11.1 HYDROCARBON DERIVATIVES
Not all atomic combinations possible on paper can actually occur in nature. One of the most important jobs of the organic chemist is to synthesize compounds which have been predicted theoretically.
Many compounds that occur in nature have not been classified and analyzed. Another job of the organic chemist is to isolate and analyze natural organic substances. If the chemist can find the structure of a natural compound, he can then attempt to synthesize the material in the laboratory.
We have already seen how complex such jobs can be, even when carbon and hydrogen are the only elements with which the chemist is concerned. When atoms of other elements are introduced into hydrocarbon molecules, literally hundreds of thousands of organic compounds can be (and are) formed.
11.2 HALOGEN DERIVATIVES
If atoms other than carbon and hydrogen are substituted for part of a hydrocarbon molecule, the chemical reactivity of the hydrocarbon is generally increased. The nonhydrocarbon part of the molecule is called a FUNCTIONAL GROUP. Most of the chemical reactivity of the substituted hydrocarbon is due to the functional group attached to it.
One family of substituted hydrocarbon molecules has a halogen atom substituted for a hydrogen atom. For example, if we substitute a bromine atom for a hydrogen atom on methane, we obtain
In subsequent discussions of compounds, we will represent any hydrocarbon radical by R- and any halogen atom by -X. The general formula for the halogen-substituted hydrocarbon compounds is R-X. As one might expect, it is possible to have more than one hydrogen atom replaced by a halogen atom. In the compound
Tetrachloromethane (carbon tetrachloride)
tetrachloromethane, more commonly called carbon tetrachloride, four chlorine atoms are substituted for the four hydrogen atoms in a methane molecule. Trichloromethane (CHCl3), or chloroform, which is used extensively as a solvent and formerly was employed as an anesthetic, is another example of a multisubstituted hydrocarbon.
We number the carbon atoms to avoid any ambiguity in naming the compounds. Thus,
In multisubstituted aromatic compounds, it is necessary to indicate the relative positions of the various substituent groups on the ring. If only two substituent groups are attached, the compound can be named using a prefix to designate the position of the substituents. The three possible relative positions of two substituent groups, and the corresponding prefixes are:
For example, the molecule
may be called ortho-dichlorobenzene or 1,2-dichlorobenzene.
If more than two substituents are attached to the benzene ring, it is necessary to assign position numbers to the carbon atoms of the ring. The atoms in the benzene ring are numbered so as to give the smallest position numbers to the substituents. For example:
is 1,3-dibromobenzene rather than 1,5-dibromobenzene. In the naphthalene molecule, the 1-position is next to the atom without a hydrogen atom attached. There are four 1-positions possible in each molecule of naphthalene. The 1-position which gives the lowest numbers to substituents is always used. The numbering system for naphthalene requires that carbon number 1 must begin at one of four positions. Those positions are either one of the two uppermost positions or lower most positions as represented on the molecule below. Once that position is identified all carbons furthest from the center of the molecule are number in order that carbons 9 and 10 are always those which form the bridge creating the "appearance" of two benzene rings.
Several more examples which illustrate the naming of substituted hydrocarbons follow.
11.3 HYDROXY COMPOUNDS - ALCOHOLS
A very large and important class of organic compounds is the group with the general formula R-OH. The R- stands for a hydrocarbon chain and -OH is the hydroxyl functional group. These compounds are called ALCOHOLS. The inclusion of a hydroxyl functional group in a chain compound is indicated in the name by adding the suffix -ol. For instance, the compound CH3-OH is named methanol. It is also known as methyl alcohol or wood alcohol, since it can be obtained by the distillation of wood. It can be used as a fuel. Methanol is a poisonous compound, however; intoxication, blindness, and death may result when its vapors are breathed in quantities or when the liquid is taken internally.
Methanol is used in the manufacture of formaldehyde and other organic products; as an antifreeze; as a solvent for resins, gums, and shellac; and as a denaturant (to make unfit for human consumption) for ethanol.
Methanol produced today is synthesized from carbon monoxide or carbon dioxide.
CO + 2 H2 CH3-OH
CO2 + 3 H2 CH3-OH + H2O
Ethanol, CH3-CH2-OH, the next member of the hydroxyl-substituted alkanes, is also known as ethyl alcohol or grain alcohol, because large quantities of it are obtained by the fermentation of grain. It is widely used in alcoholic beverages and in many industrial processes as a solvent and raw material for making other organic compounds.
C6H12O6 2 CH3-CH2-OH + 2 CO2
glucose ethanol + carbon dioxide
Solutions of alcohol resulting from fermentation contain from 8 to 12 per cent alcohol, but by fractional distillation 95 per cent alcohol can be obtained. Removal of the residual water by distillation over calcium oxide or barium oxide results in the production of ABSOLUTE ALCOHOL (100%).
Ethyl alcohol is a colorless liquid with a characteristic and some-what pleasant odor. It is miscible with water in all proportions. The boiling point of the pure alcohol is 78.37oC but it forms a constant boiling mixture with water that contains 95.57 per cent alcohol by weight and boils at 78.15oC. Ethanol is the least toxic of all the alcohols and is present in all alcoholic beverages. Of all organic compounds, ethanol ranks first in quantity and value of production. It is used as a solvent in the preparation of tinctures, essences, extracts, and varnishes. It is used in the preparation of iodoform, ether, medicinals, dyes, perfumes, vinegar, and solvents for the lacquer industry. It is used as an antifreeze and has been used to some extent as a motor fuel.
The third member of the alcohol family is propanol which exists in two isomeric forms, 1-propanol and 2-propanol, due to the two possible locations to which the -OH may attach on the carbon chain. 2-propanol is the alcohol commonly used as rubbing alcohol in hospitals.
The structural formula of the first three hydroxyl-substituted alkanes are:
Just as it is possible to replace more than one hydrogen atom of a hydrocarbon with halogen atoms, it is also possible for more than one hydroxyl group to be substituted in an organic compound. The most common dihydroxy substituted alkane is 1,2-ethanediol, better known as ethylene glycol. Ethylene glycol is used in large quantities as the base for antifreeze mixtures for the protection of automobile cooling systems in cold weather.
Glycerol, a trihydroxy substituted alkane, has the substitutive name 1,2,3-propanetriol. Glycerol is used in medicine, cosmetics and in the manufacture of nitroglycerin and dynamite.
Derivatives of benzene which contain one or more hydroxyl groups attached directly to carbon atoms of the benzene ring are called PHENOLS. The hydroxyl group in these compounds is markedly acidic in character, and PHENOL itself, C6H5-OH, is commonly known as Carbolic Acid. Phenol is the principal aromatic alcohol with the structural formula
The two naphthols, 1-naphthol and 2-naphthol which are isomeric derivatives of naphthalene, are represented as :
Two alcohol molecules will react under the proper conditions to produce compounds with the general formula R-O-R'. These compounds are called ETHERS. The other product of the reaction is water which is taken up by a dehydration agent. A typical reaction is:
2 CH3-CH2-OH --H2SO4 CH3-CH2-O-CH2-CH3+ H2O
The ethers are named by naming the radical on either side of the oxygen atom, and adding the word ether. The two side chains can be the same, of course, in which case the name is mentioned just once, but preceded by the prefix di-. Thus, in the preceding reaction, 2 moles of ethanol will react to form 1 mole of diethyl ether.
Ethers may also be named as OXY DERIVATIVES of the hydrocarbons.
The ether radicals for the first four alkanes are:
Another name for diethyl ether is ethoxyethane. Dimethyl ether is methoxymethane, and methylheptyl ether is 1-methoxyheptane.
|dimethyl ether||methylheptyl ether|
The structural formula for 1,4-dimethoxy-2-pentene is
Many ethers are used in industry as solvents. Diethyl ether has found widespread use as an anesthetic since 1846, in which application it is referred to simply as ether. It and other ethers are valuable solvents for gums, fats, waxes, and resins.
In a great many classes of organic compounds, a carbon atom is part of the functional group. One such group of compounds contains the ALDEHYDE group, represented by the general formula R-CHO. The structural formula for this functional group is
Note that the aldehyde group must occur at the end of a carbon chain designated by R- in the general formula R-CHO. The simplest aldehyde, formaldehyde, has the formula HCHO. It is used to make embalming fluid and plastics. The substitutive name for formaldehyde is methanal. Aldehydes are named by adding the ending -al to the parent hydrocarbon stem name. Since the aldehyde functional group must be at the end of the chain, it will always be position number 1 in the parent chain. No number designation is necessary. If it is not part of the parent chain, but attached as a side chain, it may be designated by the prefix aldo-.
Whenever a compound contains both an alcohol group and an aldehyde the aldehyde takes precedence.
Formaldehyde or methanal is sold in an aqueous solution which contains about 37 per cent formaldehyde by weight and is known as FORMALIN.
Acetaldehyde or ethanal (CH3-CHO) is colorless, water-soluble, and has an odor like that of freshly cut green apples. It is used in the manufacture of aniline dyes, synthetic rubber, and other organic materials.
The odor of almonds is due to the presence of an aldehyde in almond oil. In this case, the aldehyde is benzaldehyde,
Another class of organic compounds is the group of compounds known as KETONES. They have the general formula R-CO-R'. In the general structural formula of ketones, the R- groups may be the same or different. The oxygen atom is doubly bonded to a carbon atom and is known as the CARBONYL GROUP, C=O. Ketones are named by adding the ending -one to the hydrocarbon stem name. The simplest possible ketone is propanone, CH3-CO-CH3, better known as acetone. It is also called dimethyl ketone. Among the uses of acetone are: as a solvent for cellulose acetate, cellulose nitrate, acetylene, plastics, and varnishes; as a remover of paints, varnishes, and fingernail polish; and as a solvent in the manufacture of drugs, chemicals, smokeless powder, and the high explosive CORDITE.
A few representative examples of ketones are:
Butanone is also known as methylethyl ketone.
11.7 CARBOXYLIC ACIDS
Organic acids with the general formula R-COOH are called CARBOXYLIC ACIDS. The acid group -COOH can be substituted more than once in one molecule giving rise to what are called POLYCARBOXYLIC ACIDS. The simplest organic acid is methanoic acid, also known as formic acid. It is found in ants. The Latin word for ant, formica, gives the acid its name.
Carboxylic acids are weak acids as demonstrated by ethanoic acid (acetic acid) with an ionization constant (Ka) of 1.8 x 10-5. Pure, anhydrous acetic acid is a liquid boiling at 118.1o. It freezes at 16.6o forming a solid resembling ice in appearance; for this reason the pure acid is usually called GLACIAL ACETIC ACID. A 5 per cent solution of acetic acid with water is VINEGAR.
|methanoic acid||ethanoic acid|
|(formic acid)||(acetic acid)|
Oxalic acid is a dicarboxylic acid represented by the formula H2C2O4. Its molecule consists of two carboxyl groups bonded together. Undiluted it is poisonous. Lactic acid contains a hydroxyl group as well as a carboxyl group, and is produced by the fermentation of milk sugar or glucose by the lactic acid bacillus. It is formed when milk sours and imparts the sour taste. Benzoic acid is a colorless crystalline solid composed of a benzene ring with a carboxyl group attached. It is used technically in the form of its sodium salt in the preservation of certain foods, such as tomato ketchup and fruit juices.
|Oxalic Acid||Lactic Acid||Benzoic Acid|
The chemical method of naming these compounds is to use the ending -oic, followed by the word acid. 3-carboxy-3-hydroxypentanedioic acid is the chemical name for citric acid.
Esters have the general formula R-COOR', and are formed from organic acids and alcohols. The other product of the reaction is water. The esterification process will proceed more nearly to completion if a substance which removes water without reacting with the acid or the alcohol is added to the reaction. For example:
CH3COOH + CH3CH2OH --H2SO4 CH3COOCH2CH3+ H2O
acetic acid + ethanol --H2SO4 ethyl acetate + water
The concentrated H2SO4 removes water from the products and is a dehydrating agent. Most esters have very pleasant odors. Many flavoring and scenting agents are made from esters. Esters are volatile liquids which are not ionized and they are soluble in organic solvents but not in water.
|methyl salicylate||C6H4(OH)COOCH3||oil of wintergreen|
Among the most important of the natural esters are fats such as lard, tallow, and butter, and oils such as linseed, cottonseed, and olive. These are made up of esters of such complex acids as palmitic, C15H31COOH, stearic, C17H35COOH, and oleic, C17H33COOH, with the trihydroxyl alcohol, glycerol, C3H5(OH)3.
Esters are named in the same manner as salts (although esters and salts have completely different properties): two-word names are used. Note that in the general formula, R-COOR', the alkyl group (R') is always attached to an oxygen atom. The alkyl group (R') is named as the first word of the two-word name. The second word is derived by adding the ending -oate to the stem of the acid name (-oic in the acid name is replaced by -oate). For example the name of the compound
is methyl propanoate. Similarly,
is named ethyl-2-chloro-3-butenoate.
Soaps are made by boiling natural fats and oils with strong bases such as sodium and potassium hydroxides. When animal fat is treated with sodium hydroxide, glycerol and sodium salts of the fatty acids, palmitic, stearic, and oleic acid, are formed. The reaction in the case of glyceryl stearate (the fat) is given by
(C17H35COO)3C3H5+ 3 NaOH 3 C17H35COONa + C3H5(OH)3
glyceryl stearate + sodium hydroxide sodium stearate + glycerol
To obtain the soap (the sodium salt of the fatty acid) free from glycerol and water, sodium chloride is added which causes the soap to precipitate, or "salt out". The soap, being lighter, collects as a crust on the top where it is removed, partially dried, and pressed into cakes for use.
A very great variety of organic compounds of biological importance contain nitrogen. Because of the great variety of oxidation levels available to nitrogen, it can combine with organic radicals in numerous ways.
AMINES ARE NITROGEN-CONTAINING ORGANIC DERIVATIVES OF AMMONIA, NH3. They are classified as primary, secondary, or tertiary depending on the number of alkyl groups attached to the nitrogen atom.
|ammonia||primary amine||secondary amine||tertiary amine|
There are several ways of naming amines. The common names of the simple amines are derived by adding the suffix -amine to the name(s) of the attached alkyl groups. For example, CH3CH2NH2 is named ethylamine and CH3CH2NHCH2CH3 is named diethylamine. The widely used, common name for the simplest aromatic amine is aniline,
also known as phenylamine or aminobenzene.
Amines may also be named as amino-substituted hydrocarbons. The compound
is named 1,4-diaminobutane.
Primary amines can be prepared by the alkylation of ammonia. This reaction involves the substitution of an alkyl group for a hydrogen atom in ammonia. The most common alkylating agents are the alkyl halides. The reaction of ammonia with an alkyl halide yields an ammonium salt which can be converted to an amine by reaction with NaOH. The synthesis of ethylamine beginning with ethyl chloride is represented below.
NH3 + C2H5Cl C2H5NH3++ Cl-
C2H5NH3+Cl-+ NaOH C2H5NH2+ H2O + NaCl
Ions, such as ethylammonium chloride (C2H5NH3+Cl-), which carry both a positive and negative charge are sometimes called ZWITTERIONS. In aqueous solution this ion may behave as an acid and donate a proton to water or it may behave as a base and accept a proton from water.
Amides are generally considered to be derivatives of carboxylic acids. The general formula for the amides is RCO-NH2. Their names are derived from the name of the carboxylic acid by changing the -oic of the acid name to the suffix -amide. For example, CH3CONH2 is called ethanamide (common name is acetamide from acetic acid).