Reactivity of the Quinoline and Isoquinoline Ring

Introduction of heteroatoms in benzene ring results in an irregular distribution of electron density, and this strongly influences reactivity and physical properties of quinoline and isoquinoline heterocycles. The 

For C-NMR, quinoline presents a further down-field signal with a chemical shift of 150.23 ppm for C2 due to the inductive effect of nitrogen atom. C8a has a chemical shift of 148.16 ppm, whereas C4 has the value of 135.81 ppm. At a higher field of 129.32 ppm and 129.26 ppm C7 and C8 can be found, followed by the values of C4a, C5 and C6 at 128.11 ppm, 127.63 ppm, and 126.35 ppm, respectively. Finally, further high field C4 is observed with a chemical shift of 120.88 ppm 

On the other hand, isoquinoline shows a peak at 152.34 ppm for Cl, followed by a 142.86 ppm peak for C3 representing the two carbons, which are poor shielded due to the inductive effect fix)m nitrogen atom. C4a gives a resonance value of 135.51 ppm, whereas C6, C-8a, C8, C7, and C5 give values between 130.06 to 126.23 ppm. Once again, C4 provides the higher field signal at 120.21 ppm. 

Following the dipole moments presented, quaternary alkylations on nitrogen take place readily. But unlike pyridine, both quinoline and isoquinoline heterocyclic nitrogen promote the reaction with nucleophiles. What is more, electrophilic substitution takes place much more easily than in pyridine, and the substituents are generally located in carbocyclic ring preferentially in the more activated positions of the benzene ring, with a positional selectivity in the case of quinolines in the order of C8 C5 other positions. 

Some other physical properties are summarized in the Table below. Physical Properties of Quinoline and Isoquinoline 

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