Rot. spec.
English translation: specific rotation
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| 17:21 Jul 7, 2007 |
| Italian to English translations [PRO] Science - Chemistry; Chem Sci/Eng / Material Safety Data Sheet | |||||||
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|  Selected response from: Zareh Darakjian Ph.D. United States Local time: 19:03 | ||||||
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| 5 +4 | specific rotation |
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| Discussion entries: 1 | |
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Answers
7 mins confidence: 
peer agreement (net): +4
peer agreement (net): +4specific rotation Explanation: This is the rotation caused by chiral molecules -------------------------------------------------- Note added at 9 mins (2007-07-07 17:30:11 GMT) -------------------------------------------------- In molecules, carbon atoms connected to four different groups rotate plane polarized light rightward or leftward by a certain number of degrees. To come up with a definite property, a certain path length is usesd with w certain concentration. The D stands for light emitted by a sodium atom (one particular wavelength). -------------------------------------------------- Note added at 10 mins (2007-07-07 17:31:47 GMT) -------------------------------------------------- Molecules having this property are known as "chiral molecules" (i.e. molecules with a handedness. Similar to a screw, can be left handed right handed. -------------------------------------------------- Note added at 11 mins (2007-07-07 17:33:04 GMT) -------------------------------------------------- http://cns.uni.edu/~macmilla/mcmurry/mcmurry_chapter_9/sld01... -------------------------------------------------- Note added at 12 mins (2007-07-07 17:33:44 GMT) -------------------------------------------------- The specific rotation is represented by the formula 100α [α]xt = , l c where t = the temperature of measurement, x = the wavelength or the name of the specific monochromatic light of spectrum used (when D line is used, indicate as D), α = the angle, in degrees, of the rotation of plane of the polarized light, l = the thickness of the layer of the measured solution, i.e., the length of the polarimeter tube (mm), c = the number of grams of a sample in 1 ml of the solution. Hereinafter in the Monographs, such a specification as “[α]20D = +20.5 - +21.5 ゚(1 g, freshly boiled and cooled water, 10 ml, on the dried basis)”indicates that the specific rotation of the substance is +20.5 - +21.5 ゚, when determined on the dried basis for the solution which is prepared by weighing accurately about 1 g of the test substance and dissolving in newly boiled and cooled water to make exactly 10 ml. -------------------------------------------------- Note added at 13 mins (2007-07-07 17:34:10 GMT) -------------------------------------------------- You might also say "optical specific rotation". -------------------------------------------------- Note added at 16 mins (2007-07-07 17:37:41 GMT) -------------------------------------------------- Between optical specific rotation and specific rotation, the latter (specific rotation) is better. ** specific rotation ** The ** specific rotation ** of a chemical compound [α] is defined as the observed angle of optical rotation α when plane-polarized light is passed through a sample with a path length of 1 decimeter and a sample concentration of 1 gram per 1 millilitre. The specific rotation of a pure material is an intrinsic property of that material at a given wavelength and temperature. Values should always be accompanied by the temperature at which the measurement was performed and the solvent in which the material was dissolved. Often the temperature is not specified; in these cases it is assumed to be room temperature. The formal unit for specific rotation values is deg cm² g-1 but scientific literature uses just degrees. A negative value means levorotatory rotation and a positive value means dextrorotatory rotation. Some examples: Sucrose +66.47° cholesterol −31.5° Camphor +44.26° Penicillin V +223° taxol −49° -------------------------------------------------- Note added at 18 mins (2007-07-07 17:39:18 GMT) -------------------------------------------------- Why ** specific rotation ** and not simply ** rotation **? ------------------------------------------------------------------------ he specific rotation can only be used to characterize compounds that are chiral - those that can exist as a pair of enantiomers - two compounds with structures that are related as nonsuperimposable object and mirror image. Chiral substances are the only type of substances that exhibit optical activity. The presence of optical activity shows that the substance is chiral. In fact the specific rotation is used to characterize each individual enantiomer. There seems to be another aspect to the question: Why the specific roation rather than another value for the rotation? The specific rotation is used because it is the rotation that would be obtained under a "standardized" set of conditions. In this way the specific rotation of one compound can be compared to that of another without concern for whether they were obtained unde that same set of conditiond. While the sign of the rotation for a compound does not change with conditions, the numerical value changes with changes in condition. The ** specific rotation ** represents the value of the rotation one would obtain when the pathlength is one decimeter and the concentration of the substance in a solvent is 1.00 g of substance per mL of solvent. The specific rotation is also specific for a given temperature and a given wavelenght of light. |
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