Attacking Reagents: Electrophiles & Nucleophiles
The species that drive organic reactions by accepting or donating electron pairs.
In an organic reaction, the organic molecule (Substrate) reacts with an **Attacking Reagent** to form intermediates and products. Depending on their affinity for electrons, these reagents are classified as **Electrophiles** or **Nucleophiles**.
1. Electrophiles ($E^+$)
Definition: Electron-loving species (Electron-deficient). They attack the region of high electron density in the substrate.
Since they accept electron pairs, they behave as Lewis Acids.
Types of Electrophiles:
-
Positively Charged Electrophiles: Species carrying a positive charge.
Examples: $H^+$ (Proton), $Cl^+$ (Chloronium), $NO_2^+$ (Nitronium), $R^+$ (Carbocation).
Note: Not all cations are electrophiles (e.g., $Na^+, NH_4^+$ have complete octets). -
Neutral Electrophiles (Incomplete Octet): Neutral molecules where the central atom has less than 8 electrons.
Examples: $BF_3$, $AlCl_3$, $BeCl_2$, Carbenes ($:CH_2$). -
Polar Functional Groups: Molecules with polar bonds where the central atom acquires a partial positive charge ($\delta+$).
Examples: Carbonyl carbon in $>C=O$, Sulfur in $SO_3$.
2. Nucleophiles ($Nu^-$ or $Nu:$)
Definition: Nucleus-loving species (Electron-rich). They attack the region of low electron density (positive center) in the substrate.
Since they donate electron pairs, they behave as Lewis Bases.
Types of Nucleophiles:
-
Negatively Charged Nucleophiles: Species carrying a negative charge.
Examples: $H^-$ (Hydride), $OH^-$ (Hydroxide), $CN^-$ (Cyanide), $R^-$ (Carbanion), $X^-$ (Halide). -
Neutral Nucleophiles (Lone Pairs): Neutral molecules with at least one lone pair of electrons.
Examples: $H_2\ddot{O}$, $\ddot{N}H_3$, $R-\ddot{O}-H$, $R-\ddot{N}H_2$. -
Pi-Bond Nucleophiles: Molecules with loosely held $\pi$-electrons.
Examples: Alkenes ($C=C$), Alkynes ($C \equiv C$), Benzene.
3. Ambident Nucleophiles
These are nucleophiles that possess two nucleophilic centers (two different atoms with lone pairs/charge) but attack through only one center at a time.
Attacks through C $\rightarrow$ Cyanides ($R-CN$)
Attacks through N $\rightarrow$ Isocyanides ($R-NC$)
Attacks through O $\rightarrow$ Alkyl Nitrite ($R-O-N=O$)
Attacks through N $\rightarrow$ Nitroalkane ($R-NO_2$)
4. Comparison Table
| Feature | Electrophile ($E^+$) | Nucleophile ($Nu^-$) |
|---|---|---|
| Affinity | Electron loving | Nucleus loving |
| Electron Density | Electron Deficient | Electron Rich |
| Lewis Character | Lewis Acid | Lewis Base |
| Charge | Positive or Neutral | Negative or Neutral |
Knowledge Check
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Great! I literally have a test the next day.thank you
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