The Importance in Direction of Curved Arrows For Electron Transfer

Question

Using the following electrophilic reaction:

CH3--CH=CH2 + Br2 --(ether)--> CH3-CHBr-CH2Br

In the first step of electrophilic attack, where the pi bond in a carbon carbon double bond attacks an electrophile (bromine), why would it be incorrect to draw the curved arrow from the double bond going to the left direction to attack a bromine atom? What does this incorrect representation of electron flow mean intuitively? Why is the right direction correct?

-Assume you have control over where the reagents can be drawn when drawing curved arrows

Answer 1

Let's number the carbons on our starting organic molecule for clarity:

CH3--CH=CH2

1 2 3

Background:

In organic chemistry, the purpose of curved arrows is to show the flow of an electron pair from a source (a lone pair or bond) to a sink (an atom or bond).

Reasoning:

In the first step for electrophilic attack, we use the carbon carbon double bond as an electron source and the bromine atom in Br2 as a sink. When we draw a curved arrow from the double bond to bromine, a new C-Br bond is formed between one of the original double bonded carbons (carbon 2&3) and a bromine atom. We need to indicate if the new bond is formed on carbon 2 or 3. Therefore, the direction of the curved arrow contains information about predicted bond formation.

Case 1- Left Arrow:

If the curved arrow begins at the double bond and goes left to attack bromine, then a new bond is formed at carbon 2, leaving carbon 3 electron deficient and forcing it to become an unstable primary carbocation. This is an energetically unfavorable configuration and thus is very unlikely to happen

Case 2- Right Arrow:

If the curved arrow begins at the double bond and goes right to attack bromine, then a new bond is formed at carbon 3, and a stable secondary carbocation is formed at carbon 2. This configuration is more favorable and thus the reaction will occur in high yield.

Personally, if the arrow goes to the right, I think "the atom to the right is taking the electrons and participating in some reaction."