Dimethylglyoxime: Understanding DMG in Chemistry
Students may already have been introduced to DMG chemicals, and have some idea about its structure and properties. It is a white crystalline powdered solid which is used in analytical chemistry as a reagent for nickel. It was one of the initial organic reagents that had found application in analytical chemistry.
We will go into an elaborate discussion on dimethylglyoxime and also focus on its important attributes. By the end of this discussion, you will also have an idea about its structure, properties along with uses.
Let us start!
Dimethylglyoxime Structure
Dimethylglyoxime or DMG is a white-coloured powder like chemical which is substantially less soluble in water but soluble in sodium hydroxide solution or methanol. The formula of dimethylglyoxime is C4H8N2O2, and it is also known as 2,3-Butanedione dioxide.
DMG is used as a reagent in analytical chemistry, and its multiple derivatives have been synthesized. It helps in the detection of various other metal ions apart from nickel. The anionic form is termed as DMGH, and the neutral form is DMGH2 (here H indicates hydrogen). DMGH2 is used for the identification of nickel and palladium. It is also a bidentate ligand, that is, two donor arms.
Note: Ligands are neutral ions attached directly to the central metal ion through coordinate bonds.
Do You Know?
Nickel ions chelation reaction with that of organic bidentate ligand dimethylglyoxime produces a raspberry or cherry red color precipitation. This reaction finds widespread use for the purpose of detecting nickel metal ions on account of the formation of its striking color.
It is a sensitive reaction which is used as a test for confirming any presence of nickel cations, even if it is in particularly low concentration. This procedure is readily applied for any metal object that may come in contact with our skin such as a wristwatch, jewelry, coin, spectacle frames etc. The process is highly useful in the identification of a particular kind of dermatitis or skin sensitivity known as Nickel itch.
1. Physical
Properties of Dimethylglyoxime
2. Chemical
Dimethylglyoxime and nickel cation react to form red precipitation of nickel dimethylglyoxime which is insoluble in nature.
\[ Ni^{2+} + 2C_{4}H_{8}N_{2}O_{2} \rightarrow Ni(C_{4}H_{7}N_{2}O_{2})_{2}\]
Red Precipitate
red precipitate + 2H+
It is a common reagent in the gravimetric estimation of nickel. The broad principles in the gravimetric estimation are – (1) precipitation of nickel from solution by addition of dimethylglyoxime, (2) filtering out the precipitate, and (3) calculation of the mass of nickel from the precipitate mass. In this instance, the organic functional groups that precipitate with the metal ion are facilitated by chelating agents. Here, dimethylglyoxime is the chelating agent.
Note: Chelating agents are such organic compounds that form coordinate covalent bonds with metal ions through cationic side chains.
The precipitation of nickel dimethylglyoxime is obtained with the addition of alcoholic solution of dimethylglyoxime. Aqueous ammonia solution is subsequently added.
\[ NiSO_{4} + 2C_{4}H_{8}O_{2}N_{2}\] \[\rightarrow\] \[Ni(C_{4}H_{7}O_{2}N_{2})_{2}+ H_{2}SO_{4} \]
Red chelate can be noticed in the solution when the pH is in the range of 5-9. Here, the electron pairs of four nitrogen atoms are donated, as opposed to the electrons of the oxygen atoms. The ammonia solution helps in preventing the pH from falling below 5. In the case of lower pH, the formation of nickel (II) ions takes place, causing the dissolution of Ni(DMG)2.
While a little excess of the reagent will not have any substantial impact, a significant excess can cause the reagent to precipitate. It may further crystallize out with the chelate. The amount of reagent to be added is directly proportional to the quantity of ions present.
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Ferrous sulfate and ammonium hydroxide react in the presence of dimethylglyoxime to form a compound of iron and ammonium sulfate, and at the same time releasing water.
\[ FeSO_{4} + 2NH_{4}OH + 2C_{4}H_{8}N_{2}O_{2} \rightarrow Fe(C_{4}H_{7}N_{2}O_{2})_{2} + (NH_{4})_{2}SO_{4} + 2H_{2}O \]
Dimethylglyoxime helps in the precipitation of palladium which is utilized to prepare a standard palladium solution out of palladium chloride. Palladium chloride is purified with the removal of any trace of platinum in the form of ammonium chloroplatinate. Subsequently, palladium is precipitated with dimethylglyoxime.
The complex is treated with aqua regia, and the nitrate ions are eliminated from the resulting solution by concentrated hydrochloric acid. Complete precipitation takes place after the solution stands overnight.
Different Applications of Dimethylglyoxime
It may be utilized as a precipitant for palladium and nickel. In the case of palladium, the precipitation takes the form of a yellow compound after diluting it with hydrochloric acid. On the other hand, precipitation from ammoniacal nickel solution takes the form of a bright red voluminous compound.
DMG helps in the detection of nickel to dermatitis or similar skin ailment. It identifies the nickel released from a watch, jewelry etc. that come directly in contact with our skin. Various countries have such regulations where dimethylglyoxime test can be purchased over the counter in pharmacies.
Dimethylglyoxime is used extensively in analytical chemistry in the form of detecting reagent, precipitating reagent, and photometric reagent for different metal ions such as platinum, palladium and nickel among others.
It plays a role in sustainable recycling of dumped lithium-ion batteries which is considered as an environmentally hazardous substance. The hydrometallurgical process that is employed for leaching lithium, cobalt, nickel from waste cathode materials of these batteries requires precipitation by dimethylglyoxime.
DMG also has application in the investigation of iron (III) reduction for determining dissolved iron in seawater. It involves kinetic experiments for examining the efficiency of the reduction of inorganic iron with sulphite under various conditions. Transition metals such as cobalt are present in seawater which requires DMG to be added to luminol solution.
Test Yourself
Question: Red precipitation takes place when the ethanol solution of dimethylglyoxime is combined with ammoniacal nickel (II). Which of the following statements is incorrect?
(a) There is tetrahedral geometry in the red complex
(b) There is symmetrical hydrogen bonding in the complex
(c) There is square planar geometry in the red complex
(d) Dimethylglyoxime acts as a bidentate ligand
Solution
(a) There is tetrahedral geometry in the red complex
Hazards of Dimethylglyoxime
DMG can cause severe health effects in case of unregulated exposure. Few potential health hazards are–
It may cause eye and skin irritation
Ingestion can be particularly harmful causing irritation in digestive tract
Inhalation can cause irritation in respiratory tract as well as mucous membranes
Moreover, owing to the flammable nature, dimethylglyoxime is a direct fire hazard. Such a threat is aggravated in its finely divided state as it may be ignited by sparks alone. In the instance of a fire, highly toxic gasses may be emitted with combustion or thermal decomposition. Fine dust with reduced particle size can cause a dust explosion.
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Dimethylglyoxime is the chemical compound with the formula CH3OCH2. It is a colorless, toxic, and explosive liquid. Dimethylglyoxime is the dimethyl form of glyoxime, a compound used in chemical weapons. Dimethylglyoxime is a volatile liquid, and is used industrially as a source of formaldehyde. Dimethylglyoxime is used in a number of chemical reactions.
Production
Dimethylglyoxime is produced industrially by combining the reaction products of formaldehyde and acetaldehyde with hydroxylamine. It may be produced directly from formaldehyde and acetaldehyde, by reaction with hydroxylamine. It is an intermediate in the production of hydroxylamine from urea and ammonia:
\[ HCONH_{2} \rightarrow H_{2}NCO + H_{2}O \]
Reactions
Glyoxalase Pathway
In the glyoxalase pathway, an enzyme called glyoxalase I converts glyoxal to d-lactate. Glyoxalase I is found in a variety of organisms, including bacteria and yeast. Dimethylglyoxime can act as a glyoxalase inhibitor.
Synthesis
Dimethylglyoxime can be produced from acetaldehyde and formaldehyde by the following method:
Formaldehyde and acetaldehyde are combined by the Grignard reagent (the magnesium salt of the acetyl chloride formed by reaction of acetic acid with magnesium metal) to form methyl acetate. This is then reduced by hydrogenation to give methyl propanoate. This propanoate is hydrolyzed to the hydroxylamine. It then reacts with formaldehyde to give glyoxime and water. The glyoxime is reduced by lithium aluminum hydride to give dimethylglyoxime.
Dimethylglyoxime can also be obtained by treating dimethylsulfoxide with hydroxylamine:
The dimethylsulfoxide is oxidized with mercury oxide to methylsulfone. The sulfone is then reduced by treatment with borohydride to give methylsulfide. The methylsulfide is treated with the Grignard reagent to give methyl propanoate. The propanoate is hydrolyzed to give hydroxylamine. The hydroxylamine is then treated with formaldehyde to give glyoxime. The glyoxime is reduced to give dimethylglyoxime.
Oxalyl chloride, acetic acid, and dimethylsulfoxide are reacted with the sodium salt of hydroxylamine to give ethyl acetate. The ethyl acetate is treated with benzene to give benzaldehyde. The benzaldehyde is reduced with sodium cyanoborohydride to give benzyl alcohol. Benzyl alcohol is then converted to benzylamine by heating with a strong base. The benzylamine is cyclized by adding the Grignard reagent, which eliminates water.
A solution of acetaldehyde and formaldehyde is treated with acetone. The reaction produces hydrogen cyanide, which is removed by absorption on celite. The formed aldehyde reacts with hydroxylamine, which forms the product.
Uses
Dimethylglyoxime is used as a ligand in coordination chemistry.
As a reagent- Dimethylglyoxime is an effective ligand in metal complexation chemistry. Dimethylglyoxime is one of the few ligands where the deprotonated species dimethylglyoxime2− is not basic.
As a reagent for chemical warfare- Dimethylglyoxime is used as a reagent in the production of chemical warfare agents, including ethyl methylphosphonofluoridate (sarin) and the more potent isosarin (soman). Dimethylglyoxime has also been considered for future use in the production of chemical weapons by the Russian Federation, for potential use as a substitute for sarin.
Synthesis and Detection
Dimethylglyoxime is unstable and decomposes spontaneously to methyl formate. It is an explosive. Dimethylglyoxime is prepared by the reaction of formaldehyde and hydroxylamine. The dimethylglyoxime is detected by gas chromatography.
FAQs on Dimethylglyoxime
1. What is Dimethylglyoxime (DMG)?
Dimethylglyoxime, commonly known as DMG, is an organic compound that appears as a white crystalline powder. Its chemical formula is C₄H₈N₂O₂. While it is only slightly soluble in water, it dissolves in organic solvents like methanol and alcohol. It is primarily recognised in chemistry as a highly selective chelating agent used for the detection and quantification of metal ions, most notably nickel.
2. What is the chemical structure of Dimethylglyoxime?
The chemical structure of Dimethylglyoxime is represented by the condensed formula CH₃C(NOH)C(NOH)CH₃. It is a dioxime, meaning it has two oxime (=NOH) functional groups. These groups are attached to a central backbone of two carbon atoms, making it a derivative of 2,3-butanedione. The presence of these two oxime groups is critical to its ability to act as a ligand in coordination chemistry.
3. What are the main physical properties of Dimethylglyoxime?
Dimethylglyoxime has several distinct physical properties relevant to its use in a laboratory setting:
- Appearance: It is a colourless or white crystalline solid.
- Solubility: It is poorly soluble in water but readily soluble in ethanol, methanol, and acetone.
- Melting Point: It has a high melting point, typically around 240°C, where it also begins to decompose.
- Odour: It is generally odourless.
4. How is the DMG test used for the detection of nickel ions?
The DMG test is a classic qualitative analysis method for detecting nickel (II) ions. When an alcoholic solution of Dimethylglyoxime is added to a neutral or slightly ammoniacal solution containing Ni²⁺ ions, a bright, voluminous cherry-red precipitate of Nickel(II) dimethylglyoximate, [Ni(DMG)₂], is formed. This reaction is highly sensitive and specific, making it an excellent confirmatory test for nickel.
5. What are the potential safety hazards associated with Dimethylglyoxime?
While useful, Dimethylglyoxime presents several safety hazards. It is a flammable solid and its fine dust can form explosive mixtures with air. Direct contact can cause skin and eye irritation. Inhalation may irritate the mucous membranes and respiratory tract, and ingestion can be harmful, causing irritation to the digestive system. Proper safety measures, such as wearing gloves and safety glasses, are essential when handling it.
6. Why is Dimethylglyoxime classified as a bidentate ligand?
Dimethylglyoxime is classified as a bidentate ligand because each molecule has two separate donor atoms that can bond to a single central metal ion simultaneously. In DMG, these donor sites are the two nitrogen atoms of the oxime groups. This ability to bind at two points allows it to form a stable ring-like structure known as a chelate with the metal ion.
7. How does the structure of the Nickel-DMG complex explain its high stability?
The exceptional stability of the Nickel-DMG complex, [Ni(DMG)₂], is due to two key structural features. Firstly, the two bidentate DMG ligands form a square planar geometry around the central Ni²⁺ ion, creating two stable five-membered rings. Secondly, this structure is further reinforced by strong intramolecular hydrogen bonds formed between the oxygen atom of one DMG ligand and the hydroxyl hydrogen of the other. This chelate effect and hydrogen bonding make the complex very stable and insoluble.
8. What is the importance of controlling pH during the precipitation of nickel with DMG?
Controlling the pH is critical for a successful precipitation. The reaction requires a slightly alkaline or neutral medium (pH 5-9). An ammoniacal solution is typically used to provide this environment and to neutralise the H⁺ ions produced during the reaction:
Ni²⁺ + 2(DMGH₂) → Ni(DMG)₂ (s) + 2H⁺
If the solution becomes too acidic (pH < 5), the equilibrium shifts to the left. The excess H⁺ ions will protonate the ligand, causing the red precipitate to dissolve and preventing the complete precipitation of nickel.
9. Besides detecting nickel, what are some other important applications of Dimethylglyoxime?
While famous for its use with nickel, Dimethylglyoxime has other important applications in analytical and industrial chemistry. These include:
- Palladium Detection: It precipitates palladium from acidic solutions as a yellow compound, allowing for its separation and quantification.
- Hydrometallurgy: It is used in the recycling of lithium-ion batteries to selectively precipitate and recover nickel from waste cathode materials.
- Seawater Analysis: It serves as a reagent in kinetic experiments to study the reduction of iron and other trace transition metals in marine environments.
10. What is the difference between a simple ligand and a chelating agent like Dimethylglyoxime?
A simple ligand, or a monodentate ligand, is an ion or molecule that donates a pair of electrons from a single donor atom to a central metal ion. In contrast, a chelating agent like Dimethylglyoxime is a polydentate ligand, meaning it has two or more donor atoms that can bind to the same metal ion. This forms a stable, ring-like structure called a chelate. The formation of these rings, known as the chelate effect, results in a complex that is significantly more stable than one formed with similar monodentate ligands.
