Organic Compounds in Everyday Life: Risks, Benefits and Choices

Willow bark was used for pain for centuries; its active compound is converted in the body to salicylic acid, which works but badly irritates the stomach. In 1897, chemists at Bayer converted salicylic acid's phenol –OH into an ester, making acetylsalicylic acid, or aspirin, which is gentler. In the school lab the reaction uses ethanoic anhydride (more reactive than ethanoic acid) with an acid catalyst, and purity is checked with iron(III) chloride, which turns violet with any leftover phenol group. Aspirin blocks the COX enzymes that make prostaglandins (pain, fever, inflammation) and a clotting signal in platelets, which is why low-dose aspirin is prescribed to some people to prevent heart attacks, and why it can cause stomach bleeding. It is not given to children and teens with viral illnesses because of a rare but serious liver and brain condition, Reye's syndrome. Old aspirin smells of vinegar because moisture slowly hydrolyzes the ester back to salicylic acid and ethanoic acid. Ibuprofen is a carboxylic acid with a benzene ring that blocks COX reversibly; only its (S) enantiomer is active, as you saw in the chirality lesson.

Fats and oils are triglycerides: esters of glycerol with three fatty acids. In vegetable oils many fatty acid chains contain cis C=C bonds that put a kink in the chain, so molecules pack loosely and the oil is liquid. Hydrogenation, the addition of ${\mathrm{H}}_2$ across C=C bonds with a nickel catalyst, straightens chains and raises the melting point, turning oil into spreadable margarine or shortening that also resists going rancid. Stopping part way (partial hydrogenation) has a side effect: on the catalyst surface some remaining cis double bonds flip to trans. Trans chains are nearly straight and pack like saturated fat, and in the body trans fats raise LDL ("bad") cholesterol and lower HDL ("good") cholesterol, increasing heart disease risk more than any other fat. Canada banned partially hydrogenated oils in foods from September 2018. Full hydrogenation makes no trans fat, but it does make saturated fat.

Polarity explains why some vitamins dissolve in fat and others in water. Vitamin A (retinol, ${\mathrm{C}}_{20}{\mathrm{H}}_{30}\mathrm{O}$) is a long hydrocarbon chain and ring with a single –OH group: mostly nonpolar, so it dissolves in body fat, is stored in the liver and can build up to toxic levels. Vitamin C (ascorbic acid, ${\mathrm{C}}_6{\mathrm{H}}_8{\mathrm{O}}_6$) has four –OH groups on six carbons and hydrogen bonds strongly with water, so it dissolves in blood plasma and any excess leaves in urine. Vitamins D, E and K are fat-soluble like vitamin A; the B vitamins are water-soluble like vitamin C. Like dissolves like, so cooking vegetables in water leaches out vitamin C, while vitamin A is absorbed best when eaten with some fat.

Soap is made by saponification (base hydrolysis of fats) and is the sodium salt of a fatty acid: a long nonpolar tail and an ionic carboxylate head. Tails dissolve in grease, heads face the water, and the grease is lifted away inside spheres called micelles. In hard water, $\mathrm{C}{\mathrm{a}}^{2+}$ and $\mathrm{M}{\mathrm{g}}^{2+}$ ions bind the carboxylate heads and form insoluble scum. Synthetic detergents have a sulfonate head whose calcium salts stay dissolved, so they work in hard water. Early detergents had branched tails that bacteria could not break down and rivers foamed; linear tails fixed that. Phosphate "builders" added to soften water fed algal blooms; a whole-lake experiment at Ontario's Experimental Lakes Area proved phosphorus was the cause, and Canada limited phosphates in laundry detergents in the 1970s.