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SN1 - First-order Nucleophilic Substitution

Influence of the Solvent on SN1 Reactions

The solvent plays a great role in SN1 reactions. However, in contrast to SN2 reactions, it is not the stabilization of the nucleophile, but the stabilization of the first reaction step's transition state and the stability of the intermediate that influence the reaction rate, as the nucleophile does not participate in the rate-determining step. Due to its ionic structure or polar character, the intermediate and transition state can be effectively stabilized by polar and protic solvents. As a result of the transition state's stabilization by a polar (or protic) solvent, the activation energy is decreased, as the non-ionic and comparatively less polar starting product is stabilized by the solvent to a lesser degree.

Animation

Fig.1
Fig.2
Stabilization of a tertiary alkyl bromide's transition state by the solvent water in an SN1 reaction.
Fig.3
Stabilization of the intermediate carbocation and bromide anion by the solvent water in an SN1 reaction.

The ability of solvents to stabilize ions through solvation is directly associated with their polarity. Polar solvents, such as water, methanol, and dimethyl sulphoxide, can effectively stabilize ions through solvation, while non-polar solvents, such as ether and hydrocarbons, cannot. The dielectric constant is a measure of a solvent's polarity. The higher the dielectric constant of a solvent is, the higher its polarity also is and, thus, cations and anions may be separated by the solvent molecules more efficiently.

Protic (organic) solvents possess acidic hydrogens. These are usually bound to oxygen or nitrogen. In hydrogen bridge bonds, the acidic hydrogen acts as an acceptor of electron density, while the heteroatom (usually oxygen or nitrogen) acts as a donator, as the hydrogen-heteroatom bond is polarized due to the electronegativity difference of the atoms in question. Thus, this type of solvent is able to stabilize not only positively charged, but also negatively charged, ions and molecules.

Tab.1
Dielectric constants (ε, at 25°C) of some common solvents.
Aprotic solventsεProtic solventsε
Hexane 1.9 Acetic acid 6.2
Benzene 2.3 1-Methyl-2-propanol 11
Diethyl ether 4.3 Ethanol 34.3
Chloroform 4.8 Methanol 33.6
Hexamethylphosphoramide (HMPT) 30 Formic acid 58.0
Dimethyl formamide (DMF) 38 Water 80.4
Dimethyl sulfoxide (DMSO) 48    

Due to the points made above, SN1 reactions proceed much more rapidly in polar than in non-polar solvents. The nucleophilic substitution of 2-chloro-2-methylpropane in water, for instance, runs 100,000 times faster than that in ethanol.

Fig.4
Influence of the solvent on SN1 reactions of 2-chloro-2-methylpropane.

SN1 reactions proceed more rapidly in protic, polar solvents because such solvent molecules stabilize the (rate-determining) transition state and the intermediate carbocation. That is, the transition state is lower in energy than in aprotic, non-polar solvents, while the energy of the starting product is hardly influenced by the solvent. As a result, the activation energy in protic, polar solvents is lower than that in aprotic, non-polar solvents.

Transition state

Exercises

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