Ch.６ - Tartaric acid
Aims
In this chapter you will learn the stereochemistry of chain compounds with two or more asymmetric carbon atom. The topic will be the number of stereoisomers for such compounds, their Newman or Fischer projections and R, S-nomenclature for such compounds. Sugars will be studied in detail since sugars are most important compounds which have more than two asymmetric carbon atoms.
Goal of this chapter
When you will have finished this chapter, you will be able to do followings:
- To know that the number of stereoisomers for a compound with n asymmetric carbon atoms is in general 2n, but the value may vary because of the symmetry of the molecule. Such phenomena as meso form or pseudoasymmetry are related to the isomer number.
- To understand that the stereoisomers of a compound with two asymmetric carbon atoms are classified into erythro and threo forms. When the molecule is symmetrical, the isomers are classified into optical active and inactive (meso) forms.
- To classify a group of sugars with three to six carbon atoms into D- and L-series and to assign stereochemistry (R, S) to each asymmetric carbon atom.
- To understand the convention in writing Fischer projections for compounds with two or more asymmetric carbon atoms, and the relation between conformation and configuration of these molecules.
New terms and concepts
If you have learned about half of the following terms, go directly to the Question.
Else, please click the terms which you have not yet learned, and review them.
- Erythro form （エリトロ形）
- Threo form （トレオ形）
- Meso form （メソ形）
- D, L series of sugars （糖のD、L 系列）
- Pseudoasymmety （擬不斉）
- Diastereomer (ジアステレオマー)
Now, you may read through Summary to ensure the point, or go to Questions to test yourself.
Go to Questions.
6.1 Erythrose and Threose
6.2 Tartaric Acid: meso-form
6.3 Configuration and conformation
6.4 The system with three and more asymmetric carbon atoms
6.5 D- and L-series of saccharide
Go to index page (International Version)
Summary
S6.1 Erythro form, threo form
For compounds with two asymmetric carbon atoms;
Threo form: the same ligands (both horizontal) at the opposite side of Fischer projection
Erythro form: the same ligands (both horizontal) at the same side of Fischer projection
go to 6.1 Erythrose and threose
S6.2 Meso form
Optically inactive stereoisomer because of the presence of a symmetry in the molecule. Meso forms have the following Fischer projection.
go to 6.2 Tartaric acid; meso form
S6.3 Configuration and conformation
Fischer projection of compounds with more than two asymmetric carbon atoms do not necessarily represent the most stable conformation. In fact the molecules are considered to assume a staggered conformation in which the repulsion among the ligands is minimal. Note that rotation about the single bond does not affect configuration at all.
go to 6.3 Configuration and conformation
S6.4 D, L-series (sugar)
The configuration of the asymmetric carbon atom furthest from the carbonyl carbon is:
identical with D-glyceraldehyde ... D-series
identical with L-glyceraldehyde ... L-series
S6.5 Pseudoasymmetry
A carbon atom which have two choral ligands and two ligands with the same structure but different configuration is named pseudoasymmetry. As for the priority of the latter two ligands, the one with R configuration has higher priority.
go to 6.5 D, L nomenclature of sugars
S6.6 Diastereomer
The isomer which is a stereoisomer but not an enantiomer is called diastereomer. The molecules with more than one asymmetric carbon atoms have stereoisomers which are in the relations of diastereomers to each other. In the below example, one can say, “threose is the diastereomer of erythrose”.
The eight isomers of the D-series aldohexoses (below) are also in the relations of diastereomer to one another.
The E and Z isomers around the double bond can also be regarded as diastereomers to each other (Ch. 3).