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Classification:ChemicalAuxiliaryAgent	CAS No.: 101-72-4	OtherNames: IPPD
MF: C15H18N2	Purity: 96%	Place of Origin: Shandong China (Mainland)
Type:Adsorbent	AdsorbentVariety:ActivatedCarbon	Usage: Coating Auxiliary Agents, Leather Auxili...
ModelNumber: 4010NA	Form:granule

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PackagingDetail:	Polypropyrene knitted bags lined with polyethylene bags.
DeliveryDetail	within 10 working days after receiving the advance payment.

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ITEM	QUALITIED GRADE
Appearance	Dark brown to dark violet pastilles
MeltingPoint	???????
Heatingloss	???????
Ash	???????
Assay(GC)	95.0% MIN

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Mainly used in manufacture of???????.

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Polypropyrene knitted bags lined with polyethylene bags.Net weight???kg per bag.

UNIT 3

MOLECULAR SYMETRY

Reading

Section A

1. You are going to read three texts which are all connected with symmetry. These are introductory parts from different books. Read the texts and be able to tell which text is:

- the best?

- the most thought provoking?

- the most difficult to agree with?

- impossible to comprehend?

- too personal?

- weird?

- the most interesting?

- the craziest?

- wonderful?

- original?

Text A

Symmetry is a concept that we all make use of in an unconscious fashion. We notice it every time we look in our bathroom mirror. We ourselves are (approximately) bilaterally symmetric. A reflected right hand looks like a left hand, a reflected right ear like a left ear, but the mirror image of the face as a whole or of the toothbrush does not look different from the original. The hand, a chiral object, is distinguishable from its mirror image; the toothbrush is not. The toothbrush is achiral and possesses a mirror plane of symmetry which bisects it. It would not surprise us if we were to inspect the two sides of the toothbrush and find them identical in many respects. It may surprise us to note that the two sides are distinguishable when held in the hand, that is, in a chiral environment (the fingers hold one side and the thumb the other). However, the achiral toothbrush fits equally comfortably into either the right or the left hand. Chiral objects do not. They interact differently with other chiral objects and often the different interactions are known by separate words. When you hold someone's right hand in your right hand, you are shaking hands; when it is the other person's left hand in your right, you are holding hands. Similar properties and interactions exist in the case of molecules as well. The presence or absence of symmetry has consequences on the appearance of spectra, the relative reactivity of groups, and many other aspects of chemistry, including the way we will make use of orbitals and their interactions. We will see that the orbitals that make up the primary description of the electronic structure of molecules or groups within a molecule have a definite relationship to the three-dimensional structure of the molecule as defined by the positions of the nuclei. The orientations of the nuclear framework will determine the orientations of the orbitals. The relationships between structural units (groups) of a molecule to each other can often be classified in terms of the symmetry that the molecule as a whole possesses. We will begin by introducing the basic terminology of molecular symmetry. Finally we will apply simple symmetry classification: to local group orbitals to decide whether or not interaction is allowed in the construction of molecular orbitals; to molecular orbitals to determine the stereochemical course of electrocyclic reactions and to help determine the principal interactions in bimolecular reactions; and to electronic states to construct state correlation diagrams. We begin by introducing molecular point groups according to the Schoenflies notation and assigning molecular and group symmetry following Jaffe and Orchin where greater detail may be found.

Text B

It is perhaps appropriate to begin this chapter by sketching what we intend to do here. It is certainly intuitively obvious what we mean when we say that some molecules are more symmetrical than others, or that some molecules have high symmetry whereas others have low symmetry or no symmetry. But in order to make the idea of molecular symmetry as useful as possible, we must develop some rigid mathematical criteria of symmetry. To do this we shall first consider the kinds of symmetry elements that a molecule may have and the symmetry operations generated by the symmetry elements. We shall then show that a complete but nonredundant set of symmetry operations (not elements) constitutes a mathematical group. Finally, we shall use the general properties of groups to aid in correctly and system­atically determining the symmetry operations of any molecule we may care to consider. We shall also describe here the system of notation normally used by chemists for the various symmetry groups.

It may also be worthwhile to offer the following advice to the student of this chapter. The use of three-dimensional models is extremely helpful in learning to recognize and visualize symmetry elements. Indeed, it is most unlikely that any but a person of the most exceptional gifts in this direction can fail to profit significantly from the examination of models. At the same time, it may also be said that anyone with the intelligence to master other aspects of modern chemical knowledge should, by the use of models, surely succeed in acquiring a good working knowledge of molecular symmetry.

Text C

The theory of molecular symmetry provides a satisfying and unifying thread which extends throughout spectroscopy and valence theory. Although it is possible to understand atoms and diatomic molecules without this theory, when it comes to understanding, say, spectroscopic selection rules in polyatomic molecules, molecular symmetry presents a small barrier which must be surmounted. However, for those not needing to progress so far this chapter may be bypassed without too much hindrance.

The application of symmetry arguments to atoms and molecules has its origins in group theory developed by mathematicians in the early nineteenth century, but it was not until the 1920s and 1930s that it was applied to atoms and molecules. It is because of this historical development that the teaching of the subject has often been preceded by a detailed treatment of matrix algebra. However, it is possible to progress quite a long way in understanding molecular symmetry without any such mathematical knowledge and it is such a treatment that is adopted here.

Ifwe compare the symmetry of a circle, a square and a rectangle it is intuitively obvious that the degree of symmetry decreases in the order given. What about the degrees of symmetry of a parallelogram and an isosceles triangle? Similarly, there is a decrease in the degree of symmetry along the series of molecules ethylene, 1,1-difluoroethylene and fluoroethylene. However, cis- and trans-1,2-difluoroethylene, present a similar problem to the parallelogram and isosceles triangle. In fact, it will be apparent by the end of this section that di-l,2-difluoroclhylcnc and an isosceles triangle have the same symmetry as also do trans-l,2-difluoroethylene and a parallelogram.

These simple examples serve to show that instinctive ideas about symmetry are not going to get us very far. We must put symmetry classification on a much firmer footing if it is to be useful. In order to do this we need to define only five types of elements of symmetry and one of these is almost trivial. In discussing these we refer only to the free molecule, realized in the gas phase at low pressure, and not, for example, to crystals which have additional elements of symmetry relating the positions of different molecules within the unit cell. We shall use, therefore, the Schönflies notation rather than the Hermann-Mauguin notation favoured in crystallography.

In discussing molecular symmetry it is essential that the molecular shape is accurately known, commonly by spectroscopic methods or by X-ray, electron or neutron diffraction.

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

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.

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