Brockmann scale

Nitropentanal. A 100-ml two-necked flask equipped with a mechanical stirrer is charged with nitroethane (3.75 g, 0.05 mol) and cooled with an ice-water bath. Acrolein (2.8 g, 0.05 mol) is added and the mixture stirred for 5 minutes. Chromatographic alumina (activity I on the Brockmann scale lOg) is added and stirring is continued for 6 hours. The reaction is monitored by t.l.c. (ethyl acetate-hexane, 2 8 as eluent). At the end of the reaction, the alumina is washed with ether (4 x 50 ml) and the filtered extract evaporated under reduced pressure. The product is purified by distillation to give 4-nitro-pentanal (50%), b.p. 71 °C/0.4mmHg i.r. 1720 (C=0) and 1545cm-1 (N02). 

Its activity can then be lowered by adding precise amounts of water. Indeed the activity of alumina may be defined according to the Brockmann scale where alumina with 3% water added is classified as grade II and with 6%, 10% and 15% grades III, IV and V, respectively. Test dyes, namely, azobenzene, p-methoxyazobenzene, Sudan yellow, Sudan red and p-amino-azobenzene can also be used to evaluate the activity of sorbent layers, the higher the R the lower the activity and Brockmann number. Silica plates can also be assigned a Brockmann number. 

The most common approach to surface modification with transition aluminas is through control of the thermal treatment. As shown in Figures 5 and 7, it is quite easy to alter the total surface Lewis and Bronsted acidity and basicity via control of the hydroxyl concentration. To some extent this technique has been used since the early 1950s, particularly in chromatographic aluminas. The so-called Brockmann scale for rating aluminas is based on thermal history, and commercial materials are available today in the various Brockmann grades. Only recently has the concept been applied to large scale adsorption systems. 

The optimum activation temperature for alumina is not known with certainty. Heating for 30 min at 75-110°C generally produces an alumina with activity suitable for adsorption TLC. Alternatively, the most active preparation (activity I on the Brockmann scale) can be obtained by heating commercial alumina to 400-450°C, followed by the gradual addition of defined amounts of water to achieve a lower activity (III-V) for TLC. The percentage of water ranges from 0% for activity I to 15% for activity V. Test dyes are used to evaluate the activity of alumina layers as follows ... 

As for silica, a variety of alumina powders are available commercially, and layers can be prepared as aqueous slurries following manufacturers instructions. Commercial powders are available in different degrees of activity according to the Brockmann scale and can be obtained as acidic (pH 4.0-4.5), neutral (7.0— 8.0), or basic (9.0-10.0). They are also available with or without a gypsum binder or fluorescent indicator. Commercially precoated alumina plates are usually received with approximately 6-8% water content (activity 111-lV). The variations in pH, specific surface area, and pore size among various TLC aluminas lead to differences in separation properties, and the conditions used to obtain certain results must be carefully documented. 

Used alumina can be regenerated by repeated extraction, first with boiling methanol, then with boiling water, followed by drying and heating. The degree of activity of the material can be expressed conveniently in terms of the scale due to Brockmann and Schodder (Chem Ber B 74 73 1941). 

It is imperative that when transferring conditions of TLC separations to preparative columns, the conditions responsible for the TLC separation be meticulously transferred. Both CC and TLC use the same principle of separation. For normal operating conditions, a TLC layer has a chromatographic activity of II-III of the Brockmann and Schodder scale. Therefore, the sorbent used for DCC has to be brought to the same grade of activity. TLC layers often contain a fluorescent indicator in which case the DCC sorbent has to contain the same phosphor. 

Brockmann activity scale Used in the characterization of alumina support materials and based on the amoimt of water adsorbed to the support material. 

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