Monday, August 9, 2010

The Benzyne Mechanism

Unactivated halogens in aromatic compound undergo indirect nucleophilic displacement in presence of very strong base, which is clearly different from the SNAr mechanism . The mechanism followed by such reaction is known as Benzyne Mechanism.
The intresting features of benzyne mechanism is that the incoming group does not always take the position vacated by the leaving group. This reactions involved elimination on followed by addition . The intermediate molecule is called Benzyne .

Sunday, August 1, 2010

Neighbouring Group Participation

Neighbouring group participation or NGP in organic chemistry has been defined by IUPAC as the interaction of a reaction centre with a lone pair of electrons in an atom or the electrons present in a sigma bond or pi bond . When NGP is in operation it is normal for the reaction rate to be increased. It is also possible for the stereochemistry of the reaction to be abnormal (or unexpected) when compared with a normal reaction. While it is possible for neighbouring groups to influence many reactions in organic chemistry (For instance the reaction of a diene such as cyclohex-1,3-diene with maleic anhydride normally gives the endo isomer because of a secondary effect {overlap of the carbonyl group π orbitals with the transition state in the Diels-Alder reaction}) this page is limited to neighbouring group effects seen with carbocations and SN2 reactions.


NGP by heteroatom lone pairs
A classic example of NGP is the reaction of a sulfur or nitrogen mustard with a nucleophile, the rate of reaction is much higher for the sulfur mustard and a nucleophile than it would be for a primary alkyl chloride without a heteroatom.





In sugar chemistry anchimeric assistance is an example of NGP.
NGP by an alkene
The π orbitals of an alkene can stabilize a transition state by helping to delocalize the positive charge of the carbocation. For instance the unsaturated tosylate will react more quickly with a nucleophile than the saturated tosylate.





The carbocationic intermediate will be stabilized by resonance where the positive charge is spread over several atoms, in the diagram below this is shown.


SN2 Mechanism

The SN2 reaction (also known as bimolecular nucleophilic substitution or as backside attack) is a type of nucleophilic substitution, where a lone pair from a nucleophile attacks an electron deficient electrophilic center and bonds to it, expelling another group called a leaving group. Thus the incoming group replaces the leaving group in one step. Since two reacting species are involved in the slow, rate-determining step of the reaction, this leads to the name bimolecular nucleophilic substitution, or SN2.





Fig:
In an example of the SN2 reaction, the attack of OH− (the nucleophile) on a bromoethane (the electrophile) results in ethanol, with bromide ejected as the leaving group.
SN2 attack occurs if the backside route of attack is not sterically hindered by substituents on the substrate. Therefore this mechanism usually occurs at an unhindered primary carbon centre. If there is steric crowding on the substrate near the leaving group, such as at a tertiary carbon centre, the substitution will involve an SN1 rather than an SN2 mechanism, (an SN1 would also be more likely in this case because a sufficiently stable carbocation intermediary could be formed.)

SN1 Mechanism

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]


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.

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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