SN2 & SN1 Reactions: Solvent Effects Explained

[khadijha.ji]

Salam o Alikum
WHY solvent effect the SN2 reaction as it effect the SN1 reactions where, it help in generation of carbocation?Although i know that a polar solvent like water can act as nucleophile but if in SN2 type of reaction solvent which can't act as nucleophile as their is STRONG nucleophile LIKE halide ions present than how it effect the reaction?

 

[DrDu]

I still don't understand your question too well but I'll try to answer.
The solvent may ease the reaction by stabilizing the transition state more than the reactants. E.g. if the nucleophile is an anion, the charge is normally more localized in the reactant than in the transition state where it is delocalized over both the nucleophile and the nucleofug (the group leaving). Now I would argue that a very polar solvent stabilizes the nucleophile more in relation to the transition state than does a weakly polar solvent.
Hence the activation energy is greater in case of a polar solvent than in case of a non-polar solvent. According to the Arrhenius equation the reaction rate is higher in case of a non-polar solvent.
Another aspect is activation entropy. The solvatization shell around an ion has a low entropy and is the larger the higher the charge density of the ion and the larger the polarity of the solvent. So under an entropic aspect, the reaction rate should be higher in a more polar solvent. As \Delta G=\Delta H -T\Delta S, a nucleophilic substitution should be eased by a non-polar solvent at low temperatures but by a polar solvent at high temperatures.
However, my organic chemistry classes are mostly long forgotten, so I don't know whether this is correct or not.

 

[Yanick]

Going through the in's and out's of solvent effects is way beyond the scope of a forum post so to really get a good handle on the concepts I suggest you read the relevant chapter(s) in your, or any, Organic Chemistry text.

Especially in terms of Sn type reactions you have to consider polar protic and polar aprotic solvents. Things like alcohols, water etc are polar protic because these molecules have accessible partial negative and partial positive charges (they Hydrogen bond and all that jazz). When salts dissolve in these types of solvents, both the anion and the cation are able to be solvated to a high degree.

Polar aprotic solvents are a different story. For example acetone. The dipole that is created across the carbonyl double bond gives this solvent polarity and the lone pairs on the oxygen are very effective at solvating cations. However, the partial positive on the carbonyl carbon is greatly shielded by the methyl groups on each side. Therefore when you drop something like NaCl into acetone the Na+ will be solvated completely but, the Cl- will have very weak interactions with the partial positive charge of the carbonyl carbon (due to steric reasons). NaCl may be a bad idea but I can't, for the life of me, think of anything else at this moment, my brain is fried from a long day. The effect of poor solvation of negative charges means that the solvent shell is held very loosely around the nucleophile making it much more reactive.

These are but a few of the things you must consider when thinking about Sn/E type reactions. Don't get too caught up with it, just consider the concepts of solvation, hydrogen bonding (the prerequisites required), acidity/basicity (and how electro/nucleophilicity somewhat parallels these characteristics), sterics etc. As I said, its very difficult to get into every detail of these reactions on a forum so your best bet is to really sit down, read the chapter and, most importantly, to think it through and understand it fully. Memorizing is really not necessary and its a shame that so many people spend so much time memorizing instead of thinking through the concepts and tying everything together.

 

[khadijha.ji]

salam o alikum
thanks for your response i think both of you don't understand my question?ok! know i have ask you my problem by giving some example
The processes of solvation take place i SN1 reactions as by solvation carbocation is formed and than attack of nucleophile will take place its is two step mechanism but as SN2 reaction is take place in one step where attack of nucleophile and leave of leaving group take place in one step only no formation of carbocation is there and also no chance of solvation is there than how a protic solvent effect the yeild of Sn2 reaction?

 

[Yanick]

Unless I am mistaken, you are asking about the solvent effects on Sn2 reactions?

If that is the case, re-read my post above. Solvent doesn't only interact with carbocations, it interacts with all ions in solution. Nucleophiles tend to be (not always of course) negatively charged and the degree of solvation will effect their nucleophilicities. Try to re-read my post but I highly suggest you read the relevant chapters in your, or any, Organic Chemistry text. A much more thorough discussion of solvent effects is provided in any intro O-Chem text.

 

[DrDu]

... and also no chance of solvation is there than how a protic solvent effect the yeild of Sn2 reaction?

I thought I had explained at length the effects of solvation on the rate of an SN2 reaction. Obviously there is a change.

Forgive me some tips on use of english language (it's also not my mother tongue): in english try to conjugate verbs: a protic solvent "effects" something, not "effect".
Personal pronoun is "I" not "i".