Thursday, July 7, 2011

Nucleophiles and Bases


Similarities and Differences

Nucleophiles and bases are structurally similar. They both have either a lone pair or a pi bond. They differ from what they attack. bases attack protons while
nucleophiles attack other electron deficient atoms,
usually carbons.
Basicity is a measure of how readily an atom donates its electron pair to a proton.
It is characterized by an equilibrium constant in acid-base reactions
thus a thermodynamic property.
Nucleophilicity is a measure of how readily an atom donates its electron pair to another atom,
characterized by the rate constant
thus a kinetic property.

Nucleophilicity// basicity in 3 instances

1.) From two nucleophiles with same nucleophilic atom, the stronger base is the stronger nucleophile
eg :- OH- is stronger nucleophile than CH3COO- because H2O is more basic than CH3COOH ( pKa H2O > pKa CH3COOH )
2.) Negatively charged nucleophiles are stronger than its conjugated acid
eg :- OH- is a stronger nucleophile than H2O
3) In a row of the periodic table nucleophilicity and basicity increases from right to left.

Steric Hindrance

But when steric hindrance is involved nucleophilicity does not // basicity. More the steric hindrance is lesser the nucleophilicity is. eg :- (CH3)3O- has more steric hindrance than CH3Ch2O-.Out of both stronger base is (CH3)3O- while stronger nucleophile is CH3Ch2O-. This is because while a proton can move near the stericaly hindered (CH3)3O- the CH3 groups repel other larger atoms and groups.

Polar Protic Solvents
In polar protic solvents anions form strong inter molecular hydrogen bonds. Smaller nucleophiles are strongly solvated because smaller the nucleophile more concentrated the charge is.Therefore smaller nucleophiles are weaker nucleophilesin protic solvents such as water and ROH.
RS- is stronger nucleophile than RO-

Polar Aprotic Solvents

In polar aprotic solvents such as acetone,acetonitrile,... the anions form dipole - dipole inter molecular interactions and are incapable of forming hydrogen interactions. Cations form ion - dipole interactions. Because dipole -dipole interactions are not as strong as hydrogen interactions the anions are not very well solveted.. Therefore the anions in polar aprotic solvents are highly reactive as both nucleophiles and bases. Thus smaller the size of the atom stronger the nucleophilicity.

Common Nucleophiles
Negatively Charged
O : OH- ,OR-,CH3COO-
N : H3-
C : CN- , CHC-
X- : Cl-, Br-, I-
S : HS- , RS-

Neutral
O : H2O ,ROH
N : NH3 ,RNH2
S : H2S ,RSH

4 comments:

  1. Bio or Chemstry or whatever Nice work.........අර Nucleophiles ටික මගෙ ඔළුව වටෙ කෑරකෙනවා වගෙ..:D :D

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  2. hehe its organic chemistry
    :D
    thanks

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