Abstract

1,6-dibromohexane is a compound of certain value in chemical research and industrial applications. Due to the rotation of the carbon-carbon single bond, 1,6-dibromohexane can exist in multiple conformations and can form complexes with urea to study its molecular motions. At the same time, 1,6-dibromohexane is an important organic synthesis intermediate, which can act as a monomer in polymerization reactions to synthesize polymers with specific structures. Additionally, 1,6-dibromohexane can be used to investigate the electrochemical reduction mechanisms of organic halides. The electrochemical reduction reaction can alter the structure and properties of 1,6-dibromohexane, providing a pathway for synthesizing new organic compounds.

Synthesis of 1,6-dibromohexane

In a four-neck flask equipped with a reflux condenser, stirrer, thermometer, and dropping funnel, a certain amount of hexanediol and a small amount of hydrobromic acid are first added and thoroughly stirred at room temperature to ensure complete dissolution. The temperature is then raised to 40°C, and concentrated sulfuric acid is slowly added dropwise via the dropping funnel. After the addition of sulfuric acid is complete, the remaining hydrobromic acid is introduced into the reactor, and the temperature is gradually increased to reach reflux within approximately 1.5 hours. After a specified reaction time, the organic phase is washed with equal volumes of water and 5% sodium carbonate solution to neutralize the mixture. Subsequently, the product is subjected to steam distillation, dried over anhydrous calcium chloride, and then undergoes vacuum distillation to collect the fraction boiling at 143-145°C/3.9 kPa, which is identified as the desired product, 1,6-dibromohexane.[1]

Discovery

By doing 2H NMR (nuclear magnetic resonance) spectroscopy, and conjunction with X-ray diffraction experiments, the guest motions of 1,6-dibromohexane in its urea inclusion compound is characterize. These motions are characterized by alkyl chain jumps between two conformations, each approximately gauche to the terminal bromines, which remain stationary. In this distorted urea channel, one conformer is heavily preferred, but thermally activated population of the unfavorable conformer leads to reversible, temperature-dependent changes in the unit cell parameters.  In this distorted urea channel, one conformer is heavily preferred, but thermally activated population of the unfavorable conformer leads to reversible, temperature-dependent changes in the unit cell parameters.[2]

Application research of 1-Bromododecane

Copolycondensation

α,α-Dibromotoluene  was found to be polymerized by the reaction with excess Mg to give poly(phenylmethylene)s 2 , whose main chains were partially dehydrogenated to carbon–carbon double bonds. The carbon–carbon double bonds in 2 can be brominated by treatment with Br2. The polymerization mechanism was presumed to include the formation of Grignard reagents of various species with benzylic carbon-bromine bonds and the nucleophilic attacks of the Grignard reagents to various compounds with benzylic carbon-bromine bonds. Copolymerization of 1 with dichlorodimethylsilane successfully proceeded. Mg/Cu-mediated copolycondensation of 1 with 1, 6-dibromohexane proceeded to give polymers that have similar compositions to those of random copolymers of ethylene and styrene.[3]

Direct Reduction at Silver Cathodes

The electrochemical reduction pathways of 1,2-dibromohexane and 1,6-dibromohexane at silver cathodes exhibit significant differences. Specifically, the reduction of 1,2-dibromohexane shows a less negative cathodic peak potential (-0.33V), and its peak current is approximately half that of 1,6-dibromohexane, indicating that the reduction of its carbon-bromine bond is relatively easier. In terms of product analysis, the electrolytic reduction of 1,2-dibromohexane yields a single product, 1-hexene (with a coulometric n value of 2.13), which strongly supports its reduction via a concerted mechanism. In contrast, the reduction of 1,6-dibromohexane (with a coulometric n value of 2.12) produces a complex mixture, including 1-hexene (21%), n-hexane (37%), 1,5-hexadiene (22%), 5-hexen-1-ol (9%), and a trace of n-dodecane. Furthermore, when 1,6-dibromohexane is electrolyzed in the presence of a proton or deuteron donor (such as 2,2,2-trifluoroethanol or D2O), the yield of n-hexane significantly increases, while the yields of 1-hexene and 1,5-hexadiene decrease. This phenomenon further confirms that the reduction of 1,6-dibromohexane involves carbanionic intermediates, as proton donors can capture these intermediates, thereby affecting the final product distribution.[4]

References

[1]Sun, D., Gu, G., & Yin, S. (2001). Synthetic method of 1,6-dibromohexane. Chemical Industry Times, 15(4), 42-43.

[2] Werner-Zwanziger, U., Brown, M. E., Chaney, J. D., Still, E. J., & Hollingsworth, M. D. (1999). Deuterium NMR studies of guest motions in urea inclusion compounds of 1,6-dibromohexane with analytical evaluation of spectra in the fast motion limit. Applied Magnetic Resonance, 17(2–3), 265–281.

[3] Ihara, E., Wake, T., Mokume, N., Itoh, T., & Inoue, K. (2006). α,α‐Dibromotoluene as a monomer for poly (substituted methylene) synthesis: Magnesium‐mediated polycondensation of α,α‐dibromotoluene and magnesium/copper‐mediated copolycondensation of α,α‐dibromotoluene with 1,6‐dibromohexane. Journal of Polymer Science Part A: Polymer Chemistry, 44(19), 5661–5671.

[4] Martin, E. T., Strawsine, L. M., Mubarak, M. S., & Peters, D. G. (2015). Direct Reduction of 1,2- and 1,6-Dibromohexane at Silver Cathodes in Dimethylformamide. Electrochimica Acta, 186, 369–376.