1. Explain the effects of the following factors on the relative reactivities of Sn1 and Sn2 reaction
a.Nature of the carbon skeleton(primary, secondary, tertiary)
b.Steric hindrance (primary vs hindered primary)
c)Nature of the leaving groups
d)Primary and tertiary vs allylic and benzylic
e)Aliphatic halides vs. alkenyl halide vs. aromatic halide
2. A plot of log k 1/T give s a straight line with the slope equaling to negative Ea/2 3R. Discuss the validity of plotting log t versus 1/T to produce a straight line with a slope equaling to positive Ea/2.3R
1. When considering the relative reactivities of Sn1 and Sn2 reactions, several factors come into play:
a) Nature of the carbon skeleton (primary, secondary, tertiary): In general, Sn2 reactions are favored on primary carbons due to the absence of steric hindrance, allowing the nucleophile to attack the carbon atom directly. In contrast, Sn1 reactions are favored on tertiary carbons because the carbocation intermediate generated during the reaction is more stable due to the presence of neighboring alkyl groups that can stabilize the positive charge.
b) Steric hindrance (primary vs hindered primary): Steric hindrance refers to the physical congestion or blockage around a carbon atom due to neighboring groups or large substituents. In Sn2 reactions, hindered primary carbons with bulky substituents may experience steric hindrance, making the nucleophile's attack more difficult. As a result, Sn2 reactions are less favored on hindered primary carbons. On the other hand, Sn1 reactions are less affected by steric hindrance since the nucleophile does not directly attack the carbon atom.
c) Nature of the leaving groups: The nature of the leaving group can significantly influence the reactivity of both Sn1 and Sn2 reactions. Good leaving groups, such as those that can stabilize the negative charge, enhance the rate of Sn2 reactions. Examples of good leaving groups include halides like chloride or bromide. In Sn1 reactions, leaving group ability is less critical since the rate-determining step is the formation of a carbocation, not the leaving group's departure.
d) Primary and tertiary vs allylic and benzylic: Allylic and benzylic carbons refer to carbon atoms directly bonded to a carbon-carbon double bond or benzene ring, respectively. These types of carbons possess resonance stabilization, making them more reactive in both Sn1 and Sn2 reactions compared to primary or tertiary carbons. This resonance stabilization lowers the energy barrier for the reaction to occur.
e) Aliphatic halides vs. alkenyl halide vs. aromatic halide: The nature of the halide group bonded to the carbon atom also affects the reactivity of Sn1 and Sn2 reactions. Aliphatic halides (alkyl halides) can undergo both Sn1 and Sn2 reactions, depending on the conditions. Alkenyl halides (halides attached to a carbon-carbon double bond) generally prefer Sn2 reactions due to the electron density of the double bond, which facilitates nucleophilic attack. Aromatic halides, which are halides bonded to an aromatic ring, typically undergo Sn1 reactions due to the stabilization of the carbocation intermediate by the aromatic system.
2. The given plot of log k vs. 1/T is commonly used in chemical kinetics to determine the activation energy (Ea) of a reaction. The slope of the plot is related to Ea through the equation: slope = -Ea / 2.3R, where R is the gas constant.
If the plot of log t (instead of log k) vs. 1/T is considered, it is important to understand that time (t) is inversely related to the rate constant (k). As a result, the slope of the plot would change to a positive value, equal to Ea / 2.3R, rather than the negative slope observed in the original plot.
The validity of plotting log t vs. 1/T depends on the specific reaction and the rate law being studied. In some cases, it may be more appropriate to plot log t vs. 1/T to obtain a straight line and determine the activation energy using the positive slope. However, it is important to note that this approach assumes a specific relationship between time and rate, typically represented by the rate equation.
It is recommended to consult the specific experimental conditions and the rate law governing the reaction to determine the appropriate plotting variables and interpretation of the resulting slope.