NUCLEOPHILES AND LEAVING GROUPS
1. What Are Nucleophiles?
Nucleophiles are “nucleus-loving” species. This means they donate electrons and attack electron-deficient atoms.
Nucleophiles can be:
- Neutral molecules with lone pairs – e.g., water, ammonia, alcohols, dimethyl sulfide, triphenylphosphine.
- Negatively charged ions – e.g., hydroxide, halides (Cl⁻, Br⁻, I⁻), alkoxide ions.
Nucleophiles always attack electrophiles (electron-poor species).
Ambident Nucleophiles
Some nucleophiles can attack from two different atoms. These are called ambident nucleophiles.
Examples:
- Cyanide ion (can attack from C or N)
- Isothiocyanate ion
- Nitrite ion
- DMSO (has both O and S with lone pairs)
For example, when DMSO reacts with methyl iodide, S-attack is preferred because sulfur is less electronegative and more nucleophilic than oxygen.
2. Factors Affecting Nucleophilicity
(a) Charge
A negatively charged species is always a stronger nucleophile than its neutral form.
Example: OH⁻ is stronger than water; RO⁻ is stronger than alcohol.
(b) Periodic Table (Across a period)
Nucleophilicity decreases as atomic number increases across a period (similar to decreasing basicity).
(c) Down a Group
Nucleophilicity increases down the group because:
- Atoms become larger
- They are more polarizable
- They are less strongly solvated
Thus, the order is:
I⁻ > Br⁻ > Cl⁻ > F⁻
(Iodide is the best nucleophile in protic solvents.)
(d) Solvation
Anions that are strongly solvated by water become weaker nucleophiles.
Example: F⁻ is heavily solvated in water → weaker nucleophile.
(e) Polarizability
Bigger atoms distort their electron cloud easily → better nucleophiles.
Example: MeS⁻ is a better nucleophile than MeO⁻ even though MeO⁻ is a stronger base.
3. Electrophiles
Electrophiles are “electron-loving” species. They are electron deficient and attack places of high electron density.
Types of electrophiles:
- Neutral Lewis acids: BF₃, AlCl₃, ZnCl₂
- Carbenes: :CH₂
- Positive ions: H⁺, NO₂⁺, carbocations
Electrophiles accept an electron pair to complete an octet.
4. Leaving Groups
A leaving group is an atom or group that detaches from the substrate with its bonding electrons.
Good leaving groups are usually weak bases because they can stabilize the negative charge after leaving.
Typical good leaving groups:
- I⁻ (best among halides)
- Br⁻
- Cl⁻
- Tosylate (TsO⁻)
Poor leaving groups:
- OH⁻
- NH₂⁻
- OR⁻
These poor groups are often converted into better leaving groups for reactions to proceed easily.
Detailed Notes:
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