Introduction
The organic iodine substance iodoform (also known as triiodomethane and, incorrectly, carbon triiodide) has the formula \[CHI_{3}\]. It is a pale yellow, crystalline, volatile chemical with a deep and characteristic odour (the scent is frequently referred to as that of hospitals, where the compound is still commonly employed) and a sweetish taste, similar to chloroform. It's sometimes used as a disinfectant.
Applications of Iodoform
The chemical is used as a disinfectant on a modest basis. It was employed in medicine as a healing and antiseptic treatment for wounds and sores around the turn of the twentieth century, but superior antiseptics have since supplanted it. Along with zinc oxide and propionic acid, it is the active ingredient in many ear powders for dogs and cats, which intend to prevent infection as it makes hair removal easier.
Background of Iodoform
Iodoform is a tetrahedral molecular geometry organoidine molecule with the formula \[CHI_{3}\]. It's a yellow solid that's generally insoluble in water and chemically reactive in free-radical processes. Minimal quantities of iodoform may be found in disinfectants due to its antibacterial effects upon topical treatment, and it is mostly utilized for veterinary purposes. Due to its radiopacity, iodoform has also been detected in dental paste and root canal filling materials in combination with other intracanal drugs. Iodoform has been used as a healing and antiseptic dressing or powder for wounds and sores since the turn of the twentieth century, although the clinical application has been limited to this point. When it is in touch with secretions or endodontic infections, iodoform is soluble in fatty acids and decomposes, releasing iodine in its initial stage.
Preparation and Properties of Iodoform
Iodoform can be manufactured using the electrolysis of an aqueous solution that contains inorganic iodides, carbonate'>sodium carbonate, and acetone. Iodoform is also referred to as carbon tri-iodide, methyl tri-iodide, and tri-iodomethane. These names are such due to the presence of 3 iodine molecules in the compound. This dressing wound chemical causes a burning sensation when applied to open wounds. Moreover, this compound's molecular weight is given as 393.73 g/mol and a melting point of 121°C. This is a highly pure substance with a purity percentage of 98%, where this value is much higher than most of the other substances. The key features of this compound are given as high purity, effectiveness, balanced composition. It is a very skin-friendly chemical because it does not cause any irritation or itching except that it causes a burning sensation on the open wounds.
Synthesis and Reactions
The synthesis of iodoform compound was first described in 1822 by Georges-Simon Serullas, by the reactions of iodine vapour with the steam over red-hot coals, including the reaction of potassium with the ethanolic iodine compound in the presence of water; and independently by John Thomas Cooper, as well. It can be synthesized using the haloform reaction with sodium hydroxide and iodine reaction with any of these 4 kinds of organic compounds:
acetaldehyde (CH3CHO)
a methyl ketone (CH3COR)
certain secondary alcohols (CH3CHROH, R is either an aryl or alkyl group)
ethanol (CH3CH2OH)
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The iodine and base reaction with the combination of methyl ketones is more reliable compared to the iodoform test (which is in yellow precipitate appearance) can be used to probe a methyl ketone presence. This also falls under the case when testing particular secondary alcohols containing at least one methyl group in the alpha position.
A few of the reagents (for example, hydrogen iodide) converts the iodoform to diiodomethane. Besides, converting into carbon dioxide makes it possible when Iodoform reacts with aqueous silver-nitrate'>silver nitrate to form carbon monoxide. When treated with the powdered elemental silver, the iodoform gets reduced, forming acetylene. Moreover, the iodoform compound decomposes to form hydrogen iodide gas, diatomic iodine, and carbon upon heating.
Uses of Iodoform
On a small scale, iodoform can be used as a disinfectant. It was also used as a component in the 20th century in medicines for healing and antiseptic dressing of sores and wounds. It was used for sterilizing the instruments that are used for surgery. There exist many side effects associated with iodoform, and hence its use has now been suppressed with the evolution and adaption of new antiseptics. It is said as an active ingredient for making dog and cat powders along with propanoic acid and zinc oxide, which are used to prevent infection. It can also be used to facilitate the removal of ear hair.
Acetone vs. Methanol
Let us distinguish between the elements Methanol and Acetone in brief.
Acetone produces crystals with NaI (sodium iodide): Add NaI solution, Boil until dissolution, then cool it to -5°C. If we get orange crystals, it is identified as acetone. Also, we need to check the book by Armarego and Perrin on laboratory chemicals purification for the proportions. This is one of the methods to result in water-free acetone.
It is also possible that a few metal salts will provide us with differently coloured complexes, such as cobalt bromide (CoBr2), blue in acetone, and pink in methanol. There is also the fact that ethanol and CoBr2 results in a pink solution, so we can expect methanol to be the same, but it might not be the same case. Probably, CoCl2 can be used as well. It is better to use the dry salts (blue for CoCl2, green for CoBr2), but it might work with the hydrated salts (pink - in both cases).
Iodoform Test for Ketones
Iodoform test is the one that is used to detect ketones and aldehydes that have an alpha-methyl group. Alpha means that it is attached to the carbon, including the functional group. The reagents used here are given as sodium hydroxide (NaOH) and iodine.
The compounds that result in a positive iodoform test are Ethanal (Acetaldehyde), Alpha Methyl groups, and Methyl ketones.
Ethanal is given as the only aldehyde that produces a positive iodoform test. Therefore, aldehyde and ketone with structure -COCH3 also exhibit positive results.
FAQs on General Characteristics of Iodoform
1. What are the main physical and chemical characteristics of iodoform?
Iodoform, with the chemical formula CHI₃, is a pale yellow, crystalline solid. It is known for its distinct, penetrating and somewhat sweetish odour, often associated with hospitals. Key characteristics include:
- Physical State: It is a volatile solid at room temperature.
- Solubility: It is practically insoluble in water but dissolves in organic solvents like ethanol and ether.
- Structure: It possesses a tetrahedral molecular geometry, similar to chloroform.
- Chemical Reactivity: It is chemically reactive in free-radical reactions and decomposes upon heating to produce diatomic iodine, hydrogen iodide gas, and carbon.
2. What is iodoform (CHI₃) primarily used for, and why is its medical use now limited?
Iodoform is primarily used on a small scale as a disinfectant and antiseptic. Historically, it was a common dressing for wounds and sores. However, its use in human medicine is now largely superseded by more effective antiseptics that do not have its strong, lingering odour or potential for skin irritation. It is still found in some veterinary products, such as ear powders for dogs and cats, to prevent infection.
3. What is the principle of the iodoform test and which functional groups does it detect?
The iodoform test is a qualitative analysis used to identify the presence of specific functional groups in an organic compound. The principle is based on the haloform reaction. It gives a positive result (a yellow precipitate of iodoform) for compounds containing:
- A methyl ketone group (CH₃CO-R)
- Acetaldehyde (CH₃CHO), the only aldehyde to give a positive test.
- Alcohols that can be oxidised to a methyl ketone under the test conditions, specifically ethanol (CH₃CH₂OH) and secondary alcohols with a methyl group on the carbon bearing the hydroxyl group (CH₃CH(OH)-R).
4. How does iodoform work as an antiseptic?
The antiseptic property of iodoform is not due to the compound itself but rather to the slow release of free iodine when it comes into contact with skin, wounds, or bodily secretions. This liberated iodine is a powerful oxidizing agent that denatures the essential proteins within microorganisms, leading to their death. This gradual release provides a sustained antiseptic action at the site of application.
5. Why do ethanol and certain secondary alcohols give a positive iodoform test, but others like methanol or 1-propanol do not?
The iodoform reaction requires the presence of a methyl group attached to a carbonyl carbon or a carbon that can be oxidized to a carbonyl.
- Ethanol (CH₃CH₂OH) is oxidized by the reagents (iodine and NaOH) to acetaldehyde (CH₃CHO), which has the required CH₃CO- structure.
- Secondary alcohols like 2-propanol (CH₃CH(OH)CH₃) are oxidized to methyl ketones (like acetone, CH₃COCH₃), which also react.
- Methanol (CH₃OH) and 1-propanol (CH₃CH₂CH₂OH) do not give a positive test because upon oxidation, they form formaldehyde and propanal, respectively. Neither of these products contains the necessary CH₃CO- group required to form the yellow iodoform precipitate.
6. How is iodoform prepared in the laboratory?
Iodoform is prepared in the laboratory using the haloform reaction. The synthesis involves reacting a compound containing a methyl ketone group or a structure that can be oxidised to it (like ethanol or acetone) with iodine in the presence of an aqueous alkali, such as sodium hydroxide (NaOH). The reaction results in the formation of a yellow, solid precipitate of iodoform (CHI₃), which can be easily filtered and identified.
7. What is the difference in the reaction of iodoform with powdered silver versus aqueous silver nitrate?
The reaction of iodoform with these two reagents is fundamentally different and showcases its distinct chemical properties:
- With Powdered Silver (Ag): Iodoform undergoes a dehalogenation coupling reaction. When heated with powdered silver, two molecules of iodoform react to form acetylene gas (C₂H₂).
- With Aqueous Silver Nitrate (AgNO₃): Iodoform undergoes hydrolysis. It reacts with aqueous silver nitrate to form a yellow precipitate of silver iodide (AgI). The carbon-hydrogen bond is also broken, leading to the eventual formation of carbon monoxide.
