18.6  What is the most bitter compound? 

Denatonium Benzoate = Bitrex, or even in some strange chemistry circles,
N-[(2-[2,6-Dimethylphenyl)amino]-2-oxoethyl]-N,N-diethylbenzenemethan-
aminium benzoate [3734-33-6]. It is added to toxic chemicals ( such as 
methylated spirits ) as a deterrent to accidental ingestion.

18.7  What is the sweetest compound? 

Most scales use sucrose as a sweetness of 1, and compare the relative
sweetness of other sweeteners to sucrose.

Name              Relative Sweetness           Category 
D-Glucose                 0.46            Natural Food Product
Lactose                   0.68               "      "     "
D-Fructose                0.84               "      "     "
Sucrose                   1                  "      "     "
Cyclamate                30               EC Permitted, USA Prohibited
Aspartame               200               EC, USA Permitted.
Saccharin               300               EC Permitted, USA Prohibited
Sucralose               650               Au, Ca Permitted, trials elsewhere
Alitame               2,000               Undergoing trials
Thaumatin             3,000               EC permitted, US chewing gum only.
Carrelame           160,000               Guanidine sweetener
Bernardame          200,000                   "        "
Sucrononate         200,000                   "        "
Lugduname           220,000                   "        "

The guanidine sweeteners are not expected to be approved for food use.
There are several other important attributes of sweeteners, such as
low toxicity, no after-taste, whether metabolised or excreted, etc.,
that must also be considered.

The potency scale is fairly flexible, and differing publications can
assign different values. The August 1995 copy of the Journal of Chemical
Education contained several papers from a symposium on sweeteners [3,4],
and an article in Chemistry and Industry also discusses sweeteners from
both natural and artificial sources [5], and Kirk Othmer has a monograph
on sweeteners.    

The sweetener used in "diet" beverages is usually Aspartame, and they
are usually required to display a warning for phenylketurics that the
product contains a source of phenylalanine. As Aspartame slowly degrades
in acid solutions, such products also have a "use-by" date. 

Although banned by the FDA in 1970 ( because a mixture of saccharin and 
cyclamate caused tumours in test animals ), saccharin has been still 
marketed under extensions of approval,  Ironically, subsequent work 
implicated the saccharin, and the cyclamate was found not to be the 
tumour-causing agent, but it is still banned. 

18.8  What salts change the colour of flames?.

Both Vogel ( qualitative inorganic ) and the Rubber Handbook list details of 
flame tests for elements. The spectra of the alkaline earth compounds are 
relatively complex, so using filters to view the flame can change the colour 
observed as dominant lines are filtered out. In general, except for copper, 
any compound of an element can be used, however toxic salts ( such as 
cyanides ) should not be used. Halogen salts are usually readily available, 
and are reasonably volatile. In all cases, perform experiments in a 
well-ventilated area - preferably a fume hood. The emission spectra in the 
visible region is the sum of several emission lines, with dominant lines
masking others. The visible spectrum is approximately :-
Red              800 - 620 nm
Orange           620 - 600 nm
Yellow           600 - 585 nm
Green            585 - 505 nm
Blue             505 - 445 nm
Violet           445 - 400 nm           

There are also the various bead tests employing borax ( sodium tetraborate 
Na2B4O7.10H2O ), Microcosmic salt ( NaNH4HPO4 ), or sodium carbonate 
(Na2CO3), using both oxidising and reducing flames. The bead test procedures 
are detailed in Vogel ( qualitative inorganic ), and similar texts.
 
Element         Colour          Some of the contributing lines, and comments.

Arsenic         Light Blue      449.4 nm, 450.7 nm. 
  ( Arsenic is highly toxic - only perform in fume hood under supervision )
Barium          Green-Yellow    553.6 nm, 539.1 nm, 536.1nm, 614.2 nm.
                Blue (faint)    455.4 nm, 493.4 nm.
Cesium          Red-Violet      852.1 nm.
Calcium         Orange          618.2 nm, 620.3 nm.
                Yellow-Green    530.7 nm, 559.5 nm.
                Violet (faint)  422.7 nm.
                                Greenish with blue glass. 
Copper          Emerald Green   521.8 nm, 529.2 nm, 515.3 nm.
                                Not chloride, or in presence of HCl 
                Azure Blue      465.1 nm.
                                Copper chloride, or HCl present
Lead            Light Blue      500.5 nm.
  ( Lead is highly toxic - only perform in fume hood under supervision )
Lithium         Carmine Red     670.78 nm, 670.79 nm.
                Orange (faint)  610.1 nm.  
                                Violet with blue glass
Potassium       Red             766.5 nm, 769.9 nm. 
                Violet          404.4 nm, 404.7 nm.
                                Purple-red with blue glass
Rubidium        Violet          780.0 nm, 794.8 nm.  
Sodium          Yellow          589.0 nm, 589.6 nm. 
                                Invisible when viewed with blue glass
Strontium       Scarlet Red     640.8 nm, 650.4 nm, 687.8 nm, 707.0 nm.
                Violet          460.7 nm, 421.5 nm, 407.8 nm.
                                Violet with blue glass
Tellurium       Green           557.6 nm, 564.9 nm, 566.6 nm, 570.8 nm.
  ( Tellurium is highly toxic - only perform in fume hood under supervision )
Thallium        Green           535.0 nm.
  ( Thallium is highly toxic - only perform in fume hood under supervision )   
Zinc            Whitish Green   Large number of peaks between 468.0-775.8 nm. 
  ( Zinc fumes are toxic - only perform in a fume hood under supervision )

Impressive coloured flames have been obtained using chlorides and a methanol 
flame in a petri dish [6]. Even more spectacular results have been obtained
by nitrating cellulose filter paper, and impregnating it with salts prior
to ignition [7].

18.9  What chemicals change colour with heat, light, or pressure?.
 
Compounds that visibly and reversibly change colour when subjected to a 
change in their environment are known as chromogenic materials. There are 
four major categories - electrochromic, photochromic, piezochromic, and
thermochromic, all of which are extensively discussed in a recent, well 
referenced, monograph in Kirk Othmer [8]. 

Electrochromic materials exhibit a change in light transmittance or 
reflectance induced by direct current at potentials of approximately one 
volt. The change usually is an oxidation-reduction reaction, using either
inorganic or organic compounds, and the colour change can occur at either 
the anode or the cathode - which are usually thin films. There are two major
classes, the ion-insertion/extraction type - such as tungsten trioxide, and
the noninsertion group - such as the viologens, a family of halides of
quaternary bases derived from 4,4'-bipyridinium. One viologen example is 
1,1'-diheptyl-4,4'-bipyridinium bromide [6159-05-3], which changes from
clear to bluish purple. The most common application of viologens has been
the electrochromic interior rearview mirrors available for cars since 1988.
These utilise a substituted viologen as the cathode colouring material, with 
a compound like phenylene diamine as the anode colouring electrochromic
material. The mechanism details, along with a description of the ingenious 
control system, are described in a recent comprehensive review of 
electrochromic materials [9].  

Photochromic materials undergo a reversible change in light absorption that
is induced by electromagnetic radiation, however most common applications
involve reversible changes in colour or transparency on exposure to visible
or ultraviolet light. This is often seen as a change in the visible spectrum 
( 400 - 700 nm ), and can be rapid or very slow. There are two major classes
of photochromic materials, inorganic and organic. 

Examples of the inorganic type are the silver halides, which are suspensions 
of fine ( 10-20 nm ) silver halide crystals dispersed throughout a glass that
has been slowly cooled. An alternative technique involves diffusion of the 
silver halide into the surface of the glass. The cuprous ion can catalyse 
both the photochromic darking and thermal fading reactions, and the colour 
can be shifted from grey to brown by the addition of gold or palladium - 
which may be added to the glass in trace amounts. The most popular current
application for glass containing silver halide is for prescription eyewear.
  
The organic photochromic systems can be subdivided according to the type of
reaction. Geometric isomerism can result in different optical properties,
eg azobenzene ( C12H10N2 [103-33-3] ) undergoes photoisomerization, and the 
cis form [1080-16-6] has higher absorbance than the trans form [17082-12-1].
Cycloaddition can produce photochromism, such as the reversible formation of 
the colourless 4b,12b,endoperoxide ( C28H14O4 [74292-77-6] ) from the red 
parent compound dibenzo(a,j)perylene-8,16-dione ( C28H14O2 [5737-94-0] ).
Dissociation, either heterolytic ( photolysis of triphenylmethyl chloride 
[76-83-5] ), or homolytic ( photolysis of bis(2,4,5-triphenylimidazole 
[63245-02-3] to form a red-purple free radical ), may also produce 
photochromism. 

UV can excite polycyclic aromatics, such as 1,2,5,6-dibenzacridene ( C21H13N 
[226-36-8] ), to their triplet state, which has a different absorption 
spectrum. Viologens may undergo redox reactions and exhibit photochromic 
behaviour when crystalline and subjected to UV. The most popular photochromic
materials utilise reversible electrocyclic reactions, and are often indolino
spiropyrans and indolino spiroxazines, however the mechanism also covers
fulgide, stilbene, and dihydroindolizine examples. Details and structures
are provided in the Kirk Othmer monograph [8], and the Journal of Chemical
Education has published descriptions and preparation techniques for both 
inorganic [10] and organic photochromic compounds and sunglasses [11].

Piezochromic materials change colour as they are compressed. There are three
common types:- organic molecules ( such as N-salicylidene-2-chloroaniline 
[3172-42-7] ), metal cluster compounds ( such as the octahalodirhenates, 
(Re2X8)2-, where X=Cl,Br,I ), and copper (II) organic complexes with
compounds like ethylene diamine. They are still being researched, and 
interested readers should investigate the references in the Kirk Othmer 
monograph [8].

Thermochromic materials reversibly change colour as their temperature is 
changed. There are a very large number of systems, but one common example 
of thermochromic transitions in metal complexes is the transition between 
the blue tetrahedral and pink octahedral coordinations of cobalt (II) when 
cobalt chloride is added to anhydrous ethanol and the temperature changed. 
Examples of thermochromic transitions in inorganic compounds include 
Ag2HgI4 [12344-40-0] and VO2, and several inorganic sulfides also have large 
changes occurring in the infra-red range, and are being considered for IR 
imaging applications. 

There are thousands of organic thermochromic compounds, with well known 
examples including di-beta-naphthospiropyran [178-10-9] ( thermally-induced 
heterolytic bond cleavage resulting in ring opening), poly(xylylviologen  
dibromide [38815-69-9] ( charge transfer interactions resulting in hydration- 
dehydration changes ), and ETCD polydiacetylene [63809-82-5] ( thermally-
induced transitions in the unsaturated backbone resulting in rearranged side 
groups ). Information on photochromism in organic and polymeric compounds is 
available in published reviews [12,13].