W4L10: Unimolecular nucleophilic substitution SN1

Nucleophilic substitutions

In any case the electrophile must be hybridised sp3

For any nucleophilic substitution we need to keep into consideration:

• Structure of the reagent (halogeno-alkanes are very common)

• Type of solvent (polar aprotic, polar protic…)

• Strength of the nucleophile

• Number of molecules involved in the transition state (Rate of the reaction)

• Leaving group (LG)

• Stereochemistry of the substitution

Strength of the nucleophile influencing SN1

• Since nucleophilic addition occurs after formation of carbocation, reaction rate is not normally affected by nature or concentration of nucleophile

• In general, though, if there is competition between SN1 and SN2, strong nucleophiles tends to favour the SN2 mechanism, weak nucleophiles SN1

Structure of the reagent influencing SN1

Stability of carbocation influencing SN1

• The order of carbocation stability can be rationalized through inductive effects and hyperconjugation

• Hyperconjugation is the spreading out of charge by the overlap of an empty p orbital with an adjacent pi bond. This overlap (hyperconjugation) delocalizes the positive charge on the carbocation, spreading it over a larger volume, and this stabilizes the carbocation

• Inductive effects are electronic effects that occur through bonds. Specifically, the inductive effect represents the influence of the electron density through bonds caused by electronegativity of the connecting group.

• In general, the greater the number of alkyl groups attached to a carbon with a positive charge, the more stable will be the cation

Solvent influencing SN1

• The Hammond postulate relates reaction rate to stability. It provides a quantitative estimate of the energy of a transition state.

• The Hammond postulate states that the transition state of a reaction resembles the structure of the species (reactant or product) to which it is closer in energy.

• Polar Protic solvents favour the SN1 reaction. The formation of hydrogen bonding between the solvent and the carbocation stabilizes the intermediate.

• Consequence of the Hammond postulate: Any factor that stabilizes a high-energy intermediate stabilizes the transition state leading to that intermediate

Number of reactants influencing SN1

• The kinetics suggest that only the electrophile is involved in the formation of the transition state. The rate-determining step, is represented by the initial formation of the carbocation.

• The Nu attacks the E only after the LG bond is broken

• This means that increasing the concentration of the Nu will not influence the rate of reaction

• This means that via SN1 it is only possible to obtain a racemic mixture of the two products

LG influencing SN1

• Critically dependent on leaving group

• Reactivity: the larger halides ions are better leaving groups

• In acid, -OH of an alcohol is protonated and leaving group is H2O, which is still less reactive than halides

SN1 or SN2