What is the priority order of factors like resonance, hybridization, solvation, etc. in determining the relative acidity or basicity of compounds in a list?

Question

What is the priority order of factors like resonance, hybridization, solvation, etc. in determining the relative acidity or basicity of compounds in a list?

1 Answer

Impact of this question

17872 views around the world

You can reuse this answer
Creative Commons License

Answer 1 · 2016-11-26T21:12:38

WARNING! Long answer! Here are my thoughts.

Explanation:

Anything that decreases the $X-H$ $"X-H"$ bond strength in the acid or stabilizes the conjugate base will increase the acidity and decrease the basicity of the conjugate base.

I would identify the factors in the order 1. $R$ $bb"R"$ esonance; 2. $A$ $bb"A"$ tom; 3. $D$ $bb"D"$ ipole $I$ $bb"I""$ nductive effect; 4. $O$ $bb"O"$ rbitals ( $RADIO$ $bb"RADIO"$ ).

Strong acids

There are six common strong acids. In order of increasing acidity, they are:

${HNO}_{3} < H_{2} {SO}_{4} < HCl < HBr < HI < {HClO}_{4}$ $"HNO"_3 < "H"_2"SO"_4 < "HCl" < "HBr" < "HI" < "HClO"_4$

Any strong acid is stronger than most organic acids.

Some factors to consider are:

1. Resonance

I put resonance stabilization of the anion first in the list.

The more resonance contributors a conjugate base has, the stronger the acid will be.

That partly explains why sulfuric acid is stronger than nitric acid.

It also explains why acetic acid ( $p K_{a} \approx 5$ $"p"K_"a" ≈5$ ) is a stronger acid than
ethanol ( $p K_{a} \approx 16$ $"p"K_"a" ≈ 16$ ).

employees.csbsju.edu

2. Atom

(a) Acidity of H-X increases from left to right on the Periodic Table.

$m m m l {CH}_{4} < N H_{3} < H_{2} O < H F$ $color(white)(mmml)"CH"_4 < color(green)("N")"H"_3 < color(red)("H")_2"O" < "H"color(lime)("F")$

$p K_{a} m ~ 50 m m l l 38 m m l 15.7 m m 3.2$ $"p"K_"a"color(white)(m)~50color(white)(mmll)38color(white)(mml)15.7color(white)(mm)3.2$

As the electronegativity of $X$ $"X"$ increases, the bonding electrons are puled closer to $X$ $"X"$ and the acidity increases.

The exception is $HF$ $"HF"$ , for which the conjugate base $F^{-}$ $"F"^"-"$ is stabilized by solvation by water molecules.

(b) Acidity increases from top to bottom of the Periodic Table.

$m m m l HF < HCl < HBr < HI$ $color(white)(mmml)"HF" < "HCl" < "HBr" < "HI"$
$p K_{a} m l 3.8 m l l -8.0 m m -9.0 m l l -10$ $"p"K_"a"color(white)(ml)3.8color(white)(mll)"-8.0"color(white)(mm)"-9.0"color(white)(mll)"-10"$

As you go down a group, the bond length becomes longer and weaker, and the charge density on the ion decreases.

3. $D$ $bb"D"$ ipoles and $I$ $bb"I"$ nductive Effects

Electronegative atoms draw negative charge toward themselves.

This weakens the H-X bond and stabilizes the conjugate base.

The effect decreases with distance from the acidic $H$ $"H"$ atom.

Inductive Effects
(From SlidePlayer)

Alkyl groups are electron-donating groups.

They push electron density onto the $COOH$ $"COOH"$ group and destabilize the carboxylate ion.

The more alkyl groups, the weaker the acid.

4. Orbitals

The more $s$ $"s"$ character in an orbital, the closer the electrons will be to the nucleus.

As we go from ${sp}^{3}$ $"sp"^3$ (alkane) to ${sp}^{2}$ $"sp"^2$ (alkene) to $sp$ $"sp"$ (alkyne) hybridization, the stability of the negative charge increases.

Alkynes are remarkably acidic compared to alkanes.

11452-presscdn-0-51.pagely.netdna-cdn.com

Here's a summary of the discussion.

cssac.unc.edu

Science

Math

Humanities

... and beyond

What is the priority order of factors like resonance, hybridization, solvation, etc. in determining the relative acidity or basicity of compounds in a list?

1 Answer

Explanation:

Related questions

Impact of this question