Molecule Projections

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Philomena Reynolds
9 years ago
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1 Key Definitions ü Stereochemistry refers to the chemistry in 3 dimensions (greek stereos = solid). This science was created by Pasteur (1860), van Hoff et LeBel (1874). ü Stereisomers are isomeric molecules that have the same structural formula but which differ only in the three-dimensional orientations of their atoms in space. ü An object is chiral when it does not present no plane symmetry (σ), no center of symmetry (C i ) and no axis of symmetry (C n & S n ) and thus when it is not superimposable to its mirror image. ü An object is achiral when it is superimposable to its mirror image. ü Optical rotation is the turning of the plane of linearly polarized light about the direction of motion as the light travels through solutions of chiral molecules. ü Racemic mixture, or racemate is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule. ü A meso compound or meso isomer is a non-optically active member of a set of stereoisomers, whose at least two of them are optically active. This means that despite containing two or more stereocenters (=chiral centers) it is not chiral. ü An absolute configuration in stereochemistry is the spatial arrangement of the atoms of a chiral molecular entity (or group) and its stereochemical description e.g. R or S (see Cahn-Ingold-Prelog priority rules)

ü An object is chiral when it does not present no plane symmetry (σ), no center of symmetry (C i ) and no axis of symmetry (C n & S n ) and thus when it is not superimposable to its mirror image.

2 Molecule Projections

3 Molecule Projections HO OH OH OH OH D-Talose CHO Frequently used for linear monosaccharides. HO HO HO H CHO H H Fisher Projection v Relative Configuration H OH CH 2 OH This projection is mostly used for cyclic sugars. Relative configuration is the configuration of any stereogenic (asymmetric) center with respect to any other stereogenic center contained within the same molecular entity. Unlike absolute configuration, relative configuration is reflectioninvariant. e.g. => 3,5-Dimethylcyclohexanone Cis/trans relationship occurs only in cyclic derivatives.

Relative configuration is the configuration of any stereogenic (asymmetric) center with respect to any other stereogenic center contained

4 Relative Configuration Syn/anti relationship occurs only in acyclic derivatives. v Erythro vs. Threo

5 Origin of Chirality The term chirality is derived from the Greek word for hand, χειρ (cheir) Chiral molecule : Molecule which is different from its mirror image (e.g. Lactic acid) Two enantiomers of a generic amino acid v Nomenclature of stereoisomers: Ø Application of Cahn-Ingold-Prelog rules v Pure & Appl. Chem. 1976, 45, v J. March. Advanced Organic Chemistry 3Ed. ISBN v IUPAC Goldbook

(e.g. Lactic acid) Two enantiomers of a generic amino acid v Nomenclature of stereoisomers: Ø Application of

6 Stereoisomery v There is stereoisomery when two compounds have the same structural formula (enantiomers & diasteromers) Ø e.g. => Substrate with two chiral carbon atoms Same structural formula

7 Meso Achiral Case A meso compound is superimposable on its mirror image, and it does not produce a "(+)" or "(-)" reading when analyzed with a polarimeter.

8 Definition: e.e. vs. d.e. v Enantiomeric excess (e.e.) Diastereomeric excess (d.e.) Ø e.e. = 95% => (R or S) = 97.5% and (S or R) = 2.5% Ø Alternative notations: e.r. (Enantiomeric Ratio) - d.r. (Diastereomic Ratio) e.r. = 19:1 means e.e. = 90% d.e = 98% means d.r. = 99:1 v Topicity Homotopic groups are equivalent groups. Two groups A and B are homotopic if the molecule remains the same (including stereochemically) when the groups are interchanged with the remaining parts of the molecule fixed. Homotopic atoms are always identical, in any environment. Homotopic NMR-active nuclei have the same chemical shift in an NMR spectrum. If such groups are not equivalent, they are considered as heterotopic groups.

Two groups A and B are homotopic if the molecule remains the same (including stereochemically) when the groups are interchanged with the remaining parts of the molecule fixed.

9 Quizz v Enantiotopic refers to the relationship between two groups in a molecule which, if one or the other were replaced, would generate a chiral compound. The two possible compounds resulting from that replacement would be enantiomers. There s talk of diastereotopic relationship if diastereomers are generated from this substitution. What is the topicity (stereochemical relationship) between the underlined substituents (homotopic, heterotopic, enantiotopic or diastereotopic) Considering the substrate herein, indicate the absolute configurations of chiral centers. If aldehyde functional group is reduced, is this molecule still chiral? Justify.

There s talk of diastereotopic relationship if diastereomers are generated from this substitution.

10 Prochirality v Prochirality Some definitions Ø Prochiral molecules are those that can be converted from achiral to chiral in a single step. If two identical substituents are attached to an sp 3 -hybridized atom, the descriptors pro-r and pro-s are used to differentiate them from each other. Promoting the pro-r substituent to higher priority than the identical alternative would result in an R chirality center at the original sp 3 -hybridized atom, and vice versa (see below e.g. with a prochiral carbon). *

If two identical substituents are attached to an sp 3 -hybridized atom, the descriptors pro-r and pro-s are used to

Isomers Have same molecular formula, but different structures

Isomers Have same molecular formula, but different structures Isomers ave same molecular formula, but different structures Constitutional Isomers Differ in the order of attachment of atoms (different bond connectivity) Stereoisomers Atoms are connected in the same