Question

In the acylium ion, the structure $R-C\equiv {{O}^{+}}$ is more stable than $R-{{C}^{+}}=O$.

Answer 1

Hint: The concept of resonance as well as octet rule is to be used in this question. Out of the resonance structures, the most stable structure is determined by the number of valence electrons.

Complete Solution :
- In order to answer our question, we need to learn about the octet rule, and resonance. Before we take up the concept of resonance or mesomeric effect, let us try to understand resonance or mesomerism. In many organic compounds, a single structural formula for a particular compound is not in a position to explain all the characteristics of that compound. In such cases, we make use of resonance or mesomerism. According to this, If a compound having a certain molecular formula can be represented by different structural formulae which differ only in the arrangement of the electron pairs and not of the atoms, such structures are called resonating or contributing or canonical structures. The compound as a whole cannot be represented by any of these structures but as a hybrid called resonance hybrid with characteristics of all contributing structures.
- Atoms of various elements try to gain, lose or share valence electrons during the formation of molecules in such a way that there are eight electrons or octets in their valence shells. This octet rule or electronic theory of chemical bonding was proposed by Kössel and Lewis in 1916. The maximum number of electrons an atom can hold is $2{{n}^{2}}$, where n is the number of shells.
- Now, let us come to our question. In our first structure, $R-C\equiv {{O}^{+}}$, every atom follows the octet rule and has 8 electrons each, be it of itself or shared. However, in our next compound $R-{{C}^{+}}=O$, the octet of oxygen is incomplete. It has 6 electrons and it violates the octet rule. Hence, the stability of $R-C\equiv {{O}^{+}}$ is more.

Note: For resonating structures, the difference in the internal energies of the most stable contributing structure for a particular compound and its resonance hybrid is known as Resonance Energy.