SN1 Reaction Mechanism

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 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. 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] (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 –

  • Step 1. Formation of carbocation

As bromide ion is highly electronegative so C-Br bond is already polar which allows easy formation of carbocation. In this step carbocation is formed by breaking the C-Br bond. As breaking of bond is involved in this step, so it's an endothermic process. This is a slow step therefore rate determining step of the reaction mechanism.

  • Step 2. Attack of nucleophile

In this step nucleophile attacks on carbocation. -OH group of water acts as nucleophile due to lone pairs on the oxygen atom which attacks on electrophilic carbocation and gives oxonium ion. This is a fast step.

  • Step 3. Deprotonation

In this step removal of proton takes place in oxonium ion by water. This deprotonation yields alcohol and hydronium ion as products. If a nucleophile is a neutral molecule then only this step is required.

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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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