A student wanted to know whether the greater proximity of...

A student wanted to know whether the greater proximity of the nucleophile to the $C$ -2 carbon in the transition state is what causes the 1,2 -addition product to be formed faster when 1,3 -butadiene reacts with HCI. Therefore, she decided to investigate the reaction of 2 -methyl- 1,3 -cyclohexadiene with HCl. Her friend told her that she should use 1-methyl-1,3-cyclohexadiene instead. Should she follow her friend's advice?

Key Concepts

Kinetic versus Thermodynamic Control

This concept distinguishes between products formed under kinetic control, which are generated more quickly from a lower-energy transition state, and those formed under thermodynamic control, which are more stable but may form slower. In the case of conjugated diene reactions, factors that favor fast formation of less stable products (kinetic control) versus slower formation of more stable products (thermodynamic control) must be considered, as they determine the product distribution based on reaction conditions and substituent positioning.

Substituent Effects

Substituent effects refer to the impact that various groups attached to a conjugated system have on its reactivity and selectivity. The position and nature of a substituent (like a methyl group) can alter both the electronic and steric environment of the diene. This alteration influences the stability of potential intermediates, such as carbocations, and the ease of nucleophilic attack, thereby affecting the preference for either 1,2- or 1,4-addition.

Transition State Theory

Transition state theory explains how the energy and structure of the transition state determine the reaction rate. In the context of electrophilic additions, the positioning of reactive centers, such as the proximity of a nucleophile to a particular carbon during the transition state, has a significant influence on the overall speed of product formation. The relative alignment and distance between the nucleophile and the electrophilic center in the transition state are key determinants in directing the reaction pathway.

Electrophilic Addition to Conjugated Dienes

This concept covers how conjugated dienes react with electrophiles like HCl through different addition pathways. The reaction can proceed via a 1,2-addition or a 1,4-addition mechanism, each leading to distinct products. The pathway taken depends on the structure of the diene and the reaction conditions, making it crucial to understand the interplay between molecular orbital interactions and intermediate stability when analyzing these reactions.

Transcript

00:01 In this question, we have a student who has observed that in the reaction of 1 -3 butodyne with hcl, the 1 -2 addition product is formed faster.

00:12 So the 1 -2 addition products would look like this.

00:16 And that's formed faster than the 1 -4 product, which looks like this.

00:20 And she wants to know if this is because of the proximity of the nucleophile to the c2 carbon in the transition state.

00:27 So basically she's investigating the proximity effect.

00:29 And so she decides that she's going to do a reaction of two methyl, one three cyclohexidine with hcl.

00:37 So that looks like this.

00:39 So, oops, this is our cyclohex.

00:44 And then she has one, three, dine, and two methyl.

00:49 This is the compound she wants to react with hcl.

00:53 But her friend tells her that she should use one methyl, one three cyclohexidine instead.

00:58 So that would look like this.

00:59 So there's our 1 -3 cyclohexidine and one methyl.

01:05 So her friend says that she should do this reaction instead of this one to really find out if the transition state, or sorry, the proximity is what's causing the 1 -2 to be formed faster.

01:17 So let's think about what the 1 -2 product would look like in both of these and what the intermediate would be.

01:24 So in this first one, we would number it one, two, three, four like this because we want the carbocadions to be as stable as possible.

01:36 So if we go this direction, we get a tertiary and number two and a secondary and number four.

01:41 If i go on the other direction, they both would have been secondary.

01:44 So our one two carbocadion is going to look like this.

01:51 So is our one, two carbocation here.

01:53 And then our one four would be a resonance form of that and it would look like this.

01:58 This would be the one for carbocadion here...

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