Chapter 24 – Infographic descriptions

24.0a The Chemistry Behind the Smell of Wet Dogs

Wet dog smell stems from microorganisms living in the dog hair. They produce bad-smelling volatile organic compounds. Adding water helps these compounds break from the hair as the water evaporates, increasing the concentration in the air.

The smell of dogs is complex: multiple chemical compounds contribute which individually do not have odours associated with dog smell, but produce it in combination. A pilot study found emitted concentrations of some compounds increased when dog hair was wet.

Greater increases: Benzaldehyde (almond-like smell); Phenylacetaldehyde (honey/floral smell); Acetaldehyde (fruity/musty smell); Phenol (medical smell); 2-Methylbutanal (musty/nutty smell).

Lesser increases: p-Cresol (faecal smell); Dimethyl Trisulfide (sulfurous smell); 2-Nonanone (fruity smell); 2,3-Diethyl-5-Methylpyrazine.

Not all compounds increased in concentration – a small selection decreased including several straight chain aldehydes (hexanal and heptanal). The concentration changes between wet and dry hair suggesting a probable chemical or biochemical reaction.

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24.1a What Causes The Smell of Fresh-Cut Grass?

Grass naturally emits volatile organic compounds (VOCs). However, when cut the emissions increase significantly. The compounds released are also known as green leaf volatiles (GLVs) and the major contributors have been shown to be a mixture of aldehydes and alcohols containing 6 carbon atoms.

• Grass cut. Enzymes break down fats.
• Linoleic and linolenic acids formed.
• Enzyme breaks into smaller fragments.

According Kirtsine et al. (1998) the emission of VOCs from pasture is:

• 39.5% (Z)-3-Hexenyl acetate
• 12.3% (Z)-3-Hexenal
• 9.4% Methanol
• 8.9% (Z)-3-Hexen-1-ol
• 7.5% (E)-2-Hexenal
• 3.6% Ethanol
• 18.8% other organic compounds.

(Z)-3-Hexenal and cut grass smell: (Z)-3-Hexenal is the main compound that gives fres-cut greass its smell. It has a low odour threshold (the amount required for the human nose to detect it) of 0.25 parts per billion. It is unstable and quickly rearranges to form (E)-2-hexenal (‘leaf aldehyde’).

(E)-2-hexenal (‘leaf aldehyde’): It has been suggested that the release of these compounds induces defense responses in other neighbouring plants. They also stimulate formation of new cells at the site of the wound, whilst some act as antibiotics, preventing infection.

References

Kirtsine, W., Galbally, I., Ye, Y., & Hooper, M. (1998). Emissions of volatile organic compounds (primarily oxygenated species) from pasture. Journal of Geophysical Research 103(D9), 10605-10619.  https://doi.org/10.1029/97JD03753

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24.1b The Chemistry of Plums & Prunes: Constipation & Chewing Gum

The aroma of plums is down to a number of volatile compounds which include: benzaldehyde, linalool, ethyl nonate, methyl cinnamate, and y-Decalactone. Several six-carbon alcohols, aldehydes and esters also contribute.

‘Wax bloom’ is the dusty white coating visible on plums. It consists of a long chain of alkanes and alcohols (mainly containing 29 carbon atoms)  and adds flavour to the plum by trapping compounds such as nonanal.

Prunes are dried plums, often cited as home remedy for constipation due to their relatively high natural levels of laxative compound sorbitol (approx. 15g per 100g). Sorbitol also responsible for laxative effect of some chewing gum (approx. 30g per 100g). Phenolic compounds (i.e. neochlorogenic acids) and the high fibre content of prunes may also aid laxative effect.

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24.3a Guarding Against Toothache & Premature Ejaculation

The essential oil of cloves is often touted as remedy for dental pain composed mainly of: 70-85% eugenol, 15% eugenyl acetate, 5-10% beta-caryophyllene.

Eugenol has antiseptic, anti-inflammatory properties, and anesthetic properties (due to ability to inhibit movement of sodium ions in peripheral nerves). It can also act as an antifungal and antibacterial agent. FDA believes there is not enough evidence of its effectiveness to recommend for tooth pain treatment. Some research shows it may be useful in creams for the treatment of premature ejaculation.

Eugenol can also have toxic side effects in larger quantities, as little at 5-10ml of undiluted essential oil can cause damage to the liver and respiratory systems.

The aroma of cloves is partially influenced by eugenol and minor compounds, such as 2-heptanone and methyl salicylate.  2-heptanone also compound secreted by honeybees when they bite intruders in their hives, the anesthetic effect paralyses the intruding creature and allows it to be removed.

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24.3b Why Can Coriander Taste Soapy? – The Chemistry of Coriander

A range of aldehyde compounds largely responsible for the coriander leaves: aldehydes with 6-10 carbon atoms, particularly decyl (10) and nonyl (9) aldehydes. Other major constituents: 2-decenoic acid, decanoic acid (also known as capric acid), tetradecenoic acid.

Chemical composition of coriander seeds slightly different, with alcoholic linalool being the major constituent.

Coriander can taste ‘soapy’ because it’s leaves contain high levels of organic compounds (aldehydes), which are the same/similar aldehydes are often found in soaps or lotions.

Scientists also discovered that dislike for coriander taste may also be influenced, to some extent, by genetic factors. Studies also suggest crushed coriander leaves ,ay lead to faster breakdown of aldehydes, diminishing soapy taste.

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24.3c The Chemistry Advent Calendar 2023: Day 6 South Korea: Kimchi

Kimchi is a common Korean side dish all year round and is also present at the Christmas table.  It consists of salted, seasoned and fermented vegetables, most commonly napa cabbage.  Lactic acid bacteria from the raw ingredients are the dominant bacteria that ferment sugars and starches in the vegetables, producing lactic acid and other compounds.  Some key flavour compounds are shown below.

• Dimethyl trisulfide; from onions and garlic (structure contains 2 carbon, 3 sulfur, and 6 hydrogen atoms)
• 2,3-butanedione; buttery flavour (structure contains 4 carbon, 2 oxygen, and 6 hydrogen atoms)
• β-phenethyl acetate: IUPAC name: phenethoxyethanoate; rose, honey, sweet flavour (structure contains 10 carbon, 2 oxygen, and 12 hydrogen atoms)
• Lactic acid: IUPAC name: 2-hydroxypropanoic acid (structure contains 3 carbon, 3 oxygen, and 6 hydrogen atoms)

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24.4a Making silver mirrors using chemistry

Glass surfaces can be given a coating of silver with a particular chemical reaction.

The reagents:

• Silver nitrate: [latex]AgNO_{3}[/latex]
• Ammonia: [latex]NH_{3}[/latex]
• Sodium hydroxide: [latex]NaOH[/latex]
• Dextrose: [latex]C_{6}H_{12}O_{6}[/latex]

Tollens’ reagent: [latex]Ag(NH_{3})_{2^{+}}[/latex] contains silver nitrate, sodium hydroxide and ammonia.

Tollens’ reagent is made of mixing silver nitrate, ammonia, and an alkaline solution (commonly a hydroxide). It is a colourless solution of a diamminesilver(1) complex. Due to the risk of explosive silver nitride forming, it must be used shortly after preparation and then disposed of safely.

The reaction:

When an aldehyde is added to Tollens’ reagent the aldehyde is oxidized to form a carboxylic acid, and the diamminsilver(1) ions reduced to metallic silver. The diamminesilver(1) ions are more difficult to reduce than silver ions, producing a silver coating in a controlled manner.

[latex]{\small \text{Dextrose} + 2Ag(NH_{3})_{2^{2}} + 2OH^{-} \small}[/latex] where Dextrose is oxidized and Tollen’s reagent is reduced to silver results in the silver mirror forming [latex]{\small \text{D-Gluconic acid} + 2Ag +4NH_{3} + H_{2}O \small}[/latex].

Using silver nitrate without ammonia leads to a colloidal suspension of silver, giving a black, cloudy appearance. Basic conditions are used because dextrose is more easily oxidized under these conditions.

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Attribution & References

Compound Interest infographics are created by Andy Brunning and licensed under CC BY-NC-ND

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