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.
a) Nature of the carbon skeleton: The reactivity of Sn1 and Sn2 reactions is greatly affected by the nature of the carbon skeleton. Sn1 reactions typically occur faster with secondary or tertiary carbon skeletons, while Sn2 reactions are generally favored with primary carbon skeletons. This is because in Sn1 reactions, the stability of the resulting carbocation intermediate is crucial, and secondary or tertiary carbocations are more stable than primary carbocations due to increased hyperconjugation and inductive effects.
b) Steric hindrance: Steric hindrance refers to the presence of bulky groups near the reacting carbon center. In Sn2 reactions, steric hindrance adversely affects the reaction rate because it hinders the nucleophile from approaching the carbon center. Therefore, hindered primary carbon centers typically exhibit slower Sn2 reactions compared to primary carbon centers with less steric hindrance.
c) Nature of the leaving groups: The nature of the leaving group significantly affects the reaction rate of both Sn1 and Sn2 reactions. Good leaving groups are stable and have a propensity to dissociate, whereas poor leaving groups are less stable and have a higher tendency to remain associated with the carbon center. Generally, good leaving groups like halides (e.g., Cl, Br, I) and tosylate (OTs) favor faster reactions. However, the impact of leaving groups on Sn1 and Sn2 reactions can vary, and it is essential to consider other factors as well.
d) Primary and tertiary vs. allylic and benzylic: Primary and tertiary carbon centers are less reactive in Sn2 reactions due to steric hindrance and/or stability of the carbocation intermediate in Sn1 reactions, respectively. In contrast, allylic and benzylic carbon centers, which are adjacent to a double bond or an aromatic ring, exhibit increased reactivity compared to their non-allylic/non-benzylic counterparts. This enhanced reactivity is due to the resonance-stabilized allylic or benzylic carbocations that can form during the Sn1 reaction.
e) Aliphatic halides vs. alkenyl halide vs. aromatic halide: The reactivity of halides in Sn1 and Sn2 reactions depends on the nature of the halide group. Aliphatic halides (halides attached to non-aromatic carbon centers) generally undergo both Sn1 and Sn2 reactions. Alkenyl halides, which contain a double bond, exhibit poor reactivity in Sn2 reactions due to the limited access of the nucleophile to the carbon center. Aromatic halides, containing halogens attached to aromatic rings, typically undergo Sn1 reactions due to the stability of the intermediate carbocation formed.
2. The plot of log k (the rate constant) versus 1/T (the reciprocal of temperature) is commonly used to determine the activation energy (Ea) of a reaction using the Arrhenius equation. According to the Arrhenius equation, the slope of the plot is equal to -Ea/2.3R, where R is the gas constant.
If the plot of log k versus 1/T produces a straight line with a negative slope (-Ea/2.3R), it indicates that the activation energy is positive. This is because the natural logarithm of the rate constant decreases with decreasing temperature, which leads to a negative slope in the plot.
On the other hand, if the plot of log k versus 1/T produces a straight line with a positive slope (+Ea/2.3R), it would imply a negative activation energy, which is not physically meaningful. A negative activation energy implies that the reaction rate increases as the temperature decreases, which violates the basic principles of thermodynamics and kinetic theory.
Therefore, plotting log k versus 1/T to produce a straight line with a positive slope is not valid in determining the activation energy of a reaction. The negative slope obtained from the plot provides accurate information about the activation energy.