The methyl or methyl group is an alkyl substituent whose chemical formula is CH 3. It is the simplest of all the carbon substituents in organic chemistry, it has a single carbon and three hydrogens; derived from methane gas. Because it can only bind to another carbon, its position indicates the end of a chain, its termination.
In the image below there is one of the many representations for this group. The sinuosities to its right indicate that behind the H 3 C- bond there can be any atom or substituent; an alkyl one, R, aromatic or aryl, Ar, or a heteroatom or functional group, such as OH or Cl.
The methyl group is the simplest of the carbon substituents in organic chemistry. Source: Su-no-G
When the functional group attached to the methyl is OH, we have the alcohol methanol, CH 3 OH; and if it is Cl, then we will have methyl chloride, CH 3 Cl. In organic nomenclature it is simply referred to as 'methyl' preceded by the number of its position in the longest carbon chain.
The methyl group CH 3 is easy to identify during elucidations of organic structures, especially thanks to carbon 13 nuclear magnetic resonance spectroscopy (13 C NMR). From it, after strong oxidations, acidic COOH groups are obtained, being a synthetic route to synthesize carboxylic acids.
Representations
Possible representations for the methyl group. Source: Jü via Wikipedia.
Above we have the four possible representations assuming that CH 3 is linked to an alkyl substituent R. All are equivalent, but while going from left to right the spatial aspects of the molecule are evident.
For example, R-CH 3 gives the impression that it is flat and linear. The representation that follows shows the three CH covalent bonds, which allow the methyl to be identified in any Lewis structure and give the false impression of being a cross.
Then, continuing to the right (the penultimate), the sp 3 hybridization is observed at the CH 3 carbon due to its tetrahedral geometry. In the last representation, the chemical symbol for carbon is not even written, but the tetrahedron is kept to indicate which H atoms are in front of or behind the plane.
Although it is not in the image, another very recurrent way when representing CH 3 consists of simply placing the dash (-) “naked”. This is very useful when drawing large carbon skeletons.
Structure
Structure of the methyl group represented by the spheres and bars model. Source: Gabriel Bolívar.
The top image is the three-dimensional representation of the first. The glossy black sphere corresponds to the carbon atom, while the white ones are the hydrogen atoms.
Again, carbon has a tetrahedral environment as a result of its sp 3 hybridization, and as such is a relatively bulky group, with its CR bond rotations sterically hindered; that is, it cannot rotate because the white spheres would interfere with the electronic clouds of their neighboring atoms and feel their repulsion.
However, the CH bonds can vibrate, just like the CR bond. Therefore, CH 3 is a group of tetrahedral geometry that can be elucidated (determined, ascertained) by infrared radiation (IR) spectroscopy, as can all functional groups and carbon bonds with heteroatoms.
The most important thing, however, is its elucidation by 13 C-NMR. Thanks to this technique, the relative amount of methyl groups is determined, which makes it possible to assemble the molecular structure.
Generally, the more CH 3 groups a molecule has, the more "clumsy" or inefficient its intermolecular interactions will be; that is, the lower their melting and boiling points will be. The CH 3 groups, because of their hydrogens, "slide" against each other when they approach or touch.
Properties
The methyl group is characterized by being essentially hydrophobic and apolar.
This is due to the fact that their CH bonds are not very polar due to the low difference between the electronegativities of carbon and hydrogen; Furthermore, its tetrahedral and symmetric geometry distributes its electron densities almost homogeneously, which contributes to a negligible dipole moment.
In the absence of polarity, CH 3 "runs away" from water, behaving as a hydrophobic. Therefore, if it is seen in a molecule, it will be known that this methyl end will not interact efficiently with water or another polar solvent.
Another characteristic of CH 3 is its relative stability. Unless the atom that is bound to it removes electron density, it remains practically inert against very strong acidic media. However, it will be seen that it can participate in chemical reactions, mainly with regard to its oxidation, or migration (methylation) to another molecule.
Reactivity
Oxidations
CH 3 is not free to oxidize. This means that it is susceptible to forming bonds with oxygen, CO, if it reacts with strong oxidizing agents. As it oxidizes, it transforms into different functional groups.
For example, its first oxidation gives rise to the methiol (or hydroxymethyl) group, CH 2 OH, an alcohol. The second, derives in the formyl group, CHO (HC = O), an aldehyde. And the third, finally, allows its conversion into the carboxyl group, COOH, a carboxylic acid.
This series of oxidations is used to synthesize benzoic acid (HOOC-C 6 H 5) from toluene (H 3 C-C 6 H 5).
Ion
CH 3 during the mechanism of some reactions can gain momentary electrical charges. For example, when methanol is heated in a very strong acid medium, in the theoretical absence of nucleophiles (seekers of positive charges), the methyl cation, CH 3 +, is formed, since the CH 3 -OH and OH bond are broken comes out with the electron pair of the bond.
The CH 3 + species is so reactive that it has only been determined in the gas phase, since it reacts or disappears at the slightest presence of a nucleophile.
On the other hand, an anion can also be obtained from CH 3: methanide, CH 3 -, the simplest carbanion of all. However, like CH 3 +, its presence is abnormal and only occurs under extreme conditions.
Methylation reaction
In the methylation reaction, a CH 3 is transferred to a molecule without producing electrical charges (CH 3 + or CH 3 -) in the process. For example, methyl iodide, CH 3 I, is a good methylating agent, and can replace the OH bond of various molecules with an O-CH 3 bond.
In organic synthesis this does not entail any tragedy; but yes when what is methylated in excess are the nitrogenous bases of DNA.
References
- Morrison, RT and Boyd, R, N. (1987). Organic Chemistry. 5th Edition. Editorial Addison-Wesley Interamericana.
- Carey F. (2008). Organic Chemistry. (Sixth edition). Mc Graw Hill.
- Graham Solomons TW, Craig B. Fryhle. (2011). Organic Chemistry. Amines. (10th edition.). Wiley Plus.
- Rahul Gladwin. (November 23, 2018). Methylation. Encyclopædia Britannica. Recovered from: britannica.com
- Danielle Reid. (2019). Methyl Group: Structure & Formula. Study. Recovered from: study.com
- Wikipedia. (2019). Methyl group. Recovered from: en.wikipedia.org