SN 2 reactions take place with inversion of configuration, and SN 1 reactions take place with ra

SN 2 reactions take place with inversion of configuration,...

Key Concepts

Double Inversion Leading to Retention of Configuration

When a reaction involves neighboring group participation, the formation of a cyclic intermediate and its subsequent opening by a nucleophile can occur with two sequential inversion steps. The first inversion occurs when the internal nucleophile forms the cyclic intermediate, and the second inversion takes place when this intermediate is opened by the external nucleophile, resulting in an overall retention of the original stereochemical configuration.

Anchimeric Assistance (Neighboring Group Participation)

Anchimeric assistance occurs when a neighboring group in the molecule, such as an oxygen, nitrogen, or a ?-system, temporarily forms a bond or cyclic intermediate with the reaction center after the leaving group departs. This participation can stabilize the intermediate and influence the reaction pathway, often leading to unusual stereochemical outcomes.

SN1 Mechanism

The SN1 mechanism involves a two-step process where the leaving group first departs, forming a planar carbocation intermediate. This intermediate can be attacked from either side by the nucleophile, generally leading to racemization rather than retention of the original configuration.

SN2 Mechanism

In an SN2 reaction, the nucleophile attacks the electrophilic center in a single concerted step, typically from the side opposite to the leaving group, causing an inversion of configuration. This mechanism inherently does not lead to retention of configuration.

Transcript

00:01 So we are given the following substitution reaction, and we want to propose an arrow -pushing mechanism to explain why we have retention of configuration, since we know that s &2 reactions have inversion of configuration, and s -n -1 reactions have rastomization.

00:15 So how is it that we're having a substitution reaction that has retention of configuration? so i'm going to go ahead and propose an arrow -pushing mechanism for us.

00:24 So we see that our first step here is the sodium hydroxide in the presence of water, and so what's going to happen here is that i'm just going to go ahead and draw our alcohol group like this.

00:38 So we're going to have our hydroxide ions.

00:42 They're going to come in and they're going to deprotonate this hydrogen, since it is the most acidic hydrogen.

00:49 And then the electrons are going to get pushed onto the oxygen.

00:53 And this is going to give us the following.

01:04 So we have the formation of an oxygen ion here.

01:07 And this is pretty cool because we have, we're going to have our oxygen ion.

01:22 It's going to come in, and it's going to attack the same molecule here, but it's going to attack at the bromide, the carbon connected to the bromine.

01:31 So what we see here is a nucleophile and a leaving group within the same molecule, which is pretty neat.

01:41 We're going to have attack here.

01:44 Bromide is going to leave.

01:45 And since this is kind of an sn2 -like reaction, we are going to have inversion of configuration, but we're also going to have the formation of a ring.

01:55 It's actually going to look like an alpha lactone.

01:58 So we're going to say that an s &2 reaction has occurred here, and our resulting product is going to look like this.

02:17 So whereas this particular hydrogen was coming towards us, now it was going away from us, and now it is coming towards us.

02:24 And then we have the oxygen here.

02:28 It's going away from us.

02:35 This is pretty neat.

02:36 So this is an alpha lactone...

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