E1 Reaction : First-Order Elimination

First-Order Elimination : The E1 Reaction

– An elimination involves the loss of two atoms or groups from the substrate, usually with formation of a pi bond.

– Elimination reactions frequently accompany and compete with substitutions.

– By varying the reagents and conditions, we can often modify a reaction to favor substitution or to favor elimination.

– First we will discuss eliminations by themselves.

– Then we consider substitutions and eliminations together, trying to predict what products and what mechanisms are likely with a given set of reactants and conditions.

– Depending on the reagents and conditions involved, an elimination might be a first-order (E1) or second-order (E2) process.

– The following examples illustrate the types of eliminations:

E1 Reaction : First-Order Elimination

Mechanism and Kinetics of the E1 Reaction

– The abbreviation E1 stands for Elimination, unimolecular.

– The mechanism is called unimolecular because the rate-limiting transition state involves a single molecule rather than a collision between two molecules.

– The slow step of an E1 reaction is the same as in the SN1 reaction. unimolecular ionization to form a carbocation.

– In a fast second step, a base abstracts a proton from the carbon atom adjacent to the C+

– The electrons that once formed the carbon–hydrogen bond now form a pi bond between two carbon atoms.

– The general mechanism for the E1 reaction is shown in rhe following Key Mechanism:

KEY MECHANISM: The E1 reaction

– The E1 reaction requires ionization to a carbocation intermediate like the SN1 , so it follows the same order of reactivity:

3° > 2° >> 1°.

– A base (usually weak) deprotonates the carbocation to give an alkene.

Step 1: Unimolecular ionization to give a carbocation (rate-limiting).

E1 Reaction : First-Order Elimination

Step 2: Deprotonation by a weak base (often the solvent) gives the alkene (fast).

E1 Reaction : First-Order Elimination

EXAMPLE: E1 elimination of bromocyclohexane in methanol.

Step 1: Ionization gives a carbocation and bromide ion in a slow step.

E1 Reaction : First-Order Elimination

Step 2: Methanol abstracts a proton to give cyclohexene in a fast step.

E1 Reaction : First-Order Elimination

– Because the rate-limiting step involves unimolecular ionization of the alkyl halide, the rate equation is first-order.

– The rate depends only on the concentration of the alkyl halide, and not on the strength or concentration of the base.

E1 rate = kr [RX]

– The weak base (often the solvent) takes part in the fast second step of the reaction.

Competition with the SN1 Reaction

– The E1 reaction almost always competes with the SN1 reaction.

– Whenever a carbocation is formed, it can undergo either substitution or elimination, and mixtures of products often result.

– The following reaction shows the formation of both elimination and substitution products in the reaction of tert-butyl bromide with boiling ethanol.

E1 Reaction : First-Order Elimination

– The 2-methylpropene product results from dehydrohalogenation, an elimination of hydrogen and a halogen atom.

– Under these first-order conditions (the absence of a strong base), dehydrohalogenation takes place by the E1 mechanism: Ionization of the alkyl halide gives a carbocation intermediate, which loses a proton to give the alkene.

– Substitution results from nucleophilic attack on the carbocation.

– Ethanol serves as a base in the elimination and as a nucleophile in the substitution

Step 1: Ionization to form a carbocation.

Step 2: (by the E1 mechanism): Basic attack by the solvent abstracts a proton to give an alkene.

E1 Reaction : First-Order Elimination

or, step 2 (by the SN1 mechanism): Nucleophilic attack by the solvent on the carbocation.

– Under ideal conditions, one of these first-order reactions provides a good yield of one product or the other.

– Often, however, carbocation intermediates react in two or more ways to give mixtures of products.

– For this reason, and E1 reactions of alkyl halides are not often used for organic synthesis.

– They have been studied in great detail to learn about the properties of carbocations, however.

Orbitals and Energetics of the E1 Reaction

– In the second step of the E1 mechanism, the carbon atom next to the C+ must rehybridize to sp2 as the base attacks the proton and electrons flow into the new pi bond

E1 Reaction : First-Order Elimination

– The potential-energy diagram for the E1 reaction (see Figure) is similar to that for the SN1 reaction.

Reaction-energy diagram of the E1 reaction. The first step is a ratelimiting ionization. 

– The ionization step is strongly endothermic, with a rate-limiting transition state.

– The second step is a fast exothermic deprotonation by a base.

– The base is not involved in the reaction until after the rate-limiting step, so the rate depends only on the concentration of the alkyl halide.

– Weak bases are common in E1 reactions.

Rearrangements in E1 Reactions

– Like other carbocation reactions, the E1 may be accompanied by rearrangement.

– Compare the following E1 reaction (with rearrangement) with the SN1 reaction of the same substrate.

– Note that the solvent acts as a base in the E1 reaction and a nucleophile in the SN1 reaction.

MECHANISM: Rearrangement in an E1 Reaction

– Like other reactions involving carbocations, the E1 may be accompanied by rearrangement.

Step 1: Ionization to form a carbocation. (slow)

Step 2: A hydride shift forms a more stable carbocation. (fast)

Step 3: The weakly basic solvent removes either adjacent proton. (fast)

SUMMARY Carbocation Reactions

A carbocation can:

1. React with its own leaving group to return to the reactant:

2. React with a nucleophile to form a substitution product (SN1):

3. Lose a proton to form an elimination product (an alkene) (E1):

4. Rearrange to a more stable carbocation, then react further.

– The order of stability of carbocations is: resonance-stabilized

3° > 2° > 1°.

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