In a Automatic Transmission vehicle, changing gear to Neutral at all possible times like,  


before breaking for stopping for a color light 
or driving downhill


will save the fuel in a considerable amount? or is it a bad practice?


Question: Energy required to break single bond vs double bond vs triple bond of a carbon atom in an organic molecule
Answer: 365 kJ/mol for the single bond, 598 kJ/mol for the double bond, and 813 kJ/mol for the triple bond. These are usually listed as bond enthalpy or heat of formation in scientific table books. If you don't find the one you need you can calculate it in steps according to Hess' Law.

If you want the energy required to break one bond as opposed to a mol of bonds, you divide the number I gave by Avogadro's number, which is the number of atoms or molecules in a mol of a compund or element.