A nucleophile is the nucleus-loving species that always bear a lone pair, sometimes having a negative charge as well, and readily attacks the electrophilic center mostly carbon to initiate a chemical reaction whereas, the Base abstracts a proton from the substrate. A single species can be a base as well as a nucleophile depending upon its ability to attack the target, an electrophile, or a proton.
For example, The amine can be as a base in the first reaction and a nucleophile in the other two given below
Nucleophile vs base
| Nucleophile | Base |
| A nucleophile is nucleus loving species | A base attacks to get acidic hydrogen. |
| Nucleophilicity is the ability to attack electrophiles. | Basicity is the ability of a species to remove protons from an organic molecule. |
| They attack electron-deficient centers of organic molecules mostly carbon. | Base attack on the acidic proton |
| Nucleophilic reactions are kinetically controlled processes. | Basic reactions are thermodynamically controlled process |
| Nucleophiles form weak bonds | Base form strong bonds with acidic protons |
| Nucleophiles are lewis bases i.e. they have more tendency to share electron pair | Bases are bronsted bases, i.e., the attack on a substrate to get proton |
| Good electrophiles are those which has less electronegativity, are bigger in size, and can easily oxidize | Good bases are those which has higher electronegativity, are smaller in size, and cannot easily oxidize |
| Nucleophile attack is fast | Base attacks are slow or intermediate. |
| Examples of a few good nucleophiles are:CN-, Br-, RO-, I-, H2O, HS-,NH3 | Examples of a few good bases are:NH2-, F-, OH-, RO-, OH-, |
Nucleophile
A nucleophile seeks a positive center i.e., carbon in many organic compounds. Al nucleophiles are lewis bases (electron-pair donors). For example in a chemical reaction, Nu:- nucleophile attacks an electrophile the carbonyl carbon te pi bond between carbon and oxygen shifts to oxygen as a negative charge.
Significance
In organic synthesis, the terms nucleophile and electrophile are used for lewis bases and lewis acids respectively. This is due to the special behavior of attacking electrophilic carbon in these reactions.
Nucleophilicity and the measurement of nucleophilic strength
It is defined as the strength of nucleophiles to attack or initiate a chemical reaction. Generally, Nucleophilic strength is measured in terms of the relative reaction rate mostly with Nucleophilic substitution reaction Sn2. this is done because the Rate expression of Sn2 reaction is given by
K = [Substrate] [Nu:]
It shows that the nucleophile takes part in the rate-determining step Hence the strong nucleophile is the one that relatively reacts rapidly in the Sn2 reaction.
For example, if a chemist wants to compare two nucleophiles i.e., Ehoxide versus methanol both are treated separately with CH3I Methoxy iodide provides all other parameters remain the same for these reactions and the rate is measured.
Relative strength of Nucleophiles
The relative strength of nucleophiles can be correlated on the basis of the structural features as explained below
- Charge nucleophiles and their conjugate acids
Charged nucleophiles are always stronger nucleophiles than their conjugate acid, For example, OH- Is a stronger nucleophile than H20
- Presence of electron donating groups
The electron donating group enhances the power of a nucleophile.
Factors affecting the strength of nucleophile
The two main factors which may affect the strength of nucleophile (Nucleophilicity)
- Steric hindrance if the nucleophile or a substrate has a bulky environment it will not be able to attack electrophiles Steric and electronic effects are two of the main factors dominating the reactivity of nucleophiles i.e ter. butoxide is a good nucleophile and targets acidic protons
- Solvation If the reaction takes place in a solvent that has the ability to solvate the Nu:– may reduce its strength.
Nucleophilicity Vs, Basicity
Nucleophilicity is the strength of the species to behave prominently as a nucleophile in chemical reactions. This happens when a substance loses its electrons (lewis base). On the other hand, the base is the proton seeker to become neutral.
When these two entities are related they are measured by two different methods I.e., Basicity is measured by the position of acid-base equilibria in a reaction while the nucleophilicity is measured by the relative rates of Substitution Sn2 reactions.
For example, OH- is the relatively stronger base compare to CN- at equilibria because OH- ions have more tendency to attach with Proton. Moreover, in CN– ion is a much stronger Nucleophile to attack an electrophilic center.
Correlation of Nucleophilcity and basicity
The trend of nucleophilicity and basicity is the same for the group of nucleophiles with the same nucleophilic atom. But when that atom is different the trend does not remain the same.
Concepts Berg
What is a better nucleophile, Iodide or Flouride
Iodide is considered a better nucleophile because it has an available lone pair. It is less basic character compared to Floride due to the delocalized charge on the large surface area. it can attack more frequently at the electrophilic Carbon to initiate the reaction.
What Is A “Charged” Nucleophile or Base?
A nucleophile or a base both have lone pairs and if they have free electrons with a negative charge they are termed as changed species.
What’s the difference between nucleophilicity and basicity?
Nucleophilicity is the ability of a chemical species to attack the electrophilic center whereas, a Base is the proton seeker and its ability to do so is called basicity.
What is a good nucleophile but weak base?
Phenoxide ion is an example of a strong nucleophile but a weak base.
Does nucleophile mean base?
Nucleophile and base are parallel capabilities of some chemical substances substance in the c reactions. Nucleophile means nucleus loving.
Is nucleophile base or acid?
All nucleophiles are bases as well if they have more tendency to lose electrons than a lewis base we call them nucleophiles.
nucleophile vs electrophile?
A nucleophile is an electron-rich species whereas an electrophile is an electron-deficient electron deficient.
Are all nucleophiles Lewis bases?
All nucleophiles are lewis bases i.e. they have electron pairs available to radially donate to an electron-deficient center.
How can we identify base and nucleophiles from a number of given compounds?
Good nucleophiles are mostly good bases with few exceptions, i.e., solvation of nucleophiles may affect the nucleophilicity and steric hindrance.
Which is a better nucleophile an ethoxide ion or a hydroxide ion and why?
The presence of the electron-donating group in the ethoxide ion makes it more susceptible to losing electron pairs as compared to the hydroxide ion hence a better nucleophile
What are soft and hard nucleophiles?
Hard and soft nucleophiles are the categories based upon the charges i.e., H2O and OH- both have nucleophilic oxygen but in OH- a negative change shows one extra lone pair and increases the nucleophilic strength of the same oxygen termed as hard nucleophiles
Which is better nucleophilic, OH- or NH2-?
Amine is the better nucleophile in comparison. Since
Is PhO- a better Nucleophile than OH-?
The ability to donate a lone pair is defined as a nucleophile. A phenoxide ion has a lone pair spread delocalized on the benzene ring due to resonance effect, Hence this ability makes it a good nucleophile and a weak base. On contrary, OH- is a good base as well as a good nucleophile.
Why is CN- a better nucleophile than CH3O-?
CN- is considered a better nucleophile as compared with methoxide Because the electronegativity of the Oxygen atom is greater than Nitrogen. Nucleophilicity is defined as the ability to donate lone pair, Hence Nitrogen being less electronegative CN- is more convenient to lose electrons.
Why is RMgX nucleophilic?
RMgX is grignard reagent. It has highly reactive R-. It attacks electrophiles due to an unusual -ive charge on the alkyl R group.
Why does fluoride ion have high basicity?
Due to the highest electronegativity, fluoride ion does not allow its electron to go away but does not attack an acidic proton.
References
- Organic Chemistry (4th edition), By Francis A. Carey (University of Virginia)
- Organic chemistry (9th edition) By T. W. G. Solomons (Department of Chemistry, University of South Florida) & C. B. Fryhle (Chair and Professor of Chemistry at Pacific Lutheran University)
