Organic chemistry / by Frank C. Whitmore.

  • Frank C. Whitmore
Date:
1937
Catalogue details

Licence: In copyright

Credit: Organic chemistry / by Frank C. Whitmore. Source: Wellcome Collection.

    45/1104 page 29
    In the olefins there is possible not only isomerism due to dif- ferent carbon skeletons but also due to differences in the position of the double bond. Thus the possible numbers of structural isomers are: C4, 3; C5, 5; C6, 13; C7, 27; Cg, 68. All of these have been made through the heptylenes (Boord 1933). Occurrence and Formation. Because of their great reactivity, olefins are not found in natural products. They are formed in the thermal decomposition of almost all carbon compounds. Thus the cracking of cottonseed oil gives about 30% unsaturated compounds (Egloff 1932). The total amount of olefins obtained as by-products in cracking processes for making gasoline is enormous. In 1936 the amount of the first three olefins formed in this way was about 5 X 1010 cu. ft. Pintsch gas obtained by cracking the gas oil fraction of petro- leum contains 35-40% olefins. The 'physical properties of the alkenes closely resemble those of the alkanes. Ethylene boils some 15° below ethane but the corresponding higher members in the two series boil at nearly the same points. In general the 1-olefin is lower boiling than the isomers with the double bond farther from the end of the chain. Thus the boiling points of n-heptane, heptene-1, and heptene-2 are 98.4°, 93.5°, and 98.3° respectively. The melting points of the olefins are somewhat lower than those of the corresponding paraffins. The presence of a double bond increases the density about 0.015 for straight chain compounds. For branched chain compounds the increase varies widely. Thus the values of d420 for 2-Me-pentane, 2-Me-pentene-l and 2-Me-pentene-4 are 0.652, 0.682 and 0.665. In the 1-olefin the branching of the chain and the double bond each strengthens the effect of the other while in the 4-olefin they are apparently independent. The density of pentene-1 is 0.641 while that of nonene-1 is 0.731. The double bond increases the index of refraction (n^) about 0.011 for 1-olefins. The increase is greater when the double bond is nearer the middle of the molecule and also in branched molecules. The chemical properties of the alkenes are in strong contrast to those of the alkanes. Not only do they add a great variety of reagents with the greatest ease but they also react with them- selves to form polymers.
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