The Arrow-Pushing Formalism
There are few things in organic chemistry that have no exception. I can only think of one rule (and subject of this section) that is never violated and only because its an agreed-upon rule (convention) or a general consensus. However, these are formal or artificial rules, conduct and other managerial aspects agreed upon in committee. thing. It’s also the subject of this article. These agreed upon formalisms, like formal charge, like nomenclature, etc., styles of drawing are not as important as arrow-pushing. You should be doing this fluidly and incessantly as you take an exam or are (seriously) studying.
The double barbed-arrow is a pair of electrons. There are two electrons on the arrowhead. The singly-barbed-arrow is a single electron (a “free” radical) we’ll see those in the sections that are relevant. For now, focus on the following:
Take note of where the electrons are coming from and going to. They are coming or moving from negative charge (something electron dense) to something positively-charged, even partially so, as it must be electron deficient for this attraction to exist. The arrow is logical. You are going from the source of electrons (since, where else would they come from?) to the sink or acceptor of electrons where a new bond will form that used to be this arrow.
The negative charge (electron-source) can be a lone pair. Or it can be a bond. The majority of what you’ll be doing with reaction mechanisms is making these arrows in the direction mentioned. It is no different than drawing resonance structures except, one species will alter the other. In resonance structures, all you do is move the electrons around, never nuclei.
Once you draw an arrow, move on by redrawing the structure and creating a bond from the arrow you just drew onto the newly drawn structure. You will continue doing this with an aim to showing what is going on and how aspects of the reaction like regiochemistry and stereochemistry occur. Sometimes, you’ll reach the end before even realizing it, at others you’ll not be sure. Either way, you are doing organic chemistry studying.
The election-movement arrows were originally coined “CURLY ARROWS” as Robert Robertson called them when he came up with the convention. It’s literally the most important skill to both envision and perform in learning organic chemistry.