Alcohols are the family of compounds that contain one or more hydroxyl (-OH) groups. Alcohols are represented by the general formula R-OH. Alcohols are important in organic chemistry because they can be converted to and from many other types of compounds. Reactions with alcohols fall into two different categories. Reactions can cleave the R-O bond or they can cleave the O-H bond.
ethanol (ethyl alcohol, or grain alcohol) is found in alcoholic beverages, CH3CH2OH.
In the Alkenes section, we already covered a few methods for synthesizing alcohols. One is the hydroboration-oxidation of alkenes and the other is the oxymercuration-reduction of alkenes. But there are a great many other ways of creating alcohols as well.
A common source for producing alcohols is from carbonyl compounds. The choice of carbonyl type (ketone, aldehyde, ester, etc) and the type of reaction (Grignard addition or Reduction), will determine the product(s) you will get. Fortunately, there are a number of variations of carbonyls, leading to a number of choices in product.
There are primarily two types of reactions used to create alcohols from carbonyls: Grignard Addition reactions and Reduction reactions. We'll look at each type of reaction for each type of carbonyl.
As we learned previously, Grignard reagents are created by reacting magnesium metal with an alkyl halide (aka haloalkanes). The magnesium atom then gets between the alkyl group and the halogen atom with the general reaction:
R-X + Mg → R-Mg-X
In our examples, we'll be using bromine in our Grignard reagents because it's a common Grignard halogen and it will keep our examples a little clearer without the need for X.
The general mechanism of a Grignard reagent reacting with a carbonyl (except esters) involves the creation of a 6-membered ring transition state. The pi bond of the oxygen attacks a neighboring magnesium bromide which in turn, releases from its R group leaving a carbocation. At the same time, the magnesium bromide ion from another Grignard molecule is attacked by the carbocation and has its magnesium bromide ion stolen (restoring it to its original state as a Grignard reagent). The second molecule's carbocation is then free to attack the carbanion resulting from the vacating pi bond, attaching the R group to the carbonyl.
At this point, there is a magnesium bromide on the oxygen of what was a carbonyl. The proton from the acidic solvent easily displaces this magnesium bromide ion and protonates the oxygen, creating a primary alcohol with formaldehyde, a secondary alcohol with an aldehyde and a tertiary alcohol with a ketone.
With esters, the mechanism is slightly different. Two moles of Grignard are required for each mole of the ester. Initially, the pi bond on the carbonyl oxygen attacks the magnesium bromide ion. This opens up the carbon for attack from the R group of the Grignard. This part of the reaction is slow because of the dual oxygens off of the carbon providing some resonance stabilization. The oxygen's pi bond then re-forms, expelling the O-R group of the ester which then joins with the magnesium bromide, leaving R-O-MgBr and a ketone. The R-O-MgBr is quickly protonated from the acidic solution and the ketone is then attacked by Grignard reagent via the mechanism described earlier.
The image above shows the synthesis of an alcohol from formaldehyde reacted with a Grignard reagent. When a formaldehyde is the target of the Grignard's attack, the result is a primary alcohol.
The image above shows the synthesis of an alcohol from an aldehyde reacted with a Grignard reagent. When an aldehyde is the target of the Grignard's attack, the result is a secondary alcohol.
The image above shows the synthesis of an alcohol from a ketone reacted with a Grignard reagent. When a ketone is the target of the Grignard's attack, the result is a tertiary alcohol.
The image above shows the synthesis of an alcohol from an ester reacted with a Grignard reagent. When an ester is the target of the Grignard's attack, the result is a tertiary alcohol and a primary alcohol. The primary alcohol is always from the -O-R portion of the ester and the tertiary alcohol is the other R groups of the ester combined with the R group from the Grignard reagent.
We will discuss reactions with Epoxides later when we cover epoxides, but for now, we'll briefly discuss the synthesis of an alcohol from an epoxide. The nature of the reaction is different than with the carbonyls, as might be expected. The reaction of Grignard reagents with epoxides is regioselective. The Grignard reagent attacks at the least substituted side of the carbon-oxygen bonds, if there is one. In this case, one carbon has 2 hydrogens and the other has 1, so the R group attacks the carbon with 2 hydrogens, breaking the bond with oxygen which is then protonated by the acidic solution. leaving a secondary alcohol and a concatenated carbon chain. The R group can be alkyl or aryl.
As an alternative to Grignard reagents, organolithium reagents can be used as well. Organolithium reagents are slightly more reactive, but produce the same general results as Grignard reagents, including the synthesis from epoxides.
The image above shows the synthesis of an alcohol from an aldehyde by reduction.
The image above shows the synthesis of an alcohol from a ketone by reduction.
The image above shows the synthesis of an alcohol from an ester by reduction. Esters can be hydrolysed to form an alcohol and a carboxylic acid.
The image above shows the synthesis of an alcohol from an ester reacted by reduction.
Follow these rules to name alcohols the IUPAC way:
|IUPAC name||Common name|
|(CH3)2CH-OH||2-Propanol||Isopropyl alcohol (Note: Isopropanol would be incorrect. Cannot
and match between systems.)
|Multiple OH functional groups|
|(from that body fat is stored as)||1,2,3-Propanetriol||Glycerol|
|-OH can be named as a substituent hydroxyl group (hydroxyalkanes)|
|Find the longest chain of carbons containing the maximum number
In an O-H bond, the O steals the H's electron due to its electronegativity, and O can carry a negative charge (R-O-). This leads to deprotonation in which the nucleus of the H, a proton, leaves completely. This makes the -OH group (and alcohols) Bronsted acids. Alcohols are weak acids, even weaker than water. Ethanol has a pKa of 15.9 compared to water's pKa of 15.7. The larger the alcohol molecule, the weaker an acid it is.
On the other hand, alcohols are also weakly basic. This may seem to be contradictory--how can a substance be both an acid and a base? However, substances exist that can be an acid or a base depending on the circumstances. Such a compound is said to be amphoteric or amphiprotic. As a Bronsted base, the oxygen atom in the -OH group can accept a proton (hydrogen ion.) This results in a positively-charged species known as an oxonium ion. Oxonium ions have the general formula ROH2+, where R is any alkyl group.
When O becomes deprotonated, the result is an alkoxide. Alkoxides are anions. The names of alkoxides are based on the original molecule. (Ethanol=ethoxide, butanol=butoxide, etc.) Alkoxides are good nucleophiles due to the negative charge on the oxygen atom.
R-OH -> H+ + R-O-
In this equation, R-O- is the alkoxide produced and is the conjugate base of R-OH
Alcohols can be converted into alkoxides by reaction with a strong base (must be stronger than OH-) or reaction with metallic sodium or potassium. Alkoxides themselves are basic. The larger an alkoxide molecule is, the more basic it is.
Recall that haloalkanes can be converted to alcohols through nucleophilic substitution.
Conversion of a haloalkane to an alcohol
R-X + OH- → R-OH + X-
This reaction proceeds because X (a halogen) is a good leaving group and OH- is a good nucleophile. OH, however, is a poor leaving group. To make the reverse reaction proceed, OH must become a good leaving group. This is done by protonating the OH, turning it into H2O+, which is a good leaving group. H+ must be present to do this. Therefore, the compounds that can react with alcohols to form haloalkanes are HBr, HCl, and HI. Just like the reverse reaction, this process can occur through SN2 (backside attack) or SN1 (carbocation intermediate) mechanisms.
SN2 conversion of an alcohol to a haloalkane
R-O-H + H+ + X- → R-O+-H2 + X- → R-X + H2O
SN1 conversion of an alcohol to a haloalkane
R-O-H + H+ + X- → R-O+-H2 + X- → R+ + H2O + X- → R-X + H2O
As stated in the haloalkane chapter, the two mechanisms look similar but the mechanism affects the rate of reaction and the stereochemistry of the product.
Oxidation in organic chemistry always involves either the addition of oxygen atoms (or other highly electronegative elements like sulfur or nitrogen) or the removal of hydrogen atoms. Whenever a molecule is oxidized, another molecule must be reduced. Therefore, these reactions require a compound that can be reduced. These compounds are usually inorganic. They are referred to as oxidizing reagents.
With regards to alcohol, oxidizing reagents can be strong or weak. Weak reagants are able to oxidize a primary alcohol group into a aldehyde group and a secondary alcohol into a ketone. Thus, the R-OH (alcohol) functional group becomes R=O (carbonyl) after a hydrogen atom is removed. Strong reagents will further oxidize the aldehyde into a carboxylic acid (COOH). Tertiary alcohols cannot be oxidized.
An example of a strong oxidizing reagent is chromic acid (H2CrO4). An example of a weak oxidizing reagent is pyridinium chlorochromate (PCC) (C5H6NCrO3Cl).