Formula, structural

A condensed structural formula, or simply a condensed structure, shows the same information as a structural fonnula, but in a condensed form. For instance, the molecular formula, structural formula, and condensed structural formula of octane are as follows  

In the condensed structural formula, the identical adjacent groups of atoms (in this case —CH2— groups) are enclosed in parentheses and subscripted to denote their number. 

In molecules where the carbon atoms do not form a single, unbranched chain, branches are indicated using additional parentheses in the condensed structural formula. For example, 2-methylheptane has the same molecular formula (CgH,g) as octane. In effect, the chain is one C atom shorter than in octane and one of the hydrogen atoms on the second C atom has heen replaced by a methyl group. The molecular formula, structural formula, and condensed structural formula of 2-methylheptane are as follows  

To save space, structural formulas are conveniently abbreviated as omdensed structural formulas such as CH3CH(CH3)CH(C2H5)CH2CH3 for 3-ethyl-2-methylpen-tane, where the CH3 (methyl) and C Hs (ethyl) groups are placed in parentheses to show that they are branches attached to the longest continuous chain of carbon atoms, which contains 5 carbon atoms. It is understood that each of the methyl and ethyl groups is attached to the carbon immediately preceding it in the condensed structural formula (methyl attached to the second carbon atom, ethyl to the third). 

You ve learned previously how to represent molecules using molecular and structural formulas [M Section 2.6], as well as using Lewis stmctures [M Section 8.3], In this section, we will leam several additional ways to represent molecules—ways that are particularly useful in the study of organic chemistry. 

In this chapter we first briefly review the most important types of covalent bonds encountered in organic substances and the ways in which these bonds are represented in structural formulas. Next we consider the sizes and shapes of organic molecules and how structural formulas written in two dimensions can be translated into three-dimensional models that show the relative positions of the atoms in space. We also discuss models that reflect the relative sizes of the atoms and the way in which the atoms may interfere with each other when in close quarters (steric hindrance). Then we go on to further important aspects of structure — the functional group concept and position isomerism. 

Our aim is to have you become more familiar with the various kinds of organic compounds and begin to see how the practicing organic chemist visualizes molecules and correlates the diverse kinds of structures that he has to deal with in his work. 

The building block of structural organic chemistry is the tetravalent carbon atom. With few exceptions, carbon compounds can be formulated with four 

There exist, however, compounds such as ethene (ethylene), C2H4, in which two electrons from each of the carbon atoms are mutually shared, thereby producing two two-electron bonds, an arrangement which is called a double bond. Each carbon in ethene is attached to only three other atoms  

Similarly, in ethyne (acetylene), C2H2, three electrons from each carbon atom are mutually shared, producing three two-electron bonds, called a triple bond, in which each carbon is attached to only two other atoms  

How many isomers of formula CH3Y would be possible if methane were a pyramid with a rectangular base What are they  

To answer this problem one makes two identical pyramidal models of CHi, (with C at the apex), and then replace, for instance, the upper right-hand H with a different atom Y in the other model replace the lower right-hand H. Next determine whether or not, by any manipulations except bending or breaking bonds, the models can be made to coincide in all their parts. If the two models are super-imposable, then they represent two molecules of the same structure if the models are not superimposable, then they represent molecules of different structures which, since they have the same molecular formula, are by definition isomers. 

All the possible pyramidal structures of CH3Y are drawn and shown in figures A through D. 

How many isomers of formula CH2YZ would be expected from each of the following structures for methane (a) Carbon at the center of a rectangle (b) Carbon at the center of a square (c) carbon at the apex of a square base pyramid (d) carbon in the center of a tetrahedron. 

Draw the important resonance forms for the following molecules and ions. 

For each of the following compounds, draw the important resonance forms. Indicate which structures are major and minor contributors or whether they have the same energy. 

In drawing resonance forms for ions, see how you can delocalize the charge over several atoms. Try to spread a negative charge over electronegative elements like oxygen and nitrogen. 

Try to spread a positive charge over as many carbons as possible, but especially over any atoms that can bear the positive charge and still have an octet for example, oxygen (with three bonds) or nitrogen (with four bonds). 

Several kinds of formulas are used by organic chemists to represent organic compounds. Some of these formulas involve a shorthand notation that requires some explanation. Structural formulas actually show which atoms are bonded to which. There are two types of structural formulas, complete Lewis structures and condensed structural formulas. In additi on, there are several ways of drawing condensed structural formulas. As we have seen, a Lewis structure symbolizes a bonding pair of electrons as a pair of dots or as a dash (—). Lone pairs of electrons are shown as pairs of dots. 

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Overall formula Structural formula Molecular weight Bolling, point, °C (1 atm) Specific gravity < (liquid)... 

Why is potassium aluminium sulphate not soluble in benzene A compound M has the composition C = 50.0% H=12.5%o A1 = 37.5%. 0.360 g of M reacts with an excess of water to evolve 0.336 1 of gas N and leave a white gelatinous precipitate R. R dissolves in aqueous sodium hydroxide and in hydrochloric acid. 20 cm of N require 40 cm of oxygen for complete combustion, carbon dioxide and water being the only products. Identify compounds N and R, suggest a structural formula for M, and write an equation for the reaction of M with water. (All gas volumes were measured at s.t.p.)... 

In many cases, quite a few different structural formulas can be produced for one and the same empirical formula (Figure 2-39). 

The commercial 2D structure editor. Chemistry 4D-Draw, from Chemlnnovativc Software Inc., includes two additional special modules besides conventional chemical drawing tools. NamExpert provides the interpretation ofa compound name according to the lUPAC nomenclature to create the corresponding chemical structure. The latter can be represented in three different styles the shorthand. Kckulc. or semi-structural formula. In contrast to NamExpert, the Nomenclature module assigns lUPAC names to drawn structures. 

The CAS Registry contains information on all the chemical compounds published in the literature since 1957. The sources of these 21 million compounds are 9000 international journals containing chemical information. The database includes CAS Registry Numbers, the CAS name (not conforming to the lUPAC convention) with synonyms, and molecular and structural formulas. The CAS provides a weekly update (see Section 5.4). 

In the mid-nineteenth century, the empirical formula of benzene, was known but its structural formula was tiot. Two proposed structures... 

This reference work differs from Beilstein in that it is baaed upon structural formulae and compounds are grouped according to the carbon skeleton rather than the functional group the latter system has the advantage that closely related compounds are grouped together. The volumes are not published in numerical order but rather on the basis of fields of current interest. They are a valuable supplement to Beilstein. The volumes which have been published to date (1955) are ... 

The isomer proportions for the nitration of the chlorotoluenes, to be expected from the additivity principle, have been calculated from the partial rate factors for the nitration of toluene and chlorobenzene and compared with experimental results for nitration with nitric acid at o °C. The calculated values are indicated in brackets beside the experimental values on the following structural formulae. In general, it can be... 

A summary of the tables is given below, while the tables with structural formulae, names, and approximate prices follow thereafter. 

The best textbooks for the advanced student of chemistry are the scientific journals. One may, for example, take any recent issue of the Journal of the American Chemical Society and look for structural formulas. On my desk 1 have Vol. 103, No. 25 (1981). The following seven compounds are discussed in this particular issue (page numbers in brackets). 

Structural formulas of this type in which electrons are represented as dots are called Lewis structures... 

Organic chemists have devised a number of shortcuts to speed the writing of structural formulas Sometimes we leave out unshared electron pairs but only when we are sure enough m our ability to count electrons to know when they are present and when they re not We ve already mentioned representing covalent bonds by dashes In condensed structural formulas we leave out some many or all of the covalent bonds and use sub scripts to indicate the number of identical groups attached to a particular atom These successive levels of simplification are illustrated as shown for isopropyl alcohol ( rub bmg alcohol )... 

With practice writing structural formulas for organic molecules soon becomes rou tine and can be simplified even more For example a chain of carbon atoms can be rep resented by drawing all of the C—C bonds while omitting individual carbons The result mg structural drawings can be simplified still more by stripping away the hydrogens... 

Write structural formulas for al avTng the given molecular formula... 

Among structural formulas in which the octet rule IS satisfied for all atoms and one or more of these atoms bears a formal charge the most stable reso nance form is the one in which negative charge re sides on the most electronegative atom... 

Structural formula J is a Lewis structure of nitro methane K is not even though it has the same atomic positions and the same number of electrons... 

Compound Structural Formula Repulsive Electron Pairs of Electron Pairs Molecular Shape Molecular Model... 

Wnte a structural formula for the CFI3NO isomer characterized by the structural unit indi cated None of the atoms in the final structure should have a formal charge... 

Wnte structural formulas for all the constitutional isomers of molecular formula C3HgO that contain... 

An n alkane of molecular formula C28H58 has been isolated from a certain fossil plant Write a condensed structural formula for this alkane J... 

Now write structural formulas for the remaining three isomers Be sure that each one IS a unique compound and not simply a different representation of one writ ten previously... 

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