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Chemguide: Support for CIE A level Chemistry Learning outcome 29: An introduction to A level organic chemistry 29.4: Isomerism: optical Learning outcomes 29.4.1, 29.4.2 and 29.4.3 These statements build on what you already know about optical isomerism, a form of stereoisomerism, from section 13.4. Read the individual statements in the syllabus as you go along. Important background Refresh your memory by reading the page about optical isomerism. It is important to follow the link to the page about plane polarised light in the second green box. It probably isn't necessary to follow all the detail, but you must understand what it means when the syllabus talks about rotating the plane of polarisation of plane polarised light. Statement 29.4.1 This statement talks about the properties of enantiomers. If you don't immediately know what enantiomers are, then you haven't read the optical isomerism page carefully enough. Two enantiomers will have exactly the same chemical and physical properties because they contain all the same functional groups and exactly the same number of atoms. That means that their chemical reactions will be the same. All the intermolecular forces will also be the same, and so the physical properties are identical as well. However, their shapes are mirror images of each other and they will rotate the plane of polarisation of plane polarised light in opposite directions. The syllabus also talks about their potential biological activity. That was mentioned on the pages you read, but will be discussed in more detail in statement 29.4.4. Statement 29.4.2 An optically active substance will rotate the plane of polarisation of plane polarised light. (This is all getting a bit repetitive!) A racemic mixture is a 50-50 mixture of the two optical isomers (enantiomers) of a compound. It will have no effect on plane polarised light, because any rotation caused by one isomer is exactly countered by the opposite rotation caused by the other one. Statement 29.4.3 The two optical isomers of a single substance will rotate the plane of polarisation of plane polarised light in opposite directions by exactly the same amount.
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