Introduction
Haloalkanes, also known as alkyl halides, are a class of organic compounds that contain halogens (fluorine, chlorine, bromine, or iodine) attached to carbon atoms. They are derived from alkanes by the substitution of one or more hydrogen atoms with halogen atoms. Haloalkanes are an important group of compounds in organic chemistry and find applications in various fields, including pharmaceuticals, agrochemicals, solvents, and industrial processes. They have diverse chemical and physical properties that make them useful in different contexts. The general chemical formula of a haloalkane is R-X, where R represents an alkyl group (a chain of carbon atoms) and X represents a halogen atom. The halogen atom is covalently bonded to the carbon atom, resulting in the formation of a polarized carbon-halogen bond. The polarity of this bond gives haloalkanes distinct reactivity and properties compared to other organic compounds. Haloalkanes can exist as gases, liquids, or solids depending on the number of carbon atoms and the nature of the attached halogen. Their physical properties such as boiling points, melting points, and solubilities vary widely based on factors such as molecular size, type of halogen, and intermolecular forces. The reactivity of haloalkanes arises from the presence of the polarized carbon-halogen bond. They can undergo various chemical reactions such as nucleophilic substitution, elimination, and reaction with metals. These reactions can lead to the formation of new organic compounds or functional group transformations.
Nomenclature of Monohaloalkanes
The nomenclature of monohaloalkanes follows the International Union of Pure and Applied Chemistry (IUPAC) system. Here‘s a general guide to naming monohaloalkanes:
1. Identify the parent chain :Locate the longest continuous chain of carbon atoms that contains the halogen atom. This chain will serve as the parent chain for naming.
2. Number the carbon atoms :Assign numbers to the carbon atoms in the parent chain, starting from the end nearest to the halogen atom. The halogen atom is assigned the lowest possible number.
3. The halogen: Replace the ” -ine ” ending of the halogen name with “-o” and add it as a prefix before the parent chain name. The prefixes for halogens are as follows: fluoro (-F), chloro (-Cl), bromo (-Br), and iodo (-I).
4. Indicate the location of the halogen :Use the numbers assigned to the carbon atoms to indicate the position of the halogen atom in the parent chain. Include the position number as a prefix before the halogen prefix.
5. Add other substituents :If there are any other substituents or functional groups present, name and number them accordingly.
Here‘s an example to illustrate the nomenclature:
CH3-CH2-CH(Cl)-CH2-CH3 3-chloropentane
Isomerism of Monohaloalk
Isomerism occurs when two or more compounds have the same molecular formula but differ in the arrangement of atoms or connectivity. Monohaloalkanes can exhibit two types of isomerism:
1. Structural Isomerism : Monohaloalkanes can have structural isomers when the carbon atoms and the halogen atom are arranged differently. For example, 1-chlorobutane and 2-chlorobutane are structural isomers of each other.
2. Stereoisomerism : Stereoisomerism occurs when the connectivity of atoms remains the same, but the spatial arrangement of atoms differs. Monohaloalkanes can exhibit stereoisomerism if they have chiral carbon atoms (carbon atoms bonded to four different groups). In this case, two enantiomers, which are non-superimposable mirror images of each other, can be formed.
Classification of Monohaloalkanes
Monohaloalkanes can be classified based on the nature of the carbon atom to which the halogen is attached. The classification includes primary (1°), secondary (2°), and tertiary (3°) monohaloalkanes:
1. Primary (1°) Monohaloalkanes :In primary monohaloalkanes, the halogen atom is bonded to a carbon atom that is directly attached to only one other carbon atom.
2. In secondary monohaloalkanes : the halogen atom is bonded to a carbon atom that is directly attached to two other carbon atoms.
3. (3°) Monohaloalkanes : In tertiary monohaloalkanes, the halogen atom is bonded to a carbon atom that is directly attached to three other carbon atoms.
The classification of monohaloalkanes is based on the number of carbon atoms bonded to the carbon atom with the halogen atom. It plays a role in determining the reactivity and behavior of these compounds in various chemical reactions.
Methods of preparation:
Monohaloalkanes can be prepared from alkanes, alkenes, and alcohols through different methods. Here are the commonly used methods for their preparation:
1. Preparation from Alkanes : Monohaloalkanes can be synthesized from alkanes through a process called free radical halogenation. In this method, an alkane reacts with a halogen (chlorine, bromine) in the presence of heat or light. The reaction involves a chain initiation, propagation, and termination steps. For example:
CH4 + Cl2 →CH3Cl + HCl
In this reaction, methane (alkane) reacts with chlorine gas to produce chloromethane (monohaloalkane) and hydrogen chloride.
2. From Alkenes : Monohaloalkanes can be synthesized from alkenes through a reaction known as electrophilic addition. In this process, an alkene reacts with a halogen in the presence of a suitable catalyst. The double bond in the alkene breaks, and a halogen atom adds to each carbon atom, resulting in the formation of a monohaloalkane. For example:
CH2=CH2 + Br2 →CH2BrCH2Br
In this reaction, ethene (alkene) reacts with bromine to yield 1,2-dibromoethane (monohaloalkane).
3. Preparation from Alcohols : Monohaloalkanes can be prepared from alcohols through a substitution reaction called nucleophilic substitution. The reaction involves the replacement of the hydroxyl group (-OH) of the alcohol by a halogen atom (-X). The reaction is typically carried out in the presence of a strong acid, such as hydrochloric acid (HCl) or phosphorus halides (e.g., PCl5, PBr3). For example:
CH3CH2OH + HCl →CH3CH2Cl + H2O
In this reaction, ethanol (alcohol) reacts with hydrochloric acid to yield chloroethane (monohaloalkane) and water.
Physical Properties of Mono halo alkanes:
Monohaloalkanes, also known as alkyl halides, exhibit certain physical properties based on their molecular structure and the nature of the halogen atom. Here are some common physical properties of monohaloalkanes:
1. State of Matter : Monohaloalkanes can exist as gases, liquids, or solids depending on the number of carbon atoms in the molecule. Generally, those with smaller alkyl chains (fewer carbon atoms) tend to be gases or volatile liquids, while those with longer alkyl chains are more likely to be liquids or solids at room temperature.
2. Boiling Point : The boiling point of monohaloalkanes generally increases with increasing molecular weight. This is due to the increase in London dispersion forces (van der Waals forces) between molecules as the size and surface area of the molecules increase. Additionally, the strength of the dipole-dipole interactions also plays a role, with larger halogen atoms generally leading to stronger intermolecular forces and higher boiling points.
3. Melting Point : The melting points of monohaloalkanes can vary depending on the size and nature of the alkyl chain and the halogen atom. Generally, monohaloalkanes with larger halogen atoms have higher melting points due to stronger intermolecular forces. Branched monohaloalkanes tend to have lower melting points compared to their straight-chain counterparts due to reduced surface area and weaker intermolecular interactions.
4. Solubility : The solubility of monohaloalkanes depends on the polarity of the molecule and the nature of the solvent. Typically, monohaloalkanes are relatively insoluble in polar solvents such as water but are soluble in nonpolar solvents like organic solvents (e.g., ether, chloroform, hexane). This is because monohaloalkanes are nonpolar or have a low polarity due to the presence of the halogen atom, which makes them compatible with nonpolar solvents.
5. Density : The density of monohaloalkanes varies depending on the molecular weight and packing of the molecules. Generally, monohaloalkanes are denser than water, and their density increases with increasing molecular weight.
6. Odor : Monohaloalkanes often have distinctive odors, which can vary depending on the specific compound. For example, chloroalkanes tend to have a characteristic sweet or chloroform-like odor.Chemical Properties:
Chemical properties of Monohaloalkanes : Monohaloalkanes, or alkyl halides, exhibit various chemical properties based on the presence of the halogen atom and the nature of the alkyl group. Here are some key chemical properties:
1. Nucleophilic Substitution Reactions:
Monohaloalkanes are highly reactive towards nucleophiles, which are species with a lone pair of electrons that can attack the carbon atom bonded to the halogen. Nucleophilic substitution reactions, such as SN1 (unimolecular) and SN2 (bimolecular) reactions, are common chemical transformations of monohaloalkanes.
2. Elimination Reactions:
Monohaloalkanes can undergo elimination reactions, where the halogen atom is removed along with an adjacent hydrogen atom, resulting in the formation of an alkene or a double bond. Elimination reactions are typically favored under basic conditions.
3. Reaction with Metals:
Monohaloalkanes can react with certain metals, such as magnesium or zinc, to form organometallic compounds. These organometallic compounds are valuable intermediates in organic synthesis and can participate in various subsequent reactions.
4. Reaction with Alcohols:
Monohaloalkanes can undergo a substitution reaction with alcohols in the presence of a base to form ethers. This reaction is known as the Williamson ether synthesis.
5. Reaction with Amines :
Monohaloalkanes can react with amines to form alkyl amines through nucleophilic substitution reactions. This reaction is known as amine synthesis or amine alkylation.
SN2 Reactions (Basic Concept) :
SN1 and SN2 reactions are two fundamental types of nucleophilic substitution reactions that occur with monohaloalkanes.
1. SN1 Reaction (Unimolecular Nucleophilic Substitution) :
In an SN1 reaction, the substitution occurs in two steps. First, the halogen atom leaves the alkyl group, forming a carbocation intermediate. Then, the nucleophile attacks the carbocation, resulting in the substitution product. The rate-determining step in an SN1 reaction is the formation of the carbocation, and the reaction rate depends only on the concentration of the alkyl halide. SN1 reactions are favored when the alkyl halide is tertiary or secondary, as the stability of the carbocation intermediate increases with increasing alkyl substitution.
2. SN2 Reaction (Bimolecular Nucleophilic Substitution) :
In an SN2 reaction, the substitution occurs in a single step. The nucleophile attacks the alkyl halide while the leaving group is still bonded, resulting in the substitution product directly. The rate-determining step in an SN2 reaction involves the collision of the nucleophile with the alkyl halide. SN2 reactions are favored when the alkyl halide is primary or methyl, as steric hindrance is minimal and allows for effective nucleophilic attack.
Chemical Reactions of the haloalkanes :
Haloalkanes (alkyl halides) can undergo various reactions to form different functional groups. Here are some examples of reactions of haloalkanes leading to the formation of alcohols, nitriles, amines, ethers, thioethers, carbylamines, nitrites, and nitroalkanes:
1. Formation of Alcohols : Haloalkanes can be converted into alcohols through nucleophilic substitution reactions using appropriate nucleophiles such as hydroxide ions (OH-) or alkoxides (RO-). For example: R-X + OH- →R-OH + X- (where X is the halogen atom)
2. Formation of Nitriles : Haloalkanes can be transformed into nitriles through nucleophilic substitution reactions with a cyanide ion (CN-). This reaction is known as nucleophilic substitution by the SN2 mechanism. For example: R-X + CN- →R-CN + X- (where X is the halogen atom)
3. Formation of Amines : Haloalkanes can be used as starting materials for the synthesis of amines through nucleophilic substitution reactions with primary or secondary amines. This reaction is known as nucleophilic substitution. For example: R-X + NH3 →R-NH2 + HX (primary amine) R-X + R‘NH2 →R-NHR‘ + HX (secondary amine)
4. Formation of Ethers : Haloalkanes can undergo nucleophilic substitution reactions with alkoxide ions (RO-) to form ethers. This reaction is known as Williamson ether synthesis. For example: R-X + R‘O- →R-O-R‘ + X- (where X is the halogen atom)
5. Formation of Thioethers : Haloalkanes can react with thiols (R-SH) in the presence of a base to form thioethers. This reaction is known as nucleophilic substitution. For example: R-X + R-SH →R-S-R‘ + HX (where X is the halogen atom)
6. Formation of Carbylamines : Haloalkanes can react with primary amines in the presence of a strong base to form carbylamines. This reaction is known as Hofmann reaction. For example: R-X + RNH2 + KOH →R-N=C=NR‘ + KX + H2O (where X is the halogen atom)
7. Formation of Nitrites : Haloalkanes can react with sodium or potassium cyanide (NaCN or KCN) followed by hydrolysis to form nitrites. For example: R-X + NaCN →R-CN + NaX R-CN + H3O+ →R-CONH2
8. Formation of Nitroalkanes : Haloalkanes can react with silver nitrite (AgNO2) in the presence of heat to form nitroalkanes. For example: R-X + AgNO2 →R-NO2 + AgX (where X is the halogen atom)
Elimination Reaction( dehydrogenation – Saytezeff’s rule ) , Reduction ReactioWurtx reactions:
1. Elimination Reaction (Dehydrogenation – Saytzeff‘s Rule) :
Elimination reactions involve the removal of atoms or groups from a molecule to form a double bond or a triple bond. In the context of haloalkanes, elimination reactions often refer to the removal of a hydrogen halide (HX) molecule from the haloalkane to form an alkene or an alkyne. Saytzeff‘s rule, also known as Zaitsev‘s rule, is a guiding principle in elimination reactions that predicts the preferred product. According to Saytzeff‘s rule, in an elimination reaction, the more substituted alkene is the major product. In other words, the hydrogen atom is more likely to be removed from the carbon atom with the fewest hydrogen atoms and the most alkyl groups attached. For example, in the dehydrohalogenation of 2-bromo-2-methylpropane (tert-butyl bromide), the hydrogen atom is removed from the carbon with fewer hydrogen atoms and more alkyl groups, following Saytzeff‘s rule:
CH3 | CH3-C-Br →CH3-C=C-H + HBr | CH3
In this reaction, the more substituted alkene, 2-methylpropene, is the major product according to Saytzeff‘s rule.
2. Reduction Reactions :
Reduction reactions involve the addition of hydrogen (H2) or hydride ions (H-) to a molecule, resulting in the increase of the number of C-H bonds or the decrease of functional groups. Reduction reactions of haloalkanes can be achieved through various methods, such as catalytic hydrogenation or reaction with reducing agents. For example, the reduction of a haloalkane, such as chloroethane (ethyl chloride), with hydrogen gas in the presence of a metal catalyst (e.g., palladium, platinum, or nickel) and heat, leads to the formation of the corresponding alkane:
CH3CH2Cl + H2 →CH3CH3 + HCl
In this reaction, the chlorine atom of chloroethane is replaced with a hydrogen atom, resulting in the formation of ethane.
3. Wurtz Reactions :
Wurtz reactions are synthetic methods used to prepare symmetrical alkanes by coupling two alkyl halides in the presence of a strong alkali metal, typically sodium (Na) or potassium (K). The reaction involves the formation of a carbon-carbon bond by the displacement of the halogen atoms with alkyl groups. For example, the Wurtz reaction between two molecules of ethyl bromide (ethyl halide) can yield butane:
2CH3CH2Br + 2Na →CH3CH2CH2CH2CH3 + 2NaBr
In this reaction, the ethyl bromide molecules react with sodium metal, resulting in the formation of butane.
It‘s important to note that while these reactions are commonly employed in organic synthesis, they may have limitations and specific reaction conditions depending on the reactants and desired products
Preparation of chloroform from ethanol:
The preparation of chloroform (trichloromethane) from ethanol involves several steps. Here‘s a detailed explanation of the process:
Step 1:
Hydrolysis of Aqueous Paste of Bleaching Powder The first step involves the hydrolysis of an aqueous paste of bleaching powder (calcium hypochlorite, Ca(ClO)2) to generate hypochlorous acid (HOCl), which is the chlorinating agent.
Ca(ClO)2 + H2O →Ca(OH)2 + 2HOCl
The bleaching powder reacts with water to produce calcium hydroxide (Ca(OH)2) and hypochlorous acid (HOCl).
Step 2:
Oxidation of Ethyl Alcohol into Acetaldehyde In this step, ethyl alcohol (ethanol, CH3CH2OH) undergoes oxidation to form acetaldehyde (CH3CHO). This reaction is typically carried out using an oxidizing agent, such as potassium dichromate (K2Cr2O7) in the presence of sulfuric acid (H2SO4).
3CH3CH2OH + K2Cr2O7 + 4H2SO4 →3CH3CHO + Cr2(SO4)3 + K2SO4 + 7H2O
Under acidic conditions, ethanol is oxidized to acetaldehyde, while potassium dichromate is reduced to chromium(III) sulfate.
Step 3:
Chlorination of Acetaldehyde into Trichloroacetaldehyde The next step involves the chlorination of acetaldehyde (CH3CHO) using hypochlorous acid (HOCl), generated from the hydrolysis of bleaching powder. This reaction leads to the formation of trichloroacetaldehyde (CHCl3CHO).
CH3CHO + 3HOCl →CHCl3CHO + 3HCl + H2O
Acetaldehyde reacts with hypochlorous acid to produce trichloroacetaldehyde, hydrochloric acid, and water.
Step 4:
Hydrolysis of Chloral into Chloroform The final step involves the hydrolysis of chloral (trichloroacetaldehyde, CHCl3CHO) to yield chloroform (CHCl3). This hydrolysis is typically carried out using an alkali, such as sodium hydroxide (NaOH), under reflux conditions.
CHCl3CHO + 2NaOH →CHCl3 + NaHCO3 + NaCl + H2O
Chloral reacts with sodium hydroxide to produce chloroform, sodium bicarbonate, sodium chloride, and water.
After the completion of these steps, the resulting mixture can be further purified and separated to isolate chloroform.
Preparation from acetone :
Step 1 :
Hydrolysis of Aqueous Paste of Bleaching Powder The hydrolysis of an aqueous paste of bleaching powder (calcium hypochlorite, Ca(ClO)2) is carried out to generate hypochlorous acid (HOCl), which acts as a chlorinating agent. The hydrolysis reaction can be represented as follows:
Ca(ClO)2 + H2O →Ca(OH)2 + 2HOCl
When the bleaching powder is mixed with water, it reacts to form calcium hydroxide (Ca(OH)2) and hypochlorous acid (HOCl).
Step 2:
Chlorination of Acetone into Trichloroacetone In this step, acetone (CH3COCH3) undergoes chlorination using hypochlorous acid (HOCl) generated from the hydrolysis of bleaching powder. The reaction can be represented as follows:
CH3COCH3 + 3HOCl →CHCl3COCH3 + 3HCl
Acetone reacts with hypochlorous acid to form trichloroacetone (CHCl3COCH3) and hydrochloric acid (HCl).
Step 3:
Hydrolysis of Trichloroacetone into Chloroform After obtaining trichloroacetone (CHCl3COCH3) from the chlorination of acetone, the next step involves the hydrolysis of trichloroacetone to yield chloroform (CHCl3). This hydrolysis reaction is typically carried out under basic conditions.
CHCl3COCH3 + 2NaOH →CHCl3 + CH3COONa + H2O
Trichloroacetone reacts with sodium hydroxide (NaOH) to produce chloroform, sodium acetate (CH3COONa), and water (H2O).
During the reaction, the hydroxide ion from sodium hydroxide acts as a nucleophile, attacking the carbonyl carbon of trichloroacetone. This leads to the cleavage of the carbon-oxygen bond, resulting in the formation of chloroform.
Chemical reactions:
1. Oxidation :
Trichloromethane can be oxidized to produce compounds like phosgene (COCl2) or carbon dioxide (CO2) in the presence of strong oxidizing agents.
Example:
Oxidation to Phosgene CHCl3 + O2 →COCl2 + HCl
2. Reduction :
Trichloromethane can be reduced to methane (CH4) or other compounds using strong reducing agents.
Example:
Reduction to Methane CHCl3 + 6H →CH4 + 3HCl
3. Haloform Reaction :
When trichloromethane reacts with silver powder in the presence of sunlight, it undergoes a haloform reaction to form a silver salt of carboxylic acid and silver chloride precipitate.
Example:
CHCl3 + 3Ag + sunlight →CHCl2COOAg + 2AgCl
4. Reaction with Concentrated Nitric Acid :
Trichloromethane reacts with concentrated nitric acid (HNO3) to form chloropicrin (trichloronitromethane) and other nitrogen-containing compounds.
Example:
CHCl3 + HNO3 →Cl3CNO2 + H2O
5. Reaction with Propanone (Acetone) :
Trichloromethane reacts with propanone (acetone) in the presence of a base to form 1,1,1-trichloro-2-propanone.
Example:
CHCl3 + CH3COCH3 + NaOH →Cl3CCOCH3 + NaCl + H2O
6. Hydrolysis with Aqueous Alkali :
Trichloromethane can undergo hydrolysis with aqueous alkali, such as sodium hydroxide (NaOH), to form sodium formate (HCOONa) and sodium chloride (NaCl).
Example:
CHCl3 + 4NaOH →HCOONa + 3NaCl + 2H2O