Unimolecular nucleophilic

Illustrate the stereochemistry associated with unimolecular nucleophilic substitution by con structmg molecular models of cis 4 tert butylcyclohexyl bromide its derived carbocation and the alcohols formed from it by hydrolysis under S l conditions... 

Section 4.9 The potential energy diagrams for separate elementary steps can be merged into a diagram for the overall process. The diagram for the reaction of a secondary or tertiary alcohol with a hydrogen halide is characterized by two intermediates and three transition states. The reaction is classified as a unimolecular- nucleophilic substitution, abbreviated as SnI. 

The noncatalytic substitution of a 4-diazonium group by nucleophiles, which has proved widely useful in this series, is probably an example of a unimolecular nucleophilic substitution. The first example... 

LIMITING UNIMOLECULAR NUCLEOPHILIC REACTIONS. KINETICS AND STEREOCHEMISTRY1... 

Because the slowest step of this reaction only involves t-butyl bromide, the overall rate of reaction only depends on the concentration of this species. This is therefore a unimolecular nucleophilic substitution, or SN1, reaction. 

Apart from overcoming coulombic repulsions, 8 2 reactions also proceed with inversion in the face of steric hindrance. By comparison, the stereochemical result of unimolecular nucleophilic substitution SN1 is variable. In fact, nucleophilic substitutions at carbon with retention invariably follow other than SN2 paths. In its broad outlines, the Hughes-Ingold approach swept away the confusions of the period 1895-1933 and has not ceased to stimulate and provoke ideas in the area of substitution reactions. Surprisingly enough, the theoretical foundations of the SN2 process require reexamination and modification, as we shall see. 

The gas-phase base-induced elimination reaction of halonium ions was thoroughly investigated in radiolytic experiments22. Radiolytically generated acids C/JH5+ (n = 1,2) were allowed to react at 760 Torr with selected 2,3-dihalobutanes to form the halonium intermediates which, in the presence of trimethylamine, undergo base-induced bimolecu-lar elimination as shown in Scheme 6. This elimination reaction occurs in competition with unimolecular nucleophilic displacement to the cyclic halonium ion and subsequent rearrangement. Isolation and identification of the neutral haloalkenes formed and kinetic treatment of the experimental results indicated that 3-halo-1 -butene is formed preferentially with respect to the isomeric 2-halo-2-butenes and that the bimolecular elimination process occurs predominantly via a transition state with an anti configuration22. 

The fact that the rate law depends only on the concentration of tert-butyl chloride means that only tert-butyl chloride is present in the transition state that determines the rate of the reaction. There must be more than one step in the mechanism because the acetate ion must not be involved until after the step with this transition state. Because only one molecule pert-butyl chloride) is present in the step involving the transition state that determines the rate of the reaction, this step is said to be unimolecular. The reaction is therefore described as a unimolecular nucleophilic substitution reaction, or an SN1 reaction. 

Sisl reaction or unimolecular nucleophilic substitution reaction (Section 8.6) A reaction in which the nucleophile replaces the leaving group at an sp3-hybridized carbon in a two-step mechanism that proceeds through a carbocation intermediate. 

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