The concept of hard and soft acids and bases.
Soft bases: the donor atoms are of relatively low electronegativity and high
polarizability and are easy to oxidize. High polarizability: they hold their valence
electrons loosely.
Hard bases: the donor atoms are of high electronegativity and low polarizability and
are hard to oxidize. They hold their valence electrons tightly.
Soft acids: the acceptor atoms are large, have low positive charge, and contain unshared
pair of electrons (p or d) in their valence shells. They have high polarizability and low
electronegativity.
Hard acids: the acceptor atoms are small, have high positive charge, and do not contain
unshared pair of electrons in their valence shells. They have low polarizability and high
electronegativity.
Saturday, July 31, 2010
Effect of diffrent factors on the strength of Acids and Bases
FIELD EFFECT:
The presence of the electron withdrawing substituents in the residues of acids in general provides stabilization of the carboxylate anion, and therefore enhances the acidity (eg. acetic acid: pKa=4.76, nitroacetic acid: pKa=1.68, fluoroacetic
acid: pKa=2.66).
This effect is, however, general (not limited to carboxyacids). We may say that groups that
withdraw electrons by the field effect increase acidity and decrease basicity, while
electron-donating groups act in the opposite direction.
OR
The effect that operates through the space but not through the bond is called field effect. It usually operates along through the inductive effect so it is difficult to remove or seperate the effect of them but the inductive depends on the nature of bonds.
2. RESONANCE EFFECT: Resonance effects that stabilize bases but not their conjugated
acids result in higher acidity (and vice versa). Example: beta-ketoesters are more acidic
than simple ketones or carboxylic esters.
In general, the electron withdrawing substituents increase acidity and decrease basicity,
while the electron donating groups act in the opposite manner.
OR
The decrease in electronegativity at one position and corresponding increases other place within the molecule due to the movement of pi electron or unshared paired of electron is known as resonance effect.
3. PERIODIC SYSTEM CORRELATION
* acidity increases / basicity decreases going from left to right in the PT (e.g. acidity
increases CH4 <> NH2- > HO- > F-).
*acidity increases / basicity decreases going from top to bottom in the PT (e.g. HF <> PH3 > AsH3).
This is related to the size of the species involved. For example, the small and hard F-
attracts a proton more than large and soft I-.
* acids that require only one electron pair to fill an outer shell are stronger acids than the
ones that need two electron pairs (e.g. GaCl3 is a stronger acid than ZnCl2).
*likewise, the acidity of MXn decreases in going down the PT (the size increases and the
attraction between a positive nucleus and a negative electron pair is weaker). BCl3 is
therefore stronger acid than AlCl3.
3. HYDROGEN BONDING:
Internal hydrogen bonding can greatly effect acidity and basicity. For example, the pKa of
ortho-hydroxybenzoic acid is pKa=2.98, while the pKa of the para-isomer is 4.58. Here the
intramolecular hydrogen bond stabilizes the carboxylate anion.
In this bond the hydrogen atom is bonded with two or more electronegative element. Internal hydrogen bonding can greatly effect the acidity and basicity ...
5. STERIC EFFECT:
The presence of the electron withdrawing substituents in the residues of acids in general provides stabilization of the carboxylate anion, and therefore enhances the acidity (eg. acetic acid: pKa=4.76, nitroacetic acid: pKa=1.68, fluoroacetic
acid: pKa=2.66).
This effect is, however, general (not limited to carboxyacids). We may say that groups that
withdraw electrons by the field effect increase acidity and decrease basicity, while
electron-donating groups act in the opposite direction.
OR
The effect that operates through the space but not through the bond is called field effect. It usually operates along through the inductive effect so it is difficult to remove or seperate the effect of them but the inductive depends on the nature of bonds.
2. RESONANCE EFFECT: Resonance effects that stabilize bases but not their conjugated
acids result in higher acidity (and vice versa). Example: beta-ketoesters are more acidic
than simple ketones or carboxylic esters.
In general, the electron withdrawing substituents increase acidity and decrease basicity,
while the electron donating groups act in the opposite manner.
OR
The decrease in electronegativity at one position and corresponding increases other place within the molecule due to the movement of pi electron or unshared paired of electron is known as resonance effect.
3. PERIODIC SYSTEM CORRELATION
* acidity increases / basicity decreases going from left to right in the PT (e.g. acidity
increases CH4 <> NH2- > HO- > F-).
*acidity increases / basicity decreases going from top to bottom in the PT (e.g. HF <> PH3 > AsH3).
This is related to the size of the species involved. For example, the small and hard F-
attracts a proton more than large and soft I-.
* acids that require only one electron pair to fill an outer shell are stronger acids than the
ones that need two electron pairs (e.g. GaCl3 is a stronger acid than ZnCl2).
*likewise, the acidity of MXn decreases in going down the PT (the size increases and the
attraction between a positive nucleus and a negative electron pair is weaker). BCl3 is
therefore stronger acid than AlCl3.
3. HYDROGEN BONDING:
Internal hydrogen bonding can greatly effect acidity and basicity. For example, the pKa of
ortho-hydroxybenzoic acid is pKa=2.98, while the pKa of the para-isomer is 4.58. Here the
intramolecular hydrogen bond stabilizes the carboxylate anion.
In this bond the hydrogen atom is bonded with two or more electronegative element. Internal hydrogen bonding can greatly effect the acidity and basicity ...
5. STERIC EFFECT:
Utility of Hammette Equation
1. I has been very successful in the treatment of the effect of group of para and meta position . It has also been in attempt to apply it to ortho position.
2. σ- determines the effect of Substituents and rho determones the sensitivity of the reactions.
3. Hammett e equation may be applied to many physical measurement including infrared frequency and NMR chemical shift.
4. It is successful to explain whether the substrate is attacked by electrophilic, nuclephilic or free radical reagent.
5. σ and rho value are also applicable for the prediction of electrophilic situation on reaction center,
2. σ- determines the effect of Substituents and rho determones the sensitivity of the reactions.
3. Hammett e equation may be applied to many physical measurement including infrared frequency and NMR chemical shift.
4. It is successful to explain whether the substrate is attacked by electrophilic, nuclephilic or free radical reagent.
5. σ and rho value are also applicable for the prediction of electrophilic situation on reaction center,
Hammette Equation
To give the numerical values of the effect of structure on reactivity the first try was taken by Hammett e and known as Hammette equation .
The basic equation is:
The basic equation is:
relating the equilibrium constant, K, for a given equilibrium reaction with substituent R and the reference K0 constant when R is a hydrogen atom to the substituent constant σ which depends only on the specific substituent R and the reaction constant ρ which depends only on the type of reaction but not on the substituent used.
The equation also holds for reaction rates k of a series of reactions with substituted benzene derivatives:
Subscribe to:
Posts (Atom)
