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03 February 2022
When an organic chemistry student, among the trickiest issues you will come across is the reaction list of substitution and elimination doubts. This is due to the common student mindset of master, memorize, and apply. Many factors has to be taken into account to fully understand these kinds of reactions. On this page I will assist you to understand how to carry out an SN2 reaction the moment faced with a poor hydroxyl forcing group.
Why don't we start with a rapid overview of the SN2 problem mechanism. SN2 stands for bimolecular nucleophilic exchange. This problem occurs any time a strong nucleophile attacks your carbon pushing out the going out of group and attaching themselves in its place.
That reaction relies on a number of factors as follows:
SN2 requires a nucleophile strong enough to attack, ideally with a harmful charge
The leaving staff or LG ELECTRONICS must be on a methyl, most important, or second carbon. SN2 cannot take place with a tertiary LG
The LG must be willing to leave and web form a weak stable molecule or ion in option
Polar aprotic solvents will be preferred as a way not to are around or 'cage' the solid nucleophile
That is easy to understand and follow when provided with a reaction sequence affecting a 1-bromopropane reacting with sodium cyanide in DMSO. We're dealing with good LG on a major carbon. Cyanide is strong enough to encounter and the extremely aprotic DMSO won't get in the way.
But Hydrobromic Acid of us wonder what about a impulse such as methanol reacting with HBr?
In cases like this we have a perfect methyl h2o and involved, on the other hand OH- is actually a terrible forcing group. So terrible actually if the impulse were to be held, OH- would definitely turn around and re-attack once it gets kicked off of the molecule.
And what good is an important leaving organisation that just won't stay gone?
This task requires a little help on the hydrobromic acidity. We initially must convert OH in a better coming out of group by protonation. Methanol is in order to attack the H on HBr. The following protonates the alcohol and kicks off the negative bromide to reside as a decent nucleophile in option.
While OH YEAH is a bad leaving organisation due to creating OH- for solution, OH2+ is a great starting group. The OH2+ is now quite unsatisfied attached to the carbon atom. Oxygen just 1 single pair and 3 sigma bonds having a positive specialized charge.
Because it desperately would like to leave, bromine is pleased to oblige. Br- attackes the carbon and kicks the actual OH2 to create a neutral water molecule during solution