3.1.3 – SN1 Reaction Mechanisms

SN1  Reaction Mechanism

The previous section established how SN2 reactions cannot occur with tertiary alkyl halides as substrates due to the steric effect. However, they can undergo another type of nucleophilic substitution reaction called SN1.

SN1 reactions occur in more than one step by a dissociative mechanism where the leaving group leaves first. This generates a carbocation intermediate, which is a compound with a positively charged carbon atom that has trigonal planar geometry. The second step of an SN1 reaction involves nucleophilic attack by the electron rich nucleophile at the carbocation centre.

An iodide (I-) reacts with tert-butyl bromide ((CH3)3CBr) to form the same compound where the Br is replaced with iodine and a bromide (Br-) is released. The first step in the mechanism is shown with a curved arrow; the arrow tail is from the Br-C bond, and the head points toward the Br. Next to an arrow to the right shows the lone pair on an iodide has a curved arrow tail pointing its head to the carbon in the carbocation intermediate at the head. To the right of the second reaction arrow is the final product that is the same molecule from the beginning but with an I instead of Br.
Figure 3.1.3.a. Overall mechanism for an SN1 reaction. The halide Br– will first leave generating a positively charged carbocation intermediate. The halide I– will perform a nucleophilic attack on the trigonal planar carbocation intermediate, generating the final products.

Let’s break down each step of the mechanism:  

Sn1 reaction between iodide (I-) and tert-butyl bromide ((CH3)3CBr). It illustrates the transition state in square brackets where Br is partially bound to the compound with dashed lines, and the carbocation intermediate where the 3 methyl groups and C are in a trigonal planar geometry and an empty p-orbital horizontally across the C.
Figure 3.1.3.b. First step of the SN1 mechanism. The C–Br bond breaks with bromine acting as the leaving group and taking a pair of electrons, generating a tetrahedral transition state. An electron deficient carbocation intermediate with an empty p-orbital forms.

Step 1. The C–Br bond breaks with bromine acting as the leaving group. The transition state has tetrahedral geometry, and includes the C–Br bond in the process of breaking (depicted with a dashed line). This generates a positively charged carbocation intermediate, which has an empty p-orbital. The incomplete octet and positively charged carbon centre makes this intermediate highly reactive.  

The same intermediate from figure 3.1.3.b has its p-orbital attacked by the iodide (I-), shown with an arrow tail at the iodide lone pair and the arrowhead at the p-orbital. The reaction’s second transition state is the I partially bonded, then the final product is shown.
Figure 3.1.3.c. Second step of the SN1 mechanism. Iodide attacks the electron-deficient carbocation, donating a pair of electrons into the empty p-orbital, forming a new C–I covalent bond. A tetrahedral transition state yields the products tert-butyl iodide and bromide.

Step 2.  The carbocation intermediate acts as an electrophile while the electron-rich iodide acts as the nucleophile. The iodide attacks the electron-deficient carbocation, donating its electron density into the empty p-orbital and forming a new C–I bond. The transition state has tetrahedral geometry and includes the C–I bond in the process of forming (depicted with a dashed line). This generates the final products: tert–butyl iodide and bromide. 

Rate Law and Energy Diagram of SN1 Mechanism

The SN1 reaction proceeds through a two-step mechanism. This is shown in an energy profile diagram with two energy maxima, corresponding to the two transition state species. The local energy minimum represents the carbocation intermediate.  

An energy profile diagram for Sn1 reactions, depicted with potential energy as the y-axis and reaction progress on the x-axis. Unlike a Sn2 bell-shape curve, Sn1 has 2 peaks and one trough to indicate the transition states and intermediate respectively, almost shaped like a camel’s back, or a mountain with a valley in between to create 2 peaks. A vertical line starting from the lowest point of the curve to the top of the first peak is labelled Ea step 1. On top of peak 1 in square brackets is a transition state of C bonded to 3 methyl groups and a dotted line bonded to Cl. At the trough, which is higher than the starting point, is the intermediate carbocation of C with 3 methyl groups. Lastly, on the second peak, which is lower than the first peak, in square brackets is I bonded to C with 3 methyl groups by a dotted line. Another vertical line starting from the trough to the second peak is labelled Ea step 2.
Figure 3.1.3.d. Energy profile diagram for the SN1 reaction (CH3)3C–Br + I– –> (CH3)3C–I + Br–

The first maximum represents the first step of the reaction: C–Br bond breaking and loss of the halide leaving group. This is the step with the largest activation energy barrier (Ea, step 1) in the mechanism. The activation energy barrier for the first step is largest because the initial reactant is stable as a neutral molecule with a complete octet. Bond breaking of the carbon-halide bond is endothermic, leading to a less stable, more reactive carbocation intermediate. The first step of an SN1 reaction is therefore the rate-determining step of the reaction.

The second maximum represents the second step of the reaction: nucleophilic attack by iodide to form the final product. This step has a lower activation energy (Ea, step 2) compared to the first step. The carbocation intermediate will readily accept electrons from the electron-rich nucleophile in an exothermic bond forming event. This yields the rate law shown below:

 

Rate = kobs[Alkyl Halide]

 

Since the first step is the slow step, the rate of an SN1 reaction only depends on the rate at which the carbon-halide breaks in the alkyl halide reactant. The reaction rate does not depend on the nucleophile. The overall rate law for an SN1 reaction is therefore first order, hence the name SN1 (Substitution Nucleophilic Unimolecular). 

(The full solution to this problem can be found in Chapter 5.2)

 

The Effect of the Substrate Structure on the SN1 Reaction Rate 

The alkyl halide substrate plays a key role in determining the rate of an SN1 reaction. It is experimentally observed that tertiary substrates most readily undergo an SN1 reaction, while methyl and primary alkyl halides are rarely observed to undergo an SN1 reaction.

The relative reactivity of substrates towards the SN1 reaction can be explained by the activation energy of the first step of the mechanism. This is in turn correlated to the energy of the carbocation intermediate: the more stable the carbocation intermediate, the lower the activation energy barrier to form that intermediate, and the faster it will form. In an SN1 reaction, tertiary substrates have the lowest activation energy barrier, and will undergo an SN1 reaction faster than secondary, primary and methyl substrates respectively.

An energy profile diagram of the four differently substituted alkyl halides (methyl, primary, secondary, and tertiary) is shown below:

An energy profile diagram where reaction progress is on the x-axis and potential energy is on the y-axis. There are 4 curves overlapping each other with the lowest ones starting at the bottom and the following ones increasing in height as you go to the top. The curves are different from Sn2 as they are like 2 bell-curves merged together, almost like a valley between 2 peaks where the first peak is higher than the second. The valley/trough of the curve is representative of the carbocation intermediates, and the peaks indicate the transition states. On the left of the curves are 4 vertical lines increasing in height labelled (from shortest to tallest): Ea3, Ea2, Ea1, and EaM indicating different activation energies.
Figure 3.1.3.e. Energy profile diagram for the SN1 reaction involving four different substrates: tertiary, secondary, primary, and methyl halides. Ea3 corresponds to the activation energy for a tertiary alkyl halide to form a tertiary carbocation intermediate. Ea2 corresponds to the activation energy for a secondary alkyl halide to form a secondary carbocation intermediate. Ea1 corresponds to the activation energy for a primary alkyl halide to form a primary carbocation intermediate. EaM corresponds to the activation energy for a methyl halide to form a methyl carbocation intermediate. The methyl carbocation intermediate is the least stable of all the carbocations and will have the highest activation energy barrier.

This trend is explained by the stability of the carbocation intermediates, which is in turn explained by considering the electron densities of surrounding groups. The greater the electron density surrounding the electron-deficient carbocation center, the greater the ability to dissipate the positive charge, and the more stable the carbocation. Alkyl groups possess larger electron clouds than hydrogen atoms and can donate their electron density towards the carbocation, and thereby dissipate the positive charge to a larger degree than hydrogen atoms.    

A depiction of different carbocation stability with a horizontal arrow at the bottom pointing towards the right labelled “increasing potential energy”. The left side is labelled most stable, and the right is least stable. From left to right, the most to least stable carbocations depicted are isobutyl which is tertiary, propyl which is secondary, ethyl which is primary, and methyl.
Figure 3.1.3.f. Experimentally observed relative stability of carbocations.

In the tertiary carbocation, the electron-deficient carbon center is surrounded by three alkyl groups, which donate some of their electron density towards the carbocation centre. This allows for dissipation of the positive charge, which stabilizes the carbocation. In the primary and secondary carbocations, there are fewer alkyl groups surrounding the carbocation center. As a result, these intermediates have less electron density surrounding the carbocation, which limits their stabilizing capacity. The methyl carbocation has only hydrogen atoms bound to the carbocation, resulting in limited electron density available to stabilize the carbocation center, making the methyl carbocation the least stable.

 

On the left side, Carbon bound to 3 hydrogens and has a positive charge, in line bond structure and 3D model below. Similarly on the right side, C bound to 3 methyl groups and has a positive charge. The left 3D model is surrounded in a red-yellow bubble, whereas the right model has a green-blue bubble and is physically larger in size. On the absolute right is a vertical rainbow scale with Red on top to blue at the bottom. The top (red) is labelled plus and the bottom is minus.
Figure 3.1.3.g. Electrostatic potential maps for methyl, and tertiary carbocation intermediates, respectively, from left to right. As shown in the red and yellow colouring, the methyl carbocation (left) has limited dissipation of positive charge, making this an unstable species. The tertiary carbocation (right) has three methyl groups to donate electron density towards the carbocation (shown in green), allowing the positive charge to be dissipated. The stability of the carbocations increases from left (least stable) to right (most stable).

Nucleophilic Substitution – Making Alcohols and Ethers

An SN1 mechanism can also be used to synthesize alcohols and ethers, by reacting a tertiary alkyl halide substrate with either water or an alcohol, respectively, as the nucleophile. This reaction occurs in three steps, these being the two-steps involved in the SN1 mechanism, followed by a proton transfer step.

Tert-butyl chloride in the first step of an SN1 reaction. A blue curved arrow is drawn such that the tail end is at bond between carbon and Cl, and the arrowhead points to Cl. A horizontal arrow to the right points to a tert-butyl carbocation (C(CH3)3+) in a trigonal planar form.
Figure 3.1.3.h. First step of the SN1 mechanism to generate alcohols and ethers. The C–Cl bond breaks, generating a tertiary carbocation.
2 SN1 reactions’ second steps where a tertiary carbocation in a trigonal planar geometry has its central carbon attacked from H2O for the first reaction and ROH for the second reaction to form oxonium cation intermediates. There are 2 curved arrows where the tail end starts from the lone pairs on Oxygen for both water and alcohol, and the heads both point towards the positively charged carbon in the carbocation. Horizontal arrows to the right show the respective oxonium cations, oxygen bonded to the central carbon, and is also bonded to either 2 H, or R and H. The oxygen carries the positive charge.
Figure 3.1.3.i. Second step of the SN1 mechanism to generate alcohols and ethers. On top, water acts as a nucleophile while alcohol acts as a nucleophile in the reaction on the bottom. These nucleophiles will attack the electrophilic carbocation center, generating oxonium cation intermediates.

Step 2, top. Water acts as a nucleophile to attack the electrophilic carbocation center, generating an oxonium cation intermediate.

Step 2, bottom. An alcohol acts as a nucleophile to attack the electrophilic carbocation center, generating an oxonium cation intermediate.

 

Based on the structures from 3.1.3.i, a chloride (Cl-) deprotonates the oxonium cations to generate a tertiary alcohol (deprotonation of the compound attacked by water) and a tertiary ether (deprotonation of the compound attacked by an alcohol). A red curved arrow has the tail end start at chloride have the heads pointing towards a Hydrogen bonded to the Oxygen.
Figure 3.1.3.j. Third step of the SN1 mechanism to generate alcohols and ethers. The chloride leaving group from step 1 will act as a base and deprotonate the oxonium intermediate, generating a tertiary alcohol and ether.

Step 3, top.  Chloride (the halide leaving group from Step 1) acts as a base to deprotonate the oxonium intermediate resulting in HCl and a tertiary alcohol.  

Step 3, bottom. Chloride (the halide leaving group from Step 1) acts as a base to deprotonate the oxonium intermediate, resulting in HCl and a tertiary ether.

The mechanism above in which the tertiary alkyl halide reacts with water or an alcohol, requires three steps. In comparison, the SN1 reaction examined earlier, in which the tertiary alkyl halide reacts with a halide such as iodide, requires two steps. This is because reaction with a neutral nucleophile (water or an alcohol) requires an extra deprotonation step (Step 3 in Figure 3.4.1.j.) to generate a final product; this step is not needed when using an anionic nucleophile (such as I–).

This reaction yields the energy profile diagram shown below:

Energy profile diagram with reaction progress of Sn1 reaction with 2 intermediate stages and 3 transition states. Illustrated by 3 cascading peaks (from taller to shorter) joined by troughs/valleys.
Figure 3.4.1.k. Energy profile diagram for the SN1 reaction (CH3)3C–Cl + H2O → (CH3)3C–OH + HCl. This energy profile diagram also illustrates the formation of ethers, but instead of water acting as a nucleophile, an alcohol acts as the nucleophile. 

The first step of the reaction (loss of the halide leaving group) is the slow step and requires the greatest amount of energy. This will generate a carbocation intermediate. Then, water or an alcohol attacks the carbocation to form the corresponding oxonium intermediate. The second step has a lower activation energy than the first step because forming a full octet at carbon is favourable and will lead to stabilization. The third step involves deprotonation of the oxonium intermediate, yielding the neutral alcohol or ether. 

The following video includes a worked example from a previous CHEM 1AA3 test or exam that students struggled with. Try solving it on your own before looking at the solution.

Key Takeaways

  • SN1 stands for substitution nucleophilic unimolecular reaction
  • This reaction is two steps, with the first step being the loss of a leaving group forming a carbocation intermediate, and the second step being the attack from a nucleophile
  • The first step forms an unstable cationic intermediate, so it is the slow, rate limiting step with a large activation energy
  • Tertiary alkyl halides form the most stable carbocation intermediate, as more alkyl groups will make it so more of their electron density is donated to the carbocation. This is why they are the most favoured for SN1 mechanisms, including making ethers and alcohols

Key terms in this chapter:

Key term Definition
Oxonium A cation intermediate that is typically formed when water performs a nucleophilic attack on an electrophilic substrate. The oxygen atom will have a formal charge of +1.

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Organic Chemistry and Chemical Biology for the Students by the Students! (and the Profs...) Copyright © 2023 by Emma Abreu; Anumta Amir; Anthony Chibba; Jim Ghoshdastidar; Sharonna Greenberg; Angela Liang; Layla Vulgan; and Shuoyang Wang is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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