How Can the Energy Stored in Functional Groups be Determined?

[samblohm]

How would I find the amount of energy that is stored in a particular functional group? I know things like Azide, Nitro, Alkynyl, Cyanides, etc. would all store a lot of energy.

 

[zhermes]

It would (approximately) just be the sum over all of the bond energies in the group.

 

[alxm]

What kind of energy? The total energy (which chemists never use)? The energy relative free atoms? The heat of formation (energy relative pure compounds)?

It's also only approximately the same for various functional groups. Otherwise, for instance, every carboxylic acid would have the same pKa. (whereas, say, the side-chain pKa's of aspartic and glutamic acid differ by 0.2 pH units, despite being about as similar as they could get) The difference in energy is small relative any number for the total energy, but it's a chemically relevant difference.

 

[samblohm]

By energy I meant the amount of energy that is released (or absorbed) during the dissociation of the bond.

 

[LudusRex]

The amount of energy stored in a particular functional group can be determined by measuring the enthalpy of formation of the molecule containing that functional group. This can be done experimentally by using techniques such as calorimetry or theoretically by using computational methods. The enthalpy of formation is the amount of energy released or absorbed when a molecule is formed from its constituent elements.

In general, functional groups that contain more highly electronegative atoms, such as nitrogen, oxygen, or halogens, tend to store more energy. This is because these atoms have a greater ability to attract and hold onto electrons, resulting in more stable and higher energy bonds. In comparison, functional groups with less electronegative atoms, such as carbon and hydrogen, tend to have lower energy bonds.

To specifically determine the energy stored in functional groups such as Azide, Nitro, Alkynyl, and Cyanides, one could look at the enthalpy of formation values for molecules containing these groups. For example, the enthalpy of formation for nitromethane, which contains a nitro group, is -76.2 kJ/mol, while the enthalpy of formation for acetonitrile, which contains a cyanide group, is -97.1 kJ/mol. These values indicate that the nitro group stores more energy than the cyanide group.

It is important to note that the amount of energy stored in a functional group can also depend on its chemical environment and neighboring functional groups. For example, a nitro group in a highly substituted molecule may have a different enthalpy of formation compared to the same nitro group in a less substituted molecule.

In summary, the energy stored in a particular functional group can be determined by measuring its enthalpy of formation, which is influenced by the electronegativity of the atoms involved and the chemical environment of the group.