Stereochemistry of Organic Compounds

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Weblinks

 https://en.wikipedia.org/wiki/Claisen_rearrangement  https://en.wikipedia.org/wiki/Axial_chirality  https://en.wikipedia.org/wiki/Allene  http://www.du.edu/nsm/departments/chemistryandbiochemistry/organic/org anic2ed/allene.html

CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module No. 30: Chirality Axis- Stereochemistry of Allenes

Suggested readings

 Stereochemistry of Organic Compounds

by Ernest L. Eliel ,Samuel H. Wilen

 Stereochemistry of Organic Compounds : Principles and Applications

by D. Nasipuri

CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module No. 30: Chirality Axis- Stereochemistry of Allenes

 Organic Reactions Stereochemistry and Mechanism (Through Solved Problems) By: P.S.Kalsi

Glossary

Allene: An allene is a compound in which one carbon atom has double bonds with each of its two adjacent carbon centres. Allenes are classified as polyenes with cumulated dienes. The parent compound of allene is propadiene.

Asymmetric allenes: Allenes having an asymmetric carbon or C1 point group is classified as asymmetric allene.

Axial chirality: Axial chirality is a special case of chirality in which a molecule does not possess a stereogenic center (the most common form of chirality in organic compounds) but an axis of chirality – an axis about which a set of substituents is held in a spatial arrangement that is not superposable on its mirror image.

Desymmetric allenes: Allenes having C2 point group known as desymmetric allenes.

Desymmetrization: Desymmetrization in stereochemistry is the modification of a molecule that results in the loss of one or more symmetry elements. A common application of this class of reactions involves the introduction of chirality. CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module No. 30: Chirality Axis- Stereochemistry of Allenes

Enantiomers: Enantiomers are stereoisomers that are non-superimposable mirror images. A molecule with 1 chiral carbon atom exists as 2 stereoisomers termed enantiomers. Enantiomers differ in their configuration (R or S) at the stereogenic center.

Hybridization: Hybridization is the idea that atomic orbitals fuse to form newly hybridized orbitals, which in turn, influences molecular geometry and bonding properties.

Optical activity: Optical activity is the ability of a chiral molecule to rotate the plane of plane-polairsed light, measured using a polarimeter. A simple polarimeter consists of a light source, polarising lens, sample tube and analysing lens.

Stereochemistry: Stereochemistry focuses on stereoisomers. Stereoisomers are isomeric molecules that have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space.

Did You Know

Cahn–Ingold–Prelog priority rules

The Cahn–Ingold–Prelog priority rules, CIP system or CIP conventions (after the scientists;Robert Sidney Cahn, Christopher Kelk Ingold and Vladimir Prelog) are a set of rules used in organic chemistry to name the stereoisomers of a molecule. A molecule may contain any number of stereocenters and any number of double bonds, and each gives rise to two possible configurations. The purpose of the CIP system is to assign an R or S descriptor to each stereocenter and an E or Z descriptor to each double bond so that the configuration of the entire molecule can be specified uniquely by including the descriptors in its systematic name. The key article by the three authors setting out the CIP rules was published in 1966.

The Cahn–Ingold–Prelog rules are distinctly different from those of other naming conventions, such as general IUPAC nomenclature, since they are designed for the CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module No. 30: Chirality Axis- Stereochemistry of Allenes

specific task of naming stereoisomers rather than the general classification and description of compounds.

Steps for naming

The steps for naming molecules using the CIP system are often presented as:

 Identification of stereocenters and double bonds  Assignment of priorities to the groups attached to each stereocenter or double- bonded atom  Assignment of R/S and E/Z descriptors

Faces

Stereochemistry also plays a role assigning faces to trigonal molecules such as ketones. A nucleophile in a nucleophilic addition can approach the carbonyl group from two opposite sides or faces. When an achiral nucleophile attacks acetone, both faces are identical and there is only one reaction product. When the nucleophile attacks butanone, the faces are not identical (enantiotopic) and a racemic product results. When the nucleophile is a chiral molecule diastereoisomers are formed. When one face of a molecule is shielded by substituents or geometric constraints compared to the other face the faces are called diastereotopic. The same rules that determine the stereochemistry of a stereocenter (R or S) also apply when assigning the face of a molecular group. The faces are then called the re-faces and si-faces. In the example displayed on the right, the compound acetophenone is viewed from the re face. Hydride addition as in a reduction process from this side will form the S-enantiomer and attack from the opposite Si face will give the R-enantiomer. However, one should note that adding a chemical group to the prochiral center from the re-face will not always lead to an S stereocenter, as the priority of the chemical group has to be taken into account. That is, the absolute stereochemistry of the product is determined on its own and not by considering which face it was attacked from. In the above-mentioned example, if chloride (Cl-) was added to the prochiral center from the re-face, this would result in an R-enantiomer.

CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module No. 30: Chirality Axis- Stereochemistry of Allenes