Topic 2 - Heterocyclic and Aromatic Chemistry

Question 1

What is produced when a primary amine reacts with a ketone?

Answer 1

An imine.

Question 2

What is produced when a secondary amine reacts with a ketone?

Answer 2

An enamine.

Question 3

What is tautomerism?

Answer 3

A reaction that only involves the intramolecular transfer of a proton.

Question 4

What are the two forms of tautomerism for a ketone?

Answer 4

The ketone form (keto) and the enol form.

Question 5

What are the two forms of tautomerism for an imine?

Answer 5

The imine form and the enamine form.

Question 6

What are the names of the positions for substituted atoms in benzene?

Answer 6

1 - Ipso
2 - Ortho
3 - Meta
4 - Para

Question 7

How are benzene structures named?

Answer 7

Multiple substituents are quoted in alphabetical order.

Priority is assigned using Cahn-Ingold-Prelog rules.

Question 8

What is the bonding like in benzene?

Answer 8

All carbons are sp² hybridised.

The p-orbitals overlap to form a continuous π system that is delocalised equally over each carbon atom.

Question 9

How does the number of resonance forms link to stability?

Answer 9

The more resonance structures that can be drawn for a cation/anion, the more stable it is.

Question 10

What are the four conditions for Hückel’s rule of aromaticity?

Answer 10

Planar

Cyclic

Fully conjugated

(4n + 2) π electrons where n is an integer (including zero).

Question 11

What is pK_a?

Answer 11

The measure of acidity and basicity.

It describes how much of the acid has been deprotonated and exists in its conjugate base form. The lower the pK_a, the stronger the acid.

Question 12

What is an acid?

Answer 12

A species with a tendency to lose a proton.

Question 13

What is a base?

Answer 13

A species with a tendency to gain a proton.

Question 14

What three factors affect the strength of an acid?

Answer 14

The stability of the conjugate base: electronegative atoms and inductively and mesomerically withdrawing groups will stabilise the conjugate base.

The H-A bond strength: H-A bond strength decreases down the periodic table due to size mismatch of atomic orbitals.

The solvent: The better the solvent is at solvating/stabilising ions, the easier it is for the acid to be deprotonated.

Question 15

What is pK_aH?

Answer 15

The measure of the strength of a base. A higher pK_aH represents a stronger base.

Question 16

What is the main factor that affects the strength of a base?

Answer 16

The availability of its lone pair of electrons.

Electronegative atoms and inductively and mesomerically withdrawing groups make the lone pair less available. This reduces the basicity.

Question 17

How does the basicity of aniline compare with the basicity of cyclohexylamine?

Answer 17

Aniline is much less basic than cyclohexylamine.

This is due to the lone pair in aniline being <i> partially </i> delocalised into the aromatic ring.

Question 18

How does the acidity of phenol compare with the acidity of cyclohexanol?

Answer 18

Phenol is much more acidic than phenol.

This is due to the resonance stabilisation of the conjugate base.

Question 19

What are the features of the acidity of benzoic acid?

Answer 19

The benzoic acid anion cannot be stabilised through resonance. However, its acidity is still affected by substituents.

Question 20

What is the bonding like in pyridine?

Answer 20

All carbon and nitrogen atoms are sp² hybridised.

1 electron in the nitrogen p-orbital contributes to the π system.

The nitrogen lone pair is orthogonal to the ring and it is <b>not</b> delocalised.

The nitrogen atom causes lower HOMO and LUMO orbital energies than benzene.

Question 21

What is the pK_a for the protonation of alcohols?

Answer 21

0.

Question 22

What is the pK_a for the deprotonation of carboxylic acids?

Answer 22

5.

Question 23

What is the pK_a for the deprotonation of phenols?

Answer 23

10.

Question 24

What is the pK_a for the deprotonation of alcohols?

Answer 24

15.