Influence of electronic and structural effects on acidity Flashcards
what happens when there’s an decrease in pKa (increase in electronegativity)?
increase in acidity
more stabilization of conjugate base
what are the inductive effects of electron-withdraw groups equilibirum?
stabilizing of the molecule
what is the inductive effect of electrons being donated in an equilibrium?
destabilizing of the molecule
what do electron withdrawing groups do to the acidity?
increase it
what does increasing the halogen atom do to the acidity?
increase in acidity
one chlorine increases acidity by a factor of 100
increasing the inductive effect
what is trichloroacetic acid?
strong acid
what are the electron donating groups?
opposite effect
increases electron density
destabilizing conjugate base
therefore forming weaker acid
what are the most common electron donating group?
alkyl groups
how does hybridization effect acidity?
the hybridization of carbon atom attached to acidic H effects acidity
the more S orbital an atom has the closer it is to the nucleus so more electronegative, and more acidic and more stable
which are more electronegative: sp, sp2, sp3?
sp most
sp2
sp3 lowest
how is delocalization of charge in base a stabilising factor?
through resonance
effect of halogen group in a benzene?
the closer it is to the carboxyl group the lower the pKa
effect of nitrophenols in benzene with ortho or para names?
further delocalization of acidic charge
increasing acidity
is m-nitrophenol less acidic for having a higher pKa than o-nitrophenol and p-nitrophenol?
no because inductive effect helps stabilize the anion
effect of more nitroxide groups on a phenol?
if there are more nitroxide groups on a phenol the more acidic it is
what is a base?
will accept a proton by donating a pair of electrons
how do we measure the strength of a base?
using the pKa of its conjugate base
what is the difference between a strong base and a weak base?
strong base has a small Ka and large pKa
weak base has a large Ka and small pKa
what is bascicity?
‘ability of a compound to use its non bonding electrons to bond with a proton’
what makes an acid stronger?
increasing stability of conjugate base
favouring loss of proton from acid
what is a more stable base?
a weaker base
what is more ready to donate electrons to a proton, anionic base or neutral base?
anionic base
what happens to basicity as atom becomes more electronegative in a compound?
basicity decreases as atom becomes more electronegative
what makes a base stronger?
increasing stability of conjugate acid
what does H-bonding do to the positive charge on nitrogen compounds?
H-bonding stabilizes cations
what does increasing acidity do?
increases stability of conjugate base
how does stabilizing the conjugate base make a weaker base?
discourages protonation making it a weaker base
what does more stability mean about the base?
the more stable a base the weaker the base
what does delocalized elctrons mean about the basicity
if electrons are delocalized the less basic the compound is
what is the inductive effect of methyl addition to nitrogen?
increases basic strength
what happens when 2nd and 3rd methyl groups are added to ammonium?
2nd methyl group added causes a smaller effect on the pka
3rd methyl group added reduces basicity
what is the effect of electron withdrawing groups?
decrease electron density which destabilizes conjugate acid making it less likely to pick up a proton making the base weaker
relationship between acidity and p character?
the acidity of the of C-H decreases as the p character increases
why is it harder to protonate in sp orbital than sp3?
electrons held closer to nucleus in sp orbital so harder to protonate than if in sp3 orbital
what happens in the delocalization of charge of conjugate base of anions?
stabilizes anion
increase in ionization of anion
what happens when delocalization of base happens?
stabilizes base reducing basicity
when does resonance stabilization occur with an amide?
with O-protonated amide (where H has attached to oxygen)
not with N-protonated amide (where H has attached to nitrogen)
how does resonance stabilization occur in O -protonates amides?
lone pair on nitrogen is able to overlap into p system
how does resonance stabilization occur in amidines?
the nitrogen replacing oxygen becomes protonated allowing stabilization of cation
what are the properties of amidines?
strong bases
what are guanidines?
stronger bases than amidines
what makes aniline a weak base?
resonance occurs in an uncharged amine
how does the effect on aromatic ring increase
effect increased if electron-withdrawing group on ortho or para position on the aromatic ring
why do aromatic amine are a weaker base than aniline?
improved delocalization in free base
what effect do electron-donating groups have?
increase basicity
why does pyridine have a lower basicity than piperidine?
pyridine is aromatic with 6p electrons
planar
lone pair of electrons held in sp2 orbital so increased s orbital than piperidine
meaning lone pair of electrons are held closer on nitrogen so less readily bonded to a proton
why is pyrrole is a stronger acid than pyridine?
the lone pair is part of the delocalized p-system and not available to bond to a proton
what happens when a halogen is more electronegative?
greater inductive effect
will effect acidity more making the molecule more acidic
what effect does placing the electron-withdrawing group have on pka when its located further from the negative charge?
less inductive effect because the electrons have to transmitted through more bonds
how is the acidity of C-H groups adjacent to carbonyl increased?
resonance stabilization
why does adding a 3rd methyl group to nitrogen reduce basicity?
because the pka is measured in aqueous solution so there’s H-bonding which stabilizes the positive charge on the nitrogen and the more alkyl groups added the less this has an effect
