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Alkyl Halides

Substitution versus Elimination Reactions

Since SN1, SN2, E1, and E2 reactions all depend on having a leaving group, the reactions are always in competition with each other, and reactions will often form more than one product based on the competing reactions. Substrate, reagents, solvent, and temperature can affect the distribution of products.

Elimination (E1 and E2) reactions and nucleophilic (SN1 and SN2) substitution are two types of reactions. The kinetics of elimination and substitution reactions both depend on a leaving group. Since the two sets of reactions depend on the same thing, there is always competition between the two reactions, and often, a reaction will yield a mixture of products. The distribution and type of products (substitution versus elimination) will depend on factors such as substrate, reagent, solvent, and temperature.

In general, substitution reactions predominate over elimination reactions (but this is only a general trend). Elimination reactions are favored over substitution during the following conditions:

  • Steric hindrance around the α\alpha-carbon increases.
  • Stronger base is used.
  • Temperature increases (increase entropy).
  • Base is a very weak nucleophile.

Methyl leaving groups only undergo SN2 reactions. Primary leaving groups undergo SN2 reactions with anything but a strong, sterically hindered base (KOtBu), which instead undergo E2 eliminations. For primary alkyl halides, SN1 and E1 reactions do not occur, because carbocations are too unstable.

Secondary leaving groups undergo SN1, SN2, E1, and E2 reactions, depending on the solvent and reagent. Tertiary leaving groups undergo SN1, E1, and E2 reactions, depending on the solvent and reagent. SN2 reactions do not occur in tertiary compounds, because tertiary leaving groups are too sterically hindered for backside attack.

Reagents that are only nucleophilic will only undergo SN1 or SN2 reactions. Basic reagents will only undergo E1 or E2 reactions. Other reagents can act as either a nucleophile or a base, depending on other conditions, such as substrate, solvent, and temperature.

SN2 reactions are favored with high concentrations of nucleophiles at room temperature. E2 reactions are favored with a strong base at high temperature. However, SN2 and E2 are usually in competition with each other. Low concentrations of a weak nucleophile with tertiary or allyl/benzyl leaving groups favor SN1 reactions. E1 reactions occur with a very weak base or solvent only and high temperature.

Solvents can affect all substrates but are especially important for substitution reactions with secondary leaving groups. Polar protic solvents lead to SN1 reactions with secondary leaving groups. Polar aprotic solvents lead to SN2 reactions with secondary leaving groups. Eliminations are run almost exclusively (but not always) in polar protic solvents.

Increasing the temperature increases the ratio of elimination to substitution products but will depend on substrate, reagent, and solvent.

Comparison of Substitution and Elimination Reactions

Leaving Group Substrate SN1 SN2 E1 E2
Methyl Never Yes Never Never
Primary Never Yes, nucleophiles and bases (except KOtBu) Never Only with sterically hindered bases (KOtBu)
Secondary Yes, with nucleophilic reagents and polar protic solvents Yes, with nucleophilic reagents and polar aprotic solvents Usually not, unless weak base and heat Yes, with strong bases
Tertiary Yes, with nucleophilic reagents or solvolysis, low heat Never Yes, with very weak bases or solvent and heat Yes, with strong bases
Regiochemistry No effect No effect Usually Zaitsev Hofmann with sterically hindered base (KOtBu); Zaitsev with all other bases
Stereochemistry Racemic Inversion of configuration Usually, no effect Usually, no effect

Determining the distribution of products of competing substitution and elimination reactions is related to the leaving group substrate. The regiochemistry and stereochemistry of the products will be determined by the type of mechanism through which the reaction occurs.