Lucas reagent and test for alcohols

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What is lucas reagent?

Lucas reagent is a mixture of anhydrous zinc chloride and concentrated hydrochloric acid, used to identify and classify primary, secondary, and tertiary alcohol. To prepare lucas reagent, both HCl and zinc chloride are taken in equimolar quantities and mixed together. Lucas reagent reacts with different alcohol to give different results based on the stability of carbocation intermediate formed during the reaction.

The lucas test was first proposed by Howard Lucas in the year 1930, from then on it was popularly used in organic chemistry for qualitative analysis until recently. But with the advancement at the technological front, qualitative analysis in organic chemistry moved towards use of spectroscopic and chromatographic methods. The lucas test is no longer popular in today’s modern era and is restricted only for teaching in educational institutions.

Lucas test

The lucas test is used to show the difference in reactivity of different types of alcohol, namely primary, secondary, and tertiary. It also displays the difference in the ease at which the corresponding carbocations of the alcohol is formed. This can be explained with an example, primary alcohol doesn’t show any quick reaction at room temperature when the lucas reagent is added to it, while the tertiary alcohol reacts to lucas reagent almost immediately. Hence, it can be summarized that the lucas test can be used to differentiate different types of alcohols based on their reaction speed, which is done by measuring the time taken for the clear solution to develop turbidity.

The lucas test observation for different types of alcohol is discussed below.

Primary AlcoholPrimary alcohol is identified when the solution remains colorless unless subjected to heat. The solution develops an oily layer when heated. Example, 1-pentanol
Secondary AlcoholIt is identified as a secondary alcohol when the solution develops an oily layer within a few minutes. It usually takes about 3-5 minutes to form the oily layer based on the solubility. Example, 2-pentanol
Tertiary AlcoholIt is identified as tertiary alcohol when the solution turns turbid and develops an oily layer immediately. Example, 2-methyl-2butanol

Lucas test mechanism

The reaction that normally occurs in the lucas test mechanism is the SN1 nucleophilic substitution. The SN1 reaction is substitution reaction in organic chemistry that involves a nucleophile replacing the leaving group. In the lucas reaction, chloride ion of HCl substitutes the hydroxyl group of the alcohol. This reaction takes place in two steps.

Step1

Formation of carbocation

The proton (H+) belonging to the hydrochloric acid will protonate the -OH group of the alcohol. Now, since chlorine is a stronger nucleophile than water attached to the carbon, it replaces the H2O group forming a carbocation. This step of formation of the carbocation is the slowest step of the reaction, hence, it is the rate-determining step. Therefore, the rate of this reaction depends on the formation of carbocation and its stability.

Step 2

Nucleophilic attack

In this step, the Cl- attacks the carbocation to form alkyl chloride. The alkyl chloride formed is insoluble in water and turns the solution turbid.

The lucas mechanism can be explained with the following reaction.

Preparation of lucas reagent

Lucas reagent can be easily prepared in the laboratory using the following method and calculations.

  • To prepare lucas reagent, you will require concentrated hydrochloric acid (HCl) and anhydrous zinc chloride (ZnCl2).
  • Measure out 47 ml of concentrated HCl and pour it in a 100 ml beaker.
  • Place the beaker in the ice bath. This is done to control and absorb the heat which will be generated further during the dissolution of ZnCl2.
  • 62.5 grams of anhydrous ZnCl2 should be carefully weighed and allowed to dry in an oven for minimum of 2 hours.
  • After about 2 hours, ZnCl2 is allowed to cool in a desiccator to prevent air contact.
  • Add the ZnCl2 mixture to the HCl kept in the ice bath. Slowly add the mixture with mild stirring.
  • Stir the mixture well until ZnCl2 is completely dissolved.
  • Your lucas reagent is now ready. Store it in a cool and dry place for further use.

Lucas reagent MSDS

Lucas reagent is a toxic and corrosive solution; hence, it should be used with utmost safety. The toxicity is due to the presence of the constituents present in the solution.

Concentrated hydrochloric acid

Concentrated hydrochloric acid is a highly corrosive chemical and may cause burn on eyes, skin, digestive tract, and respiratory tract. It is fatal even if inhaled or swallowed.

Potential health risks

Eye: Exposure to fumes causes severe irritation and redness of the eyes. Contact with the solution causes severe burns and may also lead to an irreversible eye injury in extreme cases.

Skin: Direct contact of the liquid on the skin causes severe burns. The severity of the burns depends on the concentration of the solution and duration of exposure.

Ingestion: Mild exposure may cause abdominal pain and vomiting, while severe cases may lead to burns in the digestive tract and sometimes even death.

Inhalation: Inhalation of the fumes, vapors, and mist may cause severe irritation of the respiratory tract with coughing and shortness of breath. It also causes burns and corrosion of the respiratory tract in extreme cases of exposure.

Chronic effect: Long-term and repeated exposure can dermatitis and erosion of teeth. Cases of bronchitis and gastritis have also been reported.

Anhydrous zinc chloride

Zinc chloride is toxic and causes burns by all kinds of exposure.

Potential health risk

Eye: Causes eye burns.

Skin: Causes skin burns.

Ingestion: Causes gastrointestinal tract burns.

Inhalation: Causes chemical burns to the respiratory tract.

Chronic: No information found yet.

Since the constituents of the lucas reagent are highly toxic and corrosive, use the necessary PPEs while dealing with them. Wear a mask, gloves, goggles, and apron without fail. In the case of chemical exposure, give the necessary first aid, and immediately take for medical help.

Handling and storage

The freshly prepared Lucas reagent should be carefully transferred in a brown storage bottle and kept in a cold, dry place away from sunlight. The bottle should be labeled immediately so that it doesn’t get mislabeled. Due to the corrosive nature of the reagent, it is advised to use them with utmost safety. Never pipette out the solution by mouth, use a pipette bulb, and wear all the required PPE to avoid any kind of misfortune.