Category: Chemistry Notes PPT

An ionic bond is defined as that bond between a metal and a non-metal which is responsible to hold the oppositely charged ions by the strong electrostatic force of attraction. The bond formed as a result of the transference of electrons from the outermost shell of metal to the outermost shell of a non-metal is alternatively known as an electrovalent bond.

There are two essential factors for Ionic Bond formation: 1. the metals participating in an ionic bond formation should have low Ionization Potential and 2. the non-metals participating in an ionic bond formation should have high Electron Affinity. Ionization Potential is the energy required to remove an electron from the outermost shell of an isolated gaseous atom. And finally, electron affinity is defined as the energy released in addition of an electron to the outermost shell of an isolated gaseous atom.

What are Electrovalent Compounds?

The chemical compounds formed as a result of the transfer of electrons from the outermost shell of metal to the outermost shell of a non-metal are called electrovalent compounds.

What are the Properties of Ionic or Electrovalent Compounds?

1. Ionic Compounds are Hard Solids. 

This is because their constituent particles are ions which are held by the strong electrostatic force of attraction and hence they cannot be separated easily.

2. Ionic Compounds Have A High Melting Point and High Boiling Point. 

They are non-volatile solids. As in these compounds, ions are held by the strong electrostatic force of attraction, so a large amount of energy is required to overcome these forces of attraction between the ions.

3. Ionic Compounds Do Not Conduct Electricity in Their Solid-State. 

However, they can conduct electricity in their fused, molten and in their aqueous solution. In solid-state, they do not conduct electricity as the ions are not free but held by the strong electrostatic force. But infused or molten state, these forces of attraction get weakened and thus the ions become free to conduct electricity. In aqueous solution, the high dielectric constant overcomes the strong electrostatic force of attraction, thus making the ions free to conduct electric current.

4. Ionic Compounds Act as Strong Electrolytes. 

As on dissolving in water, ionic compounds allow the passage of electric current through them due to the presence of free ions.

5. Ionic Compounds are Soluble in Water but Insoluble in Organic Solvents like Benzene and Phenol. 

As water has maximum dielectric constant, therefore it decreases the force of attraction between the ions and thus it forms free ions and hence they dissolve in water. But in organic solvents like Phenol, as the dielectric constant is minimum, it increases the force of attraction making the ions more strongly held by the electrostatic force which makes the compounds insoluble.

6. On passing an electric current through fused, molten and aqueous solution of electrovalent compounds, the ions dissociate and migrate towards electrodes.

7. Ionic compounds undergo fast reactions in their aqueous solution.

What is the Principle of Ionic or Electrovalent Compounds Formation?

An Ionic compound is formed when atoms of metals from Groups 1 to 3 in the periodic table loses electrons to the atoms of the non-metals from Groups 5 to 7 in the periodic table to complete their stable electronic configuration according to Duplet or Octet rule. The protons of these atoms remain constant during these transfers of electrons.

What is the Duplet Rule?

Elemental atoms generally lose, gain, or share electrons with other atoms in order to achieve the same electron structure as the nearest rare gas with two electrons in the outer level.

What is the Octet Rule?

Elemental atoms generally lose, gain, or share electrons with other atoms in order to achieve the same electron structure as the nearest rare gas with eight electrons in the outer level.

Example: Sodium Chloride Formation by Ionic Bond?

There is only one electron in the outermost shell of Sodium (Na) atom and if it loses that electron from its outermost shell i.e. the M shell then the L shell becomes the outermost shell and it has a stable octet. There are eleven protons in the nucleus of this atom but the number of electrons has become ten, this gives the sodium atom a positive charge and creates a sodium cation Na+. Whereas, on the other hand, there are seven atoms in the outermost shell of the Chlorine (Cl) atom then it needs one more electron to complete its octet. If Sodium and Chlorine reacted then the electron lost by sodium would be taken by Chlorine. After this process, the chlorine atom would have a negative charge, because it would have seventeen protons in its nucleus and there would be eighteen electrons in its K, L and M shells. This gives the Chlorine atom a negative charge. Hence, the negatively charged Chlorine and positively charged Sodium creates an Ionic bond or an Electrovalent compound called Sodium Chloride, commonly known as Common Salt.

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What are Cations and Anions

Ionic bonds involve a cation and an anion. The bond is formed when an atom, typically a metal, loses an electron or electrons and becomes a positive ion, or cation. Another atom, typically a non-metal, is able to acquire the electron(s) to become a negative ion or anion. Another example of an ionic bond is the formation of sodium fluoride, NaF, from a sodium atom and a fluorine atom. In this reaction, the sodium atom loses its single valence electron to the fluorine atom, which has just enough space to accept it. The ions produced are oppositely charged and are attracted to one another due to electrostatic forces.

What are the Differences between Electrovalent Compounds and Covalent Compounds?

1. Ionic compounds are solids whereas covalent compounds are liquids or very soft waxy solids.

2. Ionic compounds have a high melting point and high boiling point but covalent compounds have a low melting point and low boiling point.

3. Ionic compounds are soluble in water but covalent compounds are soluble in organic solvents like Benzene and Phenol.

4. Ionic compounds are insoluble in organic solvents like Benzene and Phenol but covalent compounds are insoluble in water.

5. Ionic compounds undergo fast reactions whereas covalent compounds proceed with slow reactions.

6. Ionic compounds conduct electricity in the molten, fused or aqueous solution due to the presence of free ions whereas covalent compounds do not conduct electricity because the ions are absent and only molecules are present.

What are Oxidation and Reduction?

Oxidation is the process of loss of electrons. It takes place at the anode. During oxidation, anion losses electrons and get converted to a neutral particle. 

Reduction is the process of gain of electrons. It takes place at the cathode. During reduction, cation gains electrons and get converted to a neutral particle.

Isoprene is a volatile and colourless liquid hydrocarbon. The compound is formed as a by-product of processing coal tar or petroleum. The compound is commonly used as a chemical raw material. Its molecular formula is C5H8.

Isoprenes are known as building blocks. They are responsible for the biosynthesis of the common terpenes. The primary role of isoprene is as a plant metabolite. It is usually termed as an alkadiene, hemiterpene, and volatile organic compound. The IUPAC name is 2-methyl-1,3-butadiene.

Introduction to Polyisoprene

Polyisoprene is a polymer of isoprene. It is a primary chemical constituent of natural rubber. Polyisoprene is a natural compound that also occurs in resins, balata and gutta-percha, and synthetic equivalents of the three materials. 

Based on the compound’s molecular structure, polyisoprene can be an elastic or a resilient polymer. For instance, natural rubber is a milky liquid, while isoprene rubber is a tough, leathery resin in natural and synthetic balata or gutta-percha.

Polyisoprene is built up from the linking of multiple isoprene molecules leading to four isomers, out of which the most important are the cis and trans isoprene.

What is Cis-Isoprene?

Natural rubber comprises exclusively of cis-1,4 polymer, produced in the milky latex of certain plants. The uniqueness of the natural rubber lies in its physical property of toughness and extensibility. 

In the absence of tensile stress, the polymer chains of the cis-1,4 polymer assume a disordered or amorphous arrangement. 

Natural rubber is said to be self-reinforcing. However, it gets greatly affected by temperature resulting in a sticky and inelastic substance. 

The polymerization of synthetic isoprene manufactures isoprene rubber. The rubber is acquired from the thermal cracking of the naphtha fraction of petroleum. The polymerization is conducted in solutions making use of the Ziegler-Natta and anionic catalysts.

The polymerization leads to cis-1,4 polyisoprene with irregular structure, which does not crystallize readily, and which is not tacky or firm as the raw material.

Isoprene rubber is a complete substitute for natural rubber, and its principal usage is in tires, rubber springs, and mountings. Footwear is an essential application of isoprene rubber.

What is Trans-Isoprene?

Trans-1,4 polyisoprene is the dominant isomer in the two materials that imitate natural rubber- balata and gutta-percha. These materials are derived from the milky exudate of certain plants or trees. 

Unlike the cis-1,4 polymer, the trans-1,4 polymer is highly crystalline. This property leads to the formation of tough, complex, and leathery materials. Their stealth properties, balata, and gutta-percha were commonly used in the 19th century as sheathings for underwater cables and golf balls. 

Ziegler-Natta catalysts can also be synthesized to obtain trans-1, 4 polymers, yielding a synthetic balata of similar properties employed in golf-ball covers and orthopedic devices braces and splints.

Introduction to Isoprene Terpene

Isoprene terpenes are the single largest class of compounds found in essential oils known as isoprenoids made up of isoprene molecules. Each isoprene molecule comprises five C atoms with double bonds. 

The simplest forms of terpenes are monoterpenes that consist of two isoprene molecules. Sesquiterpenes consist of three isoprene molecules, and diterpenes consist of four isoprene molecules. 

Terpenes are – cyclic and acyclic groups, which indicate their structure. Cyclic terpenes usually form a ring, while Acyclic terpenes include linear. A few terpenes occur as essential oils like the monocyclic, bicyclic, and tricyclic monoterpenes.

Terpene hydrocarbons are thermally labile and can be easily oxidized, and hold excellent anti-inflammatory, antiseptic, antiviral, analgesic properties, and antibacterial properties.

What is Isoprene Uses and Isoprene Price?

Due to the outstanding mechanical properties and low cost, isoprene rubber is the most preferred material for several engineering applications. The typical isoprene uses include tires, adhesives, anti-vibration mounts, springs, drive couplings, and bearings. The most significant portion of the produced NR and IR is used for tires as it is a usual blend with SBR and PBD rubber to achieve superior performance.

Polyisoprenoids are used for numerous rubber applications, including medical equipment, shoe soles, baby bottle nipples, elastic films, toys, and threads for golf balls or textiles. Other uses of isoprene include paints, adhesives, and coatings. Styrene-isoprene rubber is a copolymer whose primary use is in pressure-sensitive bonds.

Crude isoprene is used in electrical insulating tapes, cement, and cable wrapping.

The industrial uses of isoprene include processing aids- specific to petroleum, feedstock, processing aids, and intermediates. 

The standard and the latest isoprene price is ₹ 155 per  Kilogram. However, the other derivatives of isoprene vary based on the constituents and the method of production. 

Kerosene, or spelled kerosine is also called paraffin oil or paraffin or kerosene oil, is a flammable hydrocarbon liquid used commonly as a fuel. Typically, Kerosene looks pale yellow or colorless and exhibits a not-unpleasant characteristic odor. It is obtained from petroleum and can be used for burning in kerosene lamps and furnaces or domestic heaters, as a fuel component or fuel for jet engines, and as a solvent for insecticides and greases.

About Kerosene Oil

It was discovered in the late 1840s by a Canadian physician named Abraham Gesner. Initially, Kerosene was manufactured from shale oils and coal tar. However, in 1859, following the first oil well drilling in Pennsylvania by E.L. Drake, petroleum quickly became the primary source of Kerosene. Due to its use in lamps, Kerosene was considered as the major refinery product for many decades until the advent of the electric lamp reduced its lighting value. And, the production further declined as the rise of the automobile established gasoline as an essential petroleum product. Nevertheless, in several parts of the world, still, Kerosene is a common cooking fuel and heating fuel for lamps. Standard commercial jet fuel is importantly high-quality straight-run kerosene, and several military jet fuels are blends based on the kerosene chemical.

Properties of Kerosene

Chemically, Kerosene is a mixture of hydrocarbons. The chemical composition completely depends upon its source, however, it usually consists of about 10 different hydrocarbons, each containing ranging from 10 to 16 carbon atoms per molecule. The saturated straight-chain and branched-chain paraffin and ring-shaped cycloparaffin are the main components (which is also known as naphthenes). Kerosene is less volatile than gasoline. Its flash point (the temperature, where it generates a flammable vapor near to its surface) is 38 °C or higher, whereas that of gasoline is as low as −40 °C. This property makes Kerosene a relatively safe fuel to store and handle.

Kerosene oil is known to be one of the so-called middle distillates of crude oil along with diesel fuel, with a boiling point between around 150 and 300 °C. It can be formed as “straight-run kerosene,” which is physically separated from the other crude oil fractions by distillation, or it can be formed as “cracked kerosene,” by cracking, or chemically decomposing the heavier oil portions at elevated temperatures.

Illuminating Oil From Coal and Oil Shale

Although “coal oil” was well known by industrial chemists at least as early as the 1700s as a byproduct of making coal tar and coal gas, it burned with a smoky flame that can prevent its use for indoor illumination. In cities, most of the indoor illumination was provided by piped-in coal gas. Whereas, for spotlighting within the cities and outside the cities, the lucrative market for fueling indoor lamps was supplied by whale oil, particularly that from sperm whales, which burned cleaner and brighter.

Kerosene From Petroleum

Samuel Martin Kier began selling lamp oil in 1851 to local miners, in the name of “Carbon Oil”. He distilled this using a process on his own invention from the crude oil. He also invented a new lamp that helps to burn his product. He has been dubbed the American Oil Industry’s Grandfather by historians. The salt of Kier wells was becoming fouled with petroleum since the 1840s. At first, Kier just dumped the useless oil into the nearby Pennsylvania Main Line Canal, but later, he began experimenting with multiple crude oil distillates, along with a chemist from eastern Pennsylvania.

Applications of Kerosene

As Fuel

Heating and Lighting

The fuel, which is also called heating oil in Ireland and the UK, remains widely used in lanterns and kerosene lamps in the developing world. Although it replaced the whale oil, the Elements of Chemistry edition in 1873 said, “The vapor of kerosene substance mixed with air is explosive, the same as gunpowder.” This may have been because of the widespread method of adulterating Kerosene with hydrocarbon mixtures, such as naphtha, that are cheaper but more volatile. In 1880, Kerosene was a significant fire risk, where nearly two of every five New York City fires were caused by defective kerosene lamps.

Cooking

In countries such as Nigeria and India, Kerosene is the essential fuel used for cooking, especially by the poor. Also, Kerosene stoves have replaced the appliances of traditional wood-based cooking. As such, an increase in kerosene prices can have a primary environmental and political consequence. Also, the Indian government subsidizes the fuel to keep the price low, to around 15 US cents per liter as per 2007, February, as lower price discourages the dismantling of forests for cooking fuel. In Nigeria, an attempt made by the government to remove a fuel subsidy, including Kerosene, met with strong opposition.

Engines

Kerosene or tractor vaporizing oil (TVO) was used in the early to mid-20th century as a cheap fuel for tractors and hit ‘n miss engines. These engines would start on gasoline; then, it switches over to Kerosene once the engine warmed up. On a few generators, by heating kerosene to its vaporized point, a heat valve on the manifold will route exhaust gases near the intake pipe which could be ignited by an electric spark.

Law of Definite Proportion, also called Proust’s Law or Law of Constant Composition, defines that the elements that make up a chemical compound are usually arranged in a specified mass ratio regardless of the source or preparation. The law of definite proportion can also be expressed in another way.

How does the Law of Definite Proportions work?

Using the law of definite proportions, the composition of compounds will always be the same by mass. In Chemistry, stoichiometry is based on this law.

Statement: Chemical compounds consist of elements that are always present at fixed ratios (in terms of their mass) according to the law of definite proportions as well as the law of constant proportions. In this ratio, neither the source nor the method of preparation of the chemical compound is relevant.

Explanation: Chemical compounds, according to the law of constant proportions, are made of elements present in a fixed ratio by mass. The concentration of each element in a compound will always be the same by mass regardless of the source of the sample.

Nitrogen and oxygen atoms are always in a 1:2 ratio in the nitrogen dioxide molecule (NO2). Consequently, Nitrogen has the same structure as oxygen.

A Brief History of the Law of Definite Proportions

Proust’s Law

In the period between 1798-1804, French chemist Joseph Proust experimented with copper carbonate and water to develop a law of definite composition or proportions. Proust formulated his observations in what is now known as Proust’s Law in 1806. As determined by mass, chemical compounds are composed of constant and defined ratios of elements. As an example, carbon dioxide consists of one carbon atom and two oxygen atoms. As a result, carbon dioxide can be described by the fixed ratio of 12 (mass of carbon):32 (mass of oxygen), or simplified as 3:8.

Disagreements with the Proust’s Law

Several chemists disagreed with Proust’s theory at the time, particularly another French chemist, Claude Louis Berthollet. The French scientist believed that elements could mix in any ratio. A chemist called John Dalton proposed that chemical compounds were composed of atoms belonging to different elements. This idea was supported at an atomic level, however, as Dalton proposed that chemical compounds were composed of set formulas of atoms. In Dalton’s law of multiple proportions, elements can combine to yield new combinations of elements in a compound. In such a scenario, the ratio of the elements within a compound can be expressed as a whole number, which is an extension of Dalton’s law of definite composition.

Non-Stoichiometric Compounds/Isotopes

The Law of definite proportions is not true universally, despite its considerable usefulness in modern chemistry. Different samples of a compound may have different elemental compositions due to non-stoichiometry. Compounds like these are subject to the Law of Multiple Proportions. As an example, the iron oxide wüstite, which may contain anywhere between 23 and 25 oxygen atoms by mass, holds 0.83 to 0.95 iron atoms for each oxygen atom. It is given as FeO, but the crystallographic vacancies result in FeO.95O being the ideal formula. The measurements of Proust were generally not accurate enough to detect these differences.

Furthermore, the composition of the element can differ depending on its source; therefore, the mass of the element can differ even within a pure, stoichiometric compound. Due to processing in the atmosphere, astronomy, crust, oceans, and deep Earth that tend to concentrate few environmental isotopes, one can use this variation in radiometric dating. Except for hydrogen and its isotopes, most of the time, the effect is small, but the instrumentation of today allows us to measure it.

Polymers

Additionally, the composition of several natural polymers differs, even when they are considered “pure”. As a rule, polymer molecules are not considered pure chemical compounds except when their molecular weights are uniform (which is mono distribution), and their stoichiometry is constant. They still might be in violation of the Law in these rare cases due to the isotopic variations.

Do You Know the Ligand Definition?

A ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex in coordination chemistry. Formal donation of one or more of the ligand’s electron pairs, often via Lewis Bases, is necessary for bonding with the metal. Metal ligand bonding can be either covalent or ionic in nature. The metal-ligand bond order can also vary from one to three. Lewis bases are considered ligands, while Lewis acidic “ligands” have been found in rare cases.

As we already defined ligands, now let’s take a look at examples of neutral ligands (neutral molecule), cationic ligands, and neutral ligands.

What is Ligand in Chemistry?

Ligand Examples

Occasionally ligands can be cations (NO⁺, N₂H₅⁺) and electron-pair acceptors. F^–, Cl^–, Br^–, I^–, S₂^–, CN^–, NCS^–, OH^–, NH₂^– are examples of anionic ligands, while NH₃, H₂O, NO, and CO are examples of neutral ligands. 

Metals and metalloids are almost always bound to ligands, while gaseous “naked” metal ions can be formed in a high vacuum. Ligand substitution rates, ligand reactivity, and redox are all factors that affect the reactivity of the central atom in a complex. In several practical fields, such as bioorganic and medicinal chemistry, homogeneous catalysis, and environmental chemistry, ligand selection is important.

Types of Ligands 

1. Monodentate Ligands

Monodentate ligands are also known as “one-toothed” ligands because they only bite the metal atom in one place.

2. Bidentate Ligands

Bidentate ligands are Lewis bases that donate two lone pairs of electrons to the central metal atom. Chelating ligand is a term used to describe them. Chelates refers to a complex containing chelating ligands.

3. Tridentate Ligands and Polydentate Ligands

To the central metal atom or ion, tridentate ligands have three lone pairs of electrons. Tetradentate molecules have four donor atoms, pentadentate molecules have five donor atoms, and hexadentate molecules have six donor atoms. Polydentate ligands are a common term for them.

4. Trans- Spanning Ligands

Bidentate ligands that can span coordination positions on opposite sides of a coordination complex are known as trans-spanning ligands.

5. Ambidentate Ligand

Ambidentate ligands, unlike polydentate ligands, can bind to the central atom in two separate ways. Thiocyanate, SCN, is a clear example of this, since it may bind to either the sulfur or nitrogen atom. Linkage isomerism is caused by such compounds. Polyfunctional ligands, especially proteins, may form isomers by bonding to a metal centre via different ligand atoms.

6. Bridging ligand

A bridging ligand is a molecule that binds two or more metal centres. Coordination polymers, which are made up of metal ion centres connected by bridging ligands, make up virtually all inorganic solids with simple formulas. Both anhydrous binary metal ion halides and pseudohalides fall into this category. In solution, bridging ligands are also present. Since polyatomic ligands like carbonate are ambidentate, they often bind to two or three metals at the same time. The prefix “” is often used to denote atoms that bridge metals. The presence of many bridging ligands makes most inorganic solids polymers. Because of their possible use as building blocks for the fabrication of practical multimetallic assemblies, bridging ligands, which are capable of coordinating multiple metal ions, have attracted a lot of attention.

7. Binucleating Ligand

Binucleating ligands are molecules that bind two metal ions together. Binucleating ligands usually involve bridging ligands like phenoxide, or pyrazine, as well as other donor groups that only bind to one of the two metal ions.

8. Metal–Ligand Multiple Bonds

Some ligands can bind to a metal centre by using the same atom but a different number of lone pairs. The metal-ligand bond angle (MXR) can be used to determine the bond order of the metal-ligand bond. This bond angle is often referred to as either linear or bent, depending on the degree to which the angle is bent. In its ionic form, an imido ligand, for example, has three lone pairs. One lone pair serves as a sigma X donor, while the other two serve as L-type pi donors. The MNR geometry is linear when both lone pairs are used in pi bonds. If either or both of these lone pairs are nonbonding, the MNR bond is bent, and the degree of the bend is determined.

9. Spectator Ligand

A spectator ligand is a closely coordinating polydentate ligand that eliminates active sites on metal but does not participate in chemical reactions. The reactivity of the metal centre to which they are bound is influenced by spectator ligands.

10. Chiral Ligands 

Chiral ligands can be used to create asymmetry in the coordination sphere. In several cases, the ligand is used as an optically pure group. Asymmetry results from coordination in some situations, such as secondary amines. For homogeneous catalyzes, such as asymmetric hydrogenation, chiral ligands are used.

Did You Know?

The number of times a ligand binds to a metal via noncontiguous donor sites is referred to as denticity (represented by). Many ligands may bind metal ions at multiple sites, which is typically due to the presence of lone pairs on multiple atoms. Chelating ligands are those that bind to more than one molecule. Bidentate ligands bind to two sites, while tridentate ligands bind to three sites. The angle formed by the two bonds of a bidentate chelate is known as the “bite angle.” 

Chelating ligands are frequently made by joining donor groups together with organic linkers. Ethylenediamine, which is made by connecting two ammonia groups with ethylene (CH2CH2) linker, is a classic bidentate ligand. The hexadentate chelating agent EDTA, which can bind through six sites and fully surround certain metals, is an example of a polydentate ligand.

The Lucas Test is the test that is performed by using Lucas reagent with alcohols to distinguish primary, secondary and tertiary alcohols. In this carbocation is formed as intermediate and it follows a unimolecular nucleophilic substitution reaction mechanism. 

As primary, secondary and tertiary alcohols differ in their reactivity with Lucas reagent, so they give different results as well and it forms the base for Lucas Test. A positive test indicates the change in colour of the sample from clear and colourless to turbid signalling formation of a chloroalkane. 

Lucas test is performed to distinguish primary, secondary and tertiary alcohols and which alcohol gives the fastest alkyl halide. Lucas test is based on the difference in reactivity of alcohols with hydrogen halide. Primary secondary and tertiary alcohols react with hydrogen halide (hydrochloric acid) at different rates. It follows the SN1 reaction mechanism. 

The Lucas test was given by Howard Lucas in 1930. After that, it soon became popular in organic chemistry for qualitative analysis. Although with the discovery of spectroscopic and chromatographic methods of qualitative analysis in organic chemistry, this test has taken a back seat and is generally used for teaching purposes in schools and colleges.

 

The Lucas test is an important topic of Class XII Chemistry. Every year many questions are asked about this topic in the final exam. So, you need to give special attention to the preparation of this topic. In this article, we will discuss the Lucas test in detail with its mechanism.

What is Lucas Reagent? 

The solution of concentrated hydrochloric acid with zinc chloride is called Lucas reagent. Thus it can also be defined as a solution of anhydrous zinc chloride present in the concentrated Hydrochloric acid. It is used to classify the alcohols that have a lower molecular weight. It is a substitution reaction where the chloride finally replaces the hydroxyl group. The reaction results from the clear and colourless solution to the turbid that indicates the formation of chloroalkanes. In this reaction, the tertiary alcohols from their respective halides are much faster than the primary or the secondary alcohols because the intermediate tertiary carbocation is much stable in tertiary alcohol. Both concentrated HCl and ZnCl₂ are taken in equimolar quantities to make the reagent. 

Since the test was first conducted in the year 1930, the test became the standard test for identification of primary, secondary and test the test was first conducted in the year 1930 and since then it became the standard method for the identification of primary, secondary and tertiary alcohol. But with the development of new testing methods like spectroscopy and various chromatography, the Luca’s test is becoming less popular.   In the Lucas test, Lucas reagent reacts with alcohols and gives different results on the basis of stability of carbocation intermediate formed during the reaction. The Chloride ion of hydrochloric acid reacts with an alkyl group of alcohol and forms alkyl chloride while zinc chloride is used as a catalyst. The rate of reaction of primary, secondary, and tertiary alcohols with Lucas reagent differ which forms the base of the Lucas Test. The simple reaction involved is represented below –

ROH + HCl →  RCl + H₂O

(In presence of  ZnCl₂)

Lucas Test

Lucus test is performed to differentiate between primary, secondary and tertiary alcohols. This test is based on the difference in the reactivity of the primary, secondary and tertiary alcohol with hydrogen halide by SN1 reaction. 

ROH + HCl →  RCl + H₂O

(In presence of  ZnCl₂)

The difference in the reactivity of the degrees of alcohol provides the differing ease of formation of the corresponding carbocations. The primary carbocation is least stable followed by secondary carbocation and the tertiary alcohols form the most stable tertiary carbocations. 

The reagent for the test is the equimolar mixture of ZnCl₂ and concentrated HCl. the alcohol becomes protonated and the water molecule that is formed leaves, that makes the carbocation, and the nucleophile Cl⁻ (which is present in excess) readily attacks the carbocation, forming the chloroalkane. Due to the low solubility of the organic chloride in the aqueous mixture, the tertiary alcohol reacts immediately within five minutes that becomes evident by turbidity.  

How to Perform Lucas Test? 

Lucas test is performed by following steps –

• Preparation of Lucas Reagent – Take equimolar quantities of zinc chloride and concentrated HCl and make a solution.

• Take a very small quantity of the given sample in a test tube.

• Now add ~2ml of the Lucas reagent in the test tube containing the given sample and mix them.

• Record the time until the solution become turbid or cloudy.

Result of the Lucas Test if Sample Contains 1° Alcohol

If the sample contains primary alcohol, then it will not give a turbid or cloudy solution as a result at room temperature. If we give heat to the solution, then after 30-45mins turbidity comes. General reaction can be represented as follows –

Sample containing primary alcohol + Lucas Reagent 🡪 No turbidity in the solution

After giving heat/ 30-45min –

Sample containing primary alcohol + Lucas Reagent 🡪 Turbidity in the solution

For example, if an ethanol solution reacts with Lucas reagent at room temperature, then it doesn’t give any turbid solution. 

Result of Lucas Test if Sample Contains 2° Alcohol 

If the sample contains secondary alcohol, then the test will give a turbid or cloudy solution as a result at room temperature after 3-5minutes. General reaction can be represented as follows –

Sample containing secondary alcohol + Lucas Reagent 3-5min.→ Turbidity in the solution

For example, if isopropyl alcohol is present in the sample solution then after adding Lucas reagent in it, it will give a turbid solution after 3-5min. The reaction is given below –

(CH₃)₂CHOH + HCl+ZnCl2→ (CH₃)₂CHCl + H₂O + ZnCl₂

Isopropyl alcohol                 2°alkyl chloride (turbid solution)

Result of the Lucas Test if Sample Contains 3° Alcohol 

If the sample contains tertiary alcohol, then the test will instantly give a turbid or cloudy solution as a result at room temperature. General reaction can be represented as follows:-

Sample containing tertiary alcohol + Lucas Reagent Instantly→ Turbidity in the solution

For example, if tertiary butyl alcohol is present in the sample solution then after adding Lucas reagent in it, it will give a turbid solution instantly. The reaction is given below –

(CH₃)₃COH + HCl + ZnCl2 → (CH₃)₃CCl + H₂O + ZnCl₂

Explanation of Difference in Reactivity of 1°,2° & 3° Alcohols with Lucas Reagent 

The reaction of primary, secondary and tertiary alcohols with Lucas reagent takes place through a unimolecular nucleophilic substitution reaction mechanism. Lucas reagent forms carbocation as intermediate with all three alcohols. But the stability of carbocation intermediate differs in all three reactions. Tertiary alcohol gives instant results with Lucas reagent as its carbocation is highly stable. While secondary alcohol gives results with Lucas reagent after 3-5mins as its carbocation intermediate is moderately stable and primary alcohol don’t give any result with Lucas reagent at room temperature because its carbocation is highly unstable. Thus, we can write stability of carbocation intermediate of primary, secondary and tertiary alcohol is –

3°>2°>1°

Mechanism of Lucas Test Reaction

The reaction generally occurs in the SN1 nucleophilic reaction which is a two step reaction. The alcohols that are highly reactive become carbocation intermediates and then exhibit an immediate reaction. The SN1 nucleophilic reaction is exhibited by the primary and the secondary alcohols.

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The two-step is generally as follows:-

1. In the first step, the H⁺ proton from the hydrogen halide will first protonate the negatively charged hydroxyl ion that is OH⁻ in the alcohol. Thus the water, thus, attached to the carbon atom is a weaker nucleophile than the chlorine ion and thus the chlorine ion replaces the water. This results in the formation of a carbocation. 

2. In the second step the Cl⁻ attacks the carbocation and thus forms alkyl chloride.

Loss of leaving group and formation of carbocation – In this step zinc chloride reacts with alcohol and forms carbocation intermediate and loss of leaving group takes place. ZnCl₂ behaves as lewis acid. Zinc gains electrons from the oxygen atom and gets bonded with it. Thus, zinc gets a negative charge while oxygen atom gets a positive charge. Now the electron-deficient oxygen atom being an electronegative element gains electrons from the alkyl group. It leads to the formation of a carbocation. This is the slowest step of the reaction. So, it is the rate-determining step. Thus, we can say the rate of reaction depends on the formation of carbocation and its stability. The reaction is given below –

Nucleophilic attack – Cl⁻ acts as nucleophile and attacks on carbocation and forms alkyl chloride. Due to the higher entropy of water, H⁺ of HCl reacts with the hydroxyl group and forms water. Catalyst zinc chloride gets removed as it is. 

Applications of Lucas Test

Lucas test has the following applications –

It is used to distinguish primary, secondary and tertiary alcohols in the sample.

• It gives information about which alcohol gives the fastest alkyl halides. By Lucas test, we can write the order of giving alkyl halides by primary, secondary and tertiary alcohols. Tertiary alcohol gives the fastest alkyl halide.

 

3°>2°>1°