Tell whether each of the following reactions is likely to be SN 1, SN 2, E 1, E 1 c B, or E2: (a

Tell whether each of the following reactions is likely to be...

Key Concepts

E2 Reaction Mechanism

The E2 mechanism is a single-step, concerted elimination reaction where a strong base abstracts a proton while the leaving group departs simultaneously, leading directly to the formation of a double bond. The E2 pathway is bimolecular and its rate depends on both the substrate and the base concentrations. It generally requires a strong base and is favored in situations where the geometry of the substrate allows for an anti-coplanar arrangement between the leaving group and the proton.

E1cB Reaction Mechanism

The E1cB mechanism involves a two-step elimination where the first step is the formation of a carbanion intermediate by the removal of a proton, followed by the expulsion of the leaving group. This mechanism is characterized by the stabilization of the carbanion, often through resonance or the presence of electron-withdrawing groups, and is distinct from the traditional E1 path that proceeds via a carbocation intermediate.

E1 Reaction Mechanism

The E1 mechanism is a two-step unimolecular elimination process that begins with the departure of the leaving group to form a carbocation intermediate, followed by the loss of a proton to generate an alkene. It shares the initial step with the SN1 mechanism and is likewise favored by substrates that can form stable carbocations, often observed in polar protic solvents and under conditions with a weak base.

SN2 Reaction Mechanism

The SN2 mechanism is a single-step reaction where the nucleophile attacks the substrate from the opposite side of the leaving group, leading to a concerted displacement. This bimolecular process occurs simultaneously with bond formation and bond cleavage, resulting in second-order kinetics. The SN2 mechanism is typically favored by primary or secondary substrates and occurs best in polar aprotic solvents that enhance the nucleophile’s reactivity.

SN1 Reaction Mechanism

The SN1 mechanism is a two-step nucleophilic substitution process in which the leaving group departs first to form a carbocation intermediate, followed by the rapid attack of the nucleophile. This mechanism is characterized by first-order kinetics, where the rate-determining step depends only on the substrate concentration, and it tends to be favored with tertiary substrates and polar protic solvents which stabilize the carbocation intermediate.

Transcript

00:01 So we have different reactions involving alkaliads, and we want to know, given certain reaction conditions, which substitution or elimination reaction is more likely to occur.

00:12 So we have to remember that there are generally four major characteristics to substitution elimination reactions that we have to analyze when trying to predict products for those.

00:19 The first characteristic is the substrate.

00:22 The second characteristic is the leaving group.

00:24 The third characteristic is a nucleophile, and the fourth characteristic is the solvent.

00:30 So let's go ahead and look at an example.

00:33 We have example a here.

00:36 And so we're going to go ahead and analyze each characteristic.

00:40 So here we have a primary alkohalide.

00:44 We have to remember that primary alkyol halides tend to favor sn2 reactions.

00:49 And we can further analyze.

00:51 We see that bromine is a good leaving group.

00:54 We have an azide as our nucleophile, which is a very strong nucleophile.

00:59 And we also have thf, which is known as a polar a -protech solvent.

01:03 So given all these characteristics, this is generally describes an sn2 reaction, and this is confirmed by seeing the product on the right side there.

01:16 Now for example b, we're given this.

01:20 Looks like we have a secondary alkali halide, and we have chlorine, which is a great leaving group, and we also have potassium hydroxide, which is a strong base, chance to favor elimination reactions, and we also have a polar protein.

01:39 Solvent.

01:41 So given all the reaction conditions here, it looks like we have an e2 elimination reaction, given that we have a strong base...

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