Good morning
The question is, The bond dissocaition energies for CH bond of methyl group in propane and propene is 89kj and 98kj respectively. Provide explanation for the relating lower value for propene over propane.
Thanks

Ask seun

Tell us say u know know chemistry goat

Vasgas:
Good morning
The question is, The bond dissocaition energies for CH bond of methyl group in propane and propene is 89kj and 98kj respectively. Provide explanation for the relating lower value for propene over propane.
Thanks

First, bond dissociation energy (BDE) relates to the amount of energy required by a molecule to generate radicals/uncharged atoms. In other words, it is the amount of energy needed by a molecule to undergo homolytic fission.

The lower bond dissociation energy for propene (89kJ) relative to that for propane (98kJ) can be explained using the concept of hybridization. Simply put, hybridization involves the mixing/combination of orbitals to produce a new set of orbitals. For instance, when a white man (take as an s-orbital) lays with, and impregnates a black woman (take as a p-orbital), they produce an half-caste child (an hybrid orbital) in form of any of an sp-/sp2-/sp3- orbital.

Basically, increasing hybridization corresponds to increasing BDE. Hence, sp>sp2>sp3. The propene molecule is sp2 hybridized while the propane molecule is sp3 hybridized. Understand?

For the propene molecule, the CH bond energy of the methyl group is lower than that of propane due to the influence/presence of the adjacent double bond/pi electrons in the (propene) molecule, CH3CH=CH2. These pi electrons tend to stabilize the electron-donating strength of the methyl (CH3) group in the molecule. In other words, the pi electrons or double bond stimulate(s) the withdrawal of electrons from the electron-rich site of the methyl group. Thus, resulting in the reduction of the CH bond strength of the methyl group in the propene molecule.

Conversely, the propane molecule neither has a double bond nor an electron-withdrawing species to effect the withdrawal of electrons from the electron-rich site(s) of the methyl group(s). There are two methyl groups in the propane molecule when you draw the structure. Hence, the CH bond energy of the methyl group in the (propane) molecule tends to remain on the higher side relative to the propene molecule.