Of the two procedures just described, preparation and carboxylation of a Grignard reagent or for

Of the two procedures just described, preparation and...

Key Concepts

Functional Group Compatibility

In organometallic reactions, especially those involving Grignard reagents, the compatibility of functional groups is essential. The presence of protic or other reactive groups (such as hydroxyl groups) can interfere with the formation and usage of Grignard reagents, as these reagents are highly reactive towards acidic hydrogens. Recognizing which functional groups may impede a reaction pathway is fundamental in choosing the appropriate synthetic route to form the carboxylic acid.

Substrate Structure and Reaction Mechanism (SN2 vs. SN1)

This key concept highlights how the structure of the substrate (primary, secondary, tertiary, or aromatic) determines the suitability of a reaction mechanism. SN2 reactions, which are required for effective nitrile formation, are favored by primary substrates and hindered by steric bulk in tertiary substrates or by poor leaving group activation in aromatic systems. Understanding this helps in selecting the appropriate procedure for converting an alkyl halide to a carboxylic acid.

Grignard Reagent Formation and Carboxylation

This concept covers the generation of organomagnesium halides from alkyl or aryl halides through reaction with magnesium in an ether solvent, and their subsequent reaction with carbon dioxide to install a carboxyl group. It is crucial to understand that the method is sensitive to the presence of acidic functional groups and that the reaction mechanism works best with substrates that readily form the Grignard reagent without side reactions. Additionally, aromatic halides are often well?suited for this technique, while substrates bearing protic or interfering groups may not be compatible.

Nitrile Formation Followed by Hydrolysis

This method involves a two-step conversion in which an alkyl halide undergoes nucleophilic substitution with cyanide ion to form a nitrile intermediate. The nitrile is then converted into a carboxylic acid via hydrolysis under acidic or basic conditions. It is important to note that this approach relies on an SN2 reaction for the cyanide substitution, which limits its use to primary (and sometimes secondary) halides. Aromatic halides or sterically hindered (tertiary) substrates typically do not undergo the necessary nucleophilic substitution efficiently.

Transcript

00:03 This is the answer to chapter 20, problem number 40, from the mcmurray organic chemistry textbook.

00:10 And in this problem, we're given four reactions in which carboxylic acids are produced from alkali halides.

00:20 And we're asked for each of these would we use greenyard carboxillation or nitriolysis in order to prepare the carboxylic acid? okay, so i've drawn the four starting materials on the left of the screen here.

00:33 Since those are going to be what we need to look at to determine this.

00:38 And basically, we're just looking at other structural features of these molecules.

00:42 So for a, we're going to see nitrile hydrolysis.

00:46 And it has to be nitrile hydrolysis because we cannot use the grignard carboxilation.

00:53 And that is due to the presence of the hydroxide here.

00:59 That functionality will destroy any green yard reagent that we form.

01:03 As we form it.

01:05 And so nitrile hydrolysis is the only one that's going to work for a.

01:10 So for b, there's no other functionality in this molecule.

01:15 And so we could actually use either here, although grinjord might work a little better, but either would work.

01:25 So let's say either.

01:27 That doesn't look good.