Half arrows

[Sarahi Garza 3k]

How do you know when to use the half arrows when representing a nucleophilic attack? In the Chapter four practice problems, for the halogens being broken off a Carbon bond (4.24), we've been using the full arrows, why is that?

[Lauren Trent 2A]

Half arrows are used when a single electron is transferred in the attack, compared to a full arrow when a pair of electrons is being transferred.

[Sylvia_Cai_3D]

You use the full arrows because two electrons from the Carbon bond are going to the halogen. Before the reaction the halogen has 3 lone pairs, which is 6 electrons and the Carbon and the halogen share electrons through a bond. During the reaction the electrons from the bond go to the halogen making it have 8 electrons now and a negative charge. It wouldn't be a half arrow because that would mean only 1 electron is transferred to the halogen which leaves the other electron on the Carbon, but that shouldn't happen since it is unstable.

[Nikhil_Madan_3C]

The practice problems deal with pairs of electrons being transferred so that's why mostly full arrows are used. Each bond consists of two shared electrons. The chlorine is more electronegative than the carbon so you can think about it like the chlorine pulling the electrons towards itself from the bond (away from the carbon). The electron rich Nucleophile is then attracted to the now positively charged carbon (electrostatics).

[Chem_Mod]

Single headed arrows are used to show single electron movement which is equivalent to radical chemistry. We will not be doing any mechanisms with radicals in this class so no need to worry about it.