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SN1 - First-order Nucleophilic Substitution

Mechanism of the SN1 Reaction

The concept of the SN1 mechanism is due to the results of countless experiments, such as the following:

1. According to the kinetics of SN1 reactions, their rate depends only on the alkyl halide's (substrate's) concentration: Reaction rate = k [RX].

2. The rate of the reaction between alkyl bromides and water decreases according to the degree of substitution of the carbon that carries a bromine atom. SN2 reactions proceed particularly slowly either if the substrate is sterically-demanding (that is, the attack of the nucleophile is prevented) or the nucelophile is electrically neutral, or if the solvent is protic. It might therefore be expected that a tertiary alkyl halide may react only very slowly with water (nucleophile and solvent). However, the reaction of 2-bromo-2-methylpropane with water, which yields 2-methylpropan-2-ol, is more than one million times faster than the corresponding reaction of methyl bromide that results in methanol.

Fig.1
Reactivity of alkyl bromides.
Tab.1
Fig.2
Fig.3
Fig.4
Fig.5
Relative reactivity <1 1 12 1 200 000
Tab.2
less reactive
Fig.6
more reactive

3. SN1 reactions that contain a chiral substrate whose electrophilic carbon is asymmetrically substituted yield a racemate. Thus, the optical activity of an enantiomerically pure chiral reagent perishes during an SN1 reaction.

A reaction mechanism of the monomolecular nucleophilic substitution containing two individual steps may be derived from the experimental results previously mentioned above:

In the first reaction step, the carbon-ligand bond is heterolytically cleaved, whereat the bonding electron pair is completely passed on to the ligand. The formation of the intermediate carbocation, through which the reaction rate is determined, requires a high amount of activation energy. Thus, an energy-rich transition state is also apparent here. Only the substrate participates in this individual step, while the nucleophile or the nucleophile's concentration is irrelevant.

The nucleophile rapidly attacks the carbocation in the second reaction step. Though two particles are involved in this individual step, it is much more rapid than the formation of the carbocation.

Fig.7
Reaction mechanism of the SN1 reaction.
Fig.8
Molecular models of a carbocation and its molecular orbitals.

The SN1 reaction energy diagram illustrates the dominant part of the substrate with respect to the reaction rate. The rate-determining step is the formation of the intermediate carbocation, or carbenium ion.

Fig.9
Reaction energy diagram of an SN1 reaction.

The experimental results corroborate the SN1 mechanism formerly proposed above:

  1. The substrate is the only particle that participates in the rate-determining step. The nucleophile and its concentration are irrelevant.
  2. A trigonal planar shape is characteristic of carbocations, as they are sp2-hybridized. In the second individual step of an SN1 reaction with a chiral substrate that contains an asymmetric substituted electrophilic carbon, the nucleophile may attack both enantiotopic sides with same probability. In consequence, the reaction yields a racemic product.
Fig.10
Attack of the nucleophile on the sp2-hybridized carbocation.

Exercise

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