Leaving group is a chemical species that departs from the rest of the molecule during a chemical reaction. It is an atom or group of atoms that is being substituted/replaced by a nucleophile or electrophile. Leaving group may leave with or without an electron pair.
It is known as nucleofuge in the case of nucleophilic substitution reactions and electrofuge
in the case of electrophilic substitution reactions.
Explanation
The term leaving group is used in substitution reactions. In the substitution reactions, an atom or group of atoms that are substituted by electrophile or nucleophile is called leaving group. leaving groups may be anions, cations, or neutral molecules. In nucleophilic substitution reactions, the leaving group leaves with a pair of electrons. whereas, in electrophilic substitution reactions leaving group leaves without pair of electrons.\
Nucleofuge
A species that depart from a molecule with electron pair during heterolytic cleavage of covalent bond is known as nucleofuge. It may be a negatively charged or neutral atom. For example,
Electrofuge
A species that depart from molecule without electron pair during heterolytic cleavage of covalent bond is known as electrofuge. It may be a positively charged or neutral atom. For example,
Leaving group as a base
In the case of nucleophilic substitution reactions, the leaving group is also a base. Because the nucleophile donates a pair of electrons whereas the leaving group accepts a pair of electrons. So both of them have an electron pair. But the main difference is the strength of the base. The nucleophile is a strong base whereas the leaving group is a weak base. So the leaving group is a nucleophile in the reverse reaction.
Rate of reaction
The rate of chemical reaction also depends on the type of leaving group. A good leaving group gives fast reactions whereas poor leaving groups give slow reactions.
Types of leaving groups
- Good leaving groups
Good leaving groups are weak bases. They can be stabilized easily. They can’t donate their electrons easily. I.e, TsO–, I–, Cl–, Br–, HSO4–, H2O etc
- Poor leaving groups
Poor leaving groups are strong bases. They can not be stabilized easily. They donate their electrons easily. For example, H–, CH2–, NH2–, F–, OH–
The ability of leaving group
The ability of leaving group is how easily an atom departs from the rest of the molecule and stabilizes itself. This will affect the rate of a chemical reaction greater the ability faster would be chemical reaction and vice versa.
Weaker the base better the leaving group
Factors affecting the ability of leaving group
The ability of leaving group depends on the following factors
- Size of atom
As the size of atoms increases, basicity decreases. This is because they have less electron density. And we know that the weaker the base the better the leaving group so the ability of the leaving group increases.
So larger size atoms are good leaving groups as compared to smaller ones.
- Electronegativity
Electronegativity is the ability of an atom or molecule to attract a shared pair of electrons toward itself. As we move from left to right electronegativity increases and basicity decreases. When the basicity decrease ie makes species good leaving groups
- Resonance
Resonance stabilized molecules are weak bases because their electron pairs are not easily available for sharing. So resonance stabilized species are the good leaving group.
Properties of a good leaving group / what makes a good leaving group
- Basicity
Weak bases are good leaving groups
- Resonance
Resonance stabilized molecules are good leaving group
- Large size
Larger sized molecules are good leaving group
- Self Stabilization
Good leaving group stabilized itself easily
| Leaving group chart/order | |||
|---|---|---|---|
| Source | Leaving group | Nature | Strength |
| tosylate | TsO– | nucleofuge | Excellent leaving group |
| iodide | I– | nucleofuge | Excellent leaving group |
| HSO4– | nucleofuge | Good leaving group | |
| NO2 | Neutral | Good leaving group | |
| bromide | Br– | nucleofuge | Good leaving group |
| chloride | CI– | nucleofuge | Good leaving group |
| water | H2O | Neutral | Good leaving group |
| -N=N (N triple bond N) | electrofuge | Moderate leaving group | |
| ammonia | NH3 | Neutral | Moderate leaving group |
| Hydrogen ion | H+ | electrofuge | Moderate leaving group |
| Hydronium ion | H3O+ | electrofuge | Moderate leaving group |
| OR– | nucleofuge | Moderate leaving group | |
| ROCO- | nucleofuge | Moderate leaving group | |
| Hydroxl ion | OH– | nucleofuge | Poor leaving group |
| Floride | F– | nucleofuge | Poor leaving group |
| amines | NH2– | nucleofuge | Poor leaving group |
| CH2– | nucleofuge | Poor leaving group | |
| Hydride | H– | nucleofuge | Poor leaving group |
Concept bergs
What is leaving group?
Leaving group is an atom or group of atoms that depart from the rest of a molecule during a chemical reaction. It is being replaced by electrophile or nucleophile .
What are the major types of leaving groups?
There are two major types of leaving groups
- Good leaving groups
- Poor leaving group
What are the best leaving group ?
Weak bases are best leaving groups i.e, Tosylate, Iodide, Chlorides, hydrogen sulfate etc.
What makes a good leaving group?
There are following factor that make good leaving group
Weak base are good leaving groups
Resonance stabilized molecule are good leaving groups
More electronegative atom are good leaving group
Atom or group of atom that stabilize itself easily are good leaving groups
Differentiate between the nucleophile and leaving group?
Nucleophiles is a strong base whereas the leaving group is a weak base.
Nucleophile donates a pair of electrons whereas the leaving group accepts a pair of electrons.
Nucleophile comes to molecule whereas leaving group leave the molecule
What are good leaving groups? Give some examples?
Good leaving groups are weak bases.
They can be stabilized easily.
They can’t donate their electrons easily.
For example, TsO–, I–, Cl–, Br–, HSO4–, H2O etc
What are poor leaving groups? Give some examples?
Poor leaving groups are strong bases.
They can not be stabilized easily.
They donate their electrons easily.
For example, H–, CH2–, NH2–, F–, OH–
What is meant by the ability of leaving group?
Ability of leaving group is defined as the ability of a molecule to depart and stabilize itself.
Good leaving groups have greater ability whereas poor leaving group have weak leaving ability.
Give some examples of neutral leaving groups?
- Water
- Ammonia
Why are weak bases good leaving groups?
Weak bases are a good leaving group because they cannot donate their electrons easily and they can stabilize themselves more quickly as compared to strong bases.
Which is better, leaving group NH2– or OH– ?
Although, both NH2– and OH– are poor leaving group but OH– is good leaving group as compared to NH2– . because O is more electronegative than N so it cannot donate its electron easily
Is I or Br a better leaving group?
I is better leaving group as compared to Br because HI is a stronger acid as compared to HBr. so the stronger the acid, the weaker the conjugate base. And we know that weak bases are good leaving groups. That’s why I is good leaving group than Br
Which is a good nucleophile and a good leaving group?
Iodide ion is a good nucleophile as well as a good leaving group; this is because it can donate its electron pairs easily and it is a weak conjugate base of stronger acid. And also it has an empty d orbital so it can easily stabilize negative charge.
Is OET a good leaving group
Is azide a good leaving group
is water a good leaving group
Water is a weak conjugate base of stronger acid hydronium ions (H3O+) and has little tendency to share its electrons; that’s why water is a good leaving group.
Does a good leaving group favor SN1 or SN2 reactions?
Yes, a good leaving group favors SN1 or SN2 reactions. Because at which reaction proceeds depends upon leaving group. And also a good leaving group is easily replaced by nucleophiles.
Reference links
Wikipedia
Master organic
