Glaser's "Chemistry is in the News"
To Accompany Wade Organic Chemistry 4/e.
Chapter 25. Lipids.
Historically, Montezuma considered chocolate an aphrodisiac and Europeans used it as medicine. More recently, efforts have been made to dispel the myth that chocolate contains a magical ingredient, attributing the chocolate "high" to a psychological association with the texture. The unique experience of eating chocolate results when cocoa butter melts in the mouth just below body temperature. However, an addiction to chocolate may be more than just this taste sensation. Scientists have found several chemical explanations for the effect. Chocolate contains small amounts of phenylethlyamine, an amphetamine-like substance, as well as cannabinoids, the active chemicals in marijuana. The stimulants caffeine and threobromine are also found in chocolate. Studies have proven that chocolate stimulates the brain to release natural painkillers.
Whatever the reason, the short-term effect of chocolate is undisputed. More interestingly, recent evidence has suggested that chocolate may have health benefits far beyond temporary relaxation. Chocolate has had a notorious reputation as "junk food" and as a dieterŐs nemesis due to its high amount of saturated fat, in the form of stearic acid. Saturated fats contribute to cholesterol formation, and are therefore particularly harmful to sufferers of atherosclerosis (build-up of cholesterol in the arteries). Nutritionists at Penn St. University have concluded that, unlike other saturated fatty acids, stearic acid does not raise cholesterol levels. If the FDA were to reclassify stearic acid as less harmful, chocolate may see an improvement in its nutritional value.
Polyphenols are another important constituent of chocolate that are now receiving attention, due to their antioxidant properties. Polyphenols prevent the oxidation and uptake of low-density-lipoproteins (LDL) which are the "bad" cholesterol molecules that contribute to plaque formation and heart disease. One company in Japan has anticipated the labeling of chocolate as a health food, and is offering chocolate fortified with extra antioxidants. This seems extreme, since the antioxidant benefit from eating chocolate can be achieved by eating fruits and vegetables, which are higher in vitamins as well.
This article brings up an interesting point. The majority of the studies
finding health value in chocolateŐs ingredients have been funded by
chocolate manufacturing companies, such as
Mars, which have an interest in promoting its value. This trend will
probably continue, since biased sources are willing to contribute the most
money. The result places critical importance on the process of peer
review and scientific scrutiny when it comes to this kind of research.
Pertinent Text References
Chapter 25. Lipids.
Chapter 25-3. Triglycerides.
Chapter 25-6. Steroids.
Chapter 20-10. Condensation of Acids and Alcohols: The Fischer Esterification.
Realistically, can you get a drug-like "high" from eating
chocolate? Explain your answer.
BR> Current research shows that attaining a high due to the stimulation of the cannabinoid receptors in the brain is not very likely because of the low concentrations of activating chemicals in chocolate. Many propose, though, that a psychological rather than a physiological high can be attained from eating chocolate.
Cholesterol is extremely insoluble and is transported in the
bloodstream bound to protein in the form of particles called
low-density-lipoproteins (LDLs). Polyphenols act as antioxidants in the
body to prevent LDLs from oxidizing and building up in arteries. Draw the
structure of cholesterol.
See Wade page 1174
Stearic acid, which is found in chocolate, and oleic acid are
both 18 carbon fatty acids, but stearic acid has a much higher melting
point. By looking at the structures
(Wade p. 1166), explain the difference in melting points with respect to
bonding and lattice formation.
Oleic acid has a cis double bond that gives it a "kink". This kink hinders tight packing, thus lowering the melting point temperature. Stearic acid is saturated, which allows formation of a solid lattice and therefore a higher melting point. Uniform zig-zag chains that contain no double bonds are responsible for the tight packing of stearic acid.
Certain ingredients in chocolate stimulate
N-acyleth-anolamines (cannabinoid receptors) to produce a feeling of
euphoria (a "high"). Anandamide mimics cannabinoids directly (by
activating cannabinoid receptors) and phenylethalamine and theobromine
mimic cannabinoids indirectly (by activating the brainŐs own anandamide
levels). A study by Nestle found that an individual would need to consume
a 22,000 pound chunk of chocolate to attain any drug-like effects.
Another study, by Christian Felder of the National Institute of Mental
Health, found that a 25 pound piece was needed to attain a "high" for a
130 pound individual. Do you think that these studies may have been
biased in any way? If so, how could this bias change the results of the
Open to interpretation. Our group agrees that the information given from these tests is most likely biased. Although the studies may have been performed under very similar conditions, those persons who display the results have the ability to choose which are the most beneficial to them.
Explain the mechanism through which stearic acid and glycerol
form tristearin by a condensation reaction. What is this reaction called?
Fischer Esterification-- acid-catalyzed nucleophilic acyl substitution by an alcohol. The acid catalyst protonates the carbonyl group of one molecule of stearic acid. The result is a protonated carboxylic acid, which has three resonance forms. The resonance form with the positive charge on the carbonyl carbon is attacked at that carbon by one of the oxygens of glycerol. Loss of a proton from the attacking oxygen of the glycerol yields an ester. One of the hydroxyl goups is protonated, and water then leaves, forming a cation with three resonance forms. Loss of a proton from the second hydroxyl group gives an ester.
Although chocolate has many potential benefits, should it be
considered a health food? Explain.
Open to interpretation. Our group agrees that although chocolate may have certain beneficial contents, it should not replace other nutritional sources.