4.12: Comparison of E1 and E2 Reactions

Learning Objective

• distinguish 1st or 2nd order elimination reactions

Elimination reactions of alkyl halides can occur via the bimolecular E2 mechanism or unimolecular E1 mechanism as shown in the diagram below.

Comparing E1 and E2 mechanisms

When considering whether an elimination reaction is likely to occur via an E1 or E2 mechanism, we really need to consider three factors:

1) The base: strong bases favor the E2 mechanism, whereas, E1 mechanisms only require a weak base.

2) The solvent: good ionizing xolvents (polar protic) favor the E1 mechanism by stabilizing the carbocation intermediate.

3) The alkyl halide: primary alkyl halides have the only structure useful in distinguishing between the E2 and E1 pathways. Since primary carbocations do not form, only the E2 mechanism is possible.

Ultimately, whether the elimination mechanism is E1 or E2 is not very important, since the product is the same alkene.  We need to remember, however, that Zeitzev´s rule always determines the most likely alkene to be formed.

 Reaction Parameter E2 E1 alkyl halide structure tertiary > secondary > primary tertiary > secondary >>>> primary nucleophile high concentration of a strong base weak base mechanism 1-step 2-step rate limiting step bimolecular transition state carbocation formation rate law rate = k[R-X][Base] rate = k[R-X] solvent not important polar protic

Exercises

1. Predict the dominant elimination mechanism (E1 or E2) for each reaction below. Explain your reasoning.

2. Which one of the following groups of compounds would eliminate $$\ce{HCl}$$ most readily on reaction with potassium hydroxide? Explain your reasoning, draw the bond-line structure and give the IUPAC name of the product.

a) $$\ce{(CH_3)_3CCl}$$ $$\ce{CH_3CH_2CH_2CH_2Cl}$$ $$\ce{CH_3CH(Cl)CH_2CH_3}$$

b) $$\ce{(CH_3)_3CCH_2Cl}$$ $$\ce{(CH_3)_2CHCH_2Cl}$$
c)

3. Specify the reaction conditions to favor the indicated elimination mechanism.

1.

2.

3. a) strong base, such as hydroxide, an alkoxide, or equivalent

b) water or alcohol or equivalent weak base with heat

c) strong base, such as hydroxide, an alkoxide, or equivalent