|
|
| Line 1: |
Line 1: |
| The '''enantiomeric excess''' of a substance is a measure of how pure it is. In this case, the impurity is the undesired [[enantiomer]] (the "opposite-handed" mirror image of a [[chiral compound]]).
| |
|
| |
|
| ==Definition==
| |
| Enantiomeric excess is defined as the [[absolute difference]] between the [[mole fraction]] of each enantiomer:<ref>{{GoldBookRef|file=E02070|year=1996|title=Enantiomer excess}}</ref>
| |
|
| |
|
| :<math>\ ee = |F_+ - F_-|</math> | | I woke up another day and [http://Realized-.net realized -] I have also been single for a while today and after much intimidation from friends I now locate myself signed up for online dating. They assured me that there are lots of sweet, regular and enjoyable visitors to meet up, so the pitch is gone by here!<br>I try to maintain as physically healthy as possible being at the gym many times a week. I enjoy [http://lukebryantickets.omarfoundation.org luke brian tickets] my sports and attempt to play or see as many a possible. I am going to often [http://lukebryantickets.asiapak.net who is luke bryan on tour with] at Hawthorn fits being winter. Note: I have noticed the carnage of wrestling matches at stocktake revenue, Supposing that you would considered buying an activity I do not brain.<br>My family and friends are amazing and spending some time together at pub gigs or meals is always [http://www.ffpjp24.org cheap luke bryan tickets] essential. As I come to realize [http://okkyunglee.com luke bryan schedule] you could never own a decent conversation using the sound I haven't ever been in to clubs. I additionally have 2 really adorable and definitely cheeky puppies who are almost always eager to meet up fresh folks.<br><br>Take a look at my web-site: [http://lukebryantickets.sgs-suparco.org luke bryant Tickets] |
| | |
| where
| |
| | |
| :<math>\ F_+ + F_- = 1 </math>
| |
| | |
| In practice, it is most often expressed as a '''percent enantiomeric excess'''.
| |
| :<math>\ ee = ([\alpha]_{obs}/[\alpha]_{max}) \times 100 </math> '''(1)'''
| |
| | |
| The enantiomeric excess can be determined in another way if we know the amount of each enantiomer produced. If one knows the moles of each enantiomer produced then:
| |
| | |
| :<math>\ ee = ((R-S)/(R+S)) \times 100 </math> '''(2)'''
| |
| <math>\ R</math> and <math>\ S</math> are the respective fractions of enantiomers in a mixture such that
| |
| | |
| :<math>\ R + S = 1 </math>
| |
| | |
| Enantiomeric excess is used as one of the indicators of the success of an [[asymmetric synthesis]]. For mixtures of [[diastereomer]]s, there are analogous definitions and uses for '''diastereomeric excess''' and '''percent diastereomeric excess'''.
| |
| | |
| As an example, a sample with 70% of ''R'' isomer and 30% of ''S'' will have an enantiomeric excess of 40%. This can also be thought of as a mixture of 40% pure ''R'' with 60% of a racemic mixture (which contributes 30% ''R'' and 30% ''S'' to the overall composition).
| |
| | |
| If given the enantiomeric excess <math>\ ee</math> of a mixture, the fraction of the major isomer, say <math>\ R</math>, can be determined using <math>\ R = +\frac{ee}{2} + 50% </math> and the minor isomer <math>\ S = - \frac{ee}{2} + 50% </math>.
| |
| | |
| A non-[[racemic]] mixture of two enantiomers will have a net [[optical rotation]]. It is possible to determine the [[specific rotation]] of the mixture and, with knowledge of the specific rotation of the pure enantiomer, the '''optical purity''' can be determined.<ref>{{GoldBookRef|file=O04310|year=1996|title=Optical purity}}</ref>
| |
| | |
| :[[File:Optical Purity V.1.svg|left|300px|Optical purity]]
| |
| | |
|
| |
| | |
| <br>
| |
| Ideally, the contribution of each component of the mixture to the total optical rotation is directly proportional to its mole fraction, and as a result the numerical value of the optical purity is identical to the enantiomeric excess. This has led to informal use the two terms as interchangeable, especially because optical purity was the traditional way of measuring enantiomeric excess. However, other methods such as [[chiral column chromatography]] and [[NMR spectroscopy]] can now be used for measuring the amount of each enantiomer individually.
| |
| | |
| The ideal equivalence between enantiomeric excess and optical purity does not always hold. For example,
| |
| * the specific rotation of (S)-2-ethyl-2-methyl succinic acid is found to be dependent on concentration
| |
| * in what is known as the '''Horeau effect''' <ref>J.P. Vigneron, M. Dhaenens, A. Horeau, ''Nouvelle methode pour porter au maximum la purete optique d'un produit partiellement dedouble sans l'aide d'aucune substance chirale'', Tetrahedron, Volume 29, Issue 7, 1973, Pages 1055-1059, ISSN 0040-4020, {{DOI|10.1016/0040-4020(73)80060-2}} </ref> the relationship between mole based ee and optical rotation based ee can be non-linear i.d. in the [[succinic acid]] example the optical activity at 50% ee is lower than expected.
| |
| * the specific rotation of enantiopure 1-phenylethanol can be enhanced by the addition of achiral acetophenone as an impurity.
| |
| | |
| The term enantiomeric excess was introduced in 1971 by Morrison and Mosher in their publication ''Asymmetric Organic Reactions''.<ref> Morrison, James D.; Mosher, Harry S.: ''Asymmetric Organic Reactions'', Prentice-Hall, Englewood Cliff, New Jersey, 1971 (ISBN 0130495514)</ref> The use of enantiomeric excess has established itself because of its historic ties with optical rotation. It has been suggested that the concept of ''ee'' should be replaced by that of ''er'' which stands for '''enantiomeric ratio''' or '''er''' (S:R) <ref>''Do the Terms "% ee" and "% de" Make Sense as Expressions of Stereoisomer Composition or Stereoselectivity?'' Robert E. Gawley [[J. Org. Chem.]]; '''2006'''; 71(6) pp 2411 - 2416; {{doi|10.1021/jo052554w}}</ref> or '''q''' (S/R) because determination of optical purity has been replaced by other techniques which directly measure R and S and because it simplifies mathematical treatments such as the calculation of equilibrium constants and relative reaction rates. The same arguments are valid for changing diastereomeric excess (de) to '''diastereomeric ratio''' (dr).
| |
| | |
| ==References==
| |
| <references/> | |
| | |
| {{chiral synthesis}}
| |
| | |
| [[Category:Stereochemistry]]
| |
I woke up another day and realized - I have also been single for a while today and after much intimidation from friends I now locate myself signed up for online dating. They assured me that there are lots of sweet, regular and enjoyable visitors to meet up, so the pitch is gone by here!
I try to maintain as physically healthy as possible being at the gym many times a week. I enjoy luke brian tickets my sports and attempt to play or see as many a possible. I am going to often who is luke bryan on tour with at Hawthorn fits being winter. Note: I have noticed the carnage of wrestling matches at stocktake revenue, Supposing that you would considered buying an activity I do not brain.
My family and friends are amazing and spending some time together at pub gigs or meals is always cheap luke bryan tickets essential. As I come to realize luke bryan schedule you could never own a decent conversation using the sound I haven't ever been in to clubs. I additionally have 2 really adorable and definitely cheeky puppies who are almost always eager to meet up fresh folks.
Take a look at my web-site: luke bryant Tickets