Refining
The SN1 reaction follows a step-by-step process in which the carbocation is formed by the removal of the leaving group. As the reaction proceeds, the carbocation is attacked by the nucleophile. In the last stage, the deprotonation of the protonated nucleophile takes place to give the required product. The step that determines the rate of this reaction depends purely on the decomposition of a single molecular species and is not affected at all by the nucleophile.
Mechanism of SN1 reaction was 1st proposed by British Chemist Christopher Ingold et al. in 1940. Many important reactions of organic chemistry take place by SN1 reaction mechanism. Before understanding the SN1 reaction and its mechanism, you need to have a basic idea of the terms like nucleophile, electrophile and leaving group. So, before talking about SN1 reaction and its mechanism in detail, we are first giving here a brief idea of all these basic terms.
Nucleophile – Nucleophile is a negatively charged or neutral and electron rich species. It can donate a pair of electrons. Nucleophile attacks positively charged species.
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Examples of Nucleophiles – Neutral Nucleophiles- ammonia (NH3), water (H2O), carboxylic acid (RCOOH) etc.
Negatively Charged Nucleophiles – bromide (Br-), iodide (I-), chloride (Cl-) etc.
Electrophile – Electrophile is an electron deficient species. It can accept a pair of electrons. It is generally a positively charged species.
Examples of Electrophile – hydronium ion (H+), nitrosonium ion (NO+) etc.
Leaving Group – A leaving group is that anion or neutral molecular fragment that departs with a pair of electrons in heterolytic bond cleavage. These can be neutral, negative or positively charged.
Examples of leaving groups – Cl-, water, H+ etc.
SN1 Reaction
SN1 reaction is a substitution reaction in organic chemistry. SN1 stands for Substitution, Nucleophilic. This type of nucleophilic substitution reactions are unimolecular. So, 1 stands for its unimolecular. As one reactant or one molecular entity is involved in the rate determining step. It means the rate determining step of the mechanism of the reaction depends on the decomposition of one reactant or single molecular entity. The reaction takes place by two steps or three steps (with a neutral nucleophile). The slow step in the reaction is called the rate determining step. In these reactions, in the 1st step leaving group gets detached from the carbon atom and forms carbocation while in the 2nd step nucleophile attacks on carbocation and in the 3rd step deprotonation takes place. The 3rd step takes place only when neutral solvent (i.e. nucleophile) has been used. The 1st step is the slow step and therefore the rate determining step. General representation of SN1 reaction –
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Where LG = Leaving Group and Nu = Nucleophile.
For SN1 reaction, rate of reaction can be expressed as – r = R−LG
R−LG (General representation) where r = rate of reaction, R = alkyl group and LG = leaving group.
Example of SN1 reaction –
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In the above reaction – leaving group – Br-
Nucleophile – OH-
Graphical Representation of SN1 Reaction Pathway
As SN1 reaction proceeds by two steps, it generates two transitional states. One, when leaving - group leaves the carbon of the alkyl group and forms carbocation and second one when nucleophile gets attached to carbocation.
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Mechanism of SN1 Reaction
SN1 reaction mechanism takes place by following three steps –
Formation of carbocation
Attack of nucleophile
Deprotonation
In SN1 reaction protic solvents are used which also act as nucleophiles in the reaction. In these reactions, substrates with strong leaving groups such as halide ions are used. For explanation of SN1 reaction mechanism we are taking an example of SN1 reaction of 2-bromopropane and water. Reaction mechanism involves following steps –
Stereochemistry of SN1 Reaction
These reactions give racemization of stereochemistry at the reaction center. In this the nucleophile attacks the planar carbocation. Reactions are given below showing how racemization occurs –
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Factor Affecting SN1 Reactions
Following components of the reaction influence the reaction pathway –
Carbocation – The more stable the carbocation is, the easier it is to form. If carbocation will be formed in an easier way, then the rate of reaction will be faster. As in SN1 reactions the step, in which carbocation is formed, is rate determining step.
Leaving Group – As removal of leaving group leads to the formation of carbocation. So, the rate of reaction gets influenced by the nature of the leaving group. If a strong leaving group such as halide ion is attached to the substrate then reaction will be faster.
Nucleophile – It does not affect the rate of reaction, but it may affect the stereochemistry of the product.
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What is an SN1 Reaction?
The SN1 reaction is a nucleophilic substitution reaction in organic chemistry where the rate determining step involves a single molecule. The SN in the reaction refers to the “Nucleophilic Substitution” and 1 refers to the rate determining step that is unimolecular. It is described by the expression rate = k [R-LG]. The first step of this reaction is slow and therefore helps in determining the rate. Thus, the rate equation (according to which the SN1 reaction is dependent on the electrophile but not on the nucleophile) holds in situations where the amount of the nucleophile is greater than the amount of the carbocation intermediate.
During the reaction a carbocation intermediate is formed. It is usually seen in the reactions of tertiary or secondary alkyl halides with secondary or tertiary alcohols under strongly acidic or strongly basic mediums. The SN1 reaction is also referred to as the dissociative mechanism in inorganic chemistry. Some examples of an SN1 type of nucleophilic substitution reaction are given below:
Effect of Solvent
Any solvent that can make the formation of the carbocation intermediate easy, will speed up the rate determining step of the SN1 reaction.
Solvents that are preferred for this type of reaction are both polar and protic in nature.
Polar nature of the solvent helps in stabilizing ionic intermediates whereas the protic nature of the solvent helps solvate the leaving group.
Some of the commonly used solvents in the SN1 reactions include water and alcohols. These solvents also act as nucleophiles.
SN1 Reaction Mechanism
Considering the hydrolysis of tertiary butyl bromide as an example, the mechanism of the SN1 reaction can be understood with the help of the following steps.
Step 1:
The carbon-bromine bond is a polar covalent bond in nature. The cleavage that is present in this bond allows the removal of the leaving group (bromide ion).
After the bromide ion leaves the tertiary butyl bromide, a carbocation intermediate is formed.
As mentioned above in the article, this is the rate determining step of the SN1 reaction.
The breaking of the carbon-bromine bond requires the absorption of heat.
Step 2:
In the second step of the SN1 reaction mechanism, the carbocation intermediate is attacked by the nucleophile.
Since water is used as a solvent in the reaction, an oxonium ion intermediate is formed.
Since water (solvent) is neutral in nature, a third step where deprotonation occurs is necessary.
Step 3:
In the previous step, the positive charge present on the carbocation was shifted to the oxygen.
The water solvent now starts acting as a base and deprotonates the oxonium ion to yield the required alcohol along with a hydronium ion as the product.
Step 2 and Step 3 of this reaction are fast in nature.
SN1 Reaction Mechanism of Alkyl Halides with H₂O
The mechanism of the reaction SN1 on can be understood with the help of the following steps:
Step 1:
As there is a cleavage of the already polar C-Br bond which allows the loss of the leaving group, a halide ion, to give a carbocation intermediate. This is the rate determining step. The breaking of the bond requires the absorption of heat.
Step 2:
Attack of the nucleophile: the lone pairs of electrons on the O atom of the water molecule, on the electrophilic carbocation creates an oxonium species.
Step 3:
In the final step the Deprotonation by a base yields the alcohol as the byproduct of the reaction.
Students must keep in mind that this is the reverse of the reaction of alcohol with HBr.
Also known, the nucleophile here, H₂O, can be replaced with any nucleophile and in that case the final deprotonation may not always be necessary.
Stereochemistry of SN1 Reaction
The carbocation intermediate that is formed in the first step of the SN1 reaction mechanism is a hybridized carbon. Its molecular geometry is trigonal planar in nature, therefore providing two different points for the nucleophilic attack, left and right.
If the reaction takes place at a stereocenter and if no point for the nucleophilic attack is preferred, the carbocation is then attacked equally from both sides, which gives an equal ratio of left and right-handed enantiomers.
Therefore, the tertiary/secondary alkyl halides can react with tertiary/secondary alcohols to undergo a nucleophilic substitution reaction. The halide is then replaced with the nucleophile in the product at last.
