In the bromination of substituted styrenes, a ρσ^+plot is noticeably curved. If the extremes of

step 1 Okay, we have the following molecules. First, this is going to take a bit to draw, but I will dr

In the bromination of substituted styrenes, a ρσ^+plot is...

In the bromination of substituted styrenes, a $\rho \sigma^{+}$plot is noticeably curved. If the extremes of the curve are taken to represent straight lines, the curve can be resolved into two Hammett relationships with $\rho=-2.8$ for EWG substituents and $\rho=-4.4$ for ERG substituents. The corresponding $\beta$-methylstyrenes give a similarly curved plot. The stereoselectivity of the reaction of the $\beta-$ methylstyrenes is also dependent on the substituents. The reaction is stere- ospecifically anti for strong EWGs, but is only weakly stereoselective, e.g., $63 \%$ anti:37\% syn, for methoxy. Discuss a possible mechanistic basis for the curved Hammett plots and the relationship to the observed stereochemistry.

In the bromination of substituted styrenes, a $\rho \sigma^{+}$plot is noticeably curved. If
the extremes of the curve are taken to represent straight lines, the curve can be
resolved into two Hammett relationships with $\rho=-2.8$ for EWG substituents
and $\rho=-4.4$ for ERG substituents. The corresponding $\beta$-methylstyrenes
give a similarly curved plot. The stereoselectivity of the reaction of the $\beta-$
methylstyrenes is also dependent on the substituents. The reaction is stere-
ospecifically anti for strong EWGs, but is only weakly stereoselective, e.g., $63 \%$
anti:37\% syn, for methoxy. Discuss a possible mechanistic basis for the curved
Hammett plots and the relationship to the observed stereochemistry.

Key Concepts

Hammett Equation

The Hammett equation is a linear free energy relationship that correlates reaction rates or equilibria with substituent effects in aromatic systems. It uses sigma constants to represent the electronic influence of substituents on the reaction center. Deviations from linearity in a Hammett plot suggest that the reaction mechanism or the rate?determining transition state changes with different substituents, indicating the presence of multiple mechanistic pathways or effects.

Electronic Effects in Reaction Mechanisms

Substituents on an aromatic ring can either donate or withdraw electrons, thereby influencing the stability of intermediates and transition states. Electron?withdrawing groups (EWGs) and electron?donating groups (EDGs) affect the charge distribution during the reaction and can change the preferred reaction pathway. This differential electronic influence often leads to variable reaction constants (rho values) and can manifest as curvature in plots correlating substituent effects with reactivity.

Mechanistic Heterogeneity and Transition State Variability

Curvature in a Hammett plot frequently indicates a shift in the mechanism or a change in the nature of the rate?determining transition state over the range of substituents. This means that for one set of substituents (e.g., EWGs) the mechanism might involve a transition state with different electronic character or stabilization compared to another set (e.g., EDGs). The distinct rho values derived from separate regions of the plot support the idea that the reaction pathway is not uniform but rather varies with the substituent's electronic properties.

Bromonium Ion Formation and Stereoselectivity in Electrophilic Addition

In the electrophilic bromination of alkenes, a bromonium ion intermediate generally forms, which is subsequently attacked by a nucleophile in an anti fashion, leading to stereospecificity. However, variations in the electronic nature of substituents can alter the structure or stability of the bromonium ion and the nature of the subsequent nucleophilic attack. This results in differences in stereoselectivity, such as strictly anti addition in cases where the transition state is well-defined versus less stereoselective outcomes when electronic factors alter the preferred trajectory of attack.

Transcript

00:01 Okay, we have the following molecules.

00:03 First, this is going to take a bit to draw, but i will draw it.

00:27 And we're told that this molecule can be converted into this molecule.

00:57 Furthermore, the initial molecule will note is a carbocation, and so is the product.

01:05 Right.

01:06 Now, there's a few things we can note.

01:08 There are similarities in this molecule, for example.

01:13 This five -membered ring is analogous to this five -membered ring.

01:21 And we'll notice here that this positive charge ends up over here, which we can kind of think of as being a cascade of double bonds, right? and hopefully after i draw this first one, you'll see where i'm going with this.

01:40 And i'll try to color code along the way.

01:46 Consider that if we have this bond right here and that if this double bond attacks this carbon and this double bond flips down right so where is that bond that's right here right because if we count from this carbon that's one two three four five carbons away one, two, three, four, five carbons away with the bond between carbons four and five.

02:28 Now, what would the result of this be? this would have left a carbocation on this carbon, right? and so basically what we can assume is that the next step is that this double bond, which i'll color code green, would have attacked that.

02:51 Carbocadion, right? we're doing this all in one step.

02:59 So there never is really a carbocadion on a carbon four, but let me continue to number here, six, eight, for example, right? and we know that nine should have a bond to carbon four...

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