Complete Guide To Reactions Of Alcohols

Alcohols are molecules with an OH functional group. They can undergo a variety of reactions including oxidation, esterification, dehydration and substitution.


OXIDATION OF ALCOHOLS


Secondary Alcohols

Secondary alcohols (OH is on a carbon that is attached to two other carbons) can be oxidized to ketones (OH turns into a =O), using a variety of reagents including: Na2Cr2O7 with H2SO4, PCC with CH2Cl2, CrO3 with H3O+ (Jones reagent), acetone and NaOCl/TEMPO.



Primary Alcohols

Primary alcohols are molecules where the OH group is on a carbon that is attached to one carbon only. Depending on the reagent primary alcohols can be oxidized into either aldehydes or carboxylic acids. Na2Cr2O7 with H2SO4 and Jones reagent (CrO3 with H3O+) oxidize primary alcohols all the way to carboxylic acids, while PCC with CH2Cl2 stops at the aldehyde.


Oxidation Of Primary And Secondary Alcohols
Oxidation Of Primary And Secondary Alcohols

Tertiary Alcohols

Tertiary alcohols will not be oxidized.


MAKING ALCOHOL INTO A GOOD LEAVING GROUP

Alcohols are very poor leaving groups. We oftentimes need to convert an alcohol into a good leaving group to do further reactions on the molecule. The reagents that make alcohol into a good leaving group are: HCl, HBr, TsCl, PBr3 and SOCl2.


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.


Preparation Of Sulfonate Esters Using sulfonyl chloride  (OH into OTs)
Preparation Of Sulfonate Esters Using sulfonyl chloride (OH into OTs)

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.

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.


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.


Reaction Of Alcohols With PBr3 and SOCl2
Reaction Of Alcohols With PBr3 and SOCl2


MAKING ALCOHOLS INTO AN ALKENE (REDUCTION OF ALCOHOLS)

Alcohol can be reduced to an alkene via E1 elimination reaction. The reagent for the reaction is usually H2SO4/heat (sulfuric acid).


Making Alcohol Into An Alkene E1
Making Alcohol Into An Alkene E1

WILLIAMSON ETHER SYNTHESIS

Williamson ether synthesis is a reaction that converts alcohols into ethers (C-O-C). In the first step, alcohol is deprotonated using a strong base such as NaH. The alkoxide ion that is created is a strong nucleophile and can attack primary alkyl halides and tosylates in an SN2 reaction.

Note: If the alkyl halide or tosylate is not primary, E2 will happen instead of SN2.


Williamson Ether Synthesis
Williamson Ether Synthesis

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