Which alkyl halide would you expect to be more reactive in an SN 2 reaction with a given nucleop

Which alkyl halide would you expect to be more reactive in...

Key Concepts

Alkyl Halide Structure and Reactivity

The structure of an alkyl halide, particularly whether it is primary, secondary, or tertiary, is crucial in determining its reactivity in SN2 reactions. Because the SN2 mechanism requires unhindered access to the carbon that bears the leaving group, primary alkyl halides typically react faster than secondary ones, while tertiary alkyl halides are usually unreactive under SN2 conditions.

Steric Hindrance

Steric hindrance refers to the physical crowding around the reactive center that can impede the approach of the nucleophile in SN2 reactions. Alkyl halides with less steric bulk—such as primary halides—allow for easier nucleophilic attack, making them more reactive in SN2 processes compared to more hindered secondary or tertiary counterparts.

SN2 Reaction Mechanism

The SN2 reaction mechanism is a bimolecular nucleophilic substitution that occurs in a single, concerted step. In this process, the nucleophile approaches the electrophilic carbon from the opposite side of the leaving group, leading to a backside attack and inversion of stereochemistry at the reaction center.

Transcript

00:01 Okay, so for this reaction, you're given two different alkyal halides, and we're going to make the assumption that each of these alkyal halides is going to react with the same nucleophile to form our product.

00:18 So r and u plus x.

00:25 Another assumption that you want to make in this problem, which is told to you, is that assume that...

00:34 Alchial halides have the same stability.

00:37 So, for example, if you look at the second problem, you have three carbons versus two carbons.

00:44 You're going to assume that they have the same stability.

00:49 Same stability.

00:54 And for all of these problems, you're looking at them under the lens of an sn2 reaction.

01:01 And so for sn2 reactions, what you're most concerned with is the sterichendrance, or the lack thereof of your alkyl halide.

01:12 So your primary is going to be better than your secondary, which is going to be better than tertiary.

01:21 So with this in mind, just going to keep this off to the side, let's go through and look at these.

01:28 So in this first reaction, our difference comes in the identity of our leaving group.

01:35 So we know that a better leaving group is going to be the species, that is a weaker base.

01:50 So if we compare br minus versus i minus, our weaker base in the case, in this case is i minus.

02:00 So this species here is going to be more reactive in an sn2 reaction.

02:09 So i is a weaker base, and this is a stronger base.

02:19 Okay, let's look at our second reaction.

02:23 For this reaction, we want to look at how this oxygen is going to affect the ability of our c -cl bond to break.

02:36 So for example, let's just kind of summarize this c.

02:45 So in this case, we have a carbon bonded to a chlorine.

02:49 In this case, we have a carbon bonded to an oxygen, cl.

02:55 So we know that oxygen is more electron withdrawing.

02:58 So if you have, if you look at this bond here, if you pull electron density away from this carbon, that makes this carbon even more positive, which means it's more electrophilic, and because this carbon is more positive, and this chlorine is negative, that makes it easier to break this bond.

03:28 Whereas in this case, we have a carbon, and carbon have the same polarity.

03:37 Right? so that's not going to be an issue for this species on the left...

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