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1800-102-2727If you have visited a zoology laboratory, you might have observed dead animals small to big placed in glass containers covered with a transparent liquid. The liquid is not water but a dilute solution of formaldehyde.
A dilute 4% aqueous solution of formaldehyde called formalin is used in biology laboratories for preserving the decay of dead biological species. Formaldehyde polymerises to form -CH2 linkages, which bond with nitrogen atoms present in the primary amino acids of the protein chains present in biological samples.
Table of content:
Methanal is the IUPAC name for Formaldehyde, and its condensed chemical formula is CH2O. The functional group is an aldehyde -CHO, with one carbon atom, two hydrogen atoms, and one oxygen atom. Formaldehyde has the chemical formula HCHO, which is represented below.
Formaldehyde is the simplest aldehyde and is a one-carbon compound. The only carbon atom in its formula is sp2 hybridised and belongs to the aldehyde functional group.
Three sigma bonds are formed by the sp2 hybridised carbon. In the ideal case, the creation of three sigma bonds give formaldehyde a basic trigonal planar geometry with bond angles of 120o. Because only two of the three carbon p orbitals participate in hybridisation, one p orbital remains unhybridized. This unhybridized p orbital forms a pi bond with the oxygen atom's unhybridized p orbital.
(i) At 300 o C, a combination of methyl alcohol vapours and the air is passed over a platinised asbestos or copper or silver catalyst.
(ii) A mixture of methanol vapours and air is passed over an iron oxide-molybdenum oxide or silver driver at 500 o C in the industrial method of producing formaldehyde.
CH2O |
Formaldehyde |
Density |
815 kg m-3 |
Molecular Weight/ Molar Mass |
30.031 g mol-1 |
Boiling Point |
-19 o C |
Melting Point |
-92o C |
Chemical Formula |
HCHO |
Q.1. The reaction of formaldehyde with ammonia produces
(A) Bakelite
(B) Urea
(C) Triethylene tetramine
(D) Hexamethylene tetramine
Answer: (D)
Solution: Formaldehyde reacts with ammonia to produce hexamethylenetetramine, which is known as urotropine and is used as a urinary antiseptic.
6 HCHO + 4 NH3 → (CH2)6N4 + 6 H2O
Q.2. Formaldehyde on reaction with base sodium hydroxide will give sodium formate and
(A) CH3OH
(B) CH3CH2OH
(C) CH3CH2CH2OH
(D) None of the above
Answer: (A)
Solution: When formalin reacts with a base, such as sodium hydroxide, it produces sodium formate and methanol. The chemical formula is depicted below.
2 HCHO + NaOH → HCOONa + CH3OH
Q.3. Which of the following reagents is required to distinguish between formaldehyde and acetaldehyde?
(A) Tollens’ reagent
(B) Fehling’s solution
(C) Schiff’s reagent
(D) Caustic soda solution
Answer: (D)
Solution: When formaldehyde reacts with Conc. NaOH(Caustic soda), it undergoes a Cannizzaro reaction. When acetaldehyde reacts with dil. NaOH(caustic soda), an aldol condensation reaction takes place.
Q.4. Which of the following aldehydes is present in the form of gas?
(A) Acetaldehyde
(B) Formaldehyde
(C) Butyraldehyde
(D) Benzaldehyde
Answer: (B)
Solution: Methanal is a gas with a boiling point of -21o C, while ethanal has a boiling point of +21o C. That means ethanal boils at nearly room temperature. The other aldehydes and ketones are liquids, with increasing boiling points as the molecules get bigger.
Q1. Why is formaldehyde more reactive in nucleophilic addition reactions than other aldehydes?
Answer: Formaldehyde is the most reactive aldehyde because there is no electron-donating group in formaldehyde(HCHO) to diminish the density of carbon atoms in the -CHO group. Other higher members have an electron donating group, which means an alkyl group, which increases electron density at the carbon atom of the -CHO group via the positive inductive effect.
Q2. Is formaldehyde and formalin the same?
Answer: Both formalin and formaldehyde have the same chemical formula, which is HCHO. However, they differ in their physical states and chemical activities. Formaldehyde is a colourless, water-soluble, flammable gas at room temperature with a strong, unpleasant odour. Formalin, on the other hand, is a liquid that is made by combining formaldehyde gas with water. This is the key difference between formalin and formaldehyde. A saturated solution of formalin typically contains around 40 % (by volume) or 37 % (by weight) formaldehyde gas and a stabiliser to prevent formaldehyde polymerization.
Q3. Does formaldehyde have a dipole moment?
Answer: The dipole moment in formaldehyde is primarily due to the -CHO group's C=O bond. Because of the electronegativity of carbon and oxygen, the C=O bond is polar. Because the oxygen atom pulls the shared pair of electrons more strongly than the carbon atom, the molecule has a partial negative charge at one end and a partial positive charge at the other.
Because there is only one polar link, no cancelling can occur, and the formaldehyde molecule's dipole moment is found to be 2.3 Debye.
Q4. Why does formaldehyde give a Cannizzaro reaction?
Answer: The Cannizzaro reaction is a chemical reaction in which an aldehyde without a hydrogen atom in the alpha position or an alpha-hydrogen undergoes base-induced disproportionation. Because formaldehyde (HCHO) lacks alpha hydrogen, it can undergo the Cannizzaro reaction.
Related Topics
Aldehyde |
Structure of aldehyde |
Aldol condensation |
Tollens’ test |
Acetone |
Ketone |