Nucleophiles

The term “nucleophile” refers to compounds that transfer electron pairs to electrophiles in order to establish chemical bonds with them. Any ion or molecule with a free electron pair or a pi bond carrying two electrons can act as a nucleophile. As formerly acknowledged, nucleophiles are electron-rich species with the ability to contribute electron pairs. They are Lewis bases. The term “nucleophile” consists of two parts: nucleus and philos. Philos is a Greek word that means “love.” As a result, nucleophiles are known as Nucleus Loving species. These nucleophiles might have a positive or neutral charge.

Some terminologies associated to nucleophiles:

• The affinity of a species for the positively charged nucleus is defined by its nucleophilic nature.
• Nucleophilicity is a word that is used to assess the nucleophilic traits of various nucleophiles.
• Nucleophilic substitution is a method in which an electron-rich nucleophile selectively attacks a positively charged (or partially positively charged) atom in a molecule and bonds with the positively charged species to switch a leaving group.

TYPES OF NUCLEOPHILES:

Nucleophiles come in a variety of types. The following species are commonly found to be good nucleophiles:

• Halogens – A halogen’s diatomic form does not have nucleophilic properties. The anionic form of these halogens, on the other hand, is a powerful nucleophile. For example, in a polar, protic solvent, diatomic iodine (I2) does not operate as a nucleophile, whereas I– is the greatest nucleophile.
• Carbon – In many organometallic compounds, as well as in enols, carbon behaves as a nucleophile. Grignard Reagents, Organolithium Reagents, and n-butyllithium are examples of compounds in which carbon behaves as a nucleophile.
• Oxygen – The hydroxide ion is an excellent example of a nucleophile in which the oxygen atom donates an electron pair. Alcohols and hydrogen peroxide are two further examples. It’s worth noting that during the intermolecular hydrogen bonding that occurs in many compounds containing oxygen and hydrogen, no nucleophilic assaults occur.
• Sulphur – sulphur possesses several nucleophilic properties due to its enormous size, relative ease of polarisation, and easily accessible lone electron pairs. H2S (hydrogen sulphide) is an excellent example of a sulphur-containing nucleophile.
• Nitrogen – Nitrogen is known to create amines, azides, ammonia, and nitrides, among other nucleophiles. Even amides have been shown to have nucleophilic properties.

Apart from the species indicated above, it can be seen that as the ions become more basic as they travel through a row in the periodic table, their nucleophilic reactivity increases.

AMBIDENT NUCLEOPHILE:

Ambident Nucleophiles are nucleophiles that can carry out nucleophilic assaults from two or more separate locations in the molecule (or ion). These kinds of nucleophile attacks repeatedly develop in the making of various products.
The thiocyanate ion, with the chemical formula SCN-, is a case of an ambident nucleophile. This ion can aim for either the sulphur or nitrogen atoms with nucleophilic attacks. The creation of a mixture of alkyl isothiocyanates with the chemical formula R-NCS and alkyl thiocyanates with the chemical formula R-SCN is common in nucleophilic substitution reactions of alkyl halides involving this ion.
As a result, an ambident nucleophile is an anionic nucleophile in which the ion’s negative charge is delocalized across two distinct atoms due to resonance effects. Enolate ions are commonly found to have this property. A resonance structure of an ambident nucleophile is depicted in the diagram below.

NUCLEOPHILIC SUBSTITUTION REACTION:

A nucleophilic substitution reaction is one in which one nucleophile replaces another in an organic process. It’s very similar to conventional displacement reactions in chemistry, in which a more reactive element replaces a less reactive element in a salt solution. The “leaving group” is the molecule on which replacement occurs, and the “substrate” is the molecule on which the electron pair is shifted from the carbon. The departing group is a neutral molecule or a negative ion when it exits.

The nucleophilicity of a nucleophile is its reactivity or strength in nucleophilic substitution reactions. A stronger nucleophile switches a weaker nucleophile from its component in a nucleophilic substitution process. It can be nearly described as follows:

R-L+Nu- →R-Nu+L-, here:

• R is the alkyl group
• L is the leaving group, which is less nucleophilic
• Nu is the stronger nucleophile

Consider the following reaction as an example-

CH3-Br + OH- → CH3-OH + Br-

NUCLEOPHILICITY:

It is defined as the nucleophiles’ ability to associate their lone pairs with a positive centre. It’s a kinetic word that refers to the nucleophile’s rate of attack on the substrates (R – L). The following reasons can be used to compare the nucleophilicity of several nucleophiles.

MECHANISMS OF NUCLEOPHILIC SUBSTITUTION:

The pace of nucleophilic substitution processes is determined not only by nucleophiles and leaving capacities, but also by the reaction mechanism. For nucleophilic substitution reactions, two mechanisms have been postulated.

SN2 MECHANISM:

The reaction is referred to as the substitution nucleophilic bimolecular mechanism. It is governed by second-order kinetics, and the rate law for a process involving the SN2 mechanism is as follows. In the case of the SN2 reaction of the form

R-L+Nu-      →     R-Nu+L-
r=k[R-L][Nu-]

The rate of the SN2 reaction is dependent on both the substrate and nucleophile concentrations, according to the rate law. As a result, the rate of the reaction is increased by both the nucleophilicity of the nucleophile and the leaving capacity of the leaving group.

It’s a one-step method with only one intermediary. To prevent repulsions, it continues by the backside (of the L) assault of the arriving nucleophile, leading to an intermediate marked by two dotted lines between carbon – Nu and carbon-X. The C – X bond is broken and the C – Nu bond is produced at the same time, as indicated by the two dotted lines in the intermediate. Finally, the C–X Bond is totally shattered and the C–Nu Bond is fully created.

SN1 MECHANISM:

The process is known as unimolecular nucleophilic substitution. The rate law for the SN1 process with R – X as the substrate and Nu- as the entering nucleophile is as follows. 

r = k[R – X]

We can see from the preceding equation that the rate of the SN1 mechanism is solely determined by the substrate concentration and is unaffected by the concentration of the entering nucleophile. Indirectly, the rate is determined by the leaving group’s ability to leave, but it is unaffected by the nucleophilicity of the approaching nucleophile.

ELECTROPHILES: 

Electrophiles are positively charged or neutral species which have affinity towards electrons. They are also known as Lewis acids.

CONCLUSION:

Nucleophiles are those species which are proton loving. They undergo substitution reactions, which are SN1 and SN2. SN1 is unimolecular because the rate of reaction depends only on the concentration of the reacting molecule. SN2 is bimolecular because the rate of reaction depends on the concentration of substrate and the attacking nucleophile.

Electrophiles are electron loving species and undergo electrophilic substitution.