Relative Reactivity of Alkyl Halides
Relative Reactivity of Alkyl Halides
Introduction
Nucleophilic substitution of alkyl halides can proceed by two different mechanisms – the SN2 and the SN1. The purpose of the experiment was to identify the effects that the alkyl group and the halide-leaving group have on the rates of SN1 reactions, and the effect that the solvent has on the rates of SN1 and SN2 reactions. The SN1 mechanism is a two-step nucleophilic substitution, or unimolecular displacement. In the first step of the mechanism, the carbon-halogen bond breaks and the halide ion leaving group leaves in a slow, rate-determining step to form a carbocation intermediate. The carbocation intermediate is then immediately detained by the weak nucleophile in a fast, second step to give the product. A solution of ethanol with some silver nitrate may be added provided the weak nucleophile – the alcohol. If an SN1 reaction occurs, the alkyl halide will dissociate to form a carbocation, which will then react with the ethanol to form an ether. Since there is not a strong nucleophile present, the cleavage of the carbon-halogen bond is encouraged by the formation and precipitation of silver bromide. The halide ion will combine with a silver ion from the silver nitrate to form a silver halide precipitate, which will advise that a reaction has occurred. + AgBr + NO3-
Figure 1: The SN1 mechanism of 2-bromo-2-methylpropane and silver nitrate. The nucleophile would have been ethanol while the silver nitrate would have disassociated to form a silver halide precipitate.
The more stable the carbocation, the quicker the reaction. Therefore, SN1 reactions desire tertiary substrates most, followed by secondary, and lastly primary. Because the strength of the nucleophile is unimportant, an ionizing solvent is needed. Water is the best solvent, followed by methanol, ethanol, propanol, and lastly acetone. In experiment two, the tertiary 2-bromo-2-methylpropane was the most favored reactant followed by the secondary
Introduction
Nucleophilic substitution of alkyl halides can proceed by two different mechanisms – the SN2 and the SN1. The purpose of the experiment was to identify the effects that the alkyl group and the halide-leaving group have on the rates of SN1 reactions, and the effect that the solvent has on the rates of SN1 and SN2 reactions. The SN1 mechanism is a two-step nucleophilic substitution, or unimolecular displacement. In the first step of the mechanism, the carbon-halogen bond breaks and the halide ion leaving group leaves in a slow, rate-determining step to form a carbocation intermediate. The carbocation intermediate is then immediately detained by the weak nucleophile in a fast, second step to give the product. A solution of ethanol with some silver nitrate may be added provided the weak nucleophile – the alcohol. If an SN1 reaction occurs, the alkyl halide will dissociate to form a carbocation, which will then react with the ethanol to form an ether. Since there is not a strong nucleophile present, the cleavage of the carbon-halogen bond is encouraged by the formation and precipitation of silver bromide. The halide ion will combine with a silver ion from the silver nitrate to form a silver halide precipitate, which will advise that a reaction has occurred. + AgBr + NO3-
Figure 1: The SN1 mechanism of 2-bromo-2-methylpropane and silver nitrate. The nucleophile would have been ethanol while the silver nitrate would have disassociated to form a silver halide precipitate.
The more stable the carbocation, the quicker the reaction. Therefore, SN1 reactions desire tertiary substrates most, followed by secondary, and lastly primary. Because the strength of the nucleophile is unimportant, an ionizing solvent is needed. Water is the best solvent, followed by methanol, ethanol, propanol, and lastly acetone. In experiment two, the tertiary 2-bromo-2-methylpropane was the most favored reactant followed by the secondary