The SN1 reaction is a substitution reaction in organic chemistry. "SN" stands for nucleophilic substitution and the "1" represents the fact that the rate-determining step is unimolecular.
SN1 reactions take place in two steps (excluding any protonation or deprotonation). The rate determining step is the first step, so the rate of the overall reaction is essentially equal to that of carbocation formation and does not involve the attacking nucleophile.
Thus nucleophilicity is irrelevant and the overall reaction rate depends on the concentration of the reactant only.
rate = k[reactant]
Sunday, August 1, 2010
Lewis Acid and Bases
Lewis had suggested in 1916 that two atoms are held together in a chemical bond by sharing a pair of electrons. When each atom contributed one electron to the bond it was called a covalentbond.
A Lewis acid, A, is a chemical substance that can accept a pair of electrons from a Lewis base, B, that acts as an electron-pair donor, forming an adduct, AB as given by the following:
A + :B → A—B
Following are some examples of reactions of Lewis acids; acids are the leftmost reactants (e.g. H+):
1.H+ + :NH3 → NH4+
2.Fe3+ + 6 H2O → Fe(III)-(OH)63- + 6 H+ (equilibrium reaction)
3.B2H6 + 2H− → 2BH4−
4. BF3 + F− → BF4−
5. Al2Cl6 + 2Cl− → 2AlCl4−
6.AlF3 + 3F− → AlF63−
7.SiF4 + 2F− → SiF62−
8.PCl5 + Cl− → PCl6−
9.SF4 + F− → SF5−
10.Metal ions forming solvates, such as [Mg(H2O)6]2+, [Al(H2O)6]3+, etc. where the solvent is a Lewis base.
A Lewis base is an atomic or molecular species that has a lone pair of electrons in the HOMO Typical examples are
compounds of N, P, As, Sb and Bi in oxidation state 3
compounds of O, S, Se and Te in oxidation state 2, including water, ethers, ketones, sulphoxides
molecules like carbon monoxide
An easy way to remember this concept is that nearly all of the compounds formed by the transition elements are coordination compounds, wherein the metal or metal ion is a Lewis acid and the ligands are Lewis bases.
A Lewis acid, A, is a chemical substance that can accept a pair of electrons from a Lewis base, B, that acts as an electron-pair donor, forming an adduct, AB as given by the following:
A + :B → A—B
Following are some examples of reactions of Lewis acids; acids are the leftmost reactants (e.g. H+):
1.H+ + :NH3 → NH4+
2.Fe3+ + 6 H2O → Fe(III)-(OH)63- + 6 H+ (equilibrium reaction)
3.B2H6 + 2H− → 2BH4−
4. BF3 + F− → BF4−
5. Al2Cl6 + 2Cl− → 2AlCl4−
6.AlF3 + 3F− → AlF63−
7.SiF4 + 2F− → SiF62−
8.PCl5 + Cl− → PCl6−
9.SF4 + F− → SF5−
10.Metal ions forming solvates, such as [Mg(H2O)6]2+, [Al(H2O)6]3+, etc. where the solvent is a Lewis base.
A Lewis base is an atomic or molecular species that has a lone pair of electrons in the HOMO Typical examples are
compounds of N, P, As, Sb and Bi in oxidation state 3
compounds of O, S, Se and Te in oxidation state 2, including water, ethers, ketones, sulphoxides
molecules like carbon monoxide
An easy way to remember this concept is that nearly all of the compounds formed by the transition elements are coordination compounds, wherein the metal or metal ion is a Lewis acid and the ligands are Lewis bases.
Super Acids
A superacid is an acid with an acidity greater than that of 100% pure sulfuric acid, which has a Hammett acidity function (H0) of −12. Commercially available superacids include Trifluoromethanesulfonic acid(CF3SO3H), also known as triflic acid, and Fluorosulfonic acid(FSO3H), both of which are about a thousand times stronger (i.e. have more negative H0 values) than sulfuric acid. The strongest superacids are prepared by the combination of two components, a strong Lewis acid and a strong Brønsted acid. The strongest known superacid is Fluoroantimonic acid.
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