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Nucleophile

Definition:

A nucleophile is a chemical species that donates an electron pair to an electrophile (electron-deficient species) to form a chemical bond. Nucleophiles are typically negatively charged or neutral molecules with lone pairs of electrons.

Examples of Nucleophiles:

  1. Negatively Charged Nucleophiles:
  • (OH^-) (hydroxide ion)
  • (CN^-) (cyanide ion)
  • (NH_2^-) (amide ion)
  • (CH_3O^-) (methoxide ion)
  1. Neutral Nucleophiles:
  • (H_2O) (water)
  • (NH_3) (ammonia)
  • (CH_3OH) (methanol)
  • (CH_3NH_2) (methylamine)
  1. π-Bond Nucleophiles:
  • Alkenes ((C=C)) and aromatic rings ((C_6H_6)) can act as nucleophiles due to their electron-rich π-bonds.

Nucleophilicity:

Nucleophilicity refers to the ability of a nucleophile to donate an electron pair. It depends on:

  1. Charge: Negatively charged species are stronger nucleophiles than neutral ones.
  2. Electronegativity: Less electronegative atoms are better nucleophiles (e.g., sulfur > oxygen).
  3. Solvent: Nucleophilicity is higher in polar aprotic solvents (e.g., DMSO, acetone) than in polar protic solvents (e.g., water, alcohols).
  4. Steric Hindrance: Less sterically hindered nucleophiles are stronger.
  • Trends in Nucleophilicity:
  • In polar protic solvents: (I^-) > (Br^-) > (Cl^-) > (F^-).
  • In polar aprotic solvents: (F^-) > (Cl^-) > (Br^-) > (I^-).

Electrophile

Definition:

An electrophile is a chemical species that accepts an electron pair from a nucleophile to form a chemical bond. Electrophiles are typically positively charged or neutral molecules with empty orbitals or electron-deficient centers.

Examples of Electrophiles:

  1. Positively Charged Electrophiles:
  • (H^+) (proton)
  • (CH_3^+) (methyl cation)
  • (NO_2^+) (nitronium ion)
  1. Neutral Electrophiles:
  • (BF_3) (boron trifluoride)
  • (AlCl_3) (aluminum chloride)
  • (CO_2) (carbon dioxide)
  1. π-Bond Electrophiles:
  • Carbonyl compounds ((C=O)), such as aldehydes and ketones, act as electrophiles due to the partial positive charge on the carbon atom.
  1. Alkyl Halides:
  • (CH_3-Br) (methyl bromide) and other alkyl halides are electrophiles because the carbon attached to the halogen is electron-deficient.

Electrophilicity:

Electrophilicity refers to the ability of an electrophile to accept an electron pair. It depends on:

  1. Positive Charge: Positively charged species are stronger electrophiles.
  2. Electron Deficiency: Molecules with electron-withdrawing groups (e.g., (NO_2), (CF_3)) are stronger electrophiles.
  3. Empty Orbitals: Species with empty orbitals (e.g., (BF_3), (AlCl_3)) are strong electrophiles.
  • Trends in Electrophilicity:
  • (NO_2^+) > (H^+) > (CH_3^+) > (BF_3).

Key Differences Between Nucleophiles and Electrophiles:

FeatureNucleophileElectrophile
DefinitionElectron pair donor.Electron pair acceptor.
ChargeNegative or neutral.Positive or neutral.
Examples(OH^-), (NH_3), (CN^-).(H^+), (BF_3), (NO_2^+).
ReactivityDonates electrons.Accepts electrons.
TrendsNucleophilicity increases with less electronegativity and less steric hindrance.Electrophilicity increases with positive charge and electron deficiency.

Example Problems:

  1. Nucleophile Example:
  • Identify the nucleophile in the reaction: [ CH_3-Br + OH^- \rightarrow CH_3-OH + Br^- ]
    • Nucleophile: (OH^-).
  1. Electrophile Example:
  • Identify the electrophile in the reaction: [ CH_3-CHO + HCN \rightarrow CH_3-CH(OH)-CN ]
    • Electrophile: (CH_3-CHO) (acetaldehyde).

Summary:

  • Nucleophiles are electron-rich species that donate electrons to electrophiles.
  • Electrophiles are electron-deficient species that accept electrons from nucleophiles.
  • Understanding nucleophilicity and electrophilicity is crucial for predicting reaction mechanisms and designing synthetic pathways in organic chemistry.

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