hypo- + -ite
FO-, hypofluorite ClO-,
hypochlorite BrO-, hypobromite
IO-, hypoiodite
Pyro is a prefix for designated compounds formed by heating a compound, usually with the removal of water, carbon dioxide, or other simple molecule/substance. E.g. pyroglutamic acid from glutamic acid.
Meta indicates the positions of substituents in aromatic cyclic compounds. The substituents have the 1,3-positions, for example in resorcinol. (organic chemistry)
Ortho describes a molecule with substituents at the 1 and 2 positions on an aromatic compound. In other words, the substituent is adjacent or next to the primary carbon on the ring. (organic chemsitry)
The prefix thio-, when applied to a chemical, such as an ion, means that an oxygen atom in the compound has been replaced by a sulfur atom. e.g. Sodium Thiosulfate (organic chemistry)
This is a classic example of stabilization by hydrogen bonding.
In the series of hydroxy-benzoic acids the order is:
- The OH in salicylic acid (the ortho-derivative) will stabilize the anion due to hydrogen bonding.
(hydrogen bonding is also possible in the protonated form (the acid) but is stronger in the carboxylate anion.
- For the para isomer, the mesomeric effect makes the compound less acidic in comparison with benzoic acid.
- For the meta isomer a minor inductive effect operates, but no resonance effect, which makes it slightly more acidic.
Remark: in solution the carboxylic acid part of the molecule is far more acidic in comparison with the phenol part, but in the gas-phase there are exceptions.
Lets start with some information we know so far : OH- has +R and weak -I effect and we know that -I is distant dependent and +R don't Show any effect at meta position and to your note ortho substituted benzoic acids are stronger acids than benzoic acids regardless of the nature of the substituent ( be it electron donating or electron withdrawing ) due to stearic and electronic reasons.
Let's start solving :
So blindly ortho would be most acidic ( due to ortho effect as explained earlier ) now although you didn't asked but i found it quite important to share so what happens at meta is bit deep than comparing at other positions , like I told you that Oh has +R and -I and at meta +R won't show its effect so only -I left which again helps in dispersing the negative charge in other words it is more acidic than benzoic acid but less acidic than ortho one., and finally at para position -I due to its distance dependence is very weak and +R is quite strong so para is least acidic as +R intensifies negative charge.
Result : ortho hydroxy B.acid > meta hydroxy B.acid > benzoic acid > para hydroxy B. acids.
So Answer to your question should be ,
Para is least acidic.
Ortho hydroxy benzoic acid is more acidic than it's para isomer because the carboxylate ion in ortho hydroxy benzoic acid get stable with intermolecule H bonding. ….But in case of it's para isomer OH group shows +R effect ,which increase the elctron density and that's why the stability of coo - decrease …For this reason ortho hydroxy benzoic acid is more acidic than ortho hydroxy benzoic acid. ..
Cyano-group is a linear group and hence is not supposed to cause any steric hinderance to the group present at ortho-position on the aromatic ring.
However, all three mono-substituted cyano-benzoic acids are more acidic than simple benzoic acid, ortho-isomer being the strongest [similar to ortho-effect].
Acidic Character order:
o-Cyanobenzoic acid [2.8] >
p-Cyanobenzoic acid [3.55] >
m-Cyanobenzoic acid [3.60] > Benzoic acid [4.2]
(pKa values are given in brackets; less pKa means more acidic)
Here, the inductive effect of the cyano group pulls electron density away from the carboxyl group, making the acid proton more positively charged, and therefore more acidic. Additionally, the -R effect of the cyano group delocalizes the negative charge from the carboxylate ion, leading to greater stability for 2-cyanobenzoate ion.
- Due to ortho effect, in most cases,-
- “Ortho-isomer of Benzoic acid is strongest Acid as compared to Simple benzoic acid or meta & para-isomers & Ortho- isomer of Aniline is Weakest Base as compared to Simple Aniline or meta & para-isomers.”
- In case of ortho-substituted benzoic acids, due to steric inhibition, the -COOH group goes out of plane and hence decrease in resonance stabilisation of acid as compared to anion make it better acid.
- Acidic Character order:
- o-Bromobenzoic acid [3.1] > m-BromoBenzoic acid [3.93]> p-BromoBenzoic acid [4.1] > Benzoic acid [4.17]
- o-ChloroBenzoic acid [2.89] > m-ChloroBenzoic acid [3.82]> p-ChloroBenzoic acid [3.98]> Benzoic acid [4.17]
- o-FluoroBenzoic acid [3.27] > m-FluoroBenzoic acid [3.87]> p-FluoroBenzoic acid [4.14]> Benzoic acid [4.17]
- O-Toluic acid [3.89] > Benzoic acid [4.17]> m-Toluic acid [4.28] > p-Toluic acid [4.35]
- 2,6-diHydroxybenzoic acid [2.3] > o-HydroxyBenzoic acid [2.98] > m-HydroxyBenzoic acid [4.08] > Benzoic acid > p-HydroxyBenzoic acid [4.58]
- o-NitroBenzoic acid [2.17] > p-NitroBenzoic acid [3.44] > m-NitroBenzoic acid [3.45] > Benzoic acid
(Note- pKa values are given in brackets; less pKa means more acidic)
When aniline acting as a base becomes NH3 + (on top of a benzene ring), it is usually stabilised by solvation. but if there is a substituent on the ortho position, it inhibits solvation. Thus the tendency to act like a base is reduced. Also, it can be explained by SIP (steric inhibition to protonation) effect.
- Basic Character order:
- o-Nitroaniline < p-Nitroaniline < m-Nitroaniline < Aniline
- o-Toluidine [4.38] < Aniline < m-Toluidine [4.67] < p-Toluidine [5.07]
- o-Fluoroaniline [pKa = 2.96] < m-Fluoroaniline [pKa = 3.38] < p-Fluoroaniline [pKa = 4.52] < Aniline (pKa = 4.6)
- o-Chloroaniline [pKa = 2.62] < m-Chloroaniline [pKa = 3.32] < p-Chloroaniline [pKa = 3.8] < Aniline (pKa = 4.6)
- o-Bromoaniline [pKa = 2.6, pKb = 11.4] < m-Bromoaniline [pKa = 3.5, pKb = 10.5] < p-Bromoaniline [pKa = 3.9, pKb = 10.1] < Aniline (pKa = 4.6)
(Note- pKa values are given in brackets; more pKa mean more basic; pKa + pKb = 14)
An acid is called a comparatively stronger acid if its anion is stable after release of the proton.
In case of p-hydroxybenzoic acid, the anion is destabilized by the +M effect of the hydroxy group lying on the p group.
Whereas, in m-hydoxy benzoic acid, the hydroxy group does not provide +M effect to the C to which the acidic group is attached.
Hence, m-hydoxy benzoic acid is stronger than its para derivative.
You can draw the resonating structures to see where the negative charges are formed. If negative charge is formed the C where the acidic group is attached, then it will destabilize the anion formed after releasing the proton.
In most cases ortho substituted benzoic acids are most acidic than their para and meta counterparts due to ortho effect. Now the question lies whether it is stronger than benzoic acid or not?. First let us compare the acidity between meta-hydroxy benzoic acid and benzoic acid. OH group being both -I and +R group will act as - I group in meta position and will withdraw electron thus decreasing the electron density on COO- and thus its acidity will be more than benzoic acid which has no electron withdrawing effect. So meta-hydroxy benzoic acid is more acidic than benzoic acid. Compared to meta-hydroxy benzoic acid it's ortho counterpart is much more acidic due to ortho effect. Hence ortho-hydroxy benzoic acid is more acidic than benzoic acid.
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