This does not affect the stereochemistry. Secondly, strong acid is a pretty blunt instrument, like a sledgehammer. It gets the job done, but can lead to some collateral damage if you have a molecule containing functional groups with various levels of acid sensitivity esters, alkenes, alkynes.
Using a milder, more targeted reagent would help us avoid undesired side reactions in more complex situations. A harder point to address is this: why not just, for example, always make alcohols into mesylates or tosylates if we want to make them good leaving groups?
This is actually a great idea most of the time! As for exceptions, I can think of at least one situation where when you would need to make a halide. The bromide ion that was displaced from phosphorus attacks carbon via backside attack, forming C-Br and breaking C-O and we are left with a new alkyl bromide with inversion of configuration and the Br 2 P-OH leaving group. In the first step, oxygen attacks sulfur, displacing chloride ion. In the second step the chloride ion attacks carbon in an SN2 reaction, leading to inversion of configuration.
Removal of SO 2 from the reaction vessel renders this reaction irreversible and helps drive the reaction to completion. The process shown works well for primary and secondary alcohols. Ask your instructor. The bottom line for today is to learn about these two methods for converting alcohols into alkyl halides, and pay particular attention to their stereochemistry. Extremely testable! But what about elimination reactions of alcohols? How would we go about making alkenes?
Many of the steps will look familiar — but there will be new wrinkles too. Provide reagents to achieve the following transformations in one-pot synthesis use only one reagent :. Click here to Register! By joining Chemistry Steps, you will gain instant access to the answers and solutions for all the Practice Problems including over 20 hours of problem-solving videos, Multiple-Choice Quizzes, Puzzles, and t he powerful set of Organic Chemistry 1 and 2 Summary Study Guides.
Notify me of followup comments via e-mail. You can also subscribe without commenting. Thionyl chloride SOCl 2 and phosphorus tribromide PBr 3 can be used for converting primary and secondary alcohols to alkyl chlorides and alkyl bromides respectively: Both reactions have similar mechanisms with the idea of turning the OH into a good leaving group and then replacing it with the Cl — or Br — nucleophile via an S N 2 reaction.
These two reactions convert the OH group into a good leaving group and the nucleophile Cl — , generated in the same step, attacks then the intermediate in an S N 2 process: Sulfur dioxide SO 2 and Cl — are the leaving groups of this substitution reaction.
The initial reaction of the alcohol with PBr 3 turns the OH into a good leaving group which is then expelled by the Br- ion in an S N 2 process: SOCl 2 and PBr 3 do not work for tertiary alcohols because of their steric hindrance. And the answer is, it is certainly an option and hydrogen halides are used to convert alcohols to alkyl halides: There are, however, a few factors to be considered.
Now, while primary and tertiary alcohols are not risked for losing stereochemistry since the carbon is not chiral, secondary alcohols my go in the S N 1 path thus losing chirality: Third , which is related to the second factor, is the possibility of carbocation rearrangements for the secondary alcohols when the reaction goes by an S N 1 mechanism: In summary, we can say that SOCl 2 and PBr 3 are great candidates for converting primary and secondary alcohols to alkyl halides since they work in mild conditions and are suitable for chiral alcohols to prevent rearrangements and loss of stereochemistry.
This content is for registered users only. Remember Me. In the presence of a base such as pyridine, the intermediate chlorosulfite ester reacts to form an "pyridinium" salt, which undergoes a relatively clean S N 2 reaction to the inverted chloride. In ether and similar solvents the chlorosulfite reacts with retention of configuration, presumably by way of a tight or intimate ion pair. This is classified as an S N i reaction nucleophilic substitution internal.
The carbocation partner in the ion pair may also rearrange. These reactions are illustrated by the following equations. An alternative explanation for the retention of configuration, involving an initial solvent molecule displacement of the chlorosulfite group as SO 2 and chloride anion , followed by chloride ion displacement of the solvent moiety, has been suggested.
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