In this section, we will have a discussion about one of the most important functional group in organic chemistry which is carbonyl functional. Carbonyl functional groups are ketones and aldehydes which compose of C=O bond. One of the importances of carbonyl functional groups is commonly act as a precursor compound of organic synthesis. This part we will see the fundamental principles in carbonyl functional groups which are the physical properties of carbonyls and the basic reaction involving ketones or aldehydes, mainly nucleophilic addition.
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| An example of natural carbonyl compound |
To begin the introduction to carbonyl compounds is the discussion about the oxidation levels in organic chemistry. The oxidation levels can be defined is the number bonds to heteroatoms, excluding C and H. The importance of the oxidation levels in organic chemistry is to help us determining the type of reactions; if the oxidation level increases the reaction is oxidation, if the oxidation level decreases the reaction is reduction. The 0 oxidation level means the C atom in a compound does not bond to any atoms besides C and H and of the example is in alkene.
Then, oxidation level 1 means the C in a compound only has one heteroatom bonded to itself. The examples of oxidation level of 1 are alcohols, ethers, amines, and alkyl halides.
If C atom has 2 heteroatoms bonded to itself, so this C atom has oxidation level 2. In this case, C atom is double-bonded to a heteroatom will be regarded as oxidation level of 2. Hence, carbonyl functional groups are considered as oxidation level of 2. Furthermore, acetal functional group is another example of oxidation level 2.
Higher oxidation level which is 3 means C atom is bonded to 3 heteroatoms. The examples of oxidation level of 3 are nitriles, carboxylic acids, and carboxylic acid derivates.
The highest oxidation level that we can get is the oxidation level of 4 which means a C atom is bonded to 4 heteroatoms. The simplest example of oxidation level of 4 is carbon dioxide and furthermore carbamate also has oxidation level of 4.
From this concept of oxidation level, we can use it to synthesis aldehydes or ketones by using redox reaction. The first reaction to synthesis aldehydes or ketones by using oxidation reaction of alcohols as shown below.
[O] or oxidising agent that is used in this reaction is commonly Cr(VI) or CrO3. The oxidation of primary alcohols would give aldehydes, but it can be oxidised further into carboxylic acids in aqueous which is a problem. Therefore, to overcome this problem the oxidation reaction should be done in organic solvent. One of the oxidising agents that can be used in organic solvent is pyridinium chlorochromate (PCC).
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| Primary alcohol oxidation |
Besides that, the oxidation of secondary alcohols would give ketones and ketones are generally resistant to oxidation.
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| Secondary alcohol oxidation |
Furthermore, the oxidation reaction has mechanism as shown below.
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| Cr(VI) oxidation mechanism |
The second way to synthesis carbonyl compounds is by reacting alkene with ozone followed with Zn, and this reaction is called ozonolysis.
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| Ozonolysis |
The third way to synthesis ketones is by converting carboxylic acid derivates into aldehydes and this process is known as reductive process.
From the synthesis reactions we can move into physical properties of aldehydes and ketones as this will determine their behaviour and reactivity. This behaviour derives from the fact that C and O has different electronegativity which means C=O bond has polarity. The difference in electronegativity of C and O causes the electron charge distribution along C=O bond which can be drawn as resonance structure.
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| The resonance structure of propanone |
For your information, both structures are extreme different structure. The implications of this resonance structure are the C=O bond is stronger and shorter than C-O bond (C=O is 720 kJ mol-1 C-O 351 kJ mol-1 bond length of C=O is 1.21 Å, C-O is 1.43 Å). Besides that, π-bond where the p-orbital has sideway overlap determine the electrophilicity of C atom in C=O, so carbonyl compound can undergo nucleophilic addition.
The first example of nucleophilic addition in carbonyl compound is the reaction with cyanide ion to form cyanohydrin as shown below.
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| Cyanohydrin formation |
Then, the mechanism is shown below.
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| Cyanohydrin formation mechanism |
As you notice from the mechanism, the hybridisation of C atom is changed from sp2 (trigonal planar) to sp3 (tetrahedral).
Another example of this type reaction is the reaction with hydride ion and the source of hydride ion is borohydride (
BH4-) as shown below.
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| Sodium borohydride reaction and its mechanism |
From the reaction above, you may notice that there is direct attack of
H- to C but
H- is attached to B and this raise a big question. As most of us acknowledge, the π-bond in C=O involves 2
p orbital meanwhile
H- is 1
s orbital, so there is a big difference energy level which means the 1
s-2
p overlap is very poor. Hence, by attaching
H- into B atom, it helps to raise H in term of energy term as B atom is available for another electron pair. Besides that, the advantage using Na
BH4 is its selectivity with carbonyl compound because it only reduces carbonyl. Furthermore, to reduce carboxylic acids and carboxylic acid derivates Li
AlH4 can be used.
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| The selectivity of sodium borohydride |
Carbonyl compound can also be a precursor compound to form C-C bond, so C atom with partially negative is required which means C atom must bond to a more electropositive atom such as Li or Mg. The first example of this reaction is organolithium reaction and common organolithiums that are used in this reaction is shown below.
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| Common organolithium |
The preparation of organolithium compound can be done by reacting alkyl halide with 2 equivalent of Li in solvent such as THF in dry condition (no water). This condition is crucial in the formation of organolithium because it will react very fast and exothermic with any source of proton (e.g. water), so it can be potentially dangerous.
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| Organolithium formation |
Because organolithium is the source of
C-, so the example reaction is shown below.
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| Organolithium reaction and its mechanism |
As you notice from the mechanism above, the reaction is irreversible, not reversible as showed in previous reactions, and it is a reductive process.
Another organometallic reagent that commonly used is organomagnesium compound or known as Grignard reagent. Grignard reagent can be prepared in the same way such as organolithium, but using an equivalent of Mg in diethyl ether solvent. It has the same property as organolithium, so the reaction should be carried in dry condition.
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| Grignard reagent preparation |
Thus, the example reaction that involving Grignard reagent is shown below.
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| Grignard reaction and its mechanism |
Forming C-C bond can also be done by reacting with acetylide anion based on the fact that acetylide proton is quite acidic (p
Ka = 25). Therefore, acetylide anion can be prepared by using a correct base as shown below.
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| Acetylide preparation |
Thus, the reaction of carbonyl with acetylide anion would give the product as shown below.
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| Reaction of carbonyl with acetylide anion and its mechanism |
Then, oxidation level 1 means the C in a compound only has one heteroatom bonded to itself. The examples of oxidation level of 1 are alcohols, ethers, amines, and alkyl halides.
If C atom has 2 heteroatoms bonded to itself, so this C atom has oxidation level 2. In this case, C atom is double-bonded to a heteroatom will be regarded as oxidation level of 2. Hence, carbonyl functional groups are considered as oxidation level of 2. Furthermore, acetal functional group is another example of oxidation level 2.
Higher oxidation level which is 3 means C atom is bonded to 3 heteroatoms. The examples of oxidation level of 3 are nitriles, carboxylic acids, and carboxylic acid derivates.
The highest oxidation level that we can get is the oxidation level of 4 which means a C atom is bonded to 4 heteroatoms. The simplest example of oxidation level of 4 is carbon dioxide and furthermore carbamate also has oxidation level of 4.
[O] or oxidising agent that is used in this reaction is commonly Cr(VI) or CrO3. The oxidation of primary alcohols would give aldehydes, but it can be oxidised further into carboxylic acids in aqueous which is a problem. Therefore, to overcome this problem the oxidation reaction should be done in organic solvent. One of the oxidising agents that can be used in organic solvent is pyridinium chlorochromate (PCC). 
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