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E1 verses E2 Reactions, Factors Affecting E1 and E2 Reactions

E1 versus E2 reactions, Number of steps, Rate of reaction, Bulk of molecule, Base strength, Solvent, Factors affecting E1 and E2 reactions.

E1 versus E2 reactions

Number of steps

The number of steps in the mechanisms of E1 and E2 can be compared to distinguish them. An intermediate carbocation step is required for the first case, whereas only one step is required for the second.

Rate of reaction

E1 - The 1 in the name refers to a single molecular species. This means that the rate of reaction solely depends on the substrate concentration. The reaction rate increases as substrate concentration increases.

E2 - Second-order bimolecular reactions are indicated by their names. The rate of reaction is determined by both deprotonating bases and substrates.

Bulk of molecule

E1 - Carbon attachments to molecules play an important role in the E1 elimination process: the more bulk they have, the faster they undergo E1. The formation of E1 elimination will not be common because primary carbocations are too unstable. Primary carbocations will be the slowest, followed by tertiary substrates, followed by secondary substrates. A carbocation with the most substitutes is favored, since a molecule with more bonds to molecules other than hydrogen will be more stable. It is important to maintain carbocation stability.

E2 - Since primary substrates are not carbocation intermediates, E2 follows a similar order. There will be a preference for tertiary substrates, followed by secondary substrates, and then there will be a preference for primary substrates.

Base strength

E1 - Nucleophilicity of Lewis bases can be weak or strong. The hydrogen is grabbed by the base in this two-step reaction instead of the leaving group being displaced. Water is used as the weak base.

E2 - Hydroxide water is a strong base because it contains hydroxide ions. To allow displacement of the polar leaving group, the single-step reaction must be carried out on a solid base.

Solvent

E1 - This type of elimination involves the use of polar protic solvents. Carbocation intermediates can be stabilized by these solvents due to their good ionization qualities.

E2 - Any type of solvent is not necessary for this reaction.

Factors affecting E1 and E2 reactions

Factors affecting E1 reactions

Three factors are associated with E1 elimination reactions:
  • Solvent type, solvent concentration, and solvent type.
  • The type of the leaving group
  • Carbocation stability
The stabilization of carbocations is facilitated by hyperconjugation. Carbocation formation occurs at this step, affecting substitution alkyl halides reaction speed. There are also secondary carbohydrate structures that can be formed via 1,2-hydride transitions and 1,2-alkyl transitions that create more stable tertiary carbohydrate structures. The weak conjugate bases left by halide groups play an important role in E1 reactions since carbon-halogen bonds are broken. The polarity of a solvent plays a fundamental role in stabilizing intermediate carbohydrate and halide species during rate-limiting steps. So, this is an important distinction to make between protic solvents and aromatic solvents.

Factors affecting E2 reactions

  • As the strength of the base is increased, E2 reactions are accelerated.
  • E2 reactions often involve strong nonpolar bases, such as OH.
  • OH is often used to carry out E2 reactions since it is a nonpolar base.
  • A stronger leaving group results in faster E2 reactions.
  • The following order applies to reaction rates: R-I > R-Br > R-Cl > R-F
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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