Colour, of compounds

J. W. Capstick further explains the effect of temp, on the colour of compounds by assuming that (i) the molecules vibrate about certain mean positions, and that (ii) a rise of temp, produces a greater amplitude of vibration, but not a greater period, so that if the vibration be not quite harmonic, a greater amplitude may, as with a pendulum, require a longer period, (iii) A rise of temp, is also supposed to weaken the cohesion or inter-molecular attraction between the molecules, and thus lessen the force of restitution, so that the molecules vibrate more slowly and thus produce the same sequence of colour changes with rise of temp, as are observed when the mass of the molecule is increased. 

Perceived colour of compound Colour absorbed by compound Wavelength of absorbed light (nm)... 

Electronic Transitions In an electronic transition an electron is excited from an occupied to an empty molecular orbital (M.O.). The energy of such transitions normally corresponds to photons in the near IR, visible or UV region of the electromagnetic spectrum. Electronic absorption bands give rise to the colours of compounds, including ones without transition metals. 

The absorption and emission spectra of metals such as copper, lithium, caesium, calcium and potassium involve lines at different wavelengths to those observed in the sodium spectrum. As with sodium, a few lines in each spectrum dominate the colours of compounds of these metals in the gas flame. This is the basis of the flame tests which are used to indicate the presence of compounds of these metals (see page 195). 

It is interesting to see if there is any connexion between the chemical structures and the colours of compounds. Among organic compounds, we find that coloured chemicals usually contain certain groups which are called chromophores (colour-bearers). These are ... 

Colour of compound under UV light = 360 nm) after chromatography as in footnote a. 

Fe(py)4(NCS)2 black room 5.48 Gouy colour of compound 66B34... 

Odling criticised the usual classification of the elements and proposed thirteen groups of analogous elements (e.g. N, P, As, Sb, Bi Cr, Mn, Fe, Co, Ni, Cu) but dthough he pointed out that the atomic heats are either about 6 or about 3, he failed to double the values (really equivalents) of the latter, as Cannizzaro (see p. 491) did. Lenssen arranged the elements in 20 triads, some of which were very arbitrary, e.g. (0 + C)=N= (8 + 6)=7, and his so-called atomic weights were often equivalents. He attempted to relate the colours given by elements to flames and the colours of compounds. 

The most important reaction of the diazonium salts is the condensation with phenols or aromatic amines to form the intensely coloured azo compounds. The phenol or amine is called the secondary component, and the process of coupling with a diazonium salt is the basis of manufacture of all the azo dyestuffs. The entering azo group goes into the p-position of the benzene ring if this is free, otherwise it takes up the o-position, e.g. diazotized aniline coupled with phenol gives benzeneazophenol. When only half a molecular proportion of nitrous acid is used in the diazotization of an aromatic amine a diazo-amino compound is formed. 

Aminoazobenzene is a very weak base, and consequently it will not form salts with weak organic acids, such as acetic acid, although it will do so with the strong mineral acids, such as hydrochloric acid. Aminoazobenzene is a yellowish-brown compound, whilst the hydrochloride is steel blue. The colour of the latter is presumably due to the addition of the proton to the phenyl-N-atom, the cation thus having benzenoid and quinonoid forms ... 

The iodoform test. Dissolve 0 -1 g. or 5 drops of the compound in 2 ml. of water if it is insoluble in water, add sufficient dioxan to produce a homogeneous solution. Add 2 ml. of 5 per cent, sodium hydroxide solution and then introduce a potassium iodide - iodine reagent dropwise with shaking until a definite dark colour of iodine persists. Allow to... 

Dissolve 0 -1 g. of the compound in 10-15 ml. of water and add bromine water until the colour of the latter persists. A white precipitate will form if a phenol is present. 

The concept of a chromophore is analogous to that of a group vibration, discussed in Section 6.2.1. Just as the wavenumber of a group vibration is treated as transferable from one molecule to another so is the wavenumber, or wavelength, at which an electronic transition occurs in a particular group. Such a group is called a chromophore since it results in a characteristic colour of the compound due to absorption of visible or, broadening the use of the word colour , ultraviolet radiation. 

Development of extraction-free photometric procedures for the determination of traces of metals for which hygienic and environmental regulations have been established is an urgent problem. For solution of this problem we used as an organic reagent l-(2- pyridylazo)-naphtol-2 (PAN) which forms intensely coloured complex compounds with many metals and is frequently used for their extraction-photometric determination however these procedures did not find wide application in water analysis due to lack of selectivity and necessity of using organic solvents. 

The raw material has to be washed to remove impurities. Diluted sodium hydroxide allows the removal of phenols and benzonitrile, and diluted sulphuric acid reacts with pyridine bases. The resulting material is distilled to concentrate the unsaturated compounds (raw feedstock for coumarone-indene resin production), and separate and recover interesting non-polymerizable compounds (naphthalene, benzene, toluene, xylenes). Once the unsaturated compounds are distilled, they are treated with small amounts of sulphuric acid to improve their colour activated carbons or clays can be also used. The resulting material is subjected to polymerization. It is important to avoid long storage time of the feedstock because oxidation processes can easily occur, affecting the polymerization reaction and the colour of the coumarone-indene resins. 

Azo- and Azoxy-compoiinds. Both classes of compounds ore usually highly coloured and are rapidly decolorised by v3rming with a solution of stannous chloride and hydrochloric acid, forming amino-cotnpounds (see Reactions, pp. i73i i77J-... 

A. M. del Rio in 1801 claimed to have discovered the previously unknown element 23 in a sample of Mexican lead ore and, because of the red colour of the salts produced by acidification, he called it erythronium. Unfortunately he withdrew his claim when, 4 years later, it was (incorrectly) suggested by the Frenchman, H. V. Collett-Desotils, that the mineral was actually basic lead chromate. In 1830 the element was rediscovered by N. G. Sefstrom in some Swedish iron ore. Because of the richness and variety of colours found in its compounds he called it vanadium after Vanadis, the Scandinavian goddess of beauty. One year later F. Wohler established the identity of vanadium and erythro-nium. The metal itself was isolated in a reasonably pure form in 1867 by H. E. Roscoe who reduced the chloride with hydrogen, and he was... 

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