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There are many things that determine if a reaction is SN1 or SN2. The things are briefly shown below:
1 .Effect of the Nucleophile :- The nucleophile takes part in the slow step (the only step)of the SN2 reaction but not in the slow step of the Therefore, a strong nucleophilepromotes the SN2 but not the SN1 . Weak nucleophiles fail to promote the SN2reaction;therefore, reactions with weak nucleophiles often go by the mechanism if the substrateis secondary or tertiary.SN1 :- Nucleophile strength is unimportant (usually weak).SN2:- Strong nucleophiles are required.2. Effect of the Substrate :- The structure of the substrate (the alkyl halide) is an importantfactor in determining which of these substitution mechanisms might operate.Most methyl halides and primary halides are poor substrates for SN1 substitutions becausethey cannot easily ionize to high-energy methyl and primary carbocations. Theyare relatively unhindered, however, so they make good SN2 substrates.Tertiary halides are too hindered to undergo SN2 displacement, but they can ionizeto form tertiary carbocations. Tertiary halides undergo substitution exclusively throughthe mechanism. Secondary halides can undergo substitution by either mechanism,depending on the conditions.. 3. Effect of the Solvent :-The slow step of the
SN1 reaction involves formation of two ions. Solvation of these ions is crucial to stabilizing them and lowering the activationenergy for their formation. Very polar ionizing solvents such as water and alcohols are needed for the SN1 . The solvent may be heated to reflux (boiling) to provide the energy needed for ionization.Less charge separation is generated in the transition state of the SN2 reaction. Strongsolvation may weaken the strength of the nucleophile because of the energy needed tostrip off the solvent molecules. Thus, the SN2 reaction often goes faster in less polar solventsif the nucleophile will dissolve. Polar aprotic solvents may enhance the strengthof weak nucleophiles.Good ionizing solvent required.May go faster in a less polar solvent.
The difficulty of assigning (R)/(S) to a stereocenter can be highly dependent on the way the molecule is drawn. For example, assigning the stereochemistry of A should be a cakewalk if you are the least bit familiar with the rules, while B and C are a little more difficult. The thing is, you’re not always going to be given a structure with the 4th ranked substituent in the back. One thing I see students do a lot in this situation is try to redraw the structure to make it easier to assign (R)/(S) While this can be effective, it can easily lead to mistakes. Furthermore, when you’re dealing with cyclic systems like morphine [C], trying to draw the mirror image in a reasonable time frame might induce brain trauma.
Instead of redrawing the whole molecule, just leave the hydrogen (or other 4th ranked substituent) in front and figure out whether the substituents ranked 1,2, and 3 go clockwise or counterclockwise. Then toggle (i.e. reverse direction CW to CCW or vice versa) to account for the fact that the 4th ranked substituent is in the front, and assign (R) or (S) based on that. You end up with the same answer without having to redraw anything.
We also make these types of adjustments when we’re looking in mirrors. If I’m looking in a mirror at you holding out your right hand, I account for the mirror by “flipping” your image to know that you are really holding out your left. It’s the same thing with molecules.
Determining (R)/(S) eventually becomes second nature, as does the ability to flip, rotate and invert chemical structures in your head.
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