SN1 and SN2

The first set of reactions and mechanisms that are commonly taught in an organic chemistry course are the substitution and elimination reactions. Each of these can go by either a one-step (SN2 or E2) or two-step mechanism (SN1 or E1). So overall, there are four possible mechanisms (SN1, SN2, E1, or E2) as well as combinations of mechanisms.

The difficulty is that all four mechanisms have exactly the same reactants: an alkyl halide and a nucleophile/base. Therefore, deciding which of the four mechanisms or combination of mechanisms can be very confusing.

Nucleophilic substitution is the reaction of an electron pair donor (the nucleophile, Nu) with an electron pair acceptor (the electrophile). An sp3-hybridized electrophile must have a leaving group (X) in order for the reaction to take place.

In both SN1 and SN2, the nucleophile competes with the leaving group. Because of this, one must realize what properties a leaving group should have, and what constitutes a good nucleophile. For this reason, it is worthwhile to know which factors will determine whether a reaction follows an SN1 or SN2 pathway.

Halides make good leaving groups (Cl, Br, I) so we will focus on the alkyl halides in our examination of nucleophilic substitution. Other leaving groups and their relative effectiveness can be seen in the diagram below. You might be wondering why fluorine is not in the list with the other halides. The reason why is the same as why HF is not a strong acid. It is not just the polarity of the bond that determines reactivity. What is most important in determining reactivity is the strength of the bond and the C-F bond is very strong. As one proceeds from Cl to Br to I, the C-X bond becomes weaker so the rates of the alkyl halides can be ordered: Rates of reaction: RCl < RBr < RI

Good nucleophiles are those with a negative charge. Any increased stabilization of the nucleophile also increases the effectiveness of the nucleophile (it's nucelophilicity).

There are TWO reactants only. You must classify each as described above to help you decide which mechanism to use.

A. Classify the alkyl halide (R-X) as either: methyl, 1°, 2°, or 3°

B. Classify the nucleophile/base as either a strong or weak nucleophile, strong or weak base, and a bulky or not bulky base. Since nucleophilicity and basicity trends are related, there are only four possible combinations as indicated in the chart above.

C. Use the flowchart or table below to decide which mechanism or combination of mechanisms will be operative. The chart or the table should give the same answer.

If you examine the two figures above, it is obvious that SN1 is not that common. When it does occur with secondary and tertiary alkyl halides, it is in competition with E1.

For substitution reactions, the nucleophile is a key reactant. Its influence on the rate depends upon the mechanism of the substitution reaction.

SN2 mechanism -> the nucleophile strength strongly influences rate

SN1 mechanism -> the nucleophile strength does NOT influence rate

Therefore, we need to understand nucleophilicity trends (especially for the SN2 reaction.) In general, MORE STABLE = LESS REACTIVE (ie less nucleophilic)

First, stronger nucleophiles react faster than weak nucleophiles. See table above for relative strengths.

Second, there are trends going across a row of the period table (diagram below: examples (a) and (b)). The more electronegative an atom the less nucleophilic or basic it is because it wants to keep its electrons and not share them.

Trends going down the periodic table (see diagram below). Nucleophilicity increases as you go down a column in the periodic table but basicity decreases. This is the same trend as we saw above for nucleophilicity, where more electronegative atoms are less nucleophilic because they they hold on to their electrons more tightly.

Third, resonance stabilized anions are less basic and less nucleophilic than similar anions without resonance (example (c)).

Fourth, Size matters for SN2: Nucleophilicity is very sensitive to the size of the nucleophile. In contrast, basicity is not very sensitive to the size of the nucleophile (even a big base can pull off a protons that is sticking off of a molecule). Therefore, with a very strong and big (hindered) base/nucleophile will not be a good nucleophile but still can be a good base.

The rate laws for SN1 and SN2 are given in the table above labeled "key traits". From general chemistry, we know that 1st order reactions only have one reactant in the rate law. This is what the "1" stands for in SN1. In 2nd order reactions, there are either two reactants that are both first order OR there is one reactant in the rate law that is second order. The "2" in SN2 stands for the two reactants in the slow step, and, therefore, the rate law for SN2 has both the alkyl halide AND the nucleophile.

Examples:

1) Double the concentration of the nucleophile in an SN1 reaction: for an SN1 reaction, the rate only depends on the concentration of R-X ([R-X]) so doubling the nucleophile concentration will have no effect on the rate.

2) Double the concentration of the nucleophile in an SN2 reaction: for an SN2 reaction, the rate depends on the concentration of R-X ([R-X]) AND the nucleophile ([Nuc]) so doubling the nucleophile concentration will double the rate. The result would be the same if you doubled the [R-X] but left the [Nuc] constant.

3) Double both the [Nuc] and the [R-X] in SN1 and SN2: For SN1, rate would double (rate only depends on [R-X] but for SN2, rate would quadruple.

See links on E1/E2 smore for worksheets on SN and E

A good fill in worksheet for study and practice-with answers

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