Nucleophilic substitution reaction is a type of organic reaction where one nucleophile is replaced by others. It is quite the same as the displacement reactions that we find in chemistry. In a displacement reaction, a less reactive element gets replaced by a more reactive element from its salt solution. The group that takes electron pairs and displaces from the carbon is known as the leaving group, and the substitution takes place on a molecule known as substrate. The leaving group leaves as a neutral molecule.
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In nucleophilic substitution reactions, nucleophilicity is the term used for describing the reactivity and strength of nucleophiles. In a nucleophilic substitution reaction, a stronger nucleophile replaces a weaker nucleophile in its compound. It can be explained roughly as follows:
R - LG + Nu٥ →R - Nu + LG٥
R - Alkyl Group
LG - Leaving Group ( Less Nucleophilic)
Nu٥ - Strongly Nucleophilic
Consider a reaction of methyl bromide with sodium hydroxide, which gives sodium bromide as a side product with methanol as the main product.
CH3 - Br + OH¯ → CH3 - OH + Br¯
Methyl Bromide (Substrate) + Hydroxide ion (Nucleophile) → Methanol (Product) + Bromide ion
Nucleophilicity is the ability of the nucleophiles to donate their lone pairs to a positive center. It is a kinetic term that relates to the rate at which the nucleophile attacks the substrates (R - LG). The following factors are used to compare the nucleophilicity of different nucleophiles.
The Basic Strength of Nucleophiles
The basic strength of nucleophiles is the ability to donate electron pairs by a species. Both the terms basic strength and nucleophilicity are kind of similar and are related directly. There is one difference between these two terms, i.e., nucleophilicity is a kinetic term whereas basic strength is a thermodynamic term.
Electronegativity of the Nucleophilic Atom
It is a common logic that nucleophilic attack gets easily successful when the lone pair of electrons are loosely held. This clearly explains that less nucleophilic or weaker nucleophiles are those nucleophiles that have lone pairs on highly electronegative atoms.
Crowded nucleophiles also termed as highly hindered nucleophiles, cannot move, which makes them a weaker nucleophile. For example, primary alkoxide ions are stronger than tertiary alkoxide ions due to the presence of steric hindrance. You can learn this in chapter Nucleophilic substitution reaction class 12.
Charged or Uncharged
In a nucleophilic reaction, if the two nucleophiles have the same nucleophilic atoms, the one which is negatively charged is more nucleophilic or has more strength than the neutral one because negative charges are more attracted towards a positive center.
Polar Solvent Effects
In the case of polar solvents like alcohol acids, H₂O, etc., the nucleophilicity of the nucleophiles depends on the effect of solvation. Heavily hydrated ions have a decreased ion mobility which in turn decreases the nucleophilicity. Hydration is a phenomenon that occurs when all the water molecules around the ion get crowded in one place. The order of nucleophilicity is not the only thing important in the case of polar solvents because hydration also plays a very important part in the determination of the strength of the nucleophiles.
Leaving Capacity of the Leaving Group
In a nucleophilic substitution reaction, the rate of the nucleophilic reaction depends on the incoming nucleophile’s nucleophilicity or its strength and the leaving group’s capacity to leave. The reaction can be faster if the leaving group has more leaving capacity. The formula to evaluate leaving capacity is that 'Weaker bases are better leaving groups'.
F٥< cl٥< Br٥< I٥ is the order of leaving capacity.
Mechanisms of Nucleophilic Substitution
The rate of nucleophilic substitution reaction doesn't only depend on the nucleophilicity of the incoming nucleophiles and the leaving capacity of the leaving group but also on the mechanism with the help of which the reaction takes place. There are two mechanisms as follows.
It is also known as the substitution nucleophilic bimolecular mechanism. While a reaction takes place, this mechanism follows 2nd order kinetics and the rate law for the reaction. The rate law explains that the SN₂ mechanism reaction depends on the concentration of the incoming nucleophile as well as the substrate. Therefore the concentration nucleophilicity of both can increase the rate of reaction. This mechanism is there in the nucleophilic substitution reaction class 12 syllabus.
It is also known as unimolecular nucleophilic substitution reaction. Here, the mechanism is independent of the nucleophilicity of the incoming nucleophile but is dependent on the leaving capacity of the leaving group. This mechanism is also covered in the nucleophilic substitution reaction class 12 syllabus. You can get detailed information on these mechanisms from the nucleophilic substitution reaction PDF.