Synthesis Topic 3

Question 1

What is heteroaromatic compound?

Answer 1

Benzene ring with carbon replaced by a heteroatom (e.g. O, N, S) in the ring

Question 2

What is the name of the compound?

Answer 2

Pyridine

Question 3

What is the name of the compound?

Answer 3

Pyrrole

Question 4

What is the name of the compound?

Answer 4

Furan

Question 5

What is the name of the compound?

Answer 5

Thiophene

Question 6

What are simillarities between benzene and pyridine?

Answer 6

Benzene has all carbons sp² hybridized, pyridine has carbons and nitrogen sp² hybridized. Benzene has six p-orbitals combine for delocalized π-system. In pyrine electron in N p-orbital contributes to π-system

Question 7

What is the difference between benzene and pyridine?

Answer 7

Pyridine N lone pair in sp² orbital orthogonal to ring: not involved in delocalization (it is available)

Question 8

How N atom in pyridine affects reactivity and properties?

Answer 8

Lower orbital energies than benzene (HOMO/LUMO) as it more electronegative than C

Question 9

Why pyridine is a weak base and a reasonable nucleophile?

Answer 9

As it has lone pair that is not decocalised around the aromatic ring and can easily react.

Question 10

Why pyridine is electrophilic?

Answer 10

As it has electronegative N that attracts electron density and makes the rest of the ring electron deficient.

Question 11

Why pyridine is weaker base than piperidine?

Answer 11

Pecause in pyridine lone pairs in sp² orbitals are less available (held closer to the nucleus) than lone pairs in sp³ piperidine orbitals (more p character)

Question 12

What are the effects of electron- withdrawing groups?

Answer 12

They decrease basicity, as it puls away electron density, making molecule less negative

Question 13

What are the effects of electron- donating groups?

Answer 13

They increase basicity, as it increase electron density making molecule more available to react with protons

Question 14

What is the effect in basisity if we substitute more N atoms to benzene ring?

Answer 14

Basicity decreases as we add more electron withdrawing N.

Question 15

Why 3-hydroxy and 4- hydroxypyridine are less basic than 3-hyrdoxypyridine?

Answer 15

Bevause 3 and 4 exist as two tautomeric forms where H can be attached to N or O

Question 16

Why pyridine is used as catalyst?

Answer 16

N is less electronegative than O thus making a better nucleophile than ROH. Intermidiate formed has poisitive N leaving group.

Question 17

When used as a nucleophilic catalyst in ether syntheses, stoichiometric amounts of pyridine are necessary. DMAP, however, can be used in catalytic amounts. Why do you thing this is the case?

Answer 17

The dimethylamino substituent in DMAP is electron donating, increasing electron density on the nitrogen and making lone pair more available for nucleophilic attack than that of pyridine. DMAP is therefore a stronger nucleophile than pyridine and more efficient nucleophilic catalyst.

Question 18

What is the difference in N of pyridine and pyrrole?

Answer 18

Pyridine has N sp³hybridized a,d pyrrole has N sp² hybridized meaning lone pair is in the 2p orbital parallel to the carbon 2p orbitals and thus contributes 2 electrons to the π system

Question 19

Why pyrrole nitrogen is non-basic and non-nucleophilic lyke pyridine?

Answer 19

Because the lone pair is delocalised around the aromatic ring: it is unsvailable for reactivity

Question 20

Why pyrrole has more electron-rich ring compared to pyridine?

Answer 20

Becaue pyrrole has two extra electrons from lone pair delocalised in the ring.

Question 21

What is the difference of resonance stabilisation of pyrrole and furan compared to benzene?

Answer 21

O and N are more electronegative than C, meaning poor orbital overlap and electron delocalisation, decreased resonance stabilisation

Question 22

What is the difference of resonance stabilisation of thiophene compared to benzene?

Answer 22

Electronegativity of S is comparable to C, meaning good orbital oeverlap, lone pair is fully delocalised into the π-system, strong resonance stabilisation

Question 23

Why imidazole is more reactive than pyrrole?

Answer 23

Because imidazole has one pyrroli-like nitrogen, the lone pair which is delocalised into the π-system and one pyridine like nitrogen, the lone pair of which is orthomonal to the aromatic π-system

Question 24

How can you protonate pyrrole?

Answer 24

Using very strong acid, but it protonates on the C not N