acid_base_organic Flashcards
Define Bronsted/Lowry (BL) acid.
A BL acid is a molecule or ion that can donate a proton.
In organic chemistry, it is useful to consider the “acidity” of very poor (even so-called negligible acids), as this helps to inform about which protons are likely to participate in chemical reactions.
Define Bronsted/Lowry (BL) base.
A BL base is a molecule or ion that can accept a proton.
In organic chemistry, it is worth considering that anything with a lone pair of electrons is a potential BL base. Again this helps pinpoint where chemical transformations are likely to occur.
Define conjugate acid/base pair.
A conjugate acid/base pair are a pair of molecules/ions that differ by the presence of one proton. For example H3O+ and H2O are a conjugate acid/base pair.
Define Ka and pKa.
These symbols represent a constant ratio of products to reactants at equilibrium at a particular temperature.
Showing how these terms are calculated is perhaps the best way to define them. For the generic acid HA, the reaction with water would be:
HA + H2O ⇔ H3O+ + A-
and the Ka can be calculated as:
Ka = [H3O+][A-]/[HA]
The pKa is calculated as:
pKa = -log Ka
What information does a pKa tell us about conjugate acid/base pair.
For the generic acid HA, the reaction with water would be:
HA + H2O ⇔ H3O+ + A-
The pKa tells us how readily the HA can lose a proton. The larger the pKa, the harder it is for the proton to be lost.
The pKa also tells us about the strength of the conjugate base. For instance, a high pKa means a relatively poor acid (HA), but correspondingly it means a relatively strong conjuagate base (A-).
While we can report the pKb for a base, the more common practice is to provide the pKa for an acid and then make the necessary conclusion for the corresponding conjugate base.
Why is it possible to compare the relative acidities of the following without knowing their pKa’s?
CH3NH-, CH3NH3+, CH3NH2
We are able to compare the relative acidities because they represent a series of conjugate acid/base pairs.
CH3NH2 is always more acidic (lower pKa) than its conjugate base CH3NH-; in turn CH3NH3+ is always more acidic (lower pKa) then its conjugate base (CH3NH2).
How does comparing the relative stabilities of conjugate bases help us compare the relative acidities of two or more acids.
For the generic acid HA, the reaction with water would be:
HA + H2O ⇔ H3O+ + A-
For the generic acid HB, the reaction with water would be:
HB + H2O ⇔ H3O+ + B-
The more stable the conjugate base is, the more acidic (lower pKa) will be the conjugate acid. A conjugate base that is more stable means that more of the base will exist at equilibrium, which in turn, means the acid has been more effective at donating a proton. A more effective acid is a stronger acid.
Explain the significance of the “A” term in ARIO, when comparing the relative stabilities of conjugate bases.
In most examples, the conjugate base has a negative charge and therefore we are looking at to what degree the negative charge can be stabilized. The “A” in ARIO refers to the “ATOM” effect. That is, to what degree can the atom, which bears the negative charge, stabilize the negative charge.
Relative to other factors, the atom effect is usually the most important stabilizing factor.
Across the periodic table, the more electronegative atom can be better stabilize the negative charge (C- < N- < O- < F-).
Down the periodic table, the larger atom can better stabilize the negative charge (F- < Cl- < Br- < I-).
Explain the significance of the “R” term in ARIO, when comparing the relative stabilities of conjugate bases.
In most examples, the conjugate base has a negative charge and therefore we are looking at to what degree the negative charge can be stabilized. The “R” in ARIO refers to the “RESONANCE” effect. That is, to what degree does resonance stabilize the negative charge.
Relative to other factors, the resonance effect is usually the second most important stabilizing factor. This is because its effect can be relatively long range in “spreading” out the negative charge.
In relation to a negative charge, resonance will play a stabilizing role if there is an allylic lone pair (that is, the atom which bears the negative charge has a lone pair and is adjacent to carbon or nitrogen with a double bond). Or in other words if the lone pair is delocalized.
Explain the significance of the “I” term in ARIO, when comparing the relative stabilities of conjugate bases.
In most examples, the conjugate base has a negative charge and therefore we are looking at to what degree the negative charge can be stabilized. The “I” in ARIO refers to the “INDUCTIVE” effect. That is, to what degree does induction stabilize the negative charge.
Relative to other factors, the inductive effect is usually the third most important stabilizing factor. This is because it is a relatively short range effect.
In relation to a negative charge, induction will play a stabilizing role if there are electronegative atoms in the vicinity of the atom which bears the negative charge. It is important to distinguish this from the atom effect. The inductive effect is not about the electronegativity of the atom with the negative charge, but rather the electronegativity of the surrounding atoms.
Explain the significance of the “O” term in ARIO, when comparing the relative stabilities of conjugate bases.
In most examples, the conjugate base has a negative charge and therefore we are looking at to what degree the negative charge can be stabilized. The “O” in ARIO refers to the “ORBITAL” effect. That is, to what degree does the type hybridized orbital stabilize the negative charge.
Relative to other factors, the orbital effect is usually the least important stabilizing factor and should be considered last. There are, however, important exceptions. For example, the C- in acetylide anion (HCC-) is more stable than the N- in the amide anion (NH2-). According to the ATOM effect, this should be the opposite. Chemists justify this observation by noting that the C- in acetylide is sp hybridized, whereas the N- in the amide is sp3 hybridized.
Theory suggests that a negative charge will be more stabilized with a greater “s” character hybridization (e.g. sp vs sp3). The greater “s” character will allow the electron density associated with the negative charge to more fully experience the effect of the positive charge of the nucleus.
Consider the structural representation of vitamin B5 Which of the indicated hydrogens would be the most acidic?
The most acidic hydrogen would be the one marked with the letter d).
The conjugate bases when the proton is lost from a), b) and d), would all have a negative charge on an oxygen, which according to the atom effect is more stable than when the negative charge is on a nitrogen (conj base for proton c).
Of the conjugate bases with a negative charge on an oxygen, the one derived from proton d) would be the most stable b/c this conjugate base also has stabilization by resonance.
On side note, conjugate bases from b),c), and d) also have additional stabilization by induction and c) and d) conjugate bases also have additional stabilization due orbital effect,
Especially considering the atom effect and the resonance effect, the proton labelled d) is the most acidic.
The pKa for the most acidic proton of ascorbic acid is 4.17. Explain what happens to the population of ascorbic acid molecules when the pH is changed from 2.17 and is increased to 6.17.
At a pH of 2.17, the population of ascorbic acid molecules are predominantly in their conjugate acid form (by a ratio of 100:1). At a pH of 4.17, the population are in a 1:1 ratio of conjugate acid molecules and conjugate base anions. At a pH of 6.17, the conjugate base anions will predominate. by a ratio of 100:1
