A. When a carboxylic acid is dissolved in isotopically...

a. When a carboxylic acid is dissolved in isotopically labeled water $\left(\mathrm{H}_{2} \mathrm{O}^{18}\right),$ the label is incorporated into both oxygens of the acid. Propose a mechanism to account for this. b. If a carboxylic acid is dissolved in isotopically labeled methanol $\left(\mathrm{CH}_{3}^{18} \mathrm{OH}\right)$ and an acid catalyst is added, where will the label reside in the product?

a. When a carboxylic acid is dissolved in isotopically labeled water $\left(\mathrm{H}_{2} \mathrm{O}^{18}\right),$ the label is incorporated into both oxygens of the acid. Propose a mechanism to account for this.
b. If a carboxylic acid is dissolved in isotopically labeled methanol $\left(\mathrm{CH}_{3}^{18} \mathrm{OH}\right)$ and an acid catalyst is added, where will the label reside in the product?

Key Concepts

Acid?Catalyzed Oxygen Exchange

This concept involves the mechanism by which the oxygen atoms in a carboxylic acid can undergo exchange with solvent molecules under acidic conditions. Protonation of the carbonyl oxygen increases the electrophilicity of the carbonyl carbon, facilitating the nucleophilic attack by water to form a tetrahedral intermediate. This intermediate permits the reversible exchange of oxygen atoms, ultimately allowing isotopic labels from the water to be incorporated into both oxygen positions of the acid.

Tetrahedral Intermediate Formation

A key intermediate in many carbonyl reactions, the tetrahedral intermediate forms when a nucleophile (such as water or methanol) adds to the electrophilic carbonyl carbon after protonation. The structure and reactivity of this intermediate enable the subsequent loss of a leaving group and the reformation of the carbonyl. In the context of isotopic labeling, the formation of this intermediate is crucial as it provides a pathway for the exchange or incorporation of labeled oxygen atoms.

Fischer Esterification Mechanism

This mechanism describes the acid-catalyzed reaction of a carboxylic acid with an alcohol to form an ester and water. In the presence of an acid catalyst, the carboxylic acid is protonated, and after nucleophilic attack by the alcohol, a tetrahedral intermediate is formed. The intermediate then undergoes elimination, leading to the formation of an ester. When isotopically labeled methanol is used, the labeled oxygen is incorporated into the ester, typically residing on the alkoxy side of the product.

Transcript

00:01 Starting off with some acetic acid, i believe.

00:06 Okay, and if we dissolve it in some labeled water, okay, where the label is oxygenation.

00:18 What will basically happen is we get some very nice incorporation.

00:23 Okay, so what's basically happening here is we're getting some nucleophilic atal substitution.

00:30 The water molecule could preferentially, you know, a type, at that site.

00:38 Okay, so assuming, okay, even without this being, you know, typically acid callized, it is possible for this oxygen to come up here and basically form some this intermediate, a tetrahedral intermediate with the oxygen 18 incorporated.

01:01 Okay, so if we just, right now, i was in a slightly acidic solution where we can can form this kind of a compound, okay, allowing water to be somewhat nucleophilic.

01:21 You will have some 18 at that site.

01:27 Then here we have basically three hydroxyl surrounding one electrophilic site.

01:35 So what happens here is we basically get either of the oxygen to party.

01:44 Okay.

01:46 So this one, for example, let's say kid leave to pick up a proton as this one comes down.

01:56 Okay.

01:57 And a water molecule could pick up this proton basically then to now form our 018 carboxinic acid.

02:09 Okay, so then we can get incorporation into that site...