l4 - electron density, inductive effects & electronegativity Flashcards
electronegativity
ability of an atom to attract a shared paired of electrons/ electron density towards itself. It is a chemical property
electropositivity
measure of an atom’s ability to donate electrons
what is electronegativity determined by?
- nuclear charge (more protons = more pull of electrons to nucleus)
- location of electrons in orbitals
- electron shielding
how does nuclear charge increase in the periodic table
as you go from
- left to right
- bottom to top
(more protons = more attractive forces)
electronegativity of fluorine
3.98
electronegativity of hydrogen
2.20
why is fluorine the most electronegative atom?
exerts the most pull on the sigma bond electrons to carbon
increased e- shielding leads to?
decrease in electronegativity
inductive effect
- effects of electronegativity on charge distribution of a molecule
- dipoles arise due to differences of electronegativity in atoms
what is a stronger inductive effect caused by?
stronger electronegativity
groups that are more electronegative than a carbon atom (delta negative)?
- halogens
- NO2
- OH
- SH
- SR
- NH2
- NHR
- NR2
- CN
- carboxylic acids
- aldehydes
- ketones
groups that are LESS electronegative than carbons (delta positive)?
- alkyl R groups
- metals (Mg)
pKa =
pKa = -log (ka)
what does a smaller pKa mean?
the stronger the conjugate acid is (acid more likely to lose H+ ions). stronger acids have more stable anions
why does the pka decrease when more chlorines are added to a carbon next to a carboxylic acid?
- cl is electronegative
- so electron density is being pulled towards the chlorine
- this causes the negative charge to stabilise across the anion
- decreasing the pKa
- so molecule with 3Cl is more stable due to e- being pulled towards all 3
what are resonance forms?
when electrons are delocalised into the model. remember to show that when you make a bond, you break a bond somewhere else.
bond migration is?
bond migration is concerted (happens all at same time)
resonance forms in a carboxylate ion
curly arrow from o- to bond, curly arrow from double bond to o.
switches it around.
in reality negative charge is stabilised/ spread across the anion, non localised.
mesomeric effect
lone pair next to a pi bond
(overlap of lp of e- in a p orbital with adjacent pi bonding systems)
electron withdrawing substituents and the mesomeric effect
-M
electrons are withdrawn from the pi bond.
orbital overlap causes the p orbital be low in electron density,
decrease in pKa, stronger acid, more stable.
electron donating substituents and the mesomeric effect
+M
lp donate electrons to the pi system.
causes pKa to increase so weaker acid.
so it is less stable
phenol and the mesomeric effect.
- weak acid
- once H+ is dissociated
- neg charge is delocalised around the ring
- so neg charge is spread out and stabilised
- this is a resonance form
what does it mean if the pH of a sol is equal to the pKa?
the solution will be 50% protonated
alkyl groups in alkyl amines
- alkyl group is slightly electron donating
- so e- is pushed towards the N
- makes N more negative and more attractive to H+ than just ammonia
why can the lone pair on a N atom of phenylamine not pick up a proton?
- the lone pair is sp2 hybridised
- so can overlap with the aromatic system
-which reduces the availability of N to pick up a proton (makes it a weaker base)
why can lp on N atom of pyridine accept H+?
- lp is pointing away from pi system
- lp is in an sp2 orbital
- still has low pka tho
pKa of amides
- N is sp2 hybridised
- lp in p orbital so overlaps with carbonyl =O bond
- so lp not available to pick up proton
- so pKa very low (-0.5)
- in order to protonate the N, pH must be very low
carbonyl reactivity
- polar oxygen is electronegative
- so pulls electron density towards it
- groups attached to carbonyl determine size of charge
- strongly donating group (OH) reduces delta plus on carbon so reactivity towards nucleophile is reduced
- electron withdrawing groups (acyl chlorides)| increase delta plus on carbon, increasing reactivity
