Mechanisms show how a reaction proceeds through a series of fundamental steps. Curved arrows show the movement of electrons in those steps. The arrows always move from nucleophile to electrophile, never electrophile to nucleophile.
A curved arrow is an arrow that indicates the direction of movement of the electrons in a reaction. This is sometimes referred to as electron pushing. Electron pushing is the flow of electrons from an electron-rich site to an electron-poor site. The movement will proceed from an electron-rich site, the tail of the arrow, to an electron-poor site, the head of the arrow. Double-headed (or double-barbed) arrows are used for all reactions except radical reactions, which use fish-hook arrows (single-headed or single-barbed arrows). Double-headed arrows are the traditional arrow where the head of the arrow looks like a triangle, and fish-hook arrows have an arrowhead that looks like a fishhook. The arrows indicate the direction of the flow of electrons. The tail of the curved arrow will always start at a bond or a lone pair. This often involves the breaking of a covalent bond. This site is rich with electrons and nucleophilic. The head of the arrow usually terminates at an atom, where a new bond forms and often another bond breaks. Curved arrows are drawn from a nucleophile (electron-rich species) toward an electrophile (electron-poor species). Chloride ion is nucleophilic and carries a negative charge. The chloride nucleophile will seek an electrophilic center, such as electron-deficient carbon. The chloride will form a covalent bond to the carbon, causing a covalent bond next to the carbon to break. Every bond-breaking and bond-making step is shown using curved arrows. Because the addition of the chlorine to the carbon will create a five-bonded carbon, which would exceed the octet rule, another curved arrow must be drawn to show the breaking of the carbon-oxygen bond. A curved arrow can never be written to exceed the octet/duet rule on period 2 atoms. If an arrow points to an atom that would exceed the octet/duet rule, another arrow showing the breaking of an existing bond must be drawn to prevent exceeding octet/duet rule.