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Does bromine bond to the carbon in unsaturated fatty acids and break the double bonds between oxygen and carbon? Does this increase the viscosity of olive oil? Could someone give an elaborated explanation?

This question arose from a lab experiment. Viscosity goes up because less double bondings makes the fatty acids more “packable”. But why would bromine want to break the bonds?

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    $\begingroup$ I briefly contemplated inquiring whether you are going to use olive oil in the lab or bromine in the kitchen, but ultimately decided against it. Yes, the addition to the double bond seems very likely. How and why would that affect viscosity is not immediately clear to me, though. $\endgroup$ – Ivan Neretin Apr 30 '19 at 10:33
  • $\begingroup$ Well, its in the lab. Viscosity goes up because less double bondings makes the fatty acids more “packable”. But why would bromine want to break the bonds? $\endgroup$ – Almin Mekić Apr 30 '19 at 11:42
  • $\begingroup$ So are you interested in the rationale for increased viscosity? Or in the chemical reaction that takes place? I would think the bromine would add to the carbon-carbon double bonds rather than the carbon-oxygen double bonds. $\endgroup$ – Karsten Theis Apr 30 '19 at 14:45
  • $\begingroup$ Viscosity is one of those bulk properties where the factors that determine it can be quite difficult to figure out based on a microscopic view and first principles. $\endgroup$ – Zhe Apr 30 '19 at 17:28
  • $\begingroup$ I meant the carbon.carbon bonds. Why would bromine bond there? $\endgroup$ – Almin Mekić Apr 30 '19 at 18:49
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Olive oil is a triglyceride, and the main fatty acid is oleic acid below.

Oleic acid structure

Now the bromine does not add to the double bond between oxygen and carbon; instead the bromine reacts with carbon-carbon double bond and add itself to it. If you use e.g. coconut oil which has minimal unsaturated (meaning having carbon-carbon double bond) fatty acids (and assume you don't shine light onto it, heating it or otherwise forcing the reaction to start) there should theoretically be no reaction.

But why would bromine want to break the bonds?

Simply put, bromine loves electron. The carbon-carbon double bond happens to be rich in electrons, so the bromine binds to it. While the carbon-oxygen double bond also has electrons, oxygen loves electron more than bromine and tends to keep the electron for itself, so bromine cannot add there.

Does this increase the viscosity of olive oil?

The individual molecules that make up olive oil is held together by weak forces, that arise from van der Waals' forces, which is attraction due to small, unevenness in electron distribution (which leads to very small positive and negative charges on the molecules that subsequently attract other molecules). This is overly simplified concept though so it's not entirely accurate.

Bromine atom is very large (hence bromine, which molecules consists of only two bromine atom, exists as a liquid in room temperature). The more the electron, the more likely there are some unevenness in electron distribution, and hence stronger attraction between molecules. Also, bromine loves electron so it tends to pull electron closer to itself, creating a small but rather permanent positive and negative charge on the molecule. This also strengthen the force between brominated oil molecules, hence the viscosity goes up.

Viscosity goes up because less double bondings makes the fatty acids more “packable”

This is rather about melting point of the fat / oil, as when freezing, the molecules in liquid need to "pack" into a solid structure. In liquid however, molecules tends not wanting to maintain the "packing", or ordered structure. Adding bromine does increase the melting point but that's another story.

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