Classes of organic compounds

Student Annotation Recall that isomers are different compounds with the same chemical formula [Mt Section 9.2], 

Carbon s ability to form chains by bonding with itself 

The presence of elements other than carbon and hydrogen 

In this section, we will discuss several types of organic compounds and how we represent them. Consider two isomers of C3H6O  

Although the two isomers contain exactly the same atoms, their different arrangements of atoms result in two very differerrt compounds. The first is an aldehyde called propanal. The second is a ketone called acetone. Aldeltydes and ketones are two classes of organic compormds. The classes of organic compormds that we will discuss in this chapter ate alcohols, carboxylic acids, aldehydes, ketones, esters, amines, and amides. 

Carbon s valence electrons are in the second shell (n = 2), where there are no d orbitals. The valence electrons of silicon, on the other hand, are in the third shell (n = 3) where there are d orbitals, which can be occupied or attacked by lone pairs on another substance—resulting in a reaction. This reactivity makes silicon compounds far less stable than the analogous carbon compounds. Ethane (CH3—CH3), for example, is stable in both water and air, whereas disilane (SiH3—SiH3) is unstable—breaking down in water and combusting spontaneously 

A functional group is a group of atoms that determines many of a moleculets properties [W Section 2.6]. 

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On pp. 410—415 Tables are given showing some of the properties of the simpler members of certain classes of organic compounds. These tables should be useful to the elementary student working with a limited range of compounds. It must be emphasised that these tables serve primarily to summarise some of the detailed descriptions given in the foregoing sections, and should be used w-hen the student is familiar with these sections. 

The analyses which follow are arranged in the order in which they would be applied to a newly discovered substance, the estimation of the elements present and molecular weight deter-minations(f.e., determination of empirical and molecular formulae respectively) coming first, then the estimation of particular groups in the molecule, and finally the estimation of special classes of organic compounds. It should be noted, however, that this systematic order differs considerably from the order of experimental difficulty of the individual analyses. Consequently many of the later macro-analyses, such as the estimation of hydroxyl groups, acetyl groups, urea, etc. may well be undertaken by elementary students, while the earlier analyses, such as estimation of elements present in the molecule, should be reserved for more senior students. 

The common drying agents that are suitable for various classes of organic compounds are listed in Table II, 39. 

Picrates, Many aromatic hydrocarbons (and other classes of organic compounds) form molecular compounds with picric acid, for example, naphthalene picrate CioHg.CgH2(N02)30H. Some picrates, e.g., anthracene picrate, are so unstable as to be decomposed by many, particularly hydroxylic, solvents they therefore cannot be easily recrystaUised. Their preparation may be accomplished in such non-hydroxylic solvents as chloroform, benzene or ether. The picrates of hydrocarbons can be readily separated into their constituents by warming with dilute ammonia solution and filtering (if the hydrocarbon is a solid) through a moist filter paper. The filtrate contains the picric acid as the ammonium salt, and the hydrocarbon is left on the filter paper. 

This reagent affords compounds (1 1) with aromatic hydrocarbons and other classes of organic compounds (heterocyclic compounds, aromatic ethers, etc.). 

Qualitative analysis for the elements. This includes an examination of the effect of heat upon the substance—a test which inter alia will indicate the presence of inorganic elements—and quahtative analysis for nitrogen, halogens and sulphur and, if necessary, other inorganic elements. It is clear that the presence or absence of any or all of these elements would immediately exclude from consideration certain classes of organic compounds. 

Study of the solubility behaviour of the compound. A semi-quantitative study of the solubility of the substance in a hmited number of solvents (water, ether, dilute sodium hydroxide solution, dilute hydrochloric acid, sodium bicarbonate solution, concentrated sulphuric and phosphoric acid) will, if intelligently apphed, provide valuable information as to the presence or absence of certain classes of organic compounds. 

Location of the compound within a class (or homologous series) of compounds. Reference to the literature or to tables of the physical properties of the class (or classes) of organic compounds to which the substance has been assigned, will generally locate a number of compounds which boil or melt within 6° of the value observed for the unknown. If other physical properties e.g., refractive index and density for a hquid) are available, these will assist in deciding whether the unknown is identical with one of the known compounds. In general, however, it is more convenient in practice to prepare one, but preferably two, crystalhne derivatives of the substance. 

The most common interfering substance, especially with alcohols of low mole cular weight, is water this may result in an inaccurate interpretation of the test if applied alone. Most of the water may usually be removed by shaking with a little anhydrous calcium sulphate,. though dry ethers (and also the saturated aliphatic and the simple aromatic hydrocarbons) do not react with sodium, many other classes of organic compounds do. Thus ... 

The methods of preparation of some of the more important derivatives of a number of classes of organic compounds are described in the various Sections dealing with their reactions and characterisation. These Sections conclude with tables incorporating the melting points and boiling points of the compounds themselves, and also the melting points of selected derivatives. For convenience, the references to the various tables are collected below. 

Functional Groups in Some Important Classes of Organic Compounds... 

The major classes of organic compounds common to living systems are lipids pro terns nucleic acids and carbohydrates Carbohydrates are very familiar to us— we call many of them sugars They make up a substantial portion of the food we eat and provide most of the energy that keeps the human engine running Carbohy drates are structural components of the walls of plant cells and the wood of trees Genetic information is stored and transferred by way of nucleic acids specialized derivatives of carbohydrates which we 11 examine m more detail m Chapter 28... 

Ketones are a class of organic compounds that contain one or more carbonyl groups bound to two aUphatic, aromatic, or aUcycbc substituents, and are represented by the general formula... 

For convenience, many of the above drying agents are listed in Table 18 under the classes of organic compounds for which they are commonly used. 

GENERAL PROCEDURES FOR THE PURIFICATION OF SOME CLASSES OF ORGANIC COMPOUNDS... 

Many completely conjugated hydrocarbons can be built up from the annulenes and related structural fragments. Scheme 9.2 gives the structures, names, and stabilization energies of a variety of such hydrocarbons. Derivatives of these hydrocarbons having heteroatoms in place of one or more carbon atoms constitute another important class of organic compounds. 

We first encountered nucleophilic substitution in Chapter 4, in the reaction of alcohols with hydrogen halides to fonn alkyl halides. Now we ll see how alkyl halides can themselves be converted to other classes of organic compounds by nucleophilic substitution. 

Although acyl fluorides, bromides, and iodides are all known classes of organic compounds, they are encountered far- less frequently than are acyl chlorides. Acyl chlorides will be the only acyl halides discussed in this chapter. 

Aromatic substances, more than any other class of organic compounds, have acquired a large number of nonsystematic names. The use of such names is discouraged, but 1UPAC rules allow for some of the more widely used ones to be retained (Table 15.1). Thus, methylbenzene is known commonly as toluene hydroxybenzene, as phenol ami nobenzene, as aniline and so on. 

Very powerful initiators of carbonium-ion polymerization were recently reported by Plesch.32b They belong to the salt-like class of organic compounds and dissociate readily into C104 ions and carboxonium positive ions. The latter are sufficiently reactive to initiate carbonium-ion polymerization of styrene. 

Motpholines are a chemical class of organic compounds with the six-membered ring tetrahydro-1,4-oxazine, as their basic structure. 

Mass Spectra and Chemical Structure While there are a number of books (Refs 16, 30, 49 64) already referred to, which deal with details of the instrumentation and techniques of mass spectrometry, there are several concise introductory texts (Refs 10, 21 52) on the interpretation of mass spectra. Still other recent books deal comprehensively with organic structural investigation by mass spectrometry. One of these (Ref 63) discusses fundamentals of ion fragmentation mechanisms, while the others (Refs 7, 15, 20, 28 29) describe mass spectra of various classes of organic compounds. In the alloted space for this article methods of interpretation of mass spectra and structural identification can not be described in depth. An attempt is, therefore, made only to briefly outline the procedures used in this interpretation... 

The hydrocarbons are the basic framework for all organic compounds. Different classes of organic compounds have one or more of the hydrogen atoms replaced by other atoms or groups of atoms. All we need to be aware of at this stage are the three classes of compounds known as alcohols, carboxylic acids, and haloalkanes ... 

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