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Sci.chem FAQ - Part 4 of 7
Section - 18. Sensory properties of chemicals

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18.1  How do light sticks work?, and how can I make one?

From: perks@umbc.edu (Mark Perks)    Date: 15 Sep 1994 
Subject: Re: Chemiluminescence Sticks

  Chemical Demonstrations [[1] v.1 p.146- ], by Bassam Shakhashiri, offers a 
  thorough discussion of Cyalume lightsticks. Professor Shakhashiri is at 
  the University of Wisconsin, Madison, I believe.

        "The Cyalume lightstick contains dilute hydrogen peroxide in a
  phthalic ester solvent contained in a thin glass ampoule, which is
  surrounded by a solution containing a phenyl oxalate ester and the
  fluorescent dye 9,10-bis(phenylethynyl)anthracene...When the ampoule is
  broken, the H2O2 and oxalate ester react.."
        
From: chideste@pt.Cyanamid.COM (Dale Chidester)  Date: Mon, 13 Mar 1995
Subject: Re: How to make chemical light ?

  The following produce rather spectacular results.  Chemicals are 
  available through Fluka and Aldrich.  The dyes are expensive.
 
  Dyes:-
  9,10-bis(phenylethynyl)anthracene (BPEA) (yellow) [10075-85-1] Fluka 15146
  9,10-diphenylanthracene (DPA)             (blue)  [1499-10-1]  Fluka 42785
  5,6,11,12-tetraphenylnaphthacene (rubrene) (red)  [517-51-1]   Fluka 84027
  
  Other reagents required:-
  bis(2-carbopentyloxy-3,5,6-trichlorophenyl)oxalate (CPPO) 
                                              [75203-51-9]  Aldrich 39,325-8
  bis(2-ethylhexyl)phthalate (DOP) (solvent)        [117-81-7]   Fluka 80032
  sodium salicylate (catalyst)                      [54-21-7]    Fluka 71945
  35% hydrogen peroxide                             [7722-84-1]  Fluka 95299

  Saturate solvent with dye and CPPO. Sonicate to help solvation. Start with 
  about 50 mg dye (BPEA, DPA or rubrene) in 10 g solvent with 50 mg CPPO and 
  5 mg sodium salicylate.  CPPO is limiting reagent.
  Put small quantity (20 drops) in a small vial and add equal volume of 
  hydrogen peroxide.  Mix vigorously. There will be two phases. Avoid skin 
  contact! Don't cap tightly!  

  The following explanation of the chemistry was provided:-
  From: sbonds@jarthur.claremont.edu (007)

  All of the material below is taken from a chemical demonstrations book 
  [[1], v.1, p.146 ].

  The oxidant is hydrogen peroxide contained in a phthalate ester solvent.  
  The concentration is very low, less than 0.5%.  The fluorescing solution 
  consists of a phenyl oxalate ester and a fluorescent dye.  The dye used is 
  9,10-bis-(phenylethynyl)anthracene (for green) or 9,10-diphenylanthracene 
  (for blue).

  Here is the reaction sequence:

  1)  (Ph)-O-CO-CO-O-(Ph) + H2O2 --> (Ph)-O-CO-CO-O-OH + (Ph)-OH

  2)          (Ph)-O-CO-CO-O-OH  -->  O-O
                                      | |    +  (Ph)-OH
                                     OC-CO

  3)                 C2O4 + Dye  --> Dye* + 2CO2

  4)                        Dye* --> Dye + hv

  In 1) The hydrogen peroxide oxidizes the phenyl oxalate ester to a 
  peroxyacid ester and phenol. The unstable peroxyacid ester decomposes to 
  the cyclic peroxy compound and more phenol in step 2). The cyclic peroxy 
  compound is again unstable and gives off energy to the dye as it decomposes
  to the very stable carbon dioxide. The dye then radiates this energy as 
  light.

An alternative chemiluminescence demonstration involves the H2O2 oxidation
of lucigenin ( bis-N-methylacridinium nitrate [2315-97-1] Aldrich B4,920-3 ),
[ [1] v.1 p.180-185 ] which has recently been modified to provide a slow
colour change across the visible spectrum [2]. One of the reagents in that
lucigenin oxidation ( Rhodamine B ) is a mutagen and suspected carcinogen.  

18.2  How do hand warmers work?, and how can I make one?

They consist of an aqueous solution of sodium acetate with a small "clicker"
disk to provide a small clean surface that initates crystallisation. The solute
is dissolved into solution by prior warming. When heat is required, the disk 
is "clicked" to scrape a small surface, and this clean surface will initiate
the sodium acetate crystallisation from the now supersaturated solution. The 
heat of crystallisation is slowly released.  

18.3  What are the chemicals that give fruity aromas? 

Most of the desirable food aromas come from low to medium molecular weight
organic compounds - usually alcohols, aldehydes, esters, ketones, and 
lactones. These may be " natural " ( extracted from natural sources ),
" nature-identical " ( synthetic, but identical to known natural compounds ), 
and " artificial " ( synthetic, not found in nature ). The perceived aroma of
molecules can change dramatically with minor isomeric or structural changes,
and common fruity aromas are usually complex mixtures of several compounds.

Because man-made chemicals are frequently made from chemicals derived from 
fossil fuels, the isotopic ratios of the carbon atoms has been used to 
discriminate between natural and nature-identical chemicals. Natural 
processes usually involve the use of enzymes that selectively produce a
specific isomer, and some man-made aromas are now produced enzymatically.
Chiral chemistry, often utilising chiral chromatography that was initially
developed for pharmaceuticals, is now also being used for the production
and testing of man-made aromas, as enantiomerically-pure aroma chemicals 
command premium prices.

Some chemicals are listed below, along with their use in either fragrances 
and/or flavours, and boiling point ( 760 mmHg, unless otherwise stated ).
Some of them are also considered toxic, and thus their use may be controlled. 
Volume A11 of Ullmann has an excellent monograph on flavours and fragrances,
and more detail can be obtained from the journal Perfumer and Flavorist. 
The catalogues of well-known suppliers such as Dragoco GmbH ( Germany ), 
L.Givaudin and Cie ( Switzerland ), and Takasago Perfumery Company ( Japan ),
also contain information on chemical composition and health and safety.  
   
Chemical                              BP        CAS RN      Application 
                                      C (mmHg)
acetoin                              148      [513-86-0]   butter
acetophenone                         202      [98-86-2]    orange blossom
benzyl acetate                       206      [140-11-4]   jasmine
butyl acetate                        125      [123-86-4]   apple
2,3-butanedione                       88      [431-03-8]   butter
(+)-carvone                          230      [2244-16-8]  caraway, dill
(-)-carvone                          230      [6485-40-1]  spearmint
citral                               229      [5392-40-5]  lemon
citronellal                          207      [2385-77-5]  balm mint
decanal                              208      [112-31-2]   citrus
dihydromyrcenol                       78 (1)  [18479-58-8] lavender
2,6-dimethyl-2-heptanol              171      [13254-34-7] freesia
ethyl butyrate                       120      [105-54-4]   pineapple
ethyl 2t-4c-decadienoate              71 (45) [3025-30-7]  pear
ethyl hexanoate                      168      [123-66-0]   pineapple
ethyl isovalerate                    132      [108-64-4]   blueberry
ethyl 2-methylbutyrate               133      [7452-79-1]  apple
geraniol                             229      [1066-24-1]  roselike
hexyl acetate                        169      [142-92-7]   pear
hexyl salicylate                     168 (12) [6259-76-3]  azalea
1-(4-hydroxyphenyl)-3-butanone                [5471-51-2]  raspberry
isoamyl acetate                      143      [123-92-2]   banana
(+)-limonene                         176      [5989-27-5]  lemon
linalool                             198      [78-70-6]    lily of the valley
linalyl acetate                      220      [115-95-7]   bergamot
8-mercapto-p-menthan-3-one            57 (8)  [38462-22-5] blackcurrant
1-p-methene-8-thiol                   40 (1)  [71159-90-5] grapefruit
3-methyl-2-cyclopenten-2-ol-1-one             [80-71-7]    caramel
4-methyl-2(2-methyl-1-propenyl)tetrahydropyran               
                                      70 (12) [16490-43-1] rose
myrcenol                              78 (50) [543-39-5]   lime
2t-6c-nonadien-1-ol                   98 (11) [28069-72-9] violet
3-octanol                            175      [20296-29-1] mushroom
1-octen-3-ol                          84 (25) [3391-86-4]  mushroom
phenethyl acetate                    238      [103-45-7]   rose
phenethyl alcohol                    220      [60-12-8]    rose
phenethyl isoamyl ether                       [56011-02-0] chamomile
2-propenyl hexanoate                                       pineapple
alpha-pinene                         156      [80-56-8]    pine 
alpha-terpineol                      217      [98-55-5]    lilac
alpha-trichloromethylbenzyl acetate           [90-17-5]    rose

18.4  What is the most obnoxious smelling compound?

Many low molecular weight sulfur-containing compounds tend to induce adverse
reactions in people, even if they have not encountered them before, eg the
glandular emissions of skunk (n-butyl mercaptan, dicrotyl sulfide).
Butyric acid reminds people of vomit, and cadaverine ( 1,5 Pentadiamine ) 
reminds people of rotten tissue, but without an earlier association, they 
may not regard them as unusually obnoxious.

18.5  What is the nicest smelling compound?

Aside from thinking about your stomach, when the smell of cooking foods
is attractive, then most people like the smell of flowers and citrus fruits.
These are volatile, aromatic, oils, whose major components are complex 
mixtures of medium volatility compounds, often derived from terpenes, eg 
Oil of Rose ( 70 - 75% geraniol = (E)-3,7-dimethyl-2,6-octadiene-1-ol ),
Oil of Bergamot ( 36 - 45% linalyl acetate = 3,7-dimethyl-1,6-octadien-3-yl 
acetate ).  Many aromatic oils are mixtures of terpene esters ( oil of 
geranium = 20 - 35% geraniol esters ) or aldehydes ( oil of lemon grass = 
75 - 85% citral = 3,7-dimethyl-2,6-octadienal ). Merck briefly describes
nearly 100 volatile oils, from Oil of Amber to Oil of Yarrow, along with 
typical applications. Flower perfumes are complex blends of compounds, and 
detailed compositions of your favourite smell are often available in the 
journal " Perfumer and Flavorist ".

Expensive flower petal perfumes, such as rose and jasmine, are produced
using extracts obtained by the traditional "enfleurage" process ( refer to
Section 24.4 ).

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