Heterocyles - directing effect

Question 1

what is a heterocyclic compound?

Answer 1

a cyclic compound in which one or more of the ring atoms is an atom other than carbon

Question 2

why is the carbon substituted atom of a 6-membered heteroaromatic system in a trigonal planar?

Answer 2

to keep the hexagonal ring flat

Question 3

where does the 5 delocalised electrons in the 6-membered heteroaromatic system stay?

Answer 3

in the P orbital

Question 4

which is the weaker base, pyridine or piperidine?

Answer 4

pyridine

Question 5

which is more stable and why, pyridine or piperidine?

Answer 5

pyridine due to its aromaticity-so the C=C bonds cause resonance increasing stability.

Question 6

does the lone pair of electrons on nitrogen in a heteroaromatic structure interact with the pi cloud?

Answer 6

no the lone pair is in a plane of its own

Question 7

what is pyridine used as in a nucleophilic substitution reaction with acyl chloride?

Answer 7

it acts as a nucleophile to help make an ester from the acyl chloride

Question 8

what does the nitrogen atom in a heteroaromatic structure do to the rest of the ring?

Answer 8

it reduces the energy of all the orbitals making the ring a less reactive nucleophile and a more reactive electrophile as the nitrogen destabilises the cationic intermediate that would form in a nucleophilic reaction

Question 9

explain how pyridines are made in the Hantzsch pyridine synthesis mechanism?

Answer 9

1. aldehyde and keto-ester react in an aldol reaction
2. addition of keto-ester to the enone, then add ammonia causing cyclisation
3. oxidation to pyridine

Question 10

what is the most common chemical reaction heterocycles with >2 nitrogen atoms undergo?

Answer 10

nucleophilic substitution

Question 11

which mechanism is more common in pyrrole’s, nucleophilic or electrophilic substitution?

Answer 11

electrophilic substitution

Question 12

what is the big difference between pyrrole and pyridine?

Answer 12

the nitrogen lone pair is delocalised around the ring in a pyrrole but in a pyridine it’s in a plane of its own and doesn’t interact with the delocalised electrons.

Question 13

what is formed when you replace a carbon atom in a 5 membered heterocycle with a nitrogen atom at the 3 position?

Answer 13

an imidazole

Question 14

which is more basic and acidic pyrroles or imidazole?

Answer 14

imidazole