The product of reacting an alkene with "Br"_2(aq) is "C"_2"H"_4"BrOH". What is its name? Why is the product NOT a geminal dibromide?

Shouldn't an alcohol form? My teacher says there is no possibility of forming an alcohol.

3 Answers
Aug 23, 2015

You're teacher is quite correct, and there is no possibility of forming of alcohol (i.e. purely R-OH, rather than a bromo-substituted (Br)R-OH species).

Explanation:

Ethylene, H_2C=CH_2, is an electron-rich species, a nucleophile that has electrons to donate, however, after it has donated its pi electrons, the intermediate will undergo reaction with nucleophilic species, such as OH_2, Br^-, OH^-, cyanide etc.

On the other hand, molecular bromine is polarizable, and can be represented as Br^(delta-)-Br^(delta+); that is the bromine nucleophile becomes polarized, and individual bromine atoms acquire partial negative or partial positive charges. The partially positive Br atom reacts with the electron-rich ethylene. We can represent this as:

H_2C=CH_2 + Br_2 rarr H_2^+ **C** -CH_2Br + Br^-

The highlighted carbon is now cationic, and will react with any nucleophile present: this could be water, or cyanide, or bromide anion, whatever is in solution. Since we used Br_2(aq), i.e. bromine solution in water, water will certainly be present in abundance, and you are likely to form a bromo-alcohol.

If, however, the olefin was treated with H^+, the first step of the reaction is to form a C-H bond; the subsequent carbocation is likely to react with water to form an alcohol.

Aug 24, 2015

"C"_2"H"_4"BrOH" is an alcohol. The formula is more commonly written as "C"_2"H"_5"BrO". It is called bromoethanol, and has two isomers.

Explanation:

The "OH"- group is a hydroxyl group that in this case forms an alcohol functional group.

The following image is the structural formula for 1-bromoethanol.
http://www.chemspider.com/Chemical-Structure.10692052.htmlhttp://www.chemspider.com/Chemical-Structure.10692052.html

The following image is the structural formula for 2-bromoethanol.
http://www.chemspider.com/Chemical-Structure.10692052.htmlhttp://www.chemspider.com/Chemical-Structure.10692052.html

Aug 24, 2015

Remember that aqueous means dissolved in water, which implies that water is present to react if its pKa is low enough (low pKa->strong acid->less stable), which it is. The conjugate acid of bromide is "HBr", whose pKa is about -9, so bromide is a very weak base, and additionally not a good nucleophile (electron-donor).

It's quite true that bromine liquid reacting with ethene gives dibromoethane. That is an early mechanism you would have learned in the first few weeks of a first-year organic chemistry class.

However, water disrupts this mechanism in the middle. What happens is:

  • In the first step, bromine makes the cyclopropane-analog bonds.
  • Then the carbon centers of each "C"-"Br" becomes electrophilic (electron-acceptor). Water, a stronger nucleophile than bromide, can attack either one (due to symmetry) from behind to bond and break one of the "C"-"Br" bonds, attaching in a "trans-" addition.

(normally, it would be the other bromide attacking from behind, not water)

  • Water finishes the mechanism by deprotonating the attached water to form hydronium and bromide in solution.

So yes, there IS the possibility of forming an alcohol. In fact, "C"_2"H"_4"BrOH" IS an alcohol. It's "2-bromoethanol".