1-Chloro-1, 2-diphenylethane can undergo E2 elimination to give either cis- or trans-1,2-dipheny

1-Chloro-1, 2-diphenylethane can undergo E2 elimination to...

Key Concepts

E2 Elimination Mechanism

The E2 mechanism involves a concerted process where a base abstracts a ?-hydrogen while the leaving group departs simultaneously, leading to the formation of a double bond. This mechanism requires a specific stereochemical arrangement, typically an anti-periplanar configuration, to enable effective orbital overlap during the transition state.

Newman Projections and Conformational Analysis

Newman projections are used to view and analyze the spatial orientations of substituents around a carbon–carbon bond. They are particularly useful in understanding the conformational requirements for reactions like E2 eliminations, where the optimal alignment (anti-periplanar) of the leaving group and the departing proton is crucial.

Anti-Periplanar Geometry

Anti-periplanar geometry refers to the arrangement where the leaving group and the hydrogen being abstracted are positioned 180° apart in the same plane. This arrangement is essential in E2 reactions because it allows the reactive orbitals to align properly, which lowers the activation energy and facilitates the simultaneous bond breaking and formation process.

Thermodynamic Stability and Stereoselectivity

Alkene products often differ in stability based on their stereochemistry. In many cases, the trans alkene is thermodynamically more stable than the cis alkene due to reduced steric hindrance between bulky substituents. This stability leads to preferential formation of the trans isomer, influencing the product distribution in elimination reactions.

Transcript

00:01 So we have one chloro, one two, diphenyl ethane, which we know undergoes an e2 elimination reaction to give us either the cis or trans isomers of one two diphenyl ethylene.

00:14 So from this statement, we see that we have two different isomers for our product.

00:20 And from this we can infer that we should have two different isomers of our reactant as well.

00:26 So i'm going to go ahead and draw the two different isomers of our reactants.

00:31 And kind of assign which is going to be the cis and which is going to be the trance.

00:35 So when i draw the following here, sorry, draw the following here.

00:48 So i'm going to draw this phenal group over here as a wedge.

00:55 And then over here, i'm going to draw this phenog group as a dash.

00:59 So notice how this particular phenog group is going away from us with the dash, and this pheno group is going towards us with the wedge.

01:08 So from this, we can conclude that when an e2 elimination reaction occurs.

01:13 They are going to be on opposite sides.

01:15 And we're going to get the trans alkene product.

01:18 And that is going to look like this.

01:23 I'm sorry, that.

01:31 All right.

01:31 So this is going to be our trans.

01:35 Oh, sorry.

01:36 This should be hydrogen right here.

01:42 So as i was saying, this is going to give us the trans isomer of our product.

01:46 And when i draw it like this, so what i did is i simply rotated this bond the carbon -carbon bond is to look like the following.

02:08 So i'm going to keep this phenogroop as a dash here.

02:12 And what i'm going to do is what happens when that bond is rotated is we're going to get another, or this phenal group is now going to become a dash as well.

02:24 So when we have our two phenogrups that are both going away from us, when we perform an e2 elimination reaction, we're going to get the following.

02:33 We're going to get the cis.

02:35 Of our product, which will look like the following.

02:44 All right.

02:45 Another question asks us to draw the newman projections that will lead to these products, and we want to explain why the transalcine is going to be the more predominant product.

02:56 So why don't we go ahead and draw the newman projections for each? so i'll go ahead and draw the first one here.

03:04 So we are going to draw that.

03:10 So for our front carbon here, what's face? facing up is our chlorine, and on our left is going to be the phenol, and on the right is going to be the hydrogen.

03:22 For the back carbon, top left is going to be hydrogen, top right is going to be phenol, and bottom is going to be the hydrogen.

03:30 Well, notice, this is going to readily undergo an e2 elimination reaction.

03:37 We see that we have a reacting chlorine and hydrogen that are anti -parriplanner to each other, which is perfect.

03:46 So a base is going to come in here, deprotonate that...

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