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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 , 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 . Even more spectacular results have been obtained by nitrating cellulose filter paper, and impregnating it with salts prior to ignition . 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 . 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 . 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 , and the Journal of Chemical Education has published descriptions and preparation techniques for both inorganic  and organic photochromic compounds and sunglasses . 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 . 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].