When is elimination favored over substitution

Look at the substrate. Elimination is typically preferred over substitution unless the reactant is a strong nucleophile , but weak base. If the carbocation were to rearrange, on which carbon would the positive charge go onto without sacrificing stability A, B, or C? False — They can be thermodynamically controlled to favor a certain product over another. By definition, an E1 reaction is a Unimolecular Elimination reaction.

This means the only rate determining step is that of the dissociation of the leaving group to form a carbocation. Skip to main content. Search for:. Elimination reactions Alkyl halides undergo elimination via two common mechanisms, known as E2 and E1, which show some similarities to S N 2 and S N 1, respectively.

These mechanisms are important in laboratory organic chemistry. As explained below, which mechanism actually occurs in a laboratory reaction will depend on the identity of the R groups ie. E1 and E2 reactions in the laboratory E2 elimination reactions in the laboratory are carried out with relatively strong bases, such as alkoxides deprotonated alcohols, — OR. Exercise A straightforward functional group conversion that is often carried out in the undergraduate organic lab is the phosphoric acid-catalyzed dehydration of cyclohexanol to form cyclohexene.

No solvent is necessary in this reaction — pure liquid cyclohexanol is simply stirred together with a few drops of concentrated phosphoric acid. In order to drive the equilibrium of this reversible reaction towards the desired product, cyclohexene is distilled out of the reaction mixture as it forms the boiling point of cyclohexene is 83 o C, significantly lower than that of anything else in the reaction solution.

Any cyclohexyl phosphate that might form from the competing S N 1 reaction remains in the flask, and is eventually converted to cyclohexene over time. Draw a mechanism for the cyclohexene synthesis reaction described above. Also, draw a mechanism showing how the undesired cyclohexyl phosphate could form.

Show Solution. Introduction E2 reactions are typically seen with secondary and tertiary alkyl halides, but a hindered base is necessary with a primary halide.

General Reaction. Key features of the E2 elimination The main features of the E2 elimination are: It usually uses a strong base often — OH or — OR with an alkyl halide Primary, secondary or tertiary alkyl halides are all effective reactants, with tertiary reacting most easily. With primary alkyl halides, a substituted base such as KO t Bu and heat are often used to minimize competition from S N 2.

The H and the leaving group should normally be antiperiplanar o to one another. Further Reading Wikipedia-Elimination reaction. References Vollhardt, K. Peter C. Organic Chemistry Structure and Function. New York: W. Breaking bonds requires an input of energy. Elimination reactions require the breakage of more bonds than substitution reactions, and therefore require a higher input of energy. This indicates that the activation energy of elimination reactions is higher than that of substitution reactions.

Increasing temperature will allow more molecules to reach the required activation energy threshold. Therefore, as temperatures is increased, elimination is favored. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Why substitution and elimination reactions are favored respectively at lower and higher temperature?

Ask Question. Asked 4 years, 4 months ago. Active 3 years ago. Viewed 6k times. There are 3 factors that must be examined to decide whether a molecule goes through an substitution vs elimination:.

Below is a figure showing the leaving group, substrate, and nucleophile in a typical substitution reaction:. For example, -OH is both a strong nucleophile and a strong base.

When the —OH is involved in elimination reactions, it is acting as a base; however, when it is involved in substitution reactions, it is acting as a nucleophile.

The leaving group is the part of the substrate that leaves during the reaction. There are many, many different leaving groups, but below is a list of the most common leaving groups. By itself, the hydroxyl group cannot leave this molecule. However, we can make a hydroxyl group into a good leaving group with a small transformation see below. On our figure of good and bad leaving groups, note that -OH is listed as a bad leaving group, while H 2 O is listed as a good leaving group.

Say that we had a molecule with an —OH, and we wanted it to leave. How could we accomplish this? What if we just turned the —OH in H 2 O? We can do this with the simple addition of acid.

Just like that, we are able to turn a bad leaving group into a good leaving group.