"This scent... vanilla?"
Toma brought the flask closer.
Milia smiled. "Vanillin. An aldehyde compound."
Kana became interested. "Aldehyde?"
"Organic compound with carbonyl group. R-CHO."
Toma drew the structural formula. "C=O?"
"Carbon-oxygen double bond," Milia explained. "This is the aldehyde characteristic."
"Similar to ketones?"
"Similar but different," Milia compared. "Aldehyde is terminal. Ketone is intermediate."
Kana wrote in her notebook. "RCHO vs RCOR'"
"Correct."
Toma asked. "Why does it smell?"
"Volatility," Milia answered. "Small molecules, so they drift in the air."
"Then bind to receptors in the nose."
Kana was surprised. "Molecules create smell?"
"Yes. Receptor proteins recognize molecular shapes."
Milia took out a perfume bottle. "Cinnamon. This is also aldehyde."
"Cinnamaldehyde," Toma said.
"Rose scent, lemon scent, many are aldehydes or ketones."
Kana was moved. "Smell was chemistry."
"Everything is chemistry," Milia said quietly.
"About aldehyde reactivity," Milia continued.
"Carbonyl group is electrophilic. Wants electrons."
Rei just entered the lab. "Talking about aldehydes?"
"Perfect timing," Toma laughed.
Rei supplemented. "Carbon in C=O is partially positively charged."
"Why?"
"Oxygen's high electronegativity. Pulls electrons."
Kana understood. "So nucleophiles attack?"
"Correct. Alcohols and amines add."
Milia showed the reaction. "Aldehydes are easily reduced."
"Becomes alcohol."
Toma experimented. "With sodium borohydride."
The aldehyde slowly reduced.
"R-CHO → R-CH2OH"
Kana asked. "The reverse reaction?"
"Alcohol oxidation," Rei answered. "Primary alcohol to aldehyde."
"And further oxidation gives carboxylic acid."
Milia pointed out. "That's why aldehydes are unstable. Gradually oxidize in air."
"Vanilla scent changing is?"
"Oxidation. Aldehyde to carboxylic acid."
Kana murmured. "Scent is fleeting."
"Molecules change. That's proof of time."
Toma opened another bottle. "This one?"
"Benzaldehyde. Almond scent."
Kana smelled it. "Indeed."
Rei explained. "Benzene ring with aldehyde group. Aromatic aldehyde."
"Aromatic?"
"Has benzene ring. Stable structure."
Milia continued. "Countless aldehydes in nature."
"Fruits, flowers, trees. All have unique scent molecules."
Toma was impressed. "Plants make them?"
"Biosynthesis. Enzymes assemble complex molecules."
Kana asked. "Aldehydes in human body too?"
Rei answered. "Formaldehyde. Metabolic byproduct."
"But toxic, so quickly oxidized."
Milia supplemented. "Aldehyde dehydrogenase. This enzyme processes aldehydes."
"When drinking alcohol?"
"Ethanol becomes acetaldehyde. That causes hangovers."
Toma grimaced. "So aldehydes are enemies."
"But as scents, they're allies," Kana said.
Milia closed the vanillin bottle. "Aldehydes are life's two faces."
"Poison and medicine, and scent."
Rei summarized. "Carbonyl group reactivity. That creates diverse functions."
Kana opened the window. Outside, plant scents.
"This scent is also aldehyde?"
"Probably," Milia smiled. "Grass scent. Leaf aldehyde."
Toma took a deep breath. "Breathing in chemistry."
"Living is doing chemistry," Rei said quietly.
The three enjoyed the invisible molecular scents. The faint scent of aldehyde filled the room.