Which of the following is paramagnetic and why?
Dear student
Out of the four structures given above, the structure 2 and 4 are both aromatic in nature as both of them are cyclic and also satisfy (4n+2) pi electrons. These there are no free electrons available in these two structures so they are not paramagnetic. Now out of structure 1 and 3, structure 1 is having a cyclic structure but has 4 pi electrons thus making it an anti aromatic compound which as such is not stable, so the double bond will then break to form a positive and a negative charge leading to the availability of free electron so it will be paramagnetic. Now in the third structure there are just 2 pi electrons so again satisfying (4n+2) pi electrons and thus is aromatic and hence stable with no unpaired electrons.
Regards
Out of the four structures given above, the structure 2 and 4 are both aromatic in nature as both of them are cyclic and also satisfy (4n+2) pi electrons. These there are no free electrons available in these two structures so they are not paramagnetic. Now out of structure 1 and 3, structure 1 is having a cyclic structure but has 4 pi electrons thus making it an anti aromatic compound which as such is not stable, so the double bond will then break to form a positive and a negative charge leading to the availability of free electron so it will be paramagnetic. Now in the third structure there are just 2 pi electrons so again satisfying (4n+2) pi electrons and thus is aromatic and hence stable with no unpaired electrons.
Regards