Draw a Potential Energy Diagram for This Reaction

9.iv: The Mechanism for an \(S_N1\) Reaction

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The S_N2 mechanism

There are two mechanistic models for how an alkyl halide tin can undergo nucleophilic substitution. In the first flick, the reaction takes place in a single step, and bail-forming and bond-breaking occur simultaneously. (In all figures in this section, 'X' indicates a halogen substituent).

This is called an 'S_Northward2' mechanism. In the term S_N2, Southward stands for 'substitution', the subscript N stands for 'nucleophilic', and the number ii refers to the fact that this is a bimolecular reaction: the overall charge per unit depends on a step in which two dissever molecules (the nucleophile and the electrophile) collide. A potential energy diagram for this reaction shows the transition state (TS) as the highest indicate on the pathway from reactants to products.

If yous look carefully at the progress of the S_N2 reaction, y'all will realize something very important virtually the event. The nucleophile, being an electron-rich species, must assault the electrophilic carbon from the back side relative to the location of the leaving group. Approach from the forepart side simply doesn't work: the leaving grouping - which is also an electron-rich group - blocks the way.

The result of this backside attack is that the stereochemical configuration at the central carbon inverts equally the reaction proceeds. In a sense, the molecule is turned inside out. At the transition state, the electrophilic carbon and the three 'R' substituents all lie on the same plane.

What this means is that S_{Due north}2 reactions whether enzyme catalyzed or not, are inherently stereoselective: when the substitution takes identify at a stereocenter, we can confidently predict the stereochemical configuration of the product. Below is an blitheness illustrating the principles we take simply learned, showing the South_{Due north}2 reaction between hydroxide ion and methyl iodide. Detect how backside attack by the hydroxide nucleophile results in inversion at the tetrahedral carbon electrophile.

Exercise
Predict the structure of the product in this SN2 reaction. Be sure to specify stereochemistry.

Influence of the solvent in an South_Northward2 reaction

The charge per unit of an SN2 reaction is significantly influenced past the solvent in which the reaction takes place. The use of protic solvents (those, such as h2o or alcohols, with hydrogen-bond donating capability) decreases the power of the nucleophile, because of potent hydrogen-bail interactions betwixt solvent protons and the reactive lone pairs on the nucleophile. A less powerful nucleophile in plough means a slower SN2 reaction. SN2 reactions are faster in polar, aprotic solvents: those that lack hydrogen-bail altruistic capability. Below are several polar aprotic solvents that are commonly used in the laboratory:

These aprotic solvents are polar but, because they practice not form hydrogen bonds with the anionic nucleophile, there is a relatively weak interaction betwixt the aprotic solvent and the nucleophile. By using an aprotic solvent we can raise the reactivity of the nucleophile. This tin sometimes have dramatic furnishings on the rate at which a nucleophilic substitution reaction tin can occur. For example, if nosotros consider the reaction between bromoethane and potassium iodide, the reaction occurs 500 times faster in acetone than in methanol.

The S_N1 mechanism

A second model for a nucleophilic substitution reaction is called the 'dissociative', or 'S_N1' mechanism: in this film, the C-Ten bond breaks first, before the nucleophile approaches:

This results in the germination of a carbocation: because the central carbon has only three bonds, it bears a formal charge of +ane. Recall that a carbocation should be pictured as sp² hybridized, with trigonal planar geometry. Perpendicular to the aeroplane formed by the three sp² hybrid orbitals is an empty, unhybridized p orbital.

In the second step of this two-step reaction, the nucleophile attacks the empty, 'electron hungry' p orbital of the carbocation to form a new bond and return the carbon to tetrahedral geometry.

We saw that S_North2 reactions outcome specifically in inversion of stereochemistry at the electrophilic carbon eye. What virtually the stereochemical result of Due south_{Due north}1 reactions? In the model S_N1 reaction shown to a higher place, the leaving grouping dissociates completely from the vicinity of the reaction before the nucleophile begins its attack. Because the leaving group is no longer in the moving-picture show, the nucleophile is free to attack from either side of the planar, sp² -hybridized carbocation electrophile. This means that well-nigh one-half the fourth dimension the production has the same stereochemical configuration as the starting textile (memory of configuration), and about one-half the fourth dimension the stereochemistry has been inverted. In other words, racemization has occurred at the carbon center. As an case, the tertiary alkyl bromide below would be expected to class a racemic mix of R and S alcohols subsequently an South_{Due north}1 reaction with water as the incoming nucleophile.

Exercise
Draw the structure of the intermediate in the ii-footstep nucleophilic substitution reaction above.

The S_Nane reaction we see an instance of a reaction intermediate, a very important concept in the study of organic reaction mechanisms that was introduced earlier in the module on organic reactivity Recall that many important organic reactions practice not occur in a single step; rather, they are the sum of two or more than discreet bail-forming / bond-breaking steps, and involve transient intermediate species that keep to react very quickly. In the Due south_N1 reaction, the carbocation species is a reaction intermediate. A potential free energy diagram for an S_N1 reaction shows that the carbocation intermediate can be visualized as a kind of valley in the path of the reaction, higher in energy than both the reactant and production but lower in energy than the two transition states.

Exercise
Draw structures representing TS1 and TS2 in the reaction in a higher place. Use the solid/dash wedge convention to show three dimensions.

Recall that the first footstep of the reaction above, in which two charged species are formed from a neutral molecule, is much the slower of the two steps, and is therefore rate-determining. This is illustrated by the energy diagram, where the activation energy for the first pace is higher than that for the second stride. Also recall that an Southward_Ni reaction has get-go order kinetics, because the rate determining step involves one molecule splitting apart, not 2 molecules colliding.

Do
Consider 2 nucleophilic substitutions that occur uncatalyzed in solution. Assume that reaction A is SNorthii, and reaction B is SNi. Predict, in each case, what would happen to the rate of the reaction if the concentration of the nucleophile were doubled, while all other conditions remained constant.

Influence of the solvent in an SN1 reaction

Since the hydrogen atom in a polar protic solvent is highly positively charged, it can collaborate with the anionic nucleophile which would negatively affect an SN2, but it does not affect an SN1 reaction because the nucleophile is non a office of the rate-determining step. Polar protic solvents actually speed upward the rate of the unimolecular substitution reaction because the large dipole moment of the solvent helps to stabilize the transition land. The highly positive and highly negative parts interact with the substrate to lower the energy of the transition country. Since the carbocation is unstable, anything that tin stabilize this even a picayune will speed up the reaction.

Sometimes in an SN1 reaction the solvent acts as the nucleophile. This is called a solvolysis reaction.The S_N1 reaction of allyl bromide in methanol is an example of what we would telephone call methanolysis, while if water is the solvent the reaction would be called hydrolysis:

The polarity and the ability of the solvent to stabilize the intermediate carbocation is very important as shown by the relative rate data for the solvolysis (see table below). The dielectric constant of a solvent roughly provides a mensurate of the solvent's polarity. A dielectric constant below xv is usually considered non-polar. Basically, the dielectric constant can be thought of as the solvent's ability to reduce the internal charge of the solvent. So for our purposes, the college the dielectric constant the more polar the substance and in the case of Due south_Ni reactions, the faster the rate.

Below is the same reaction conducted in two unlike solvents and the relative rate that corresponds with it.

Exercise
Depict a complete curved-arrow mechanism for the methanolysis reaction of allyl bromide shown to a higher place.

One more than of import signal must be made before standing: nucleophilic substitutions as a rule occur at sp³-hybridized carbons, and not where the leaving grouping is attached to an sp^two-hybridized carbon::

Bonds on sp^two-hybridized carbons are inherently shorter and stronger than bonds on sp³-hybridized carbons, meaning that information technology is harder to break the C-X bail in these substrates. S_{Due north}2 reactions of this type are unlikely also because the (hypothetical) electrophilic carbon is protected from nucleophilic assault by electron density in the p bond. S_North1 reactions are highly unlikely, considering the resulting carbocation intermediate, which would be sp-hybridized, would be very unstable (we'll discuss the relative stability of carbocation intermediates in a afterwards section of this module).

Before we look at some existent-life nucleophilic substitution reactions in the next chapter, nosotros volition spend some fourth dimension in the remainder of this module focusing more closely on the three main partners in the nucleophilic substitution reaction: the nucleophile, the electrophile, and the leaving group.

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Source: https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map%3A_Essential_Organic_Chemistry_(Bruice)/09%3A_Substitution_and_Elimination_Reactions_of_Alkyl_Halides/9.04%3A_The_Mechanism_for_an_(S_N1)_Reaction

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