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

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...

Alkynes: addition of H2 gas and Lindlar catalyst

Alkynes: addition of Hydrogen gas (H2) Performing a catalytic reduction on an alkyn will result in giving an alkane. The alkene intermediate will be formed in the process, but will immediately react into an alkane. The end result is just the formed alkane, without stacking of the alkene intermediate. The Lindlar catalyst In theory this is a "poisoned or defected" catalyst. If you use a normal catalyst you will get the above effect with your alkyne. The Lindlar catalyst contains Palladium combined with Calcium-carbonate and treated with Lead. Palladium is the actual catalyst, the calcium carbonate is the carrier of the substance and the lead is the poisonous compound. Using the Lindlar catalyst instead of another catalyst together with Hydrogen gas, will lead into forming the cis-alkene intermediate instead of the forming of the corresponding alkane.

Painkillers: brand names for paracetamol (= acetaminophen) in foreign countries

Do you ever need a medication when in a foreign country, but in this country the medication you need is not available under a certain name? It's useful to know the active compound of the medication! In this case, I will present to you a list of brand names which contain paracetamol (= acetaminophen) throughout the world. When you are in the local pharmacy, ask about the active compound ("paracetamol" or "acetaminophen") and they will know immediately what you mean! If you talk about a brand from your own country it can take a while until the pharmacist knows what you mean. Useful : in almost all countries, generic brands are available, usually under the name of the active compound, so just ask for "paracetamol" or "acetaminophen" if you would like the cheapest product. Paracetamol = Acetaminophen EUROPE Austria: Ben-u-ron, Mexalen Belgium: Algostase, Dafalgan Croatia: Daleron, Lekadol, Plicet Czech Republic/Czechia: Paralen ...

Pharmacist Blog about what I encounter and find useful/interesting

Search This Blog