Separations by physical means

Mixtures are blends of two or more elements and/or compounds that are not chemically bound to each other. Mixtures can usually be separated by physical means. 

Note 3 In principle, the constituents of a blend are separable by physical means. 

Mixture A system of two or more substances that can be separated by physical means. 

A is correct. The salts are stereoisomers because they have the same bond-to-bond connectivity, and they must be diastereomers because they can be separated by physical means (crystallization). Notice from the diagram that they are NOT mirror images of each other, and therefore cannot be enantiomers. 

In many cases, amino acids can be resolved by the methods we have already discussed (Section 5-16). If a racemic amino acid is converted to a salt with an optically pure chiral acid or base, two diastereomeric salts are formed. These salts can be separated by physical means such as selective crystallization or chromatography. Pure enantiomers are then regenerated from the separated diastereomeric salts. Strychnine and brucine are naturally occurring optically active bases, and tartaric acid is used as an optically active acid for resolving racemic mixtures. 

Homogeneous mixture (or solution) A system of two or more substances (elements or compounds) that are interspersed, such as the gases making up the air or salt dissolved in water. The individual substances have distinct chemical properties and can be separated by physical means. 

Compound A combination of two or more atoms of different elements in a precise proportion by mass. In a compound, the atoms are held together by attractive forces called chemical bonds and cannot be separated by physical means. 

Salad—uneven distribution of components can be easily separated by physical means... 

The substances in a mixture can be separated by physical means. For example, if one substance dissolves in a liquid solvent but another does not, the mixture can be filtered. The substance that dissolved will be carried through the filter by the solvent, but the other substance will not. 

There are many different types of mixtures, but they all have these common characteristics They represent physical combinations of two or more substances. The individual substances in a mixture do not lose their original properties, and the substances in a mixture can be separated by physical means. Another characteristic that all mixtures share is that their composition is variable. By this I mean that the substances that are found in the mixture can be mixed in with varying proportions or concentrations. If you mixed in five silver coins and three copper coins, or four silver coins and seven copper coins, you still end up with a mixture of copper and silver coins. 

Figure 1-6 (a) A mixture of iron and sulfur is a heterogeneous mixture, (b) Like any mixture, it can be separated by physical means, such as removing the iron with a magnet. 

Mixtures can be separated by physical means because each component retains its properties (Figures 1-6 and 1-7). For example, a mixture of salt and water can be separated by evaporating the water and leaving the solid salt behind. To separate a mixture of sand and salt, we could treat it with water to dissolve the salt, collect the sand by filtration, and then evaporate the water to reclaim the solid salt. Very fine iron powder can be mixed with powdered sulfur to give what appears to the naked eye to be a homogeneous mixture of the two. Separation of the components of this mixture is easy, however. The iron may be removed by a magnet, or the sulfur may be dissolved in carbon disulfide, which does not dissolve iron (Figure 1-6). 

Since enantiomers have identical physical properties they cannot be separated by physical means. They can be separated by resolution through diastereomers. In this method, enantiomers are converted to diastereomers by reaction with a pure optically active compound. Diastereomers have different physical properties and can be separated. After separation, the diastereomers are converted back to the original enantiomers. 

Unlike compounds, mixtures can be separated by physical means into their components. A heterogeneous mixture has a non-uniform composition with visible boundaries between the components. A homogeneous mixture (solution) has a uniform composition because the components (elements and/or compounds) are mixed as individual atoms, ions, or molecules. 

Recall that mixtures differ fundamentally from compounds in three ways (1) the proportions of the components can vary (2) the individual properties of the components are observable and (3) the components can be separated by physical means. In some cases, as in a mixture of iron and sulfur (Figure 2.19),... 

The mixture still has to be separated and, as it is a mixture of diastereoisomers, it can be separated by physical means. The chloroacetate is just a convenient crystalhne derivative. 

Mixtures are coalitions of more or less uniformly distributed chemical substances that can usually can be separated by physical means. See Terminology, Solutions, p.247. 

When we talk about two chiral diastereoisomers, we have no choice but to draw the structure of one enantiomer of each diastereoisomer because we need to include the stereochemical information to distinguish them, even if we re talking about a racemic mixture of the two enantiomers. To avoid confusion, it s best to write something definite under the structure, such as (meaning racemic) under a structure if it means this diastereoisomer but not this enantiomer of this diastereoisomer. So in this case we should say that the chemists were able to separate these two diastereoisomers, but wanted only one enantiomer of the trans diastereoisomer and that this could not be separated by physical means. 

Tme enantiomers have identical transport and other physical properties and are not separable by physical means including IMS using nonchiral or racemic media. However, enantiomeric ions should have unequal interaction potentials with chiral molecules and thus different mobihties in chiral buffers. That is analogous to a different strength of adsorption of enantiomers in solution on chiral solids or micelles that allow chiral chromatography. A molecule needs some minimum size to possess... 

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