Chromyl Chloride Test


Chromyl Chloride Test – Definition

The inorganic compound chromyl chloride (\[CrO_{2}Cl_{2}\]) has the formula \[CrO_{2}Cl_{2}\]. It's a reddish-brown liquid at ambient temperature, which is rare for transition metal complexes.


  • The action of HCl on \[CrO_{3}\] produces \[CrO_{2}Cl_{2}\].

  • Concentrated HCl is used to treat (\[K_{2}CrO_{4}\]), followed by the addition of \[H_{2}SO_{4}\] as a dehydrating agent.

  • \[CrO_{2}Cl_{2}\] separates as a dense, immiscible liquid.It can then be separated with a separatory funnel or by simple distillation.

  • Other chlorinating agents, such as \[PCl_{5} and TiCl_{4}\], can also be used to make \[Cr O_{2} Cl_{2}\].

Properties and Mechanism of Test

Chemical properties

  • \[CrO_{2}Cl_{2}\] is an aggressive oxidizing agent that is extremely electrophilic.

  • The quick hydrolysis of chromic and hydrochloric acids demonstrates its electrophilicity:

\[CrO_{2}Cl_{2} + 2H_{2}O \rightarrow  H_{2}CrO_{4} + 2HCl\]

  • The fact that \[CrO_{2}Cl_{2}\] smells in damp air further demonstrates its strong reactivity toward the water.

Chromyl Chloride Test for Chloride

The chromyl chloride test is a qualitative analysis test that determines how \[Cl^{–}\] ions are conformed.

Eg. Heat a sample of chlorine-containing salt with potassium chromate (\[K_{2}Cr_{2}O_{7}\]) and concentrated sulphuric acid ( \[H_{2}SO_{4}\]). 

If there is a chloride present, chromyl chloride will be formed and red fumes will come out. Test reaction for Chromyl chloride is;

\[K_{2}Cr_{2}O_{7} + 4NaCl + 6H­_{2}SO_{4} \rightarrow  2CrO_{2}Cl_{2} + 2KHSO_{4} + 4NaHSO_{3} + 3H_{2}O\]

Fluorides, bromides, iodides, and cyanides do not create similar compounds, hence this test is selective for chlorides.

Reagent for Oxidation of Alkenes

\[CrO_{2}Cl_{2}\] oxidizes terminal alkenes to aldehydes depending on the solvent. Alpha-chloro ketones and similar compounds are produced by internal alkenes.

It can also attack benzylic methyl groups and produce aldehydes through the Etard Reaction.

Compatible Solvents

Because \[CrO_{2}Cl_{2}\] is such a strong reagent, careful solvent selection is required. \[CrO_{2}Cl_{2}\] is expected to break down when exposed to alcohols due to its high reactivity toward the water, comparable to the behavior of other highly electrophilic chlorides such as \[VOCl_{3}, TiCl_{4}, and SO_{2}Cl_{2} \].

Chlorocarbons, particularly dichloromethane, are excellent solvents, as are other electrophilic chlorides.

\[CrO_{2}Cl_{2}\] attacks most grease, which adds to the practical complications.

FAQs on Chromyl Chloride Test

1. What is an Alternative Test for Chlorides Other than Chromyl Chloride Test?

Here is an alternate test for chlorides which is feasible for heavy metal chlorides too. We have to prepare soda extract of the given salt. NaCl is produced even for the covalent chlorides. Sometimes we need to melt them due to the insoluble chlorides. Then we need to follow with the dilution of the extract using the water and filter it. Further, add \[AgNO_{3}\] (silver nitrate) to the solution and it produces a white precipitate AgCl. For confirming chloride, we can add sodium arsenite Na3AsO3 to the solution and shake it well until a yellow precipitate is formed. This confirms the presence of chloride.

2. What are the Risks Associated with the Handling of Chromyl Chloride Tests?

We should take safety precautions while handling the chromyl chloride tests and Chromyl Chloride hydrolyses to release HCl and hexavalent chromium. The chromyl chloride vapour is known to irritate the respiratory system and eyes. Also burning of skin and eyes takes place when these come in contact with the liquid. Its ingestion can also cause severe internal damage. In chronic conditions, it can generate chromosomal aberrations and act as a human carcinogen via inhalation. Ulceration may also be caused due to frequent exposure of the skin to chromyl chloride.

3. What would happen if anyone inhales Chromyl Chloride accidentally?

Inhalation can result in a severe allergic respiratory reaction. Respiratory burns are produced by chemical burns. Internal damage would result from ingestion.

Chronic: May harm the liver and kidneys. Chronic exposure to water-insoluble hexavalent chromium compounds has been linked to lung cancer and tumors of the gastrointestinal tract.

Hydrochloric acid (HCl) and chromic acid are produced when \[Cr O_{2} Cl_{2}\] is hydrolyzed (\[H_{2}CrO_{4}\]). Although it is not combustible, chromyl chloride aids in the burning of other chemicals and reacts aggressively with water. In the event of a fire, avoid making direct contact with water.

4. What is toluene and what happens it reacts with chromyl chloride?

Toluene is a substituted aromatic hydrocarbon that is also known as toluol. It's a clear, water-insoluble liquid that smells like paint thinner. It's a phenyl group connected to a methyl group (CH₃) in a mono-substituted benzene derivative.

In the presence of non-polar solvents like carbon tetrachloride, toluene interacts with chromyl chloride to create benzaldehyde. 

Chromyl chloride forms a chromium compound by changing the methyl group. To make benzaldehyde, the chromium compound is hydrolyzed in acid.

5. What happens if Toluene is exposed to air?

The following symptoms may appear immediately or shortly after exposure to toluene levels in the air 100,000 ppbv:

  • Tiredness, dizziness, headache, loss of coordination or hearing, euphoria, sleeplessness, and nausea are some of the symptoms that can occur.

  • Irritation of the eyes and nose

  • At 4,000 ppm, there could be a rapid delay in reaction time, unconsciousness, and death.

The most severe side effects of toluene exposure have been observed among toluene addicts. The majority of studies of workers exposed to moderate amounts of toluene reveal no negative impact on their health.

6. What is Chromic acid?

Chromic acid is a chemical that comes in a variety of shapes and sizes. It's created by combining acids with chromates or dichromates, or by combining sulfuric acid with chromium(VI) oxide. 

It has high oxidizing power. It turns green as it is diminished. It's used to oxidize a variety of compounds, including alcohol. Because it contains the chromate ion, it is extremely poisonous. Either a reddish solution or red flakes are present. Chromium chloride is made from it.

7. What are Chromate and Dichromate?

The ion chromate is made up of:

  • one chromium atom (in its +6 oxidation state) and 

  • four oxide atoms 

\[CrO_{4}\]is its chemical formula. Dichromate is identical to chromium oxide, but it has two chromium atoms and seven oxide atoms (therefore, \[Cr_{2}O_{7}\]).

In acidic conditions, chromate and dichromate oxidize rapidly, but very slowly in basic conditions. Dichromates are orange or red in colour, whereas chromates are bright yellow. They can be made in a basic environment by reacting chromium(III) oxide with a metal oxide.