The Addition of a Halogen Hydride to 3-Heptene
Creators: name: Dillon, Robert Troutman
Publisher: name: California Institute of Technology
Year: 1925
Subjects: Chemistry

description: The purpose of this study is to determine the relative affinity for electrons of the ethyl and propyl groups and thereby to check the indications of the ionization constants of certain acids and amines.  The ionization constants of methyl, ethyl and propyl amines and acetic, propionic and butyric acids, indicate that the ethyl group has a smaller affinity for electrons than either the methyl or the propyl group.  Lucas and Moyse have shown this to be true with respect to the methyl group and it is the intent of this work to investigate the evidence with respect to the propyl group.

Two theories regarding the electronic character of carbon union have been proposed.  The first is the alternate polarity theory which has been supported and worked on by Cuy.  The second is the displacement theory which has been proposed by Lewis and has been used by Latimer and Rodebush in accounting for the ionization constants of some acids of phosphorus.  The latter theory has also been worked on in this laboratory by Lucas and Jameson, Lucas, Simpson and Carter and Lucas and Moyse.

According to the alternate polarity theory the carbon atoms in aliphatic chain tend to become arranged alternately positive and negative.  Therefore, 3-heptene (minus the hydrogen atoms) would appear to be: 
[See Abstract for chemical formula].  
As may be seen C-3 is preponderantly negative while C-4 is positive.  Therefore, when a strong polar compound such as hydrogen bromide is added to this unsaturated hydrocarbon, the positive hydrogen will go to C-3, while the bromine will go to C-4.  The theory consequently predicts the formation of 4-bromo-heptane on the addition of hydrogen bromide to 3-heptene.

The ionization constant, Ka of propionic acid is 1.35 x 10-5 and of butyric acid is 1.65 x 10-5, while Kb of ethyl amine is 5.6 x 10-4 and of proyl amine is 4.7 x 10-4.  These values indicate that the propyl group has a greater affinity for electrons than the ethyl group.  The validity of this apparent difference in affinity for electrons forms the basic assumption on which the displacement theory is based.  This difference results in the displacement of the electronic orbits in the chain, which may be represented by a displacement of the average position of the electron pair.  Therefore, 3-heptene (minus the hydrogen atoms) would appear to be: 
[See Abstract for chemical formula].   
Consideration of the preceding diagram indicates that C-3 is essentially positive in comparison to C-4.  When hydrogen bromide is added to this hydrocarbon the theory predicts the formation of 3-bromo-heptane.  This is opposite to that predicted by the first theory.

A factor which must be considered in dealing with this problem is that of symmetry of the molecule.  Reference to innumerable organic compounds shows that the tendency towards this symmetrical formation is exceedingly great.  In the addition of hydrogen bromide to 2-pentene as well s to 3-heptene, as mentioned above, the two theories predict different products.  In the case of 2-pentene, the symmetrical form is predicted by the displacement theory, and this isomer in the work of Lucas and Moyse was found to be formed in excess.  However, in the case of 3-heptene the unsymmetrical form is predicted.  Therefore if the unsymmetrical form is produced in excess the evidence shows unquestionably the agreement of the electron affinities of the ethyl and propyl groups with the indications of the ionization constants.  It also shows the factor of symmetry to be of minor importance compared to that of electron affinity.  On the other hand, if the symmetrical form is produced in excess the evidence shows that the factor of symmetry is not of minor importance.