Question

1. On page 410 in your Brown & Foote textbook, the following reaction is shown.

3-pentanol + HBr --heat--> 3-bromo-pentane (major product) + 4-bromo-pentane (minor product)

a. Explain how the minor product can form in this reaction and what does it tell us about the mechanism by which the reaction proceeded.

b. Is there any basis in reality to the books claim regarding this reaction? Perform a literature search to see if indeed any chemist reported such results. If you do find such a paper, give the full citation of the paper.

2. Part of the procedure this week calls for a reflux which has a water cooled condenser open on the top to the atmosphere. Why would it be a bad idea to stopper or cap the apparatus? Explain what would happen if the apparatus was capped?

Answer 1

1. In th reaction of 3-pentanol with HBr we get a major product : 3-bromopentane and a minor product : 4-bromopentane.

a. The products are formed by an SN1 reaction of the strating alcohol. The alcohol gets protonated first at the -OH to form more labile H2O group, which leaves to generate a secondary carbocation at the 3-position. The nucelophile Br- directly attacks at the carbocation center to form major product. The carbocation also rearranges to 4-position of chain by a 1,2-hydrogen shift mechanism. The Br- then attacks the newly formed carbocation center to give minor product. This proves the reaction followed an SN1 pathway.

b. References related to this can be found in "Organic Chemistry, year 2015, page 169-182 by Robert J Ouellet and J David Rawn

c. The reaction is refluxed so as to eliminate H2O and formation of carbocation occurs which then reacts with the Br- to yield the desired product. The reflux condenser is not capped but is open at the top so that no pressure is developed due to heated gas in the system. If the condenser is capped or closed at the top, risk of explosion may occur.