Heteroaromatic Chemistry- Introduction

Question 1

What is pyridine and describe its structure

Answer 1

1. Replace C-H group in benzene with N

2. There is now a lone pair perpendicular to the plane - not involved in delocalisation

Question 2

What is the structure of pyridine similar to and what does this mean about its properties

Answer 2

1. An imine
2. Pyridines are electron deficient- the C=N unit is electron withdrawing
3. Hydrogens are deshielded

Question 3

Describe deshielding of protons in pyridine

Answer 3

1. Adjacent H is most deshielded
2. Next is least deshielded as resonance or pyridine never puts +ve charge on this position
3. Para H is in middle because resonance puts positive charge on it but it is further away from EWG

Question 4

Describe structure of pyrrole

Answer 4

1. Replace HC=CH in benzene with N

2. Lone pair has to donate 2 electrons to maintain aromaticity- Nitrogen is in the aromatic ring system

Question 5

What other atoms can be used to replace HC=CH in benzene

Answer 5

1. O= furan

2. S= thiophene

Question 6

What is the structure of pyrrole similar to and what does this mean about its properties

Answer 6

1. An enamine

2. Pyrroles are electron rich- nitrogen lone pair unit is electron donating

Question 7

Describe shielding of protons in pyrrole

Answer 7

1. Adjacent H is not as shielded as closer to the N

2. Para H is more shielded as the negative charge is pushed onto it and it is not as close to the N

Question 8

Compare 5 membered systems to 6 membered

Answer 8

1. 5 are more electron rich
2. More susceptible to SeAr
3. Electrons are in the system
4. 6 membered have lone pair out of system