2. Find five things in the texts to finish the sentence: “It reminds me of…”

3. How do these texts differ? What do they have in common?

4. Tell what was said in the texts about:

chiral, achiral, left hand, toothbrush, orbitals, molecular symmetry, symmetry operations, three-dimensional models,spectroscopy,valence theory,free molecule

5. Choose one of the three texts and render it.

Section B

1. Read the text below and decide which word or words combination best fit each gap. There are two words you don’t need to use.

Achiral, stereotopic, enantiomer, homotopic groups, enantiomers, chiral, enantiotopic groups, identical molecules, enantiotopic, constitutional isomers and diastereomers, constitutionally heterotopic and diastereotopic groups, symmetry.

STEREOISOMERISM OF MOLECULES

The stereomeric relationship between pairs of substances may be derived through the sequence of questions and answers represented by the flow diagram in Figure 1. In terms of properties, three broad categorizations arise:

1. a)_________________ Not distinguishable under any conditions, chiral or achiral.

2. b)________________The same in all scalar properties and distinguishable only under chiral conditions. Only molecules of which the point groups are C_n (n≥1), D_n (n>1), T, O, or I are chiral and can exist in enantiomeric forms.

3. c)_______________ Differ in all scalar properties and are distinguishable in principle under any conditions, chiral or achiral. Geometric isomers, which are related by the orientation of groups around a double bond, are a special case of diastereomers.

Molecules are d)__________if their molecular point groups do not include any S_n (n≥1) symmetry elements. Otherwise they are e)_________. An achiral molecule is not distinguishable from its own mirror image. This is often phrased as ``an achiral molecule is superimposable on its own mirror image.'' A chiral molecule is not superimposable on its mirror image. A molecule which is identical to the mirror image of another molecule is the f) __________of that molecule. According to the definitions above, an object is either chiral or it is not, it belongs to a particular point group or it does not. However, efforts have been made to define degrees of chirality and continuous measures of symmetry. The concepts of chirality and isomerism may readily be extended to pairs or larger assemblages of molecules, hence the reference to chiral and achiral environments above. Groups may be compared by internal comparison (groups in the same molecule) or by external comparison (groups in different molecules). One can also compare faces of a molecule in the same way as groups, since the comparison actually applies to environments. Thus, the two faces of the carbonyl groups of aldehydes, unsymmetrical ketones, esters, and other acid derivatives are enantiotopic. Reaction at the two faces by a chiral nucleophile will take place at different rates, resulting in asymmetric induction.

Figure 1. Flow chart for deciding stereomeric relationships between pairs of substances.

STEREOTOPIC RELATIONSHIPS OF GROUPS IN MOLECULES

Many of the ideas espoused in this and the next section are due to the work of Mislow. The concepts used to describe relationships between pairs of molecules may readily be extended also to pairs of groups within a molecule. This is particularly useful in determining the appearance of an NMR spectrum or the possibility of selective reaction at similar functional groups. Regions (such as faces of planar portions) around molecules may be similarly classified. The same relationships could also be applied to (groups of) atomic orbitals within the molecule. These are collectively referred to as “groups” for the purpose of the flow chart in Figure 2. From the analysis of Figure 1, three broad groupings of properties emerge:

1. g)_____________Not distinguishable under any conditions, chiral or achiral. To have homotopic groups, a molecule must have a finite axis of rotation. Thus the only molecules which cannot have homotopic groups are those whose point groups are C₁;C_s;C_i , and C_∞v.

2. k)_____________The same in all scalar properties, distinguishable only under

chiral conditions.

3. l)______________Differ in all scalar properties and are distinguishable under any conditions, chiral or achiral. Asymmetric molecules cannot contain homotopic or enantiotopic groups, only diastereotopic or constitutionally heterotopic groups. Groups may be compared by internal comparison (groups in the same molecule) or by external comparison (groups in different molecules).

One can also compare faces of a molecule in the same way as groups, since the comparison actually applies to environments. Thus, the two faces of the carbonyl groups of aldehydes, unsymmetrical ketones, esters, and other acid derivatives are m)____________. Reaction at the two faces by a chiral nucleophile will take place at different rates, resulting in asymmetric induction.

Figure 2. Flow chart for deciding stereotopic relationships between pairs of groups.