving-group ability increases as you go down the column. The two reactions below is the same reaction done with two different leaving groups. One is significantly faster than the other.

This is because the better leaving group leaves faster and thus the reaction can proceed faster. When considering whether a nucleophilic substitution is likely to occur via an S N 1 or S N 2 mechanism, we really need to consider three factors:. When the leaving group is attached to a tertiary, allylic, or benzylic carbon, a carbocation intermediate will be relatively stable and thus an S N 1 mechanism is favored.

Weaker nucleophiles such as water or alcohols favor the S N 1 mechanism. Polar protic solvents favor the S N 1 mechanism by stabilizing the carbocation intermediate. S N 1 reactions are frequently solvolysis reactions. Because substitution occurs at a chiral carbon, we can also predict that the reaction will proceed with racemization.

In the reaction below, on the other hand, the electrophile is a secondary alkyl bromide — with these, both S N 1 and S N 2 mechanisms are possible, depending on the nucleophile and the solvent. In this example, the nucleophile a thiolate anion is strong, and a polar aprotic solvent is used — so the S N 2 mechanism is heavily favored.

The reaction is expected to proceed with inversion of configuration. Exercise 8. Loss to give a carbocation would not happen without significant heat. On the other hand, with a secondary bromine, E2 is a real issue to watch out for.

The substitution reaction would likely be accompanied by elimination products from E2. It certainly could, especially with secondary alkyl halides! Your email address will not be published. Save my name, email, and website in this browser for the next time I comment. Notify me via e-mail if anyone answers my comment. This site uses Akismet to reduce spam. Learn how your comment data is processed. SN2 reaction of alkoxide ions with alkyl halides to give ethers Williamson synthesis Description: Alkyl halides or tosylates will react with alkoxy ions to form ethers.

Li, Na, K Examples: Notes: Note that since this is an SN2 reaction and proceeds via backside attack, there will be inversion of configuration at the carbon note the last two examples. Test Yourself! Click to Flip. I hope I made myself clear. CH3O- is an example of an alkoxide ion.

Polar aprotic solvents have a dipole moment, but their hydrogen is not highly polarized. Polar aprotic solvents are not used in S N 1 reactions because some of them can react with the carbocation intermediate and give you an unwanted product.

Rather, polar protic solvents are preferred. Since the hydrogen atom in a polar protic solvent is highly positively charged, it can interact with the anionic nucleophile which would negatively affect an SN2, but it does not affect an S N 1 reaction because the nucleophile is not a part of the rate-determining step See S N 2 Nucleophile. Polar protic solvents actually speed up the rate of the unimolecular substitution reaction because the large dipole moment of the solvent helps to stabilize the transition state.

The highly positive and highly negative parts interact with the substrate to lower the energy of the transition state. Since the carbocation is unstable, anything that can stabilize this even a little will speed up the reaction. Sometimes in an S N 1 reaction the solvent acts as the nucleophile. This is called a solvolysis reaction see example below. The polarity and the ability of the solvent to stabilize the intermediate carbocation is very important as shown by the relative rate data for the solvolysis see table below.

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