CHEMISTRY COMMUNITY

How do you know if the arrow for a mechanism should be drawn from the actual element or from the bond?

Arrows shouldn't be drawn from an atom. Arrows are always drawn from an electron-rich source (lone pair or double bond) to an electron-deficient sink.

In electrophillic addition, an arrow is drawn from an alkene (double bonds are electron-rich) to an electrophile (partially or fully positive species).

In nucleophillic substitution, an arrow is drawn from the lone pair of a nucleophile (partially or fully negative species) to a partially positive carbon (the electrophile in this case).

Arrows can point toward an atom, if a bond is to be formed with that atom, but arrows are never drawn directly from an atom. Rather, they can be drawn from the lone pairs of an atom.

It should always be drawn from the bond or lone pairs because that is where the electrons are coming from. Arrows are always drawn from electron rich areas to electron deficient.

when there is a molecule such as Br2 why is it that the arrow goes toward the delta negative? or does it depend on the equation?

Br2 has a polar-covalent bond, so the delta negative atom is electron withdrawing, therefore attracting the electrons of the bond

Andrea Medina 1A wrote:when there is a molecule such as Br2 why is it that the arrow goes toward the delta negative? or does it depend on the equation?

The arrow goes to the δ+