CH2O is Polar in nature because of oxygen atom which is highly electronegative (3.44). The oxygen atom has the ability to gain partial negative charge and leave partial positive charge on carbon and hydrogen atoms. This charge imbalance causes the molecule to be Polar.
WHAT IS CH2O
In chemical nomenclature, preferred IUPAC name of CH2O is “Formaldehyde”, while its systemic IUPAC name is “Methanal”. Its name is originated from “formic acid (methanoic acid)”, due to its similarity. It is a naturally occurring organic compound. It is the simplest form of aldehyde (R-CHO). According to the report of 1996, the installed capacity for the production of formaldehyde was about 8.7 million tons per year. It is highly toxic; can cause a significant damage to human health. In 2011, the US National Toxicology Program called formaldehyde as “known to be a human carcinogen”.
Formaldehyde is more complicated than many other carbon compounds; these compounds can be interconverted and can be used interchangeably. Some of its forms are:
- 1, 3, 5-Trioxane is a white solid that dissolves without degradation in organic solvents. Its formula is (CH2O)3. It is trimer of molecular formaldehyde.
- Methanediol, having the formula CH2(OH)2. Depending on the temperature and concentration, it also exists in equilibrium with various oligomers (short polymers).
- Molecular aldehyde is a colorless gas with a pungent and irritating odor.
- Paraformaldehyde is a white solvent which is insoluble in most of the solvents. Its formula is HO(CH2O)nH.
WHAT ARE POLAR AND NON POLAR MOLECULES
There are two main classes of molecules; POLAR and NON POLAR. Some are highly polar or non polar, while others are only partially. This depends on the electronegativities of the two atoms sharing electrons
Polar molecule occurs when the sharing of electrons between two atoms is not equal in a covalent bond. If the electronegativity difference between two atoms is between 0.4 and 2.0, the atom forms a polar covalent bond. The electronegativity difference between the atoms higher than 2.0, the bond is ionic. Examples of Polar molecules include water (H2O), ammonia (NH3), hydrogen sulfide (H2S), sulfur dioxide (SO2), etc.
NON POLAR MOLECULES
When sharing of electrons between two atoms in a covalent bond is equal, then non polar molecule occurs. The electrons are evenly distributed in non polar covalent bond. Electronegativity less than 0.4, the bond is considered as non polar. All the homonuclear diatomic atoms (H2, O2, Cl2) are truly non polar. Other examples are; benzene (C6H6), methane (CH4), any noble gas (He, Ne, Kr, Ar), carbon dioxide (CO2), carbon tetrachloride (CCl4), hydrocarbon liquids (toluene and gasoline), ethylene (C2H4) and most organic molecules.
PHYSICAL PROPERTIES OF FORMADEHYDE (CH2O)
- Formaldehyde appears as colorless gas at room temperature.
- It has pungent and highly suffocating odor.
- The molecular weight is 30.03 g/mol.
- Formaldehyde is highly flammable gas.
- Its melting point is -92 C, while boiling point is -19 C.
- It is denser than water; the density is 0.8153 g/cm3 at a temperature of -20 C.
- CH2O is soluble in water and acetone. Solubility in water is 400 g/L.
- Its vapor pressure is greater than 1.
- The dipole moment of methanal is 2.330 D.
- It is highly toxic; can cause severe injuries to the skin.
- On evaporating its aqueous solution, a crystalline solid (paraformaldehyde) is produced.
- Formaldehyde in aqueous form is corrosive to carbon steel, while formaldehyde in gas form is not.
CHEMICAL PROPERTIES OF CH2O
Formaldehyde or methanal gives the following important reactions:
- Oxidation reaction.
- Reduction reaction.
- Self oxidation-reduction reaction.
- Reaction with Tollen’s reagent.
- Reaction with Fehling solution.
- OXIDATION REACTION:
Oxidation is defined as the gain of oxygen and loss of hydrogen.
When formaldehyde is treated with K2Cr2O7 and H2SO4 (oxidizing mixture), it is converted into formic acid (methanoic acid) due to active H2 atom of formaldehyde.
- REDUCTION REACTION:
Reduction is referred as the loss of oxygen and gain of hydrogen.
When formaldehyde is treated with some reagents, Nascent Hydrogen (Na/Hg and water, or sodium and alcohol) is liberated, then hydrogen is added to carbonyl group and it is reduced to methanol.
- SELF OXIDATION REDUCTION REACTION
Oxidation reduction reaction is also called as “redox reaction”. In this chemical reaction, oxidation states of atoms are changed. It is characterized by formal transfer of electrons between chemical species, with one species (reducing agent) undergoes oxidation, while other species (oxidizing agent) will undergo reduction.
Treatment of formaldehyde with strong alkali causes the reduction of one molecule of formaldehyde to methanol and other molecule is oxidized to formic acid. This self oxidation-reduction reaction is called “Cannizaro Reaction”.
- REACTION WITH TOLLEN’S REAGENT
The ammonical solution of silver nitrate is Tollen’s reagent. It is formed by adding AgNO3 to get white precipitate of AgOH soluble in aqueous ammonia.
Formaldehyde reacts with Tollen’s reagent to form ammonium formate.
- REACTION WITH FEHLING’S SOLUTION
An aqueous solution of tartaric acid, cupric salt and sodium hydoxide are mixed, a complex of cupric tartarate is formed which is soluble in NaOH and forms Fehling’s solution.
When formaldehyde is treated with Fehling’s solution, sodium formate is formed.
On heating, it is polymerized in the presence of sulfuric acid (H2SO4).
SUMMARY: Formaldehyde is organic in nature. It appears in four different forms. It is colorless gas, having an irritating smell. It gives a self oxidation-reduction reaction.
PREPARATION OF CH2O
Formaldehyde (CH2O) was first prepared by Hofmann by passing air laden with methanol vapor over the heated platinum wire. The process was upgraded later by Loew and Tollen who substituted a roll of copper gauze for the platinum catalyst. Than O. Blank, in 1908, claimed that, silver (precipitated on asbesttos) is better than copper. Finally, Jobbling made the statement that gold is best catalyst among all for this purpose.
1. BY THE OXIDATION OF METHANOL
By heating methanol with the mixture of K2Cr2O7 or concentrated sulfuric acid, it oxidizes to methanal (formaldehyde).
2. BY THE DEHYDROGENATION OF METHANOL
Formaldehyde is formed when methyl alcohol or methanol vapors and air is overheated in the presence of catalyst (CuO and MoO).
3. BY THE DISTILLATION OF CALCIUM FORMATE
Calcium formate is the calcium salt of formic acid, having the formula Ca(HCOO)2. On dry distillation of calcium formate; it gives formaldehyde (CH2O).
4. PREPARATION OF PARAFORMALDEHYDE
Formaldehyde solution is evaporated in the presence of concentrated sulfuric acid. The crystalline mass if filtered, washed with water and then dried. A white crystalline powder is left behind, which is called paraformaldehyde.
USES OF FORMALDEHYDE
- Formaldehyde is used to generate the products like melamine resin, urea folmaldehyde resin, 1,4 butanediol, bakelite formaldehyde resin, phenol formaldehyde resin, methylene diphenyl diisocyanate and polyoxymethyleneplastics.
- Formaldehyde-based resins are used by textile industries as finishers to make fabric crease-resistant.
- Aqueous solution of formaldehyde, along with alcohol, is used to preserve biological specimen.
- It is reacted with fuming HNO3, to give highly explosive cyclonite, which was also used in World War II.
- Methanal is used to make components for the electrical system, automobiles, door panels, axles and brake shoes.
- Its aqueous solution is used as a disinfectant, as it kills most of the fungi and bacteria.
- Formaldehyde releasers (or formaldehyde donor) are used as biocides in cosmetics.
- CH2O is used to prepare chemicals used in paints.
- It is used in photography, in low concentration, for process C-41 (color negative film) stabilizer in final wash step; also in the process E-6 (pre bleach step).
- Methanal is also used in the processing of anti polio vaccine.
- It is used in the replacement of antibiotics for the treatment of urinary tract infections.
- It is used in the silvering of mirror.
STRUCTURE OF FORMALDEHYDE
Formaldehyde consists of one carbon atom; carbon can make 4 bonds, so it shares a double bond with an oxygen atom and has 2 single bonds with two hydrogen atoms, one on both sides. It is an organic compound due to the presence of carbon atom. It is classified as aldehyde (R-CHO), where -CHO is the formyl group while R can be hydrogen or carbon.
Carbon has 4 valence electrons; three of these are sp2 hybridized and they form two C-H and one C-O bonds. The fourth carbon electron remains unhybridized, so it overlaps with an oxygen electron and form a “π bond (Pi bond)”. It forms H2C=O this arrangement. There is a double bond between carbon and oxygen; it consists of one pi-bond and one sigma-bond. Pi bond is weaker than the sigma bond. According to Valence Shell Electron Repulsion Theory (VSEPR), formaldehyde (methanal) has “trigonal planar” structure; and its bond angle is “120-degrees”.
IS CH2O POLAR OR NON POLAR?
The difference in electronegativity and the sharing of electrons between two atoms in a covalent bond, makes the molecule polar or non polar. If the sharing of electrons between two atoms is unequal than it is said to be “Polar”, and when the sharing of electrons is equal between the atoms than the molecule will be “non polar”. Formaldehyde (CH2O) is polar in nature. The unbalanced electron density makes the formaldehyde polar. The difference in electronegativity between hydrogen and carbon is negligible, while electronegativity difference between carbon and oxygen is large enough to create polarity. This means that electrons in the double bond are more drawn to oxygen, which shift the position of charges, making a “partial +” charge on the carbon and “partial -” charge on the oxygen. Another reason is that, this electronegativity difference allows polarity to make a molecule non symmetrical (asymmetrical), so the -ve charge is centralized at one side of the molecule by the oxygen.
ELECTRONEGATIVITY OF CH2O
The CH2O molecule consists of 1 carbon, 1 hydrogen and 2 oxygen atoms. The electronegativity of carbon is 2.55, of hydrogen is 2.2 and that of oxygen is 3.44. So, oxygen is the most electronegative of all in the formaldehyde molecule. This is why oxygen pulls the bonded electron pair to its side. The intensity of electrons also increases on the oxygen atoms because the oxygen is connected to carbon atom via a double bond.
HOW TO CHECK IF A MOLECULE IS POLAR OR NON POLAR?
There are some points which will help to determine whether a molecule is polar or non-polar; these points are as follows:
It is the measurement of the separation of two opposite electrical charges. Dipole moment is directly proportional to polarity of a molecule; greater the value of dipole moment, more is its polarity. The dipole moment of all non polar molecule is “0”, while the value of dipole moment for polar molecules is always “non-zero”.
Formaldehyle has a dipole moment “2.330 D”, which is non-zero value, hence it is “Polar”.
Electronegativity is referred as the tendency of an atom to attract a shared pair of electrons. Polarity of a molecule is directly proportional to the electronegativity difference of an atom.
Electrons in a bond is shifted towards the more electronegative atom; thus, oxygen in CH2O is the most electronegative, so it pulls the negative charge towards its side and gain a partial -ve charge. This causes the imbalance of charge.
The geometrical shape of a molecule is also an important point to remember while checking the polarity of a molecule. The polar molecules are asymmetrical in shape due to which the charges of the molecules are unevenly distributed among its atoms. And, the non polar molecules are seen to be symmetrical due to which the charges on all atoms remain balanced.
So, the structure of formaldehyde (CH2O) is asymmetrical; thus, it is a polar molecule.
ABSTRACT: Formaldehyde is highly useful in industries. Methanal is polar. The polarity of any molecule can be checked by its dipole moment, electronegativity and its geometrical structure.
FREQUENTLY ASKED QUESTION (FAQs)
1. Why is formaldehyde used as a preservative?
Formaldehyde is used as a preservative because it is a combination of carbon, hydrogen and oxygen and all these have anti-bacterial properties.
2. How to neutralize formic acid?
Baking soda (NaHCO3) has the ability to neutralize acids; formic acid can also be neutralized by thick paste of sodium bicarbonate water (baking soda).
3. Is formaldehyde acidic?
NO, it is not acidic, as its C-H bond is fairly stable, so it does not give H+ ions in the aqueous solution. Another reason is that it is not electron deficient so it does not accept electrons. So any of the definitions of acid (arrhenious, bronsted lowry and lewis) does not apply for formaldehyde.
CH2O is Polar; the reason of its polarity is oxygen atom which is highly electronegative and the uneven sharing of electrons. Its dipole moment is non-zero (2.330 D), which is also the reason of its polarity. The structure of formaldehyde is asymmetrical (trigonal planar), having the bond angle 120 degrees.