SN1 Reaction: Mechanism, Formula, Influences and Factors

The S_N1 reaction is a reaction which is nucleophilic substitution in nature. The rate of reaction with reference to the determination of step in an S_N1 reaction is unimolecular in nature and also depends on the electrophilic nature of the leaving group. There is no influence of the nucleophile upon the rate of reaction, even though there is nucleophilic attack on the carbocation formed when the leaving group is eliminated as far as the steps involved in the mechanism pertaining to the S_N1 reaction is concerned. 

Keyterms: Rate of reaction, Nucleophilic substitution, Carbocation, Solvent, Oxonium, Ion, Bromide, Alcohol, Hydronium, nucleophile, oxonium ion

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Steps involved in S_N1 Reaction Mechanism

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• The first step involves the formation of carbocation. It is formed by the breaking of the carbon-bromine bond. The bromide ion is eliminated. The process is endothermic in nature as it involves the breaking of the bond. In terms of the rate of reaction pertaining to the mechanism of the reaction, the step is slow. 
• The second step involves the nucleophilic attack being encountered by carbocation. Due to the usage of water as a solvent, the outcome is the formation of an oxonium ion. In terms of the rate of reaction pertaining to the mechanism of the reaction, the step is fast.
• The third step involves the elimination of the proton in the oxonium ion by water. Due to such deprotonation, alcohol and hydronium ions are yielded as products. If the nucleophile is neutral in nature in terms of molecule, there is a requirement of this step only. In terms of the rate of reaction pertaining to the mechanism of the reaction, the step is fast. 

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Formula

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As far as the formula is concerned, the reactivity order is described as follows with reference to S_N1 reactions:

(CH₃)₃C- > (CH₃)₂CH- > CH₃CH₂- > CH₃-

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Main Factors influencing the S_N1 Reactions

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The main factors influencing the S_N1 reactions are the following components:

• Carbocation - The formation of carbocation depends on its stability. Therefore, lack of stability would impede the formation of carbocation. The easier formation of carbocation would result in the rise of speed pertaining to the rate of reaction. The step pertaining to the determination of the rate of reaction in S_N1 reactions is the one which involves the formation of carbocation. 
• Leaving group - Carbocation is formed on the basis of the elimination of the leaving group. Therefore, the rate of reaction is affected through the nature of the leaving group. The attachment of a strong leaving group such as halide would result in the rise of speed pertaining to the rate of reaction. 
• Nucleophile - The rate of reaction is not influenced by nucleophile. However, the stereochemistry may be influenced through the nucleophile.

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Influence of Solvent in S_N1 Reactions

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• The solvent that has the ability for facilitation with reference to the formation of carbocation intermediate would result in the rise of speed pertaining to the rate of reaction in terms of the step that determines the rate of reaction as demonstrated earlier.
• In such kinds of reactions, the solvents which are preferred are both protic and polar in nature.
• The polar nature of the solvent facilitates the stabilization of the ionic intermediate. On the other hand, the protic nature of the solvent leads to the solvation of the leaving group
• Some examples of solvents used in such kinds of reactions include alcohols and water. Such solvents are also treated as nucleophiles in terms of the S_N1 reactions.

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Stereochemistry pertaining to S_N1 Reactions

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• The carbocation intermediate formed is an sp² carbon of hybridized nature. The molecular geometry of such carbocation intermediate is trigonal planar, as a result of which there is a facilitation for the two different points of the nucleophilic attack, right and left.
• If the reaction occurs in a stereocenter and if there is no preference of either of the avenues in terms of a nucleophilic attack, the carbocation then encounters attacks form both the sides in an equal manner, thereby resulting in the yielding of a ratio which is equal in nature that consists of enantiomers left-handed and right handed as presented below: 

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Things to Remember

• There are two side reactions relating to S_N1 reactions which commonly occur. Such reactions are carbocation rearrangement and elimination reactions. 
• If the S_N1 reaction takes place in conditions which are hot or warm or where the temperatures are high, there would be high chances of the predominance of E1 elimination, that would further lead to the formation of an alkene.
• If the S_N1 reaction occurs in the conditions which are cold or where the temperatures are low, it is so competitive in nature that it poses difficulties in terms of favouring one over the other. However, some it may result in the formation of some alkene.

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Previous Year Questions

1. The d and l enantiomers of an optically active compound differ in… (JKCET 2010)
2. In a mixture, two enantiomers are found to be present in…
3. The number of enantiomers of the compound… (BITSAT 2013)
4. Which of the following will give a pair of enantiomers… (NEET 2007)
5. Which of the following has two stereoisomers… (BITSAT 2016)
6. The process of separation of a racemic modification into d and l-enantiomers… (NEET 1994)
7. Among the following statements about the molecules… (WBJEE 2014)
8. If there is no rotation of plane polarised light by a compound… (NEET 2007)
9. In an SN1 reaction on chiral centres, there is… (NEET 2015)
10. Which of the following compounds is not chiral… (NEET 1998)
11. By Wurtz reaction, a mixture of methyl iodide and ethyl iodide gives… (BCECE 2006)
12. The Wurtz-Fittig reaction involves condensation of… (JEE Main 2013)
13. The reaction in which 2 molecules of chlorobenzene reacts with metallic sodium…
14. Which of the following alkane cannot be made in good yield by Wurtz reaction… (NEET 2020)
15. CH₃CH₂Br undergoes Wurtz reaction … (VITEEE 2011)
16. The reagent which can do the conversion CH₃COOH … (KCET 2021)
17. The alkane that gives only one mono-chloro product on chlorination … (NEET 2019)
18. The correct order of boiling points of … (KEAM)