Passage
Aldehydes are organic compounds encountered in everyday life and used as intermediates in metabolism. Common aldehydes include formaldehyde, which is used in the fixation of tissues, and cinnamaldehyde, which provides the flavor and smell of cinnamon (Figure 1) .
Figure 1 Common aldehydesThe aldol condensation shown in Figure 2 is an important carbon-carbon bond-forming reaction that has many applications in the synthesis of bioactive molecules. It also takes place in some metabolic processes, such as the citric acid cycle and gluconeogenesis. The aldol condensation begins with the aldol reaction, which can be acid- or base-catalyzed and requires nucleophilic addition to a carbonyl to provide the aldol product, which can be a β-hydroxy ketone or aldehyde. The final step of the aldol condensation consists of an acid- or base-catalyzed dehydration of the aldol product to yield an α,β-unsaturated ketone or aldehyde.
Figure 2 General aldol reaction and condensationThe reverse of an aldol reaction or condensation is known as a retro-aldol, which is important in metabolism, most notably in glycolysis. This reaction breaks a carbon-carbon bond to form aldehydes and/or ketones. In glycolysis, fructose 1,6-bisphosphate is broken down into an aldehyde and a ketone, glyceraldehyde 3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP) , respectively (Figure 3) .
Figure 3 Retro-aldol in glycolysis
-Which of the following reagents CANNOT be used to make benzoic acid from benzaldehyde?

Question

Passage
Aldehydes are organic compounds encountered in everyday life and used as intermediates in metabolism. Common aldehydes include formaldehyde, which is used in the fixation of tissues, and cinnamaldehyde, which provides the flavor and smell of cinnamon (Figure 1) .

Figure 1 Common aldehydesThe aldol condensation shown in Figure 2 is an important carbon-carbon bond-forming reaction that has many applications in the synthesis of bioactive molecules. It also takes place in some metabolic processes, such as the citric acid cycle and gluconeogenesis. The aldol condensation begins with the aldol reaction, which can be acid- or base-catalyzed and requires nucleophilic addition to a carbonyl to provide the aldol product, which can be a β-hydroxy ketone or aldehyde. The final step of the aldol condensation consists of an acid- or base-catalyzed dehydration of the aldol product to yield an α,β-unsaturated ketone or aldehyde.

Figure 2 General aldol reaction and condensationThe reverse of an aldol reaction or condensation is known as a retro-aldol, which is important in metabolism, most notably in glycolysis. This reaction breaks a carbon-carbon bond to form aldehydes and/or ketones. In glycolysis, fructose 1,6-bisphosphate is broken down into an aldehyde and a ketone, glyceraldehyde 3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP) , respectively (Figure 3) .

Figure 3 Retro-aldol in glycolysis
-Which of the following reagents CANNOT be used to make benzoic acid from benzaldehyde?

Quizplus · Accepted Answer

11ecba2d_1260_0550_a3bf_df480b11ee2d_MD0008_00 The oxidation of an aldehyde to a carboxylic acid requires an increase in the number of C-O bonds and a decrease in the number of C-H bonds. An increase in C-O bonds results in the loss of electron density around carbon and a partial positive charge, while the oxidizing agent gains electrons and becomes reduced. Aldehydes are readily oxidized to carboxylic acids by numerous oxidizing agents, including chromium reagents, in an aqueous environment. The aldehyde is believed to go through a hydrate intermediate, also known as a geminal diol, before being oxidized to a carboxylic acid. However, pyridinium chlorochromate (PCC) can only oxidize primary and secondary alcohols to aldehydes and ketones, respectively. PCC cannot oxidize aldehydes further because it is an anhydrous oxidizing reagent and does not contain the water necessary to convert the aldehyde to a hydrate, a necessary intermediate to be oxidized to the carboxylic acid. (Choices A, B, and C) CrO₃, KMnO₄, and H₂CrO₄ can all oxidize aldehydes to carboxylic acids because these reagents are all in aqueous environments, meaning they contain the water necessary to convert the aldehyde to the hydrate intermediate that is oxidized to the carboxylic acid. Educational objective: Aldehydes are readily oxidized to carboxylic acids in aqueous environments by a number of oxidizing agents, including chromium compounds. Aldehydes are believed to go through a hydrate intermediate before being oxidized to a carboxylic acid. The anhydrous oxidizing reagent pyridinium chlorochromate (PCC) cannot oxidize aldehydes because it lacks the water needed to form the hydrate intermediate.

Passage Aldehydes Are Organic Compounds Encountered in Everyday Life and Used