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
Try
http://en.wikipedia.org/wiki/SN1_reaction
and
http://en.wikipedia.org/wiki/SN2_reaction
For number 2 you just have to make a graph of ln(t) in your x axis and 1-T as your y-axis where the T is temperature in Kelvins. You could get a "linear" equation from the graph (y=mx+b) so you take the M and set it eqaul to (Ea)/R....and R is 8.314jouls/mole.K and solve for Ea
1. Factors affecting the relative reactivities of Sn1 and Sn2 reactions:
a) Nature of the carbon skeleton: The reactivity of Sn1 and Sn2 reactions depends on the nature of the carbon skeleton. In Sn2 reactions, the nucleophile directly replaces the leaving group at the same time. Therefore, the more substituted (i.e., tertiary) the carbon atom, the more hindered the reaction becomes. As a result, Sn2 reactions are more favorable with primary carbon atoms, where steric hindrance is minimal.
In contrast, Sn1 reactions involve the formation of a carbocation intermediate. The stability of the carbocation plays a crucial role in determining the reactivity. Tertiary carbocations are more stable than secondary and primary carbocations due to increased hyperconjugation and inductive effects. Therefore, Sn1 reactions are more favorable with tertiary carbon atoms.
b) Steric hindrance: Steric hindrance refers to the presence of bulky groups around the reacting carbon atom. The bulkier the groups, the more hindered the reaction becomes. In Sn2 reactions, strong steric hindrance can prevent the nucleophile from attacking the carbon atom effectively. Hence, reactions involving hindered primary or tertiary carbons are slower in Sn2 reactions. On the other hand, Sn1 reactions are less affected by steric hindrance, as the nucleophile attacks the carbocation from the backside, avoiding most of the steric interactions.
c) Nature of the leaving groups: The leaving group is the atom or group of atoms that depart from a molecule, generating a positive charge. Good leaving groups facilitate the Sn1 and Sn2 reactions. Examples of good leaving groups include bromide (Br-), chloride (Cl-), iodide (I-), and tosylate (TsO-). In general, better leaving groups are more stable and capable of accepting negative charge.
d) Primary and tertiary vs. allylic and benzylic: In both Sn1 and Sn2 reactions, primary carbon atoms are typically more reactive than tertiary carbon atoms due to the reasons mentioned earlier. However, allylic and benzylic carbocations are stabilized by resonance, making them more stable than their alkyl counterparts. This increased stability results in faster Sn1 reactions compared to primary alkyl carbocations.
e) Aliphatic halides vs. alkenyl halides vs. aromatic halides: The nature of the halide attachment also affects the reactivity of Sn1 and Sn2 reactions. Aliphatic halides, which are attached to alkanes, generally undergo both Sn1 and Sn2 reactions. Alkenyl halides, attached to alkenes, primarily undergo Sn2 reactions. Aromatic halides, attached to aromatic rings, are less reactive and undergo Sn2 reactions under specific conditions.
2. The validity of plotting log k versus 1/T to produce a straight line with a slope equaling positive Ea/2.3R is questionable. The slope of a plot of log k versus 1/T is typically equal to negative Ea/2.3R and is derived from the Arrhenius equation, where 'Ea' represents the activation energy and 'R' is the gas constant.
If we observe a straight line with a slope equal to positive Ea/2.3R, it suggests that the rate constant ('k') decreases as temperature ('T') increases, which is against the normal behavior observed in chemical reactions. Normally, reactions proceed faster as the temperature increases due to increased kinetic energy of the reacting species.
A positive slope in the plot of log k versus 1/T suggests an erroneous result or an inconsistent experimental setup. In such cases, it is necessary to reevaluate the experimental conditions, ensure accurate measurement of rate constants, and verify the reaction mechanism before coming to conclusions.