In organic chemistry it is crucial to recognize OH as a bad leaving group and remember reagents that convert it to a good leaving group.
A good leaving group, when it leaves, is very stable on its own and will not attack the substrate again. Examples of good leaving groups are I (iodine) and Cl (chlorine). As we go down group 17 on the Periodic table, the size of halogens increases making them more capable of stabilizing the negative charge. Thats it why iodine is one of the best leaving groups.
OH makes a bad leaving group since it would leave as OH-, a very strong base that is ready to attack. Therefore, we will discuss ways of making OH a good leaving group. What are they?
1. The reaction of alcohol with TsCl (tosyl chloride) produces ROTs ( alkyl tosylate). OTs is an excellent leaving group. In this reaction, the stereochemistry of OH as it gets converted to OTS remains the same. For example, if OH was on a wedge, OTs would be on a wedge as well.
2. The reaction of alcohol with HBr or HCl yields Br or Cl instead of the OH group. Please note that these reactions are capable of rearrangements and do not always give the desired product.
3. PBr3 (phosphorus tribromide) turns OH into Br (works on most primary and secondary alkyl halides). Results in INVERSION of stereochemistry due to the SN2 reaction in the second step.
4. SOCl2 (thionyl chloride) with pyridine converts OH into Cl (also works on primary and secondary alcohols). It results in inversion of stereochemistry just like PBr3.
What can we do once we turn OH into a good leaving group? Any reactions that involve a leaving group. For example an alcohol with H2SO4 usually undergoes E1 giving the most substituted and stable double bond. It can also undergo SN1, SN2, and E2 depending on which nucleophile is being used and what are the solvent conditions.
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