arrow pushing mechanisms

[ZaitsevRules]

Hey all, just wondering if anybody had any insightful tips on sitting down and completing an arrow-pushing mechanism. I know it's a weird question but I always miss points on tests because I just don't know which electrophiles get protonated, etc. Any response is appreciated! thanks a lot

[EvolvEarth]

It depends on the mechanism. If your professor discusses a mechanism, all I can say is draw it out and memorize it, because reactions do some crazy things sometimes that you can predict. Electrons are the ones moving, never protons. If a hydroxyl group grabs a hydrogen, you draw the arrow pointing to the hydrogen. Remember resonance structures as well. I can't help you any more beyond that, you simply need to draw out mechanisms over and over again. Many similar mechanisms work similarly, but you'll always find something that is a little funky and may not figure out why.

[hermanntrude]

it's a difficult thing to do, for sure. Practise, practise practise, and learn the mechanisms in your textbooks. Never forget what electrons are and how they behave, that way you can often tell where they might come from and where they might go to. Also, don't forget that in many of the complex mechanisms, the key step is when four or six electrons move in a circle (an aromatic transition state).

[psynapse]

There are only two fundamental rules you need to know:

1. Electric effects -- resonance and induction.

2. Steric effects -- Also thermo and kinetic conditions.

Everything else falls into these two categories. Remember electro neg values and "NOF" is "enough" for hydrogen bonds... yeah weak I know. When you take these simple rules and principles of O-Chem into account your mechanisms should make more sense. Try not to simply memorize but to understand although as previously mentioned there are some weird reactions out there.

As a sider don't forget methyl and proton migrations to stabilize carbions. They always do if they can if that makes sense.

These are just some points to keep in the back of your mind while doing em.

[TJames]

1) Move pairs of electrons. Never + charges.

 

2) Because you are moving pairs of electrons, the source of the electrons must be an atom with a lone pair.

 

3) Whenever you push two electrons into an atom, if that atom already had 8 electrons, you must push two out. The exception is H+.

[DeanK2]

A thorough revision of your notes will help. It is certainly something that needs to be practiced as there will usually be one to three transformations per problem.

[TJames]

1) Move pairs of electrons. Never + charges.

 

2) Because you are moving pairs of electrons, the source of the electrons must be an atom with a lone pair.

 

3) Whenever you push two electrons into an atom, if that atom already had 8 electrons, you must push two out. The exception is H+.

 

I left something important out and unfortunately I cannot edit this post.

 

The source of electrons can be either a lone pair or a double bond but (almost) never a single bond. Therefore your arrow will always begin at a lone pair of electrons, or a double/triple bond between two atoms.

 

I find it much easier to do these correctly if you always circle the electrons that will be doing the moving. Don't merely start your arrow at the atom. You don't want to get into the habit of thinking that the arrows show ATOMS moving - there are many reactions in which the atoms barely move at all.

[MulderMan]

Usually depends on who is setting your test!

Some people like the arrows to finish at the middle of the bond, and some like them to finish at the other atom.

 

The key things are arrows should start from a lone pair, or a bond, i.e the source of electrons.

 

They should also be CURLY, straight arrows are for use in dative bonding (think inorganic metal-ligand).

 

Once you have done a few of the more basic mechanisms, if you are given two reagents in an exam it becomes a sixth sense to know how they will react!

 

Know your electrophiles and nucleophiles!