Rank the following substances in order of increasing acidity. (a) (CH3)2 CHOH, HC=CH,(CF3)2 CHOH

Rank the following substances in order of increasing...

Rank the following substances in order of increasing acidity. (a) $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHOH}, \mathrm{HC}=\mathrm{CH},\left(\mathrm{CF}_{3}\right)_{2} \mathrm{CHOH}, \mathrm{CH}_{3} \mathrm{OH}$ (b) Phenol, $p$ -methylphenol, $p$ -(trifluoromethyl)phenol (c) Benzyil alcohol, phenol, $p$ -hydroxybenzoic acid

Rank the following substances in order of increasing acidity.
(a) $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHOH}, \mathrm{HC}=\mathrm{CH},\left(\mathrm{CF}_{3}\right)_{2} \mathrm{CHOH}, \mathrm{CH}_{3} \mathrm{OH}$
(b) Phenol, $p$ -methylphenol, $p$ -(trifluoromethyl)phenol
(c) Benzyil alcohol, phenol, $p$ -hydroxybenzoic acid

Key Concepts

Functional Group Influence on Acidity

Different functional groups inherently exhibit varying levels of acidity due to their structural and electronic properties. For instance, carboxylic acids, phenols, and alcohols each show a characteristic acidity profile based on how well their conjugate bases are stabilized by resonance and inductive effects.

Resonance Stabilization

Resonance stabilization involves the delocalization of the negative charge over multiple atoms in the conjugate base. This delocalization spreads out and stabilizes the charge, leading to increased acid strength, particularly in aromatic systems and conjugated molecules.

Acid Strength and pKa

Acid strength is typically measured by the acid dissociation constant (Ka) or pKa, with lower pKa values correlating to stronger acids. This concept is fundamental when ranking substances by acidity, as it reflects how readily a proton (H+) is lost from a molecule.

Conjugate Base Stability

The stability of the conjugate base formed upon deprotonation is a key determinant of acid strength. The more stabilized the conjugate base, the more favorable the loss of a proton will be, making the original molecule a stronger acid.

Inductive Effects

Inductive effects arise from the electron-withdrawing or electron-donating nature of substituents attached to the acidic center. Electron-withdrawing groups stabilize the conjugate base through the -I effect, enhancing acidity, while electron-donating groups destabilize it, reducing acidity.

Transcript

00:01 So for this question, we're given a series of molecules in parts a, b, and c, and we're asked to rank them in order of increasing pca.

00:13 And so we can start with a.

00:18 So we see that we mainly have alcohol functionalities.

00:22 So we know that for a straight chain alcohol, we expect the pca.

00:37 To be roughly 15 to 16.

00:42 And we know that since we have an alkyne in here, the pca of an alkyne is about 25.

00:56 So right off the bat, we see that we have all alcohols and one alkyne.

01:02 So we can go ahead and say that our alkyne is the least acidic.

01:07 And we know that just by knowing some general trends in pca between different functional groups.

01:17 So now we need to think about which one would go next.

01:22 So we have a straight chain alcohol here.

01:28 So this one is roughly 15 to 16.

01:31 And now we have a non -straight chain alcohol and a non -straight chain alcohol.

01:39 So for this alcohol, because we have branching from the two methyl, from a methyl group here, this is actually going to pump electron density towards our oh group.

02:02 And it's going to destabilize the anion of the conjugate base.

02:10 So branching actually increases the pca.

02:17 And so this alcohol will go next.

02:25 And now we have our straight chain alcohol, the pca of 15 to 16...

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