Skip to main content

Alkylhalides: Substitution reactions 3 (Sn2)

Alkylhalides: Substitution

(Ir)reversibility

Basic rule with substitution reactions: a Sn2 reaction always goes in the direction where the stronger base drives the weaker base away. The stronger base will the place of the weaker base, thus it is a irreversible reaction. For example have a look on following reaction: 

CH3-Cl  +   OH-     >>       CH3-OH   +   Cl-

Chloride is a weaker base than the added hydroxide ion, this means that the reaction is NOT reversible, thus irreversible.

Solvent

The problem with second row (O, N, ...) nucleophiles is that they are very small. Because they are so small, polar solvents are not fit for these nucleophiles. These nucleophiles get solvated very fast by these polar solvents, this blocks the nucleophile attacking the alkylhalide on the backside, and thus no reaction will take place. Conclusion: water and alcohol are NOT fit as solvent for 2nd row nucleophiles used with Sn2 reactions.

What solvents must be used then? The answer lies in Polar aprotic solvents. The most used of these solvents are DMF, DMSO and Acetonitril (CH3CN). These solvents can solvate a cation better than an anion, because their partial negative charge is on the surface of the molecule, the partial positive charge is covered in the molecule, thus more hidden for the nucleophile to react with. The nucleophile will be less likely solvated so this means it will be more reactive in these kind of solvents. 

Sn2 advantages

A lot of molecules with different functional groups (a very wide range of diversity) can be made with Sn2 reactions, a small overview can be found below:




Comments

Popular posts from this blog

Coca-Cola: Life - myth or truth?

"Coca-Cola Life" Recently the Coca-Cola company released a new beverage: the Coca-Cola Life drink. It should contain less sugar than the normal Coca-Cola drink; a part of the sugar is replaced with the recently approved sweetener from the Stevia plant (stevioglycosides). It's being sold as a 'natural' drink, but those stevioglycosides are as natural as the chemical sweetener aspartame found in Light and Zero drinks. However it is true that the amount of sugar is much lower (being reported as 33%, some say it's around 20%); yet approximately still 3-4 sugar cubes / 33 cl can (5-6 in a normal can). I bought it myself and found that the flavor has not really changed from the original Coca-Cola, however it does taste more "flat" - but definitely true it yourself. The outside of the can has the colour green. I personally do not associate this "healthy - natural" colour with Coca-Cola, but it's growing on me. In sum...

Erythropoietin: definition, structure, synthesis in vivo

Definition: EPO: = erythropoietin   A glycoprotein hormon-like structure, a sialoglycoprotein, which is an important factor in the survival, growth and proliferation of erythroid precursor cells (EPC) and it improves the de novo creation, differentation and growth of red blood cells (RBC). Thus, EPO controls the erythropoiesis = production of RBC. EPC: these are cells that are located in the bone marrow, will eventually form the RBC. RBC: cells responsible for the transport and distribution of oxygen throughout the body. Structure: Built out of 165 aminoacids (AA). They are all connected and form 1 polypeptide chain.  Although, within the chain, there are 2 disulfide bonds.  Respectively on positions: Cys7-161 and Cys29-33  Cys = cystein and the numbers indicate the positions these AA are located. Cystein structure. Available  sulfide  group for bonding There are also 4 positions where there is a possibility for glycosylation  Namel...

Alkynen: Elektrofiele additie

Elektrofiele additie: Waterstofhaliden op alkynen Voorbeeld van waterstofhaliden: HCl, HBr, HF, HI Gebruik van één equivalent waterstofhalide met alkyn (dus 1:1 geen overmaat geen tekort) Bij een overmaat aan het waterstofhalide zal een tweede reactie plaatsvinden op het gehalogeneerde alkyn, ter vorming van een geminaal regioisomeer.