Colour of chemical compounds

The relationship between resonance and the colour of chemical compounds has proved to be of great assistance in explaining the observed empirical laws of relationship between colour and chemical constitution. 

The classical techniques for the solvent extraction of chemical compounds from vegetable material are based upon the correct choice of solvent and conditions e. g. heating or agitation. A range of commercially important pharmaceuticals, flavours and colourants are now derived from vegetable sources. It has been shown that the solvent extraction of organic compounds contained within the body of plants and seeds is significantly improved by the use of power ultrasound [25]. 

Very little is known of the primary cause of colour in such compounds, but our knowledge is so far advanced that colour is regarded as a characteristic property of whole classes of chemical compounds and the study of the constitution of such compounds has shown that a close relationship exists between their colour and their chemical structure. 

Figure 9.8 The effect of pH upon a solution ofphenolphthalein. This compound is not coloured at pH values less than 8 though it is bright pink for those greater than 9.5. The graph presented here in 3D perspective reveals that for acid pH there is no absorption in the visible region of the spectrum. In contrast, absorptions towards 500 nm appear when the pH becomes alkaline, which are responsible for the well-known colour of the compound. Notable in this example is the modification in the chemical bonding, which is pH dependent.

Photochromic mechanism implies a transition of chemical compound from a form A into a form B, each of them of a specific absorption spectrum, implying specific colouring of the form as well. The triggering mechanism is usually ultraviolet (UV)-light induced (process of activation). As a result of incident UV light the compound (molecule) changes its colour. 

Substances such as brass, wood, sea water, and detergent formulations are mixtures of chemicals. Two samples of brass may differ in composition, colour and density. Different pieces of wood of the same species may differ in hardness and colour. One sample of sea water may contain more salt and different proportions of trace compounds than another. Detergent formulations differ... 

The absorption of light by a chemical compound is directly related to the concentration of the compound. For example, suppose you add several drops of blue food colouring to a litre of water. If you add a few millilitres of bleach to the solution, the intensity of the colour of the food dye diminishes as it reacts. You can then monitor the colour change. (Do not try this experiment without your teacher s supervision.)... 

I can say, for example, that it tends to form chemical bonds to five other atoms at a time, but can tolerate fewer and, at a push, more. It is a metal, probably quite a soft one, heavier than iron but lighter than lead. Many of its compounds - its combinations with other elements - will be coloured. It will be apt to form bonds to other niobium atoms - so-called metal-metal bonds. It will behave chemically in a similar manner to the element vanadium, but will be more similar still to tantalum. 

The Maillard reaction is inextricably linked to the desirable flavour and colour characteristics of cooked foods and this review provides an insight into some of the chemistry associated with flavour generation in the reaction and the different aromas which are involved. The chemical pathways associated with the initial and intermediate stages of the Maillard reaction are presented and routes by which the important classes of aroma compounds may be formed from Maillard intermediates are discussed. 

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