L5: Electronegativity, pKa And The Inductive Effect
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What is electronegativity?
A chemical property that describes the tendency of an atom to attract a shared pair of electrons (or electron density) towards itself)
What factors affect electronegativity?
• nuclear charge (more protons = more pull) • number and location of electrons in the atomic shells
Why does electronegativity increase across the periodic table?
More protons in the nucleus increases the attractive force available
Why does electronegativity decrease down a group
Extrea shielding of the shells of electrons?
What is electropositivity?
A measure of an element’s ability to donate electrons - the opposite of electronegativity
What is the effect of electronegativity on charge distribution of a molecule?
Where there are differences in electronegativity, atoms will become slightly positive and slightly negative - electron density is pulled towards the most electronegative element
What is polarisation of a covalent bond and what does it lead to?
• the displacement of a shared electron cloud • results in covalent bond with a partial ionic character
When is the inductive effect stronger?
When the electronegativity is stronger
What is pKa?
The negative log10 of Ka
What happens when the value of pKa is smaller?
The stronger the conjugate acid (more likelt to lose H+) and the weaker the conjugate base
What are resonance forms?
Sets of different ways of drawing Lewis structures that describe the delocalisation of electrons
What is the mesomeric effect?
where the substituents have either electron withdrawing or donating properties based on relevant resonance structures
What is at the heart of the mesomeric effect?
overlap of a lone pair of electrons in a p-orbital with adj pi-bonding system
What are the signs for mesomeric effect?
+M: electron donating mesomeric effect -M: electron withdrawing mesomeric effect
What is an example of resonance?
Phenol (WA) once the proton is lost, the negative charge is delocalised around the aromatic ring, so being spread out = stabilised
What is an example of a strong mesomeric donator?
M+ NH2
What happens to the electron density in the delocalised pi system when there’s a strong mesomeric donator present?
it increases = the negative charge from the oxygen is less well stabilised = conjugate anion less stable = higher pKa
What happens when there is a negative mesomeric group?
It becomes deficient in electron density = stability increases = lower pKa
How to understand pKa of bases
• 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
What is a good rule of thumb with the pKa?
when pH = pKa 50% will be protonated
How to understand basicity of amines
• alkyl groups are slightly electron donating • the extra electron density pushed towards the nitrogen makes it more attractive to a proton
Why is the pKa for phenylamine lower than cyclohexylamine?
• 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
Why is the pKa for pyrrole lower than pyridine?
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
How to understand pKa for amides
• 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