To make it more understandable, nitromethane is widely known as the nitro booster used in the drag-racing engine. It is a colorless liquid with a strong pungent smell that produces fumes of nitrogen oxides when decomposing violently when overheated. Moreover, the liquid is oily and inflammable making it a good choice as a fuel. Being highly toxic to the human body, nitromethane can irritate the skin and affects the central nervous system making it an important human carcinogen to study. Nitromethane is a nitroalkane that is produced when one hydrogen is replaced by a nitro group. By one laboratory method, nitromethane is produced when sodium chloroacetate reacts with sodium nitrite in the aqueous state. The reaction as follows: ClCH2COONa + NaNO2 + H2O ——–> CH3NO2 + NaCl + NaHCO3
The Lewis Structure of Nitromethane (CH3NO2)
In order to start with the Lewis structure of nitromethane, it is crucial to study the individual participating atoms first. The atomic number of Carbon is 6 which makes it electronic configuration 1s2 2s2 2p2. As we know, the p shell needs 6 valence electrons to complete its octet, there is a need for 4 more valence electrons. Due to this, the total number of valence electrons in one carbon atom is 4. Now, for the hydrogen atom, its atomic number is 1 which makes its electronic configuration 1s1. As the s shell needs 2 electrons to complete its octet, there is a need for 1 electron. Due to this, the total number of valence electrons for hydrogen is 1. In the case of the nitrogen atom, its atomic number is 7 where its electronic configuration is 1s2 2s2 2p3. Because p shell needs 6 valence electrons, there exists a scarcity of 5 electrons. Due to this, the valence electrons in the nitrogen atom are 5. Lastly, in the case of oxygen, its atomic number is 8 by which its electronic configuration is 1s2 2s2 2p4. Here, it is important to know that the valence electrons are also those, present in the highest principal energy level which are s and p for the oxygen atom. This makes the total number of valence electrons in oxygen 6. Below is the image of the lewis structure of Carbon, Nitrogen, and Oxygen atom.
Steps to Draw the Lewis structure of Nitromethane (CH3NO2)
Step 1: Find the total number of valence electrons present in a single nitromethane molecule (CH3NO2): It is 24 as 4 are coming from the carbon atom, 1 from each hydrogen atom, 5 from the nitrogen atom, and 6 from each oxygen atom. Step 2: Find how many more valence electrons are needed by a single nitromethane molecule (CH3NO2): It is 14 as 4 are needed by the carbon atom, 1 each by the hydrogen atom, 3 by the nitrogen atom, and 2 by each oxygen atom. Step 3: Find the central atom in one nitromethane (CH3NO2) molecule: From the molecular formula, it is clear that the hydrogen atoms are bonded to the carbon atom whereas the oxygen atoms are bonded to the nitrogen atom. With this information, it is clear that there will be two central atoms bonded together with a single bond. Step 4: Look for the type of bond-forming among the participating atoms in one nitromethane (CH3NO2) molecule: It is important to understand that we are talking about nitromethane, not methyl nitrite. The confusion may occur as the molecular formula of the compounds is the same. But it is important to realize how each of the participating atom form which type of bond. In methyl nitrite, one oxygen atom bonds with the carbon which is not the case in nitromethane. Each participating atom is forming a single covalent bond whereas between nitrogen and one oxygen atom, a double covalent bond is forming. Step 5: Now, assemble all the steps and draw the Lewis structure of Nitromethane:
Molecular Geometry of Nitromethane
From the Lewis structure, we have got to know about two central atoms in a single nitromethane molecule. So, there will be two molecular geometry for separate entities of -CH3 and -NO2. The bond angle between the hydrogen-carbon-hydrogen atoms is 109.5° and that in the carbon-nitrogen-oxygen atom is 120°. Here it is important to understand that the hydrogen-carbon-nitrogen bond is linear meaning 180° so it won’t hold much substance in this discussion. The molecular geometry of nitromethane (CH3NO2) can be studied with the help of the Valence Shell Electron Pair Repulsion (VSEPR) theory. It says the -CH3 end is tetrahedral whereas the -NO2 end is trigonal planar because of the bond angle they are making. Below is the attached VSEPR chart.
It might interest you that even after the presence of lone pairs of electrons, there exist distortion from the ideal shape and bond angle. This is because of the reason that the lone pairs of electrons are present in proportion due to which they cancel out overall neutralizing the effect.
Hybridization in Nitromethane (CH3NO2)
From the molecular geometry, it is clear that the nitromethane (CH3NO2) molecule is following the ideal conditions, by which it is clear that the carbon atom is sp3 hybridized whereas the nitrogen atom is sp2 hybridized. In the case of the carbon atom, one s orbital and 3p orbitals from the same shell mix and overlap to produce four new orbitals of similar energy. The newly produced orbitals have 25% characteristics of s orbital and 75% characteristics of p orbital. Whereas, in the case of the nitrogen atom, one s orbital and one p orbital from the same shell mix and overlap to produce three orbitals of similar energy. The newly produced orbitals have 33.33% characteristics of s orbital and 66.66% characteristics of p orbital.
Polarity in Nitromethane (CH3NO2)
Polarity is a concept that has arisen from the fact that the electric charge can separate within a molecule making one positively charged end and another negatively charged one. This is the case with nitromethane (CH3NO2) as it is considered one of the most polar solvents existing on the Earth. If we calculate the formal charge on the nitromethane (CH3NO2) by the formula Formal Charge = the number of valence electrons on free atom – [the number of lone pair electrons – ½ the number of bonding electrons] The overall charge will be zero. It is crucial to know that all the atoms have zero formal charges except nitrogen and oxygen. The nitrogen atom has a +1 formal charge where oxygen has -1. It is because they cancel out one another so the net dipole moment is zero. On contrary, the observed dipole moment shown by the nitromethane (CH3NO2) is a bit higher which is the reason why it is polar in nature. This anomaly is shown because of the hyperconjugation effect which leads to delocalization of those electrons only that are participating in the bond formation having sigma (σ) characteristics. Here, it is important to know that a single covalent bond has only a sigma bond, a double bond has one sigma and one pi bond whereas a triple bond has one sigma and two pi bonds. When delocalization of electrons occurs in nitromethane, the structure of it changes where the double bond shift between carbon and nitrogen atom, earlier which was in between nitrogen and oxygen atom. As we know the nitrogen is positively charged and oxygen is negative, the change in the location of the double bond strengthens the positive charge on nitrogen. Now, the formal charge on nitrogen and oxygen do not cancel out each other because of which net dipole moment persist and separation of electric charges occur. This is why nitromethane (CH3NO2) is polar in nature.
Conclusion
The behavior of nitromethane (CH3NO2) can be studied in detail with the help of its resonance structures. It is crucial to know that nitromethane and methyl nitrite are different molecules altogether having similar molecular formulas. A lot can be studied about a molecule through the Lewis structure where we studied molecular geometry, hybridization, and polarity in this piece. If a molecule can have many Lewis structures, it is crucial to select the one that can provide maximum stability. A double bond present on one leg is much more stable than present in between the central atoms. The molecular geometry of nitromethane determines the fact that the molecule is following all the ideal conditions which gives a hint of other molecular geometry parameters to be in alignment with the ideal requirements. Moreover, the hybridization and polarity are also in alignment with the expected conditions.
