|Lecture Notes: 1 March||
With this chapter we begin our discussion of the carbonyl compounds. These compounds are characterized by the presence of the C=O functional group. Note that the R-C=O part is referred to as an acyl part. Compounds containing the carbonyl group are ubiquitous in nature, occurring as key parts of proteins, carbohydrates, and fats as well as other biomolecules.
Carbonyl compounds include a number of families of organic molecules, among the most important are in the table below:
|Carboxylic acid anhydride||
Other important carbonyl containing compounds of biological interest include Lactones (cyclic esters), thiol esters (R-CO-S-R'), and mixed acid anhydrides (R-CO-OPO3-)
The properties of the carbonyl group are dictated by the double bond between the carbon and the oxygen. As seen with alkenes this system is planar, with the pi bond exposed above and below the plane of the atoms. However, due to the large difference in electronegativities between oxygen and carbon, this bond is quite polar. Thus the carbonyl carbon has a partial positive charge (its an electrophile) and will be attacked by nucleophiles. And of course the oxygen will have a partial negative charge, and thus can behave as a Lewis base or a nucleophile.
The carbonyl compounds are broken into two categories based on their chemistry. As you might expect the chemistry of the acyl group will largely involve nucleophilic attacks on the sterically open carbonyl carbon. Thus a major determinant of the possible reactions will depend on the quality of the potential leaving group:
Aldehydes and ketones have priority over alcohols etc. IUPAC names are based on alkane chains. For aldehydes the -e is replaced by -al, and the longest chain must include the aldehyde (-CHO) group, which is numbered 1.
Note the common names: formaldehyde, acetaldehyde, propionaldehyde.
When the aldehyde group is attached to a ring it is named as a -carbaldehyde:
Ketones are named by adding -one instead of the -e of the corresponding alkane. The parent chain is the longest chain containing the ketone group.
An important common name is acetone.
When used as a substituent, the -COR group is named as an acyl group (-CHO is a formyl group, -COCH3 is an acetyl group). When the =O is a substituent then it is given the name -oxo (keto- in the older literature and in frequently in biology).
Aldehydes have priority over ketones:
Priority for naming: -COOH > -CHO > -CO- > -OH > -NH2 > -SH
We have already seen two methods for synthesizing aldehydes: cleavage of alkenes by ozone, and controlled oxidation of primary alcohols, for example by pyridinium chlorochromate:
Ketone synthesis is again a matter of review for us, and is similar to aldehyde synthesis. The first two methods are identical to those seen for aldehydes, above, but done with secondary alcohols:
(Of course with secondary alcohols stronger oxidizers may be used, since we don't have to worry about continuing to a carboxylic acid.)
Aldehydes are easily oxidized to carboxylic acid (RCOH -> RCOOH), however ketones are unreactive, since oxidation would break a C-C bond.
A simple procedure for oxidizing aldehydes, which also serves as a qualitative test for the presence of aldehydes is the Tollen's test. This reaction uses silver ion complexed by ammonia as the oxidizing agent to give metallic silver. If done in a clean glass vessel a silver mirror is deposited on the glass - a positive test for aldehyde.
These are the most important reactions undergone by aldehydes and ketones. Remember that neither of these functional groups includes a good leaving group. Thus we will not see substitution reactions, nor will we see elimination reactions.
As you might expect aldehydes are in general somewhat more reactive than ketones since the carbonyl carbon is less hindered.
Nucleophilic addition to the carbonyl group gives 1,1-diols for aldehydes and n,n-diols for ketones (geminal or gem diols). This reaction is readily reversible to give back the original aldehyde or ketone. In most cases the carbonyl form is favored over the gem diol form.
Hydration of carbonyl groups can be catalyzed by either acids or bases. We will see that carbonyl reactions generally will be catalyzed by either or both. Let's look carefully at what is going on in these catalyses - we'll use hydration as our model for catalysis, and note variations as we study further.
Remember that catalysis means that we are enhancing the rate of a reaction without affecting the equilibrium position of the reaction. Remember also that the catalyst is intimately involved, but is regenerated, so its concentration is unaffected by the reaction. So how might we affect the rate of attack on a carbonyl group? Two possibilities:
For base catalysis the nucleophile is modified: the actual attacking species is hydroxide (HO:-) ion, which is more electron rich and therefore a better nucleophile that water:
For acid catalysis, on the other hand, the carbonyl group is protonated, making it more positive (electron deficient), and therefore a better electrophile:
Let's look at the base catalyzed reaction in detail:
Note that the ultimate source of the OH for synthesizing the gem diol can be considered to be water, since water was used to regenerate the attacking species, and only water is lost to the system!
Next let's look at the acid catalyzed reaction:
Once again the synthesis of the gem diol is sped up by a catalyst which is regenerated at the end of the process.
In a reaction similar to hydration we can add alcohols to carbonyls.
Addition of a single alcohol molecule gives a hemiacetal:
Hemiacetal formation is catalyzed by acid and base, as expected with their similarity to diols. Hemiacetals are not seen often in organic chemistry, but are very important in sugar chemistry - the common forms of sugars are hemiacetals stabilized by steric considerations of their ring forms. We will see later.
Hemiacetals are generally quite unstable, being easily hydrolyzed to give starting materials, or readily converted via addition of a second alcohol to acetals:
Acetals are quite stable to bases, so their formation is only catalyzed by acids (catalyzed reactions must be reversible, or would affect equilibrium, and no longer, by definition catalysis!):
Acetals are often used as protecting or blocking groups. Thus one can protect an aldehyde or ketone from reduction by derivitizing to an acetal. What makes them useful is the fact that the acetal is readily reverted to the original aldehyde or ketone in acidic solution. Note examples of selective reduction of esters in oxo-esters on pg 273.
Can convert aldehydes and ketones to imines, also known as Schiff bases (C=NR), by reacting with ammonia or primary amines:
2,4-Dinitrophenylhydazones are sometimes made from aldehydes and ketones to give solid, crystalline products with defined melting points:
We will use these compounds later in lab in identifying unknown aldehydes and ketones by melting points.
Oximes. In a similar way we can synthesize oximes by reacting aldehydes or ketones with hydroxylamine (NH2OH). Oximes are also generally crystalline solids at room temperature:
Last modified 29 June 2004