Question

The number of resonating structures of allyl radical is:
A.1
B.2
C.3
D.4

Answer 1

Hint: To answer this question recall the properties of resonance structures. Resonance structures of any molecule are in which the chemical interaction and properties remain the same, but the electrons are distributed around the structure differently.

Complete step by step answer:
We know from the theory of resonating structures that they are the multiple structures with similar energy, the position of nuclei, bonding and the nonbonding pair of electrons. They are known as canonical structures and the molecule exists in nature as a superimposition of multiple Lewis structures. Let’s see an example of resonating structures:

An example of resonating structures
From the concept of resonating structures, we can say the following about existence of resonating structures of a molecule:
-Same positions of all atoms.
-The same number of valence electrons as electrons are not created or destroyed.
-Equivalent energy.
-Opposite charges on adjacent atoms to regain stability. If like charges are present on adjacent atoms, the resonating structure will be highly unstable.
-There are two resonance structures for allyl radical, as shown below. In the two resonance forms of the allylic cation, the positive charge is located on the terminal carbon atoms and never on the middle carbon:

Hence, the correct option is B.

Note:
There are two components to evaluating resonance structures - whether it is "legal" and whether it is "significant". The key to drawing resonance structures is to make sure you are not breaking or creating any new single bonds. The connectivity between atoms should stay the same. As long as you stick to that, the structures you draw should be valid resonance structures. Determining which structures are energetically reasonable is something that requires a bit more practice.