An alcohol is a functional group that contains an –OH group.
Alcohols can often act as acids or bases, making them a very “flexible” molecule for acid and base reactions. They can also hydrogen bond, which can stabilize the molecule. They are abundantly present in nature in molecules such as carbohydrates.
But let’s be honest, when most people hear the word alcohol, they immediately think of the beverage. Ethanol, a specific alcohol molecule, is the ingredient in these drinks giving alcohol its famous (or infamous) effects.
In this chapter, we’ll learn about the nomenclature of alcohols and different reactions of alcohols, including oxidation reactions, reduction reactions (such as the PCC reaction), and Grignard reactions.
Naming alcohols is similar to naming alkanes but with a small twist. The prefixes of naming still remain the same (e.g. 3 carbons still is prop-), but the suffix changes to “-ol,” instead of the “-e” suffix of alkanes. The alcohol itself should also be named in the parent chain unless a higher priority functional group is in the molecule (we won’t see this until second semester). Here are two fairly simple naming examples to introduce you to alcohol nomenclature.
When an alcohol is a substituent, it is called a hydroxy group. This occurs when a higher priority functional group is present in the molecule. We won’t run into many situations where this occurs until later in the year, however, as most of the higher priority functional groups are covered in the second semester.
Introduction to Oxidation and Reduction
Oxidation and reduction are defined as the loss and gain of electrons respectively. However, in organic chemistry, it’s simpler to think of oxidation and reduction in different terms, specifically by looking at the number of carbon-oxygen bonds. If carbon-oxygen bonds are added (like when a single carbon-oxygen bond turns into a double bond), then this molecule has been oxidized. If carbon-oxygen bonds are removed (such as when a carbon-oxygen double bond becomes a single bond), then this molecule has been reduced.
In a reduction reaction, the number of carbon-oxygen bonds is reduced. In organic chemistry class, this generally means we start with a carbonyl group (such as an aldehyde or a ketone) and reduce this molecule down to an alcohol. 2 common reduction reagents are shown below:
LiAlH4 and NaBH4 essentially act as a source of hydride ions (H:-). Below are the mechanisms for both of these reagents:
Some professors teach that this reagents should be used in different situations (e.g. LiAlH4 should only be used for a ketone and NaBH4 should only be used on a ketone), while others teach that they can be used interchangeably. Pay attention in lecture or go to office hours to find out your professor’s preference for this.
Oxidation is the addition of carbon-oxygen bonds. Most commonly, this reaction involves an alcohol being turned into a carbonyl group. There are 2 common oxidation reagents in the organic chemistry class: PCC,CH2CI2and Na2Cr2O7, H2SO4, H2O. While these 2 reagents similarly perform oxidation, Na2Cr2O7, H2SO4, H2O overoxidizes a primary alcohol to a carboxylic acid, while PCC,CH2CI2turns a primary alcohol into an aldehyde. The main reason for this is that the Na2Cr2O7 reagent also uses acid and water, which enables the overoxidation.
Professors generally do not require students to know the mechanisms for these two reagents.
The Grignard reaction often (but not always) forms alcohols while creating carbon-carbon bonds, making it a very important reaction in the organic chemistry course. In fact, this is the only reaction we have learned up to this point that can form carbon-carbon bonds. Therefore, the Grignard reaction should be considered in any reaction you face that adds carbon atoms to a molecule.
Grignard reagents utilize either magnesium (Mg) or lithium (Li) to place a large concentration of electrons on a carbon atom, making the carbon atom highly nucleophilic.
Let’s look at the mechanism of the Grignard reaction.
Note that the Grignard reaction is very sensitive to reacting with acidic protons in a solution. For example, Grignard reacts quite quickly with water by deprotonating an acidic hydrogen, so it is important that the Grignard and the water are added in distinct steps.
Likewise, the Grignard will react with acidic hydrogen atoms (like those found in an alcohol) in the substrate via an acid-base reaction to form an alkane, rather than the intended nucleophilic attack using the Grignard.
Therefore, it’s important to use protecting groups to ensure this side reaction doesn’t occur. The group with the acidic hydrogen is protected just prior to the addition of the Grignard, ensuring that the Grignard can’t react with the acidic proton. After the Grignard reaction has completed, the protecting group can then be removed. Different professors and textbooks prefer different protecting groups, so refer to your textbook to see these exact reagents.
More Reactions of Alcohols
The following reactions will cover different ways to convert an alcohol into a halogen. The first mechanism relies on properties of molecules that we have already learned up to this point.
If either HBr or HI is added to an alcohol, the alcohol will be displaced by the halogen (either iodine or bromine). However, -OH is not a good leaving group, which poses a dilemma for the displacement. But the –OH can be transformed into a good leaving group. The acid in this reaction first protonates the alcohol, forming water, which is a good leaving group. From here, the nucleophilic halogen can attack. If the alcohol is primary, then the reaction follows an SN2 mechanism. If the alcohol is tertiary, then the reaction follows an SN1 mechanism. Textbooks differ on whether secondary alcohols follow an SN1 or an SN2 mechanism, so be sure to check with your professor on what he or she would like.
The reaction above can therefore replace alcohols with either iodine or bromine. Two more reactions are useful for replacing an alcohol with bromine or chlorine.