11.10 The E1 and E1cB Reactions

E1 Reaction Mechanism and Characteristics

Elimination reactions form alkenes by removing a leaving group and a proton from adjacent carbons. The E1 and E1cB mechanisms both accomplish this, but they differ in which bond breaks first and what intermediate forms along the way. Understanding these two pathways helps you predict when each one operates and what products to expect.

Mechanism of E1 Reactions

The E1 reaction proceeds in two steps, with the leaving group departing before the proton is removed.

1. Step 1 (rate-determining): The bond to the leaving group breaks heterolytically, generating a carbocation intermediate. Because this is the slow step, anything that stabilizes the carbocation speeds up the reaction. Tertiary substrates react fastest, secondary substrates are slower, and primary substrates essentially don't undergo E1 at all.
2. Step 2 (fast): A weak base (water, ethanol) removes a proton from a carbon adjacent to the carbocation, forming the alkene.

Because the carbocation forms before deprotonation, E1 shares several features with the SN1 reaction:

• Both are unimolecular: the rate law is $\text{rate} = k[\text{substrate}]$, depending only on substrate concentration.
• Both require substrates that can form stable carbocations (tertiary > secondary).
• Both are favored by polar protic solvents (ethanol, water) that stabilize the carbocation through solvation.
• Both can involve carbocation rearrangements (1,2-hydride or 1,2-methyl shifts) if a more stable carbocation is accessible.

Typical E1 conditions: a tertiary substrate, a polar protic solvent, a weak base, and heat (heat generally favors elimination over substitution).

E1 Stereochemistry and Regiochemistry

Unlike E2, the E1 mechanism has no strict geometric requirement for the proton and leaving group. The carbocation intermediate is planar and can rotate freely, so the base can remove a proton from any adjacent position and from either face.

This means E1 reactions often produce a mixture of alkene regioisomers. Among those isomers, Zaitsev's rule still applies: the more substituted (more stable) alkene is the major product, because the transition state leading to it is lower in energy.

By contrast, E2 reactions require antiperiplanar geometry between the leaving group and the proton being removed. This geometric constraint can sometimes override simple Zaitsev predictions, and in certain cases (bulky bases like potassium tert-butoxide), Hofmann's rule applies, favoring the less substituted alkene.

E1cB Reaction Mechanism and Conditions

E1cB Reaction Mechanism

The E1cB (Elimination Unimolecular conjugate Base) mechanism is essentially the reverse order of E1: the proton is removed first, and the leaving group departs second.

1. Step 1 (rate-determining): A strong base removes a proton from the $\beta$-carbon, generating a carbanion intermediate (the conjugate base of the substrate).
2. Step 2 (fast): The carbanion expels the leaving group, forming the alkene.

The key question is: why would a carbanion form instead of the reaction going through a concerted E2? The E1cB pathway is favored when:

• The $\beta$-proton is unusually acidic, meaning the carbanion is stabilized. This happens when electron-withdrawing groups are nearby, such as in $\beta$-dicarbonyl compounds or $\beta$-cyano compounds.
• The leaving group is poor (e.g., $\text{-OH}$, $\text{-OR}$), so it won't leave on its own in an E1 step and isn't easily displaced in a concerted E2 step. The buildup of negative charge on the carbanion is what eventually "pushes" the leaving group out.
• A strong base is present (alkoxides like $\text{NaOEt}$, amides like $\text{NaNH}_2$).
• The solvent is often aprotic (DMSO, DMF), which doesn't stabilize carbocations but doesn't interfere with carbanion formation either.

A classic example is the base-promoted elimination of $\beta$-hydroxy carbonyl compounds. The carbonyl group acidifies the $\beta$-proton enough for a strong base to remove it, and the resulting carbanion then loses the hydroxide leaving group.

Comparing E1, E1cB, and E2

A useful way to keep these three elimination mechanisms straight is to think about the order of bond-breaking events:

Additional Considerations

Kinetics

Both E1 and E1cB follow first-order kinetics because only the substrate appears in the rate-determining step. However, the identity of that slow step differs: in E1, it's leaving group departure; in E1cB, it's deprotonation. E2, by contrast, is second-order because both the substrate and the base are involved in the single concerted step.

Dehydration as an E1 Process

Acid-catalyzed dehydration of alcohols is a common example of E1 elimination. The hydroxyl group is first protonated (converting it to a good leaving group, water), then water departs to form a carbocation, and finally a base removes a proton to yield the alkene. This is why dehydration works best with tertiary alcohols and requires acid catalysis plus heat.