Until 1828, chemists believed that certain chemicals such as sugar, silk, vinegar, and oil could be produced only by living organisms. Such chemicals were called organic chemicals. Chemists knew that organic substances were easily decomposed by heat into inorganic substances, but they had not been able to convert inorganic chemicals into organic ones.
Synthetic organic chemistry was born in 1828 when Friedrich Wohler, a German chemist, converted an inorganic chemical called ammonium cyanate into urea, a chemical isolated from urine and recognized as organic by all chemists of that era. Since that time, approximately a million organic compounds have been synthesized. This is many times more than all the inorganic substances known at the present time. Modern drugs, dyes, synthetic fabrics, building materials, insecticides, and many other familiar substances are products of synthetic organic chemistry.
Organic substances have one feature in common. They all contain carbon atoms. Thus, organic chemistry is the chemistry of carbon compounds. The fact that carbon forms more compounds than all of the other elements put together suggests that carbon has some very unusual, almost unique, characteristics.
The most significant feature of the carbon atom is its ability to form covalent bonds with other carbon atoms as readily as it does with other kinds of atoms. This ability leads to the formation of chain-like molecules which may contain thousands of carbon atoms. When the ends of a chain are joined, cyclic or ring structures are obtained. In theory, there is no limit to the number of carbon compounds which can be formed.
Many organic compounds such as methyl ether (CH3-O-CH3) and ethyl alcohol (CH3CH2-OH) have the same number of respective elements (C2H6O) but entirely different properties and structure. Substances with the same formula but different structure are known as isomers. To distinguish one isomer from another, we must rely heavily on structural formulas. To expedite the study of organic compounds, we group them into relatively few categories in terms of their structures and compositions and use systematic methods for naming them.
Carbon is the first member of Group IVA of the Periodic Table; the other members of this group are silicon (Si), germanium (Ge), tin (Sn), and lead (Pb). Carbon and silicon are predominantly nonmetallic in character whereas germanium, tin, and lead are metallic elements. Each of these elements has four valence electrons and each exhibits a maximum oxidation state of +4; these elements, especially the heavier ones, also show an oxidation state of +2. The electron configuration for carbon is:
Carbon is nineteenth among the elements in terms of abundance; it constitutes only about 0.027 per cent of the earth's crust. It is found in the free state in the two allotropic forms of diamond and graphite. The term allotropy is used to designate the existence of an element in two or more forms in the same physical state.
Carbon atoms can link covalently to other carbon atoms, as well as to atoms of many other elements. It is this linking of carbon to carbon atom, called catenation, which gives rise to the tremendous number of carbon compounds.
Three types of carbon bonds exist. The single bond which consists of two electrons being shared, one from each carbon atom. The double bond which consists of four electrons being shared, two from each carbon. The triple bond which consists of six electrons being shared, three from each carbon atom. Double and triple bonds are less flexible than single bonds. Double bonds are more easily broken than are single bonds and triple bonds are more easily broken than a double bond. Because double and triple bonds are more easily broken, molecules containing double or triple bonds are usually more reactive than molecules containing only single bonds.
10.4 ORGANIC COMPOUNDS - CLASSIFIED
The strength of the C-C bond is so great that, during many reactions, the carbon skeleton of the molecule remains intact even though other groups (-Cl, -OH, -NO2) attached to the molecule may be changed. This characteristic makes it convenient to classify organic compounds in terms of their skeletal carbon structure and the reactive groups (functional groups) which are located within the molecule. There are four general skeletal structures associated with organic compounds. These are aliphatic, alicyclic, aromatic, and heterocyclic.
ALIPHATIC COMPOUNDS are composed of carbon atoms linked together in an open chain structure. The chain may have branches but does not form any closed loops. For example, butane, methylpropane, methylbutane, dimethylpropane and 2-methylpentane are aliphatic compounds.
ALICYCLIC COMPOUNDS are compounds in which the carbon atoms are arranged in a ring structure. The rings may contain three, four, five, or a larger number of atoms. Cyclopropane, sometimes used as an anesthetic, is a common example of an alicyclic compound.
All AROMATIC COMPOUNDS are compounds which have structures related to that of benzene, the simplest aromatic compound. These compounds are characterized by molecules which contain a ring of six carbon atoms. The ring structure is represented by alternating single and double bonds. More complicated aromatic compounds may contain many benzene rings, side chains, and functional groups such as -OH and -NO2.
HETEROCYCLIC COMPOUNDS are composed of molecules which incorporate atoms such as oxygen, nitrogen, sulfur, and phosphorus as part of the ring system. Nucleic acids, important components of biological cells, contain heterocyclic ring systems. Ethylene oxide, a substance used as a fumigant for foodstuffs, is an example of a heterocyclic compound.
10.5 FUNCTIONAL GROUPS
It has been found that organic compounds containing the same types of functional groups behave in much the same manner. That is, the size and shape of the carbon atom skeleton does not affect the chemical behavior of the substance to any great extent. This observation allows us to systematize the study of organic chemistry by classifying substances in terms of their reactive or functional groups. However, it should be remembered that no classification system is perfect. There are many variations in composition and structure so that it is often difficult to rigidly classify a substance as belonging to a particular group.
|Carboxylic Acids||-COOH||CH3COOH||Ethanoic Acid (Acetic Acid)|
|Esters||-COOR'||CH3COOCH3||Methyl Ethanoate (Methyl Acetate)|
One of the largest classifications of organic compounds is the group known as the hydrocarbons. These are compounds composed of carbon and hydrogen only. Hydrocarbons are simple organic compounds. Almost all other organic compounds can be named as derivatives of these simple hydrocarbons. If the carbon atoms are linked in chains, the compounds are called aliphatic compounds; if the atoms are linked in rings, the compounds are called alicyclic.
The chain compounds may be further classified on the basis of the individual carbon-to-carbon bonds. Chain compounds in which all carbon-to-carbon bonds are single bonds are called ALKANES. These compounds are also called saturated hydrocarbons, because each carbon-to-carbon bond is a single bond, and the valence of the carbon atom is, therefore, saturated. No more atoms can be bonded to the atoms in the compound, without breaking the compound into two or more fragments.
|Name||Molecular Formula||Structural Formula||Boiling Point (oC)|
Names of the higher members of this series consist of a numerical term, followed by "-ane". Examples of these names are shown in the table below. The generic name of saturated aliphatic (acyclic) hydrocarbons (branched or unbranched) is "alkane".
Examples of names:
(n = total number of carbon atoms)
|11 Undecane||22 Docosane||33 Tritriacontane|
|12 Dodecane||23 Tricosane||40 Tetracontane|
|13 Tridecane||24 Tetracosane||50 Pentacontane|
|14 Tetradecane||25 Pentacosane||60 Hexacontane|
|15 Pentadecane||26 Hexacosane||70 Heptacontane|
|16 Hexadecane||27 Heptacosane||80 Octacontane|
|17 Heptadecane||28 Octacosane||90 Nonacontane|
|18 Octadecane||29 Nonacosane||100 Hectane|
|19 Nonadecane||30 Triacontane||132 Dotriacontahectane|
|20 Icosane||31 Hentriacontane|
|21 Henicosane||32 Dotriacontane|
Each compound differs from the next by a multiple of the -CH2- (methylene group). Essentially we are building the series of compounds by removing a hydrogen atom from one of the carbon atoms and adding - CH2- to the chain and then replacing the hydrogen.
A series of compounds whose structures differ from each other by a specific structural unit (such as -CH2- in the case of alkanes) is called a Homologous Series. A general formula can be written for all of the members of a homologous series such as the alkanes. For the alkanes, the formula CnH2n+2, where n is the number of carbon atoms in the compound.
There are general trends in physical and chemical properties within homologous families which can be used to study these families as a whole. For instance, as the molecular weight of the compounds in a family increases, the boiling point increases. This can be seen from the table listing the first ten members of the alkane family above.
It is interesting to note that the first four alkanes, which all exist in the vapor state under normal atmospheric conditions, are the principle ingredients in natural gas. The members of the alkane family use the Greek (sometimes Latin) prefix for the number of carbon atoms, and the characteristic ending -ane to identify and describe their position and structure within their respective family.
If one hydrogen atom, with its associated electron, is removed from a hydrocarbon molecule, a Radical is left:
Radicals are named by substituting the ending -yl for the normal -ane ending of the parent compound.
For convenience in naming organic compounds, carbon atoms in a structural formula are given position numbers. In an unbranched chain molecule, the numbering of carbon atoms can begin at either end of the chain:
Not all alkanes have unbranched chains of carbon atoms. Complex alkanes are named by using the longest chain of carbon atoms as the basis of the compound name. The PARENT CHAIN does not necessarily occur in a straight line. This compound
has pentane (C5H12) as the parent chain since the longest chain contains five carbon atoms. The carbon atoms of the longest chain are given position numbers beginning at one end of the parent chain. The CH3- group which is attached to the main chain is called a side chain or substituent. The side chain is named as a radical. We indicate, by number, the position of the carbon atom of the parent chain to which the side chain (radical) is attached. Thus, the name 3-methylpentane for the above saturated alkane. The parent compound is pentane, the radical is methyl which attached to the number-3 carbon atom of the parent chain. The name is written with a hyphen between the substituent (radical) position number and name. The radical and parent are written as one word. Numbering of the carbon atoms of the parent chain begins at the end which will give the lowest position numbers to the radical.
It is relatively easy to see that if one does not follow the established rules the previous structure may have been mistakenly named 4-ethyl-4-methylhexane. ONE MUST BEGIN NUMBERING THE CARBONS OF THE PARENT CHAIN FROM THAT END CLOSEST TO THE ATTACHED RADICAL. You may have wondered why the structure was not named 3-methyl-3-ethylhexane. The fact of the matter is that both 3-ethyl-3-methylhexane and 3-methyl-3-ethylhexane are correct. It is permissible to choose one of two rules.
The first rule listed is generally the accepted method of representing the attached radicals and functional groups. Whatever method is chosen it is important to be consistent.
If there are two or more radicals attached which are alike, it is convenient to use prefixes (di-, tri-, tetra-, penta-, etc.) instead of writing each group separately. A comma is placed between the position numbers of the substituents which are alike.
The parent chain is the five carbon chain-pentane. The carbons of the parent chain must be numbered starting from the left because the radicals (methyl) are located closest to that end. The two similar radicals are positioned on the second and third carbons of the parent chain, thus, the 2,3- designation. Because there are two radicals of the same kind the prefix di- is utilized to indicate the number present. Again, note the presence of the comma and hyphen. For structures such as methane (CH4), ethane (CH3-CH3), and propane (CH3-CH2-CH3) only one structural diagram needs to be drawn. There is, however, an alternative structure for butane.
|Butane (C4H10)||Methylpropane (C4H10)|
As noted in section 10.1, the existence of two or more substances with the same molecular formula (C4H10 in this instance), but different arrangements of atoms and bonds, is called ISOMERISM. The two structures of butane diagrammed above are called isomers of butane. Most organic compounds have isomers but there is no known way of predicting exactly how many isomers most compounds can form. Pentane (C5H12), the next member of the alkane family, has three isomers:
Because there is only one form of methylpropane, methylbutane, and dimethylpropane it is not necessary to include the position numbers when writing their name.
Hexane (C6H14), the next member of the alkane family, has five isomers, and heptane (C7H16) has nine. Isomers are named according to the longest chain, and not according to the total number of carbon atoms in the molecule. Thus, the isomers of heptane are:
The number of isomers increases dramatically with the number of carbons in the parent chain as indicated by octane with eighteen (18), nonane with thirty-five (35), and decane with seventy-five (75).
|Fraction||Composition of carbon chains||Boiling range (oC)||Percent of crude oil|
|Natural Gas||C1 to C4||Below 20||10%|
|Petroleum ether (solvent)||C5 to C6||30 to 60||10%|
|Naphtha (solvent)||C7 to C8||60 to 90||10%|
|Gasoline||C6 to C12||75 to 200||40%|
|Kerosene||C12 to C15||200 to 300||10%|
|Fuel oils, mineral oil||C15 to C18||300 to 400||30%|
|Lubricating oil, petroleum jelly, greases, paraffin wax, asphalt||C16 to C24||Over 400||10%|
Saturated hydrocarbons occur in three forms: straight-chain forms (alkanes), branched chain forms (alkanes), and cyclic forms (cycloalkanes). The cycloalkanes contain only single bonds, and have the general formula CnH2n. They occur in natural petroleum in such forms as:
Because these diagrams are somewhat unwieldy, organic chemists use stylized drawings to represent cyclic compounds. In the cyclic compound symbols, a carbon atom is understood to be at the intersection of each pair of straight lines. Each carbon atom is understood to be bonded to a sufficient number of hydrogen atoms to produce a total number of four bonds (the number required for carbon). The standard symbols used to represent the first four cycloalkanes are:
Cyclopropane, Cyclobutane, Cyclopentane, Cyclohexane
Another hydrocarbon family is the family of compounds containing double bonds between carbon atoms. These compounds are termed the ALKENES or OLEFINS. They are named just as their parent compounds, the alkanes, are named, except the ending -ene replaces -ane. The ending -ene signifies a double bonding of carbon atoms. The first five members of the alkene family are ethene, propene, butene, pentene, and hexene. Many organic compounds contain double bonds. Such compounds are termed UNSATURATED, since they can combine with other elements or compounds without breaking the carbon chain, by adding on at the double bond. The alkenes constitute a homologous series with the general formula CnH2n. Physical and chemical properties of alkenes vary with increasing molecular weight, in the same way as do the properties of the alkanes.
With the introduction of double bonds, a new way of forming isomers is introduced. Butene can be represented as:
Compounds with double bonds, which exist in isomeric form, are named by placing the position number of the carbon atom on which the double bond begins, before the name of the parent compound.
Carbon atoms in alkenes are always numbered so that the lowest possible position number is assigned to the first carbon atom to which the double bond is attached. The parent compound is named from the longest continuous chain containing a double bond. For example:
It is required that the parent chain contain the double bond. Thus, a six carbon parent chain rather than a seven carbon chain containing only single bonds.
If more than one double bond occurs in a molecule, the endings -adiene, -atriene, and so on are used. The compound:
The double bond makes possible another kind of isomerism. The kind of isomerism which we have already studied is called STRUCTURAL ISOMERISM. The new type is called GEOMETRIC ISOMERISM. Geometric isomers are a result of the rigidity of a molecule, which prevents rotation of atoms about a bond. This rigidity is usually due to a ring structure or a double bond. Such rigidity is usually not present in singly bonded, open-chained molecules.
These two compounds can be separated physically and are separately identifiable, even though they have the same name, 2-butene. Most cases of geometric isomerism are a result of molecule rigidity which prevents rotation about a double bond. In those molecules in which the substituent groups are on the same side of the double bond, the name is preceded by the prefix cis-. In those molecules in which the groups are on opposite side of the double bond, the name is preceded by the prefix trans-.
Compounds with triple-bonded carbon atoms are called ALKYNES, and constitute a homologous series with the general formula CnH2n-2. They are important raw materials for industries producing synthetic materials such as plastics and fibers. Chemically, they are very reactive. The alkynes are named just as the alkenes, except the ending -yne replaces -ene. Acetylene is the common name for ethyne, the first member of this series. Acetylene is commercially the most important member of the family.
The corresponding alkyne radicals are as follows:
The numbering system for location of the triple bond and the substituent groups of the alkynes follows much the same pattern as was used for naming the alkenes. For example,
In naming hydrocarbons that have both double and triple bonds, A DOUBLE BOND TAKES PRECEDENCE OVER A TRIPLE BOND. The double bond is given the lower position number. For example,
10.10 AROMATIC HYDROCARBONS
To an organic chemist, one of the most important organic compounds is the cyclic hydrocarbon benzene. Its structural diagram is:
In this structural representation it is assumed that there is a carbon and hydrogen atom at each corner unless another group is shown in place of the hydrogen at one or more corners. This diagram should not be confused with the symbol
which is used to represent cyclohexane, an alkane. Note that a benzene has a conjugated system of alternating double and single bonds. It, therefore, possesses great stability. There are so many compounds derived from benzene, that the study of benzene derivatives constitutes a whole branch of organic chemistry. Since most of these compounds have rather distinctive odors, they are called AROMATIC compounds. Aromatic compounds are normally named as derivatives of benzene. Aromatic compounds occur in small quantities in some petroleum reserves, and to a large extent in the coal tar which is obtained from the distillation of coal. In some compounds, several rings may be formed together in a fused system. These compounds have properties similar to benzene. An example of a fused ring compound is naphthalene:
The radical formed by removing a hydrogen atom from a benzene ring is called the PHENYL radical.
Some examples of benzene compounds are:
Chlorobenzene, Hydroquinone, Nitrobenzene, Phenol, Picric Acid