Substitution vs elimination reaction

Substitution vs Elimination Reactions — Comparison

This page gives a concise, exam-friendly comparison between substitution and elimination reactions, covering definitions, types, mechanisms, factors affecting each, examples, and a summary table.

1. Basic Definitions

Substitution Reaction

In a substitution reaction, one atom or group in a molecule is replaced by another atom or group.

General example: R–X + Nu- → R–Nu + X-

Elimination Reaction

In an elimination reaction, two atoms or groups are removed from adjacent carbon atoms to form a double bond (an alkene).

General example: R–CH2–CH2–X → Alkene + HX

2. Types

• Substitution: SN1 (unimolecular), SN2 (bimolecular)
• Elimination: E1 (unimolecular), E2 (bimolecular)

3. Reaction Mechanisms

SN1 (Substitution, Unimolecular)

Two-step: (1) leaving group leaves forming a carbocation, (2) nucleophile attacks carbocation. Rate depends on substrate concentration.

SN2 (Substitution, Bimolecular)

One-step concerted attack by nucleophile from the back-side → inversion of configuration. Rate depends on both substrate and nucleophile.

E1 (Elimination, Unimolecular)

Two-step: formation of carbocation (same intermediate as SN1), then base removes a proton to give an alkene. Competes with SN1.

E2 (Elimination, Bimolecular)

One-step concerted removal of β-hydrogen by a base while leaving group leaves. Requires anti-periplanar geometry for the eliminated hydrogen and leaving group.

4. Nature of Substrate

Substrate vs Favored Pathway

Substrate	Substitution	Elimination
Primary	SN2 favored	E2 if strong base
Secondary	SN1/SN2 mixture	E2 strongly favored (with strong base)
Tertiary	SN1 favored	E1/E2 both possible (E2 with strong base)

5. Role of Nucleophile / Base

Substitution reactions require a nucleophile (Nu^-) such as OH-, CN-, I-. Strong nucleophiles favor SN2.

Elimination reactions require a base (B^-) such as OH-, OR-, t-BuO-. Strong, especially bulky, bases favor E2 and can lead to Hoffmann product.

6. Temperature Effect

Low temperature generally favors substitution, while high temperature favors elimination because elimination often produces more molecules (increased entropy).

7. Major Products

Substitution: new substituted compound; stereochemical consequences: SN2 → inversion, SN1 → racemization.

Elimination: alkene formed. Zaitsev's rule: the more substituted alkene is generally the major product; bulky bases give Hofmann product.

8. Solvents

• Polar protic solvents (e.g., water, alcohols) favor SN1 and E1 (stabilize carbocations).
• Polar aprotic solvents (e.g., DMSO, acetone) favor SN2 (they do not strongly solvate nucleophiles).

9. Competition Between Substitution and Elimination

Often the same substrate and reagent mixture can undergo both substitution and elimination. General trends:

• Strong base + high temperature → elimination dominates.
• Strong nucleophile + low temperature → substitution dominates.
• Bulky base (e.g., t-BuO^-) → elimination (Hofmann product).

Quick Summary Table

Feature	Substitution	Elimination
What happens?	Group replaced	Groups removed to form double bond
Typical product	Substituted compound	Alkene
Requires	Nucleophile	Strong base
Favored by	Low temp	High temp
Mechanisms	SN1 / SN2	E1 / E2
Stereochemistry	Inversion (SN2), racemization (SN1)	Anti-periplanar requirement (E2); product stereochemistry depends on alkene geometry

10. Simple Examples

SN2 example: CH3CH2Br + CN- → CH3CH2CN + Br-

E2 example: CH3CH2CH2Br + KOH (alc.) → CH3CH=CH2 + KBr + H2O

Study Tips

1. Practice mechanism arrows for SN1, SN2, E1, and E2 to understand intermediates and transition states.
2. Memorize how substrate structure (primary/secondary/tertiary) biases the pathway.
3. Use temperature and base/nucleophile strength to predict the major outcome when routes compete.

Conclusion

Substitution and elimination are often competing reactions. By examining substrate structure, nucleophile/base strength, solvent, and temperature, you can predict which pathway will dominate. Understanding the detailed mechanisms (SN1 vs SN2 and E1 vs E2) helps in predicting stereochemistry and products.

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