Relative Reactivity of Alkyl Halides

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Relative Reactivity of Alkyl Halides

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 2-bromobutane, the primary 1-bromobutane, and the primary 1-chlorobutane. This order is determined by whether the molecule is primary, secondary, or tertiary. 2-bromo-2-methlypropane + AgNO3 + (CH3)2CO AgBr + ethyl-t-butylether + isobutylene

Figure 2: The SN2 mechanism of 2-bromo-2-methlypropane with AgNO3 in (CH3)2CO. The SN2 reaction mechanism is a one-step, bimolecular displacement in which the bond-breaking and bond-making processes occur simultaneously. The SN2 reaction requires a strong nucleophile. The order of reactivity is the opposite of the SN1 reaction because the nucleophile must attack from the back, and is favored with the least steric hindrance. The halide attached to a primary carbon is easier to attack from the back. In experiment one, the 1-chlorobutane was the most favored reactant followed by the primary 1-bromobutane, the secondary 2-bromobutane, and the tertiary 2-bromo-2-methylpropane. This order is determined by whether the molecule is primary, secondary, or tertiary. “SN2 reactions are particularly sensitive to steric factors, since they are greatly retarded by steric hindrance (crowding) at the site of reaction. In general, the order of reactivity of alkyl halides in SN2 reactions is: methyl > 1° > 2°. The 3° alkyl halides are so crowded that they do not generally react by an SN2 mechanism.”1 1-chlorobutane and NaI-acetone ------> 1-iodobutane + NaCl (precipitate) In general, weaker bases make better leaving groups. SN1 and SN2 reactions show the same trends, but SN1 is more sensitive. The reactants favored in the SN2 mechanism are the opposite of the SN1 reaction. the primary 1-chlorobutane was most favored, followed by the primary 1-bromobutane, the secondary 2-bromobutane, and tertiary 2-bromo-2-methylpropane. Table 1: Table of Reagents with molecular weight, density, melting point, and boiling point. Name| Molecular Weight (g/mol)| Density (M/V)| Melting point (°C)| Boiling point (°C)| 2-bromo-2-methylpropane|...
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