# Lecture 10: Elimination Reactions: E1 and E2
## Organic Chemistry I
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## Learning Objectives
By the end of this lecture, students will be able to:
1. Define elimination reactions and identify the products (alkenes)
2. Describe the E2 mechanism and its concerted, one-step nature
3. Describe the E1 mechanism and its two-step process through a carbocation
4. Apply Zaitsev's rule to predict the major alkene product
5. Explain the stereochemical requirements of E2 reactions (anti-periplanar geometry)
6. Predict the regiochemistry of elimination products
7. Analyze how substrate, base, and solvent affect E1 vs. E2 competition
8. Recognize Hofmann elimination as an exception to Zaitsev's rule
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## Lecture Content
### I. Introduction to Elimination Reactions
Elimination reactions involve the removal of two atoms or groups from adjacent carbons to form a pi bond. The most common type is beta-elimination, also called 1,2-elimination, in which a base removes a hydrogen from the beta-carbon (the carbon adjacent to the one bearing the leaving group), the leaving group departs, and a double bond forms between the alpha and beta carbons. The general equation is: base + H-C(beta)-C(alpha)-LG yielding C=C + base-H + LG-.
Elimination reactions are in constant competition with substitution reactions (SN1 and SN2) because the same substrates and reagents can potentially follow either pathway. Learning to predict which pathway dominates under a given set of conditions is one of the most important skills in organic chemistry.
### II. The E2 Mechanism
The E2 mechanism (Elimination, Bimolecular) is a one-step, concerted process in which the base removes the beta-hydrogen, the C-H bond breaks, the C=C double bond forms, and the leaving group departs all simultaneously. The rate law is rate = k[base][substrate], making the reaction second-order overall and bimolecular. A strong base is required because it appears in the rate law and must actively participate in the rate-determining step.
The transition state involves all five key atoms: the base, the hydrogen, the beta-carbon, the alpha-carbon, and the leaving group. A critical stereochemical requirement governs the E2 mechanism: the hydrogen being removed and the leaving group must be in an anti-periplanar arrangement, meaning they are positioned 180 degrees apart in a dihedral angle. This geometry allows optimal orbital overlap for pi bond formation, as the sigma C-H and sigma C-LG orbitals align properly to overlap and form the new pi bond. In cyclic systems, this requirement translates to the condition that H and the leaving group must be trans-diaxial on the cyclohexane ring.
Panel A: The E2 mechanism shown with curved arrows: a strong base (B:-) attacks the beta-hydrogen, the C-H bond breaks, a pi bond forms between C-alpha and C-beta, and the leaving group departs simultaneously. All bond-making and bond-breaking events are shown occurring in a single step. Panel B: Newman projection showing the anti-periplanar requirement — H and LG are 180 degrees apart (anti) in the Newman projection. A second Newman projection shows the unfavorable syn-periplanar arrangement (0 degrees) that does NOT lead to E2 elimination. Panel C: A cyclohexane ring showing trans-diaxial arrangement of H and Br required for E2 elimination, with the resulting alkene product.
### III. Regiochemistry of E2: Zaitsev's Rule
When a substrate has multiple beta-hydrogens available, more than one alkene product is possible, and Zaitsev's rule (also known as Saytzeff's rule) predicts that the more substituted alkene will be the major product. More substituted means more alkyl groups attached to the carbons of the double bond, and these alkenes are thermodynamically more stable due to hyperconjugation. Alkenes are classified by their degree of substitution: monosubstituted (R-CH=CH2), disubstituted (R2C=CH2 or R-CH=CH-R with cis/trans possibility), trisubstituted (R2C=CHR), and tetrasubstituted (R2C=CR2, the most stable). For example, treating 2-bromobutane with sodium ethoxide produces 2-butene (the more substituted, disubstituted alkene) as the major product along with 1-butene (the less substituted, monosubstituted alkene) as a minor product.
The Hofmann elimination represents an important exception to Zaitsev's rule. When a bulky base such as potassium tert-butoxide or LDA is used, the less substituted alkene becomes the major product. The bulky base cannot easily access the more sterically hindered beta-hydrogen adjacent to the more substituted position, so it instead removes the more accessible hydrogen, producing the less substituted alkene. For instance, treating 2-bromobutane with potassium tert-butoxide favors 1-butene over 2-butene.
### IV. Stereochemistry of E2: E/Z Selectivity
When the E2 product can exist as E or Z geometric isomers, the anti-periplanar requirement of the mechanism controls which stereoisomer forms. The groups remaining on the double-bond carbons are fixed in their positions by the geometry of the transition state. In acyclic systems, anti-elimination of the larger groups typically produces the trans (E) alkene as the major product because this arrangement places the large groups on opposite sides, reducing steric strain.
In cyclic systems, the stereochemical constraint is even more restrictive. Only a hydrogen and leaving group that are trans-diaxial can undergo E2 elimination. This limitation can restrict which products form and can even prevent elimination entirely if the required geometry is unattainable in the preferred chair conformation.
### V. The E1 Mechanism
The E1 mechanism (Elimination, Unimolecular) is a two-step process that shares its first step with the SN1 mechanism. In the slow, rate-determining step, the substrate ionizes to form a carbocation (R-LG yielding R+ + LG:-). In the fast second step, a base removes a beta-hydrogen from the carbocation to generate the alkene (R+ + B: yielding alkene + B-H+).
The rate law is rate = k[substrate], making the reaction first-order. Base strength does not affect the rate because the base enters only in the fast second step. Because E1 shares the carbocation intermediate with SN1, these two pathways always compete with one another. Both require tertiary or secondary substrates, polar protic solvents, and weak bases. E1 is favored over SN1 at higher temperatures because elimination produces more product molecules than substitution, giving elimination a more favorable entropy change. Carbocation rearrangements, including 1,2-hydride and methyl shifts, can occur in E1 reactions just as they do in SN1.
### VI. Zaitsev's Rule in E1
The E1 mechanism also follows Zaitsev's rule. Since the carbocation intermediate can lose any of its beta-hydrogens, the proton whose removal generates the more substituted, more stable alkene is preferentially lost. For example, when 2-bromo-2-methylbutane undergoes E1 elimination in ethanol and water, the tertiary carbocation that forms preferentially loses a hydrogen from C3 to give the trisubstituted alkene (the Zaitsev product) as the major product, while loss of a hydrogen from the methyl group at C1 gives the disubstituted alkene as a minor product.
### VII. E1 vs. E2 Comparison
The E2 mechanism works with all substrate classes when a strong base is provided, while E1 requires tertiary or secondary substrates to form reasonably stable carbocations. E2 demands a strong base (NaOH, NaOEt, KOtBu, NaNH2), whereas E1 proceeds with weak bases including solvent molecules like water and alcohols. E2 is second-order (rate = k[base][substrate]), while E1 is first-order (rate = k[substrate]). Mechanistically, E2 is concerted in one step, while E1 proceeds stepwise through a carbocation intermediate. E2 is stereospecific, requiring anti-periplanar geometry, while E1 has no geometric requirement, though Zaitsev's rule still applies. Rearrangements never occur in E2 but are possible in E1.
Panel A: Energy diagram comparison — E2 shown as a single-barrier profile (one transition state, no intermediate) and E1 shown as a two-barrier profile (first barrier for ionization to carbocation, second smaller barrier for deprotonation). Both diagrams show reactants, products, and energy levels. Panel B: Summary chart with rows for mechanism steps, rate law, base requirement, substrate preference, stereochemistry, and rearrangements, with E1 and E2 in separate columns. Key distinguishing features are highlighted.
### VIII. E2 in Cyclic Systems
E2 elimination in cyclohexane derivatives imposes special requirements because the hydrogen and the leaving group must be trans-diaxial for the anti-periplanar geometry to be satisfied. The substrate must adopt the correct chair conformation for elimination to proceed.
This is dramatically illustrated by the 1-bromo-4-tert-butylcyclohexane system, where the tert-butyl group locks the ring with itself in the equatorial position. In cis-1-bromo-4-tert-butylcyclohexane, the bromine is forced into the axial position (opposite face from the equatorial tert-butyl group), placing it trans-diaxial to available beta-hydrogens and allowing rapid E2 elimination. In trans-1-bromo-4-tert-butylcyclohexane, however, the bromine occupies the equatorial position, and no anti-periplanar hydrogen is available. E2 elimination in this isomer is extremely slow because the ring would need to flip to place bromine axial, which would simultaneously force the tert-butyl group into the highly unfavorable axial position.
### IX. Temperature Effects on Elimination
Higher temperatures favor elimination over substitution for a thermodynamic reason. Elimination produces more product molecules (an alkene plus HX, versus a single substitution product), resulting in a larger positive entropy change. At elevated temperatures, the -T(delta S) term in the Gibbs free energy equation (delta G = delta H - T(delta S)) becomes more significant, making elimination increasingly favorable. This principle applies to both the SN1/E1 and SN2/E2 competitions, providing a practical guideline: low temperatures favor substitution, while high temperatures favor elimination.
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