L5: Electronegativity, pKa And The Inductive Effect

Question 1

What is electronegativity?

Answer 1

A chemical property that describes the tendency of an atom to attract a shared pair of electrons (or electron density) towards itself)

Question 2

What factors affect electronegativity?

Answer 2

• nuclear charge (more protons = more pull)
• number and location of electrons in the atomic shells

Question 3

Why does electronegativity increase across the periodic table?

Answer 3

More protons in the nucleus increases the attractive force available

Question 4

Why does electronegativity decrease down a group

Answer 4

Extrea shielding of the shells of electrons?

Question 5

What is electropositivity?

Answer 5

A measure of an element’s ability to donate electrons
- the opposite of electronegativity

Question 6

What is the effect of electronegativity on charge distribution of a molecule?

Answer 6

Where there are differences in electronegativity, atoms will become slightly positive and slightly negative
- electron density is pulled towards the most electronegative element

Question 7

What is polarisation of a covalent bond and what does it lead to?

Answer 7

• the displacement of a shared electron cloud
• results in covalent bond with a partial ionic character

Question 8

When is the inductive effect stronger?

Answer 8

When the electronegativity is stronger

Question 9

What is pKa?

Answer 9

The negative log10 of Ka

Question 10

What happens when the value of pKa is smaller?

Answer 10

The stronger the conjugate acid (more likelt to lose H+) and the weaker the conjugate base

Question 11

What are resonance forms?

Answer 11

Sets of different ways of drawing Lewis structures that describe the delocalisation of electrons

Question 12

What is the mesomeric effect?

Answer 12

where the substituents have either electron withdrawing or donating properties based on relevant resonance structures

Question 13

What is at the heart of the mesomeric effect?

Answer 13

overlap of a lone pair of electrons in a p-orbital with adj pi-bonding system

Question 14

What are the signs for mesomeric effect?

Answer 14

+M: electron donating mesomeric effect

-M: electron withdrawing mesomeric effect

Question 15

What is an example of resonance?

Answer 15

Phenol (WA)

once the proton is lost, the negative charge is delocalised around the aromatic ring, so being spread out = stabilised

Question 16

What is an example of a strong mesomeric donator?

Answer 16

M+
NH2

Question 17

What happens to the electron density in the delocalised pi system when there’s a strong mesomeric donator present?

Answer 17

it increases
= the negative charge from the oxygen is less well stabilised
= conjugate anion less stable = higher pKa

Question 18

What happens when there is a negative mesomeric group?

Answer 18

It becomes deficient in electron density
= stability increases = lower pKa

Question 19

How to understand pKa of bases

Answer 19

• Dissociation of the protonated base to give free base and a proton
• Use pKa to determine how likely a base is to be protonated at a given pH

Question 20

What is a good rule of thumb with the pKa?

Answer 20

when pH = pKa
50% will be protonated

Question 21

How to understand basicity of amines

Answer 21

• alkyl groups are slightly electron donating
• the extra electron density pushed towards the nitrogen makes it more attractive to a proton

Question 22

Why is the pKa for phenylamine lower than cyclohexylamine?

Answer 22

• the lone pair of the N of phenylamine is sp2 hybridised
• can overlap with an aromatic system
  = reduces availability of the electron density at the nitrogen to pick up a proton

Question 23

Why is the pKa for pyrrole lower than pyridine?

Answer 23

Pyridine - lone pair is in an sp2 orbital and is available to proton
Pyrrole - lone pair on the nitrogen is involved in the aromaticity = not available to hydrogen bond

Question 24

How to understand pKa for amides

Answer 24

• have N as sp2 hybridised
• lone pair is in a p-orbital
• available to overlap with the carbonyl pi-bond
  = lone pair not available to pick up a proton

Question 25

How to understand carbonyl reactivity

Answer 25

• inherently a polar bond with delta- on O and delta+ on C
• strongly donating group (OH) reduces delta+ on C
  = reactivity towards nucleophiles is reduced
• for electron withdrawing, increase delta+ on C
  = promotes reaction to take place