Question

Which species in each pair is more stable?

Short answer

a)

Less stable because the positive charge is shared by two primary allylic carbons.

More stable because the positive charge is shared by two primary allylic and secondary allylic carbons.

b)

Less stable because charge is on primary allylic carbon.

More stable because charge is on secondary allylic carbon.

Less stable because negative charge is not delocalized.

More stable because negative charge is delocalized.

Less stable, because positive charge is shared by both O and N.

More stable, because positive charge is shared by two nitrogens. Nitrogen is less electronegative than oxygen , so nitrogen is more comfortable with a positive charge.

Step-by-step solution

Reason behind this 

a)

Less stable because the positive charge is shared by two primary allylic carbons.

More stable because the positive charge is shared by two primary allylic and secondary allylic carbons.

b)

Less stable because charge is on primary allylic carbon.

More stable because charge is on secondary allylic carbon.

Less stable because negative charge is not delocalized.

More stable because negative charge is delocalized.

Less stable, because positive charge is shared by both O and N.

More stable, because positive charge is shared by two nitrogens. Nitrogen is less electronegative than oxygen , so nitrogen is more comfortable with a positive charge.

Final answer

More stable because the positive charge is shared by two primary allylic and secondary allylic carbons.

More stable because charge is on secondary allylic carbon.

c)

d)

More stable, because positive charge is shared by two nitrogens. Nitrogen is less electronegative than oxygen , so nitrogen is more comfortable with a positive charge.