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Haloalkanes and Haloarenes are hydrocarbons where one or more hydrogen atoms are replaced with halogen atoms. They are derivatives of alkyl halides and aryl halides, which are used in many medicines.
- Haloalkanes and haloarenes are more reactive than parent alkanes and aromatic compounds.
- They are made up of multiple halogen atoms.
- Haloalkanes are also popular with the name of alkyl halides.
- Haloarenes are also called aryl halides.
- Both compounds have many uses in our daily lives.
- Chloramphenicol and chloroquine are used to treat typhoid and malaria, respectively.
- Some Haloalkanes and Haloarenes are labelled as pollutants and have negative effects on our environment.
- Chlorofluorocarbons (CFCs) are haloalkanes responsible for the depletion of the ozone layer.
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Key Terms: Haloalkanes and Haloarenes, Haloalkanes, Haloarenes, Aryl Halides, Aromatic Compound, Organic chemistry, hydrocarbons, reactions, Alkyl Halides, Chlorofluorocarbons.
What are Haloalkanes and Haloarenes?
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Haloalkanes and Haloarenes are a type of hydrocarbons in which a hydrogen atom in an aromatic hydrocarbon or aliphatic compound is replaced by halogen atoms.
- Haloalkanes are derived from open-chained hydrocarbons, also known as alkanes.
- Haloarenes are derived from aromatic hydrocarbons.
- They stay in the environment for a longer period of time.
- The molecules of compounds are broken down by soil bacteria.
- Haloalkanes and Haloarenes are used as hydrophobic solvents in many chemical industries.
Examples of Haloalkanes and HaloarenesSome example of haloalkane are as follows:
Some example of haloarenes are as follows:
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Read More:
| Chapter Related Concepts | ||
|---|---|---|
| Methyl Ethyl Ketone | Oximes | Peroxydisulfuric Acid |
| Gaterman-Koch reaction | Stephen Reaction Mechanism | Reformatsky Reaction |
Haloalkanes
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Haloalkanes are hydrocarbons which are made up of aliphatic alkanes. In this, one or more hydrogen atoms are replaced by halogen elements such as chlorine, bromine, fluorine, iodine, etc.
- These halogen atoms are attached to the sp3 hybridized carbon atom of the alkyl group.
- These are saturated organic compounds.
- In this case, chemical bonds are attached to the carbon atom with single bonds.
- The single carbon atom is attached to the Halogen atom.
- Haloalkanes are made by aliphatic alkanes by the process of free radical halogenation.
- They are odourless compounds.
- The compound precipitates in SN2 substitution reactions.
Haloarenes
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Haloarenes are hydrocarbons consisting of an aromatic ring and one or more hydrogen atoms replaced by halogens. In this compound, the halogen atom is attached to the alkyl group's sp3 hybridized carbon atom.
- They differ from Haloalkanes by their method of preparation and properties.
- Let's understand the difference by changing the structure of the hybridization of carbon atoms.
- In this, you will change the C-X bond of the group.
- It will create two unique sets of compounds.
- One of the most important Haloarene is aryl chloride.
- Haloarenes are made by direct halogenation of aromatic rings.
- They have a sweet odour.
- These do not precipitate in SN2 substitution reactions.
Classification of Haloalkanes and Haloarenes
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Haloalkanes and Haloarenes can be classified based on the following points.
Classification on the basis of number of halogen atoms
Haloalkanes and Haloarenes are classified into three categories based on number of halogen atoms which are as follows:
Monohaloalkanes and Monohaloarenes
Monohaloalkanes and Monohaloarenes are compounds that contain one halogen atom.
Dihaloalkanes and Dihaloarenes
Dihaloalkanes and Dihaloarenes are compounds that contain two halogen atoms.
Trihaloalkanes and Trihaloarenes
Trihaloalkanes and Trihaloarenes are compounds that contain three halogen atoms.
Classification on the basis of sp2 hybridized carbon halogen bond
Haloalkanes and Haloarenes are classified into two categories based on sp2 hybridized carbon halogen bond which are as follows:
Vinyl Halide
The halogen atom gets attached to the sp2 hybridized carbon C=C are called Vinyl Halide
Aryl Halide
The halogen atom gets attached to the sp2 hybridized carbon in an aromatic ring are called Aryl Halide
Classification on the basis of sp3 hybridized carbon halogen bond
Haloalkanes and Haloarenes are classified into two categories based on sp3 hybridized carbon halogen bond which are as follows:
Alkyl Halide
Halogen gets attached to the alkyl chain are called Alkyl Halide.
Allylic Halide
The halogen atom gets attached to the sp3hybridized carbon which is situated adjacent to the sp2 hybridized carbon C=C are called Allylic Halide.
Benzylic Halide
The halogen atom gets attached to the sp3hybridized carbon, attached to a benzene ring are called Benzylic Halide.
Preparation of Haloalkanes and Haloarenes
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Let’s understand the preparation of haloalkanes and haloarenes by using various preparation techniques:
Preparation of Haloakanes
Suppose you are given the task of preparing haloalkanes then you can follow below methods to create haloalkane which are as follows:
Preparation by Alcohols
The easiest method to prepare haloakane is by using alcohol when R-OH is reacted with a suitable reagent it will form R-X. The reagents that help in the reactions are:
- Halogen Acids (HX)
This reaction takes place when an organic compound is a derivative of an alcohol that reacts with Halogen Acid in order to form Haloalkane as the major product. The basic reaction takes place as follows:
Halogen Acids
Example of Halogen Acids (HX)Example: By taking Hydrogen bromide as an example, we can form bromoalkanes by the reaction of hydrogen bromide with alcohol as follows: R-OH + NaBr + H2SO4 → R-Br + NaHSO4 + H2O |
- Thionyl chloride (SOCl2)
Thionyl chloride (SOCl2) is the most preferred reagent among all the three. Here alcohol reacts with SOCl2 (Thionyl chloride) to form alkyl halide and the by-product formed by this reaction is gaseous in nature.
- It easily dissipates into the atmosphere, leaving behind pure alkyl halide.
Example of Thionyl chloride (SOCl2)Example: In the following example we can see the reaction of Ethanol with Thionyl chloride R-OH + SOCl2 → R-Cl + SO2 + HCl |
- Phosphorous halides (PX5 or PX3)
In this, when a phosphorous halide reacts with an alcohol, it interchanges the functional group of alcohol –OH with the corresponding halide. The general reaction takes place as follows:
R-OH + PCl5 → R-Cl + POCl3 + HCl
Example of Phosphorous halides (PX5 or PX3)Example: We can further take the example of the reaction of ethanol with PCl3 to form chloroethane. The reaction takes place as follows: 3R-OH + PX3 → 3R-X + H3PO3 (X = Cl, Br) |
Preparation by Free Radical Halogenation
Formation of alkyl bromides and alkyl chloride is possible by the free radical halogenation, but as these radicals are highly reactive and non-specific in nature, they can result in the formation of a mixture of products.
- It is not a preferred method for preparation of haloalkanes.
Example of Free Radical HalogenationExample: Chlorination of free radical can result in the formation of a number of haloalkanes, which in turn makes it difficult to isolate a single product. If we consider the reaction of butane with chlorine in the presence of light, a mix of product formation takes place which can be seen as follows:
Preparation by Free Radical Halogenation |
Preparation by Alkenes
An electrophilic addition reaction can be used to convert an alkene into a haloalkane as alkene will react with HX to form R-X. The general reaction in this will take place as follows:
Preparation by Alkenes
Preparation of Haloarenes
Suppose you are given the task of preparing haloarenes then you can follow below methods to create haloalkane which are as follows:
Preparation by Electrophilic Substitution Reaction
Electrophilic Substitution reaction helps in the preparation of Haloarenes such as aryl bromides and aryl chlorides by using the halogens such as chlorine and bromine in the presence of Lewis Acid.
- The reaction takes place in the presence of Lewis Acid and must be carried out in dark.
- Some of the reactions with chlorine to obtain the electrophiles are:
Preparation by Electrophilic Substitution Reaction
Preparation by Sandmeyer’s Reaction
In this case, the primary aromatic amine reacts with sodium nitrate in order to form diazonium salt in the presence of cold mineral water. Here HNO2 is prepared by the reaction of sodium nitrate with HX at a temperature of 273-278 K.
Preparation by Sandmeyer’s Reaction
Properties of Haloalkanes and Haloarenes
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Some important physical and chemical properties of Haloalkanes and Haloarenes are as follows:
Physical Properties of Haloalkanes and Haloarenes
Various Physical Properties of Haloalkanes and Haloarenes are as follows:
- Haloalkanes and haloarenes are colourless and odourless compounds.
- Haloarenes are crystalline substances that are heavier than water.
- Haloalkane is a hydrophobic compound that is heavier than alkane.
- The boiling point and melting of haloalkanes and haloarenes depend upon the lattice structure.
- They have high boiling point and melting point.
- The dipole moment of haloalkanes and haloarenes depends upon the value of electronegativity of carbon and halogens.
- Haloarenes compounds are less reactive than haloalkanes compounds.
Chemical Properties of Haloalkanes and Haloarenes
Haloalkanes and Haloarenes are reactive compounds that undergoes various chemical reactions so various Chemical Properties of Haloalkanes and Haloarenes are as follows:
- Elimination Reaction
- Nucleophilic Substitution
- Reaction with Metals
- Reduction
Uses of Haloalkanes and Haloarenes
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The various uses of Haloalkanes and Haloarenes are as follows:
- Haloalkanes and Haloarenes are used as solvents for non-polar compounds, also known as hydrophobic compounds.
- It is used in the treatment of Malaria (Chloroquine).
- The compounds are used for organic synthesis.
- Haloalkanes and Haloarenes is used as an insecticide
- Some derivatives, such as chloramphenicol, are used for the treatment of typhoid.
- Dichloro-diphenyl-trichloroethane or DDT is used in insecticides.
Environment Effects
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Although haloalkane and haloarene compounds are used for various commercial purposes, they can still pollute the environment due to the pollutants and the toxins they release.
- Compounds like methyl bromide act as a very harmful fumigant to the environment.
- Chlorofluorocarbons (CFC) are a major cause of ozone depletion.
- Methyl iodide does not cause depletion of the ozone layer.
Important Topics for JEE MainAs per JEE Main 2024 Session 1, important topics included in the chapter Haloalkanes and Haloarenes are as follows:
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Things to Remember
- Haloalkanes and haloarenes are hydrocarbons which are formed by the substitution of hydrogen atoms.
- They have high melting and boiling points and are less soluble in water.
- Haloarenes are compounds that undergo electrophilic substitution reactions.
- Haloalkanes are compounds that undergo nucleophilic substitution reactions.
- They are highly soluble in organic solvents due to the presence of a nonpolar nature.
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Sample Questions
Ques. (A) State one uses each DDT and iodoform.
(B) Which compound in the following couples will react faster in SN2 displacement and why?
(i) 1-bromopentane or 2-bromopentane
(ii) l-bromo-2-methylbutane or 2-bromo-2-methylbutane? (3 Marks)
Ans: (A)DDT (DichlorodiphenylTrichloroethane) is a powerful insecticide which is widely used in sugarcane and fodder crops to kill mosquitoes, lice which carry pathogens.
Iodoform (CHI3) is used as an antiseptic for dressing wounds. Its antiseptic properties are due to liberation of iodine when iodoform comes in contact with skin but not due to iodoform itself.
(B) The reactivity depends upon steric hindrance in SN2 reactions
(i) 1-Bromopentane (1° halogen) having less steric hindrance therefore is more reactive than 2-Bromopentane hence undergoes SN2 reactions faster.
(ii) 1-Bromo-2-methylbutane having less steric hindrance is more reactive towards sn2 Reaction than 2-bromo-2-methyl butane (more steric hindrance).
Ques. Answer the following:
(A) Haloalkanes easily dissolve in organic solvents, why? ..
(B) What is known as a racemic mixture ? Give an example.
(C) Of the two bromo derivatives, C6H5CH(CH3)Br and C6H5CH(C6H5)Br which one is more reactive in SN1 substitution reaction and why? (3 Marks)
Ans: (A) Haloalkanes can easily dissolve in organic solvent because haloalkanes are non-polar in nature.
(B) The mixture which contains equal amount of optically active d(+) dextro and levo rotatory l(-) substances is called racemic mixture, e.g. racemic mixture of glucose contains equal amount of dextrorotatory glucose and laevorotatory glucose.
(C) C6H5CH(C6H5) Br will undergo SN1 substitution due to greater stability of carbocation formed.
Ques. How do you convert the following:
(i) Prop-l-ene to propan-2-ol
(ii) Bromobenzene to 2-bromoacetophenone
(iii) 2-bromobutane to But-2-ene? (3 Marks)
Ans: The conversion are as follows:
Ques: How do you convert the following:
(i) ethyl chloride is treated with Nal in the presence of acetone,
(ii) chlorobenzene is treated with Na metal in the presence of dry ether,
(iii) methyl chloride is treated with KNO2? (3 Marks)
Ans: The conversion are as follows:
Ques. Answer the following questions:
(i) What is meant by chirality of a compound? Give an example.
(ii) Which one of the following compounds is more easily hydrolysed, CH3CHClCH2CH3 or CH3CH2CH2Cl?
(iii) Which one undergoes SN2 substitution reaction faster and why?
(3 Marks)
Ans: The conversion are as follows:
Ques. Write structural formula of the following compounds
(i) 3-iodo-4-tert. butyl heptane
(ii) 4-bromo-3-methyl pent-2-ene? (2 Marks)
Ans: The conversion are as follows:
Ques. Write chemical equations when
(i) ethyl chloride is treated with alcoholic KOH.
(ii) chlorobenzene is treated with CH3Cl in the presence of anhydrous AlCl3? (2 Marks)
Answer: The conversion are as follows:
Ques. Explain as to why
(i) Alkyl halides, though polar, are immiscible with water.
(ii) Grignard’s reagent should be prepared under anhydrous conditions? (2 Marks)
Answer: (i) Alkyl halides are immiscible with water because they cannot form H-bonds with water.
(ii) Grignard’s reagent should be prepared in absence of water, i.e. anhydrous conditions because the Grignard’s reagent reacts with water to form alkane.
Ques. Solve the following question:
(A) Draw the structure of 2- Bromopentane
(B) Give the IUPAC name of the compound
? (2 Marks)
Ans: (A) The structure of 2- Bromopentane is-
(B) The IUPAC name of the compound
The IUPAC name of this compound is 3-Bromo-2-methylprop-l-ene.
Ques. Explain : (A) sp2 hybridized carbon halogen bond (B) sp3 hybridized carbon halogen bond? (2 Marks)
Ans.(A) sp2 hybridized carbon halogen bond: When one s and two p atomic orbitals mix together, they form sp2 hybridization, where the combination of such atomic orbitals creates 3 new hybrid orbitals all of equal energy. Here one electron in the s orbital is promoted to one of the 2p atomic orbitals.
(B) sp3 hybridized carbon halogen bond: When characters of one 2s-orbital and three 2p-orbitals mix together it is known as sp3 hybridization. The mixing creates four hybrid orbitals, which have similar characteristics. For an atom to get sp3 hybridized, it must contain one s orbital and 3 p orbitals.
Ques. What is the main difference between Haloalkanes and Haloarenes? (4 Marks)
Ans. The main point of difference between Haloalkanes and Haloarenes are as follows:
| Haloalkanes | Haloarenes |
|---|---|
| Haloalkanes is a type of aliphatic compound where hydrogen atom is replaced by halogen. | Haloarenes is a type of aromatic compound where hydrogen atom is replaced by halogen. |
| They are prepared by free radical halogenation. | They are prepared by direct halogenation of aromatic compounds. |
| Haloalkanes are odourless compounds. | Haloarenes are sweet odour. |
| They precipitate by SN2 substitution reactions. | They are not precipitated by SN2 substitution reactions. |
Ques. Can Haloalkanes and Haloarenes be used in the medical field? (1 Mark)
Ans. Yes, some derivations of Haloalkanes and Haloarenes can be used in treatment of ailments like Malaria and Typhoid.
Ques. Can Haloalkanes and Haloarenes be differentiated on the basis of smell? (1 Mark)
Ans. Yes, they can be differentiated on the basis of smell. Haloalkanes are odourless compounds whereas Haloarenes have a sweet odour.
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