Synthesis Topic 2

Question 1

What is the general tendency of acid?

Answer 1

To lose a proton.

Question 2

What is the general tendency of base?

Answer 2

To accept a proton.

Question 3

What is pK_a?

Answer 3

The measure of acidity and basicity.

Question 4

What pK_a tells about an acid?

Answer 4

It describes how much of the acid has been deprotonated and exists in its conjugate base form: it is a quantitative measure of acid strength in solution.

Question 5

What are the main factors that affect strength of an acid?

Answer 5

1. Stability of the conjugate base A^-: electronegative A and inductively and mesomerically electron withdrawing substituents stabilise A^- and make HA a strongger acid
2. The H-A bond strength: the weaker the bond the stronger the acid
3. The solvent: the better the solvent is at solvating/stabilising ions, the easier it is for the acid to be deprotonated.

Question 6

What pKaH tells about an base?

Answer 6

The lower the pK_aH the weaker the base.

Question 7

What is pK_aH?

Answer 7

The pK_a of conjugate acid

Question 8

What affects the strength of the base?

Answer 8

Any factor that increases the strength of an acid usually decreases the strength of the corresponding conjugate base.

The main factor that affects the strength of a base is the availability of its lone pair of electrons: electronegative B and inductively and measomerically electron-withdrawing substituents make lone pair less available for reaction with H⁺, making B a weaker base

Question 9

Why aromatic amines are weaker bases than aliphatic amines?

Answer 9

It is because in aromatic amines lone pair on N can be delocalised in aromatic ring. However, the N lone pair is in an sp³ orbital. Therefore, orbital overlap with the aromatic π-system is not optimal and delocalisation is only partial.

Question 10

What is the meaning of aliphatic compound?

Answer 10

Any chemical compound belonging to the organic class in which the atoms are connected by single, double, or triple bonds to form nonaromatic structures.

Question 11

What is mesomeric effect?

Answer 11

It is defined as the polarity produced in the molecule by the interaction of two π bonds or between a π bond and lone pair of electrons present on an adjacent atom.

Question 12

Why aromatic rings are more acidic than aliphatic ones?

Answer 12

Due to resonance stabilisation of the conjugate base.

Question 13

What is the effect of aromatic rings on radicals?

Answer 13

Aromatic rings stabilises radicals as radical can be delocalised to the aromatic ring.

Question 14

Why electron donating groups makes acid weaker?

Answer 14

Inductively electron donating group destabilises conjugate base.

Question 15

Why electron withdrawing groups makes acid stronger?

Answer 15

It stabilises conjugate base.

Question 16

Which compound is stronger base and why?

Answer 16

Oxygen is more electronegative than nitrogen, the lone pair on oxygen is held closer to the nucleus and thus less available for reaction with H⁺ making phenol a weaker base than aniline.

Question 17

What is formula for substituent constant?

Answer 17

Question 18

What positive σ means?

Answer 18

It indicates an electron-withdrawing substituent.

Question 19

What negative σ means?

Answer 19

It indicates an electron-donating substituent.

Question 20

What is the Hammett equation?

Answer 20

Question 21

What reaction constant tells us?

Answer 21

It tells us how the reaction responds to substituent effects and gives us an idea about the mechanism of the reaction.

Question 22

What positive ρ value means?

Answer 22

It means there are more electrons in the transition state than in the starting material: electron-withdrawing substituents increase rate of reaction

Question 23

Why Hammett effect does not work for ortho- substituents?

Answer 23

Because para and meta substituents have electronic effect and otrtho substituents have steric effects.

Question 24

What negative ρ value tells us?

Answer 24

It means fewer electrons are present in the transition state than in the starting material: electron-donating substituents increase reaction rate.

Question 25

Answer 25

Negative ρ means electron-donating substituents increase reaction rate. As a result RDS must involve a positively charged transition state as electron-donating substituent will stabilise positive charge.