The carbonyl group It is an organic and oxygenated functional group that resembles the gaseous carbon monoxide molecule. It is represented as C = O, and although it is considered organic, it can also be found in inorganic compounds; as carbonic acid, HtwoCO3, or in organometallic compounds with CO as a binder.
However, it is in the chemistry of carbon, life, biochemistry and other similar scientific branches where this group stands out for its enormous importance. If it weren't for him, many molecules wouldn't be able to interact with water; proteins, sugars, amino acids, fats, nucleic acids and other biomolecules would not exist were it not for him.
The image above shows what this group looks like in the general skeleton of a compound. Note that it is highlighted by the blue color, and if we removed the substituents A and B (R or R ', equally valid), a carbon monoxide molecule would remain. The presence of these substituents defines a large number of organic molecules.
If A and B are atoms other than carbon, such as metals or non-metallic elements, one can have organometallic or inorganic compounds, respectively. In the case of organic chemistry, the substituents A and B will always be either hydrogen atoms, carbon chains, lines, with or without branches, cyclic, or aromatic rings..
This is how it begins to understand why the carbonyl group is quite common for those who study natural or health sciences; it is everywhere, and without it the molecular mechanisms that happen in our cells would not occur.
If its relevance could be summarized, it would be said that it contributes polarity, acidity and reactivity to a molecule. Where there is a carbonyl group, it is more than likely that just at that point the molecule can undergo a transformation. Therefore, it is a strategic site to develop organic synthesis through oxidation or nucleophilic attacks..
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What are the structural and electronic characteristics of the carbonyl group? Above can be seen, now using the letters R1 and Rtwo instead of A and B, there is an angle of 120 ° C between the substituents and the oxygen atom; that is, the geometry around this group is a trigonal plane.
For such geometry to be the carbon and oxygen atoms must necessarily have sp chemical hybridizationtwo; thus carbon will have three sp orbitalstwo to form simple covalent bonds with R1 and Rtwo, and an orbital p pure to establish the double bond with oxygen.
This explains how there can be a C = O double bond.
If the image is observed, it will also be seen that oxygen has a higher electron density, δ-, than carbon, δ +. This is due to the fact that oxygen is more electronegative than carbon, and therefore "robs" it of electron density; and not only him, but also the R substituents1 and Rtwo.
Consequently, a permanent dipole moment is generated, which can be of greater or lesser magnitude depending on the molecular structure. Wherever there is a carbonyl group, there will be dipole moments.
Another consequence of the electronegativity of oxygen is that in the carbonyl group there are resonance structures that define a hybrid (the combination of the two structures in the upper image). Note that the pair of electrons can migrate towards the orbital p oxygen, which leaves the carbon atom with a positive partial charge; a carbocation.
Both structures are constantly succeeding, so that carbon maintains a constant deficiency of electrons; that is, for cations that are very close to it, they will experience electrostatic repulsion. But, if it is an anion, or a species capable of donating electrons, you will feel a strong attraction for this carbon..
Then what is known as the nucleophilic attack occurs, which will be explained in a future section.
When a compound has the group C = O, it is said to be carbonyl. Thus, depending on the nature of the carbonyl compound, it has its own nomenclature rules.
Although, no matter what it is, they all share a common rule: C = O has priority in the carbon chain when listing carbon atoms.
This means that if there are branches, halogen atoms, nitrogenous functional groups, double or triple bonds, none of them can carry a locator number less than C = O; therefore, the longest chain is listed as close as possible to the carbonyl group.
If, on the other hand, there are several C = O's in the chain, and one of them is part of a higher functional group, then the carbonyl group will carry a larger locator and will be mentioned as an oxo substituent..
And what is this hierarchy? The following, from highest to lowest:
-Carboxylic acids, RCOOH
-Ester, RCOOR '
-Amida, RCONHtwo
-Aldehyde, RCOH (or RCHO)
-Ketone, RCOR
Substituting R and R 'for molecular segments, a myriad of carbonyl compounds are created represented by the families above: carboxylic acids, esters, amides, etc. Each one has its traditional or IUPAC nomenclature associated with it..
The upper image shows the nucleophilic attack suffered by the carbonyl group. The nucleophile, Nu-, it can be an anion or a neutral species with available electron pairs; like ammonia, NH3, for example. It looks exclusively for carbon because, according to the resonance structures, it has a positive partial charge.
Positive charge attracts Nu-, which will seek to be approximated by a "flank" such that there is the least steric hindrance from the R and R 'substituents. Depending on how voluminous they are, or the size of the Nu-, the attack will occur at different angles ψ; can be very open or closed.
Once the attack happens, an intermediate compound will be formed, Nu-CRR'-O-; that is, oxygen is left with a pair of electrons to allow Nu to be added- carbonyl group.
This negatively charged oxygen can intervene in other steps of the reaction; protonated as a hydroxyl group, OH, or released as a water molecule.
The mechanisms involved, as well as the reaction products obtained by this attack, are very varied..
The nucleophilic agent Nu- it can be many species. For each one specifically, when reacting with the carbonyl group, different derivatives originate..
For example, when said nucleophilic agent is an amine, NHtwoR, imines originate, RtwoC = NR; if it is hydroxylamine, NHtwoOH, gives rise to oximes, RR'C = NOH; if it is the cyanide anion, CN-, cyanohydrins are produced, RR'C (OH) CN, and so on with other species.
At first it was said that this group is oxygenated, and therefore oxidized. This means that, given the conditions, it can be reduced or lose bonds with the oxygen atom by replacing it with hydrogens. For example:
C = O => CHtwo
This transformation indicates that the carbonyl group was reduced to a methylene group; there was a gain of hydrogen as a result of the loss of oxygen. In more appropriate chemical terms: the carbonyl compound is reduced to an alkane.
If it is a ketone, RCOR ', in the presence of hydrazine, HtwoN-NHtwo, and a strongly basic medium can be reduced to its respective alkane; This reaction is known as Wolff-Kishner reduction:
If, on the other hand, the reaction mixture consists of amalgamated zinc and hydrochloric acid, the reaction is known as Clemmensen reduction:
The carbonyl group can not only add nucleophilic agents Nu-, but under acidic conditions it can also react with alcohols through similar mechanisms.
When an aldehyde or ketone partially reacts with an alcohol, hemiacetals or hemicetals are produced, respectively. If the reaction is complete, the products are acetals and ketals. The following chemical equations summarize and clarify what has just been mentioned:
RCHO + R3OH g RCHOH (OR3) (Hemiacetal) + R4OH g RCH (OR3) (OR4) (Acetal)
RCORtwo + R3OH g RCORtwo(OH) (OR3) (Hemicetal) + R4OH g RCORtwo(OR3) (OR4) (ketal)
The first reaction corresponds to the formation of hemiacetals and acetals from an aldehyde, and the second of hemicetals and ketals from a ketone.
These equations may not be simple enough to explain the formation of these compounds; However, for a first approach to the subject, it is enough to understand that alcohols are added, and that their side chains R (R3 and R4) become bonded to carbonyl carbon. That is why OR are added3 and OR4 to the initial molecule.
The main difference between an acetal and a ketal is the presence of the hydrogen atom bonded to the carbon. Note that the ketone lacks this hydrogen.
Very similar as explained in the nomenclature section for the carbonyl group, its types are a function of which are the substituents A and B, or R and R '. Therefore, there are structural characteristics that share a series of carbonyl compounds beyond just the order or type of bonds..
For example, mention was made at the beginning of the analogy between this group and carbon monoxide, C≡O. If the molecule is devoid of hydrogen atoms and if there are also two terminal C = O, then it will be a carbon oxide, CnORtwo. For n equal to 3, we will have:
O = C = C = C = O
Which is as if there were two C≡O molecules joined and separated by a carbon.
Carbonyl compounds can not only be derived from CO gas, but also from carbonic acid, HtwoCO3 or OH- (C = O) -OH. Here the two OH represent R and R ', and replacing either one of them or their hydrogens, derivatives of carbonic acid are obtained.
And then there are the derivatives of carboxylic acids, RCOOH, obtained by changing the identities of R, or substituting H for another atom or chain R '(which would give rise to an ester, RCOOR').
Both aldehydes and ketones have the presence of the carbonyl group in common. Its chemical and physical properties are due to it. However, their molecular environments are not the same in both compounds; in the former it is in a terminal position, and in the latter, anywhere in the chain.
For example, in the upper image the carbonyl group is inside a blue box. In ketones, next to this box there must be another carbon or chain segment (top); while in aldehydes, there can only be one hydrogen atom (bottom).
If the C = O is at one end of the chain, it will be an aldehyde; that's the most direct way to differentiate it from a ketone.
But how do you know experimentally if an unknown compound is an aldehyde or a ketone? There are numerous methods, from spectroscopic (absorption of infrared radiation, IR), or qualitative organic tests.
Regarding qualitative tests, these are based on reactions that, when positive, the analyst will observe a physical response; a change in color, release of heat, formation of bubbles, etc..
For example, when adding an acid solution of KtwoCrtwoOR7 the aldehyde will change to carboxylic acid, which causes the color of the solution to change from orange to green (positive test). Meanwhile, ketones do not react, and therefore, the analyst does not observe any color change (negative test).
Another test consists of using the Tollens reagent, [Ag (NH3)two]+, for the aldehyde to reduce Ag cations+ to metallic silver. And the result: the formation of a silver mirror at the bottom of the test tube where the sample was placed..
Finally, a series of examples of carbonyl compounds will be listed:
-CH3COOH, acetic acid
-HCOOH, formic acid
-CH3Car3, propanone
-CH3CartwoCH3, 2-butanone
-C6H5Car3, acetophenone
-CH3CHO, ethanal
-CH3CHtwoCHtwoCHtwoCHO, pentanal
-C6H5CHO, benzaldehyde
-CH3CONHtwo, acetamide
-CH3CHtwoCHtwoCOOCH3, propyl acetate
Now, if examples of compounds that simply possess this group are cited, the list would become almost endless.
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