Frank C. Whitmore - Organic Chemistry, Volume One

Paraffins, alkanes, homologs of methane.

This simplest homologous series of organic chemistry shows the gradation in physical properties characteristic of such series. From a gas, only slightly less volatile than oxygen, the repeated increase of CH2 in the compounds produces volatile liquids at C5 and low-melting solids at C16. The increase in boiling point for an increase of CH2 decreases with the higher members (p. 4). Of isomers, the normal ( n -) (straight chain) member has the highest boiling point. In the series to C8 the n -hydrocarbons boil lower than the lowest boiling isomer of the next homolog. Thus all the octanes boil higher than n -heptane. At that point in the series, however, the spread between two successive n -hydrocarbons becomes so small and the possibility of branching, with accompanying lowering of the b.p., so great that two of the highly branched nonanes boil lower than n -octane. These are 2,2,5-trimethylhexane and 2,2,4,4-tetramethylpentane. The densities of the n -alkanes increase from 0.4 to a limiting value of about 0.78. The value 0.77 is reached by the Cn member.

The index of refraction ( n 20D) for the liquid n -alkanes ranges from 1.3577 for n -pentane to 1.4270 for n -pentadecane. A rise of 1 decreases the n D by 0.00055 for n -pentane and 0.00044 for n -dodecane. The use of the line of the hydrogen spectrum instead of the D line decreases the n 20 for n -pentane by 0.0019 and for n -dodecane by 0.0022 while the use of the line in place of the D line gives increases of 0.0044 and 0.0053 respectively.

The alkanes are practically insoluble in water but soluble in most organic liquids. In aniline their solubility is limited at ordinary temperature. The Critical Solution Temperatures (C.S.T.) in aniline and in liquid sulfur dioxide are characteristic of the individual hydrocarbons both in this and other series. The C.S.T. in aniline for some of the normal. alkanes in C. follow: C5, 71.4; C6, 69.0; C7, 69.9; C8, 71.8; C9, 74.4; C10, 77.5; C11, 80.6; C12, 83.7. The C.S.T. in liquid SO2 are as follows: C6, 10.2; C8, 26.9; C10, 37.3; C12, 47.3; C14, 55.5; C32, 110.0. The values for furfural are: C6, 92; C7, 95; C12, 112.5; C13, 115.9; C14, 119.6; C15, 122.7; C16, 125.9; C17, 129.3; C18, 132.0; C20, 138.1; C24, 147.2; C26, 150.3.

Because of their inability to add reagents the alkanes are called saturated hydrocarbons. Thus they can react with halogens only by substitution, a hydrogen being removed for each halogen which enters the molecule. They do not ordinarily react with hydrogen. Under extreme conditions, hydrogenolysis occurs with splitting of the C-C linkage. Ethane, under such conditions, gives methane. An important application is the conversion of the easily obtainable alkylation product, 2,2,3-Me3-pentane, to Me3-butane (Triptane) by hydrogenolysis.

A mixture of the higher members of the series, paraffin wax, received its name because of its inertness to acids and oxidizing agents (from parum affinis ). Because methane is inert to most reagents and because the next few normal homologs are rather inert, the name paraffin hydrocarbons has given the impression that the entire series is very inactive chemically. This is not true. Even paraffin wax is fairly reactive with oxygen at slightly elevated temperatures (preparation of acids). While reagents such as nitric acid, sulfuric acid, chromic acid mixture or potassium permanganate do not act readily with the lower normal members, some of them act with the higher members and with the branched members containing a tertiary hydrogen, R3CH. This hydrogen can be replaced by NO2, SO3H or OH with nitric acid, sulfuric acid or oxidizing agents respectively.

Paraffins react readily with chlorine in light or at slightly elevated temperature to give substitution of H by Cl (chlorination). Polychlorides are readily obtained. The reaction may become dangerously explosive if not controlled.

Vapor phase nitration of paraffins, replacement of H or alkyl by NO2, is increasingly important commercially. The high temperature necessary for nitration favors splitting of CC. Thus the nitration of propane gives not only 1- and 2-nitropropane but also nitromethane and nitroethane.

Paraffin hydrocarbons react with SO2 and Cl2 (Reed Reaction) in the presence of actinic light to give alkyl sulfonyl chlorides, RSO2Cl.

In the first part of the series the increase of CH2 makes a marked difference in the percentage composition. Successive additions of CH2 have a decreasing effect as the composition approaches that of (CH2) n . Thus an ordinarily accurate C and H determination would barely distinguish C20 from C30.

Possible and Known Isomers. Using only the conception of the tetra-valence of carbon the following numbers of structural isomers are predicted for the alkanes: 1 each for C1, C2 and C3, 2 for C4, 3 for C5, 5 for C6, 9 for C7, 18 for C8, 35 for C9 and 75 for C10. Methods of calculating the number of theoretically possible isomers have been developed. The numbers indicated are 366,319 for C20 and over 4 billion for C30. Many of the structural isomers contain asymmetric carbon atoms and can give rise to stereoisomers. Thus, of the 18 structurally isomeric octanes, 3-Me-heptane, 2,3-Me2-hexane, 2,4-Me2-hexane and 2,2,3-Me3-pentane each contains an asymmetric carbon and could exist in dextro and levo optically active forms. A fifth octane, 3,4-Me2-hexane, contains two similar asymmetric carbons and could exist in d-, l- and meso -forms. Thus the total number of isomers of the octanes becomes 24, of which 11 are stereoisomers and 13 are non-stereoisomers. Similarly for C10, the 75 structure isomers give rise to 101 stereoisomers and 35 non-stereoisomers. Soon the numbers predicted lose all physical significance, there being a total of 3,395,964 possible isomeric eicosanes (C20).

Turning from the predicted to the known we find that all the predicted structural isomers have been prepared for the first nine members of the alkane series. Of the 75 possible structurally isomeric decanes, about half have been prepared. Many optically active hydrocarbons have also been prepared (p. 22).

The preparation of the higher alkanes involves many difficulties among which are (1) the decreased activity of the larger molecules, (2) the failure of many reactions when extreme branching of the carbon chain occurs,

The effect of branching in isomeric paraffins may be seen from the melting and boiling points, C., and refractive indices, n 20D, of n -octane, 4-Me-heptane, 2,2,4-Me3-pentane (iso-octane), and 2,2,3,3-Me4-butane which are respectively: 56.8, 125.6, 1.3976; 121.3, 117.5, 1.3980; 107.3, 99.2, 1.3914; +101.6, 106.5. The effect of symmetry in raising the m.p. is notable in the last.

Occurrence of the Alkanes. The alkanes are widely distributed in nature. Methane occurs in natural gas from 75 to nearly 100%, in fire damp in coal mines and as marsh gas formed by the decay of vegetable matter. The higher homologs are found to a decreasing extent in natural gas and to an increasing extent in petroleum. A typical analysis of a natural gas from a large high pressure line supplied from many wells of various ages and from different sands gave the following percentages: methane 78, ethane 13, propane 6, butanes 1.7, pentanes .6, hexanes .3, heptanes and above .4. Gas from the Lower Oriskany Sand of Pennsylvania has 98.8% methane while a gas from Glasgow, Kentucky has been found with only 23%.

NORMAL ALKANES