Haloalkanes and Haloarenes class 12 notes
Haloalkanes are formed by replacing hydrogen atoms in aliphatic hydrocarbon with halogen atoms results in the formation of alkyl halides and in case of aromatic aryl halides it forms haloalenes respectively.
Classification of Haloalkanes and Haloarenes
Based on the number of atoms
1. Depending up on the number of one, two, or more Halogen atoms they are named as Mono, Di, or Poly Halolkanes or Haloarenes.
2. Compounds Containing sp3 C—X Bond; where X is Chlorine, Fluorine, Bromine, and Iodine.
Alkyl halides A homologous series is formed in this and is represented by Cn H2n+1X.
Allylic Halides In this halogen atom is bonded to the carbon atom in an sp3 hybridised form.
Benzylic Halides In this halogen atom is bound with a carbon atom present next to an aromatic ring.
3. Compounds containing sp2C- X bonds are either vinylic halide or aryl halide. In vinylic halide, the halogen atom is bound with carbon-carbon double bond. Whereas Aryl Halide is the compound where halogen atom is bonded to sp2 hybridised Carbon atom in an aromatic ring.
- Halogenated alkyls are called halogen-substituted hydrocarbons in the IUPAC nomenclature.
- Halogen is a common name for aryl halides and is also the name of IUPAC.
- Dihalo compounds with the same type of halogen atom are further classified as genomic halides (halogen atoms are on the same carbon atom) and vicinal halides have adjacent halogen atoms.
Nature of C-X Bond
The carbon-halogen bond of an alkyl halide is polarized because the halogen atom has a higher electronegativity than carbon. Carbon atoms are partially positively charged and halogen atoms are partially negatively charged. Going down the group in the periodic table, the size of the halogen atom increases, so the fluorine atom is the smallest and the iodine atom is the largest. As a result, the carbon-halogen bond length also increases from C-F to C-I.
Methods Of Preparation
- Easily accessible alcohols are the best and readily available for making Alkyl Halides.
- The hydroxyl groups of alcohol replace halogens when they react with concentrated halogen acids, phosphorus halides, or thionyl chloride.
- Thionyl chloride is preferred because the other two products are generated gases. Therefore, this reaction yields a pure alkyl halide.
- Phosphorus tribromide and triiodide are usually produced in situ by the reaction of red phosphorus with bromine and iodine produced in the reaction mixture.
- The decreasing order of reaction of alcohols with a haloacid is 3°>2°>1°.
Various methods of preparation of haloalkanes and haloarenes are mentioned below:
By Free radical Halogenation
Radical halogenation of alkanes results in a complex mixture of the isomers of monohaloalkanes and polyhaloalkanes, that are difficult to separate as pure compounds. In such cases, the yield of any compound is low.
By Electrophilic Substitution
Aryl Halides are prepared by electrophilic substitution of the arene with halide elements in the presence of a Lewis acid catalyst such as iron or ferric chloride. Para & Ortho isomers have a very different range of melting points and hence can be easily separated.
Treatment of primary aromatic amines suspended in cold mineral acid aqueous solution with sodium nitrite forms diazonium salts (Unit 13, Class XII). When a newly prepared solution of diazonium salt is mixed with copper (I) chloride or copper (I) bromide, the diazonium group is replaced with -Cl or -Br.
The simplest method of generating it from alkene is converted to corresponding alkyl halide by reaction with hydrogen chloride, hydrogen bromide or hydrogen iodide
In the laboratory, the addition of CCl4 in a reaction of bromine with alkene results in the discharge of a reddish-brown color of bromine constitutes an important method for the detection of the double bond in a molecule.
The process of synthesis of alkyl fluorides is achieved by heating an alkyl halide in the presence of a metallic fluoride such as CoF2 AgF, Hg2F2, or SbF3. The reaction is termed Swart’s reaction.
The reaction of alkyl halides with NaI catalysed by dry acetone results in forming alkyl iodides.
Boiling point :
- The top members are liquids or solids.
- Molecules of organic halogen compounds are generally polar.
- Due to its higher polarity and higher molecular weight than the parent hydrocarbon, the intermolecular attractive forces are stronger with halogen.
- The boiling point of alkyl halides decreases in the order of RI> RBr> RCl> RF. This is because as the size and mass of the halogen atom increase, so does the van der Waals force.
The density of haloalkanes is always greater than that of water. However, the number of carbon atoms is directly proportional to the density.
Haloalkanes are slightly soluble in water. In order to make it completely soluble, this reaction emits energy and heat making it an exothermic reaction.
There are three important types of reactions under this:
Nucleophilic Substitution Reaction
- Nucleophiles are species having lots of electrons.
- Therefore, they have a tendency to attack the electron-deficient portion of the substrate molecule.
- A reaction in which the nucleophile is replaced is referred to as a Nucleophilic substitution reaction.
- The substrate is a haloalkane having a partial positive charge which is attached by the neutrophile.
Some groups have two neutrophilic centers and are termed ambident nucleophiles. Examples are Cynides, Nitrites.
Inversion of Configuration
SN2 reaction: The composition of the attacked carbon atoms gets reversed when neutrophile attacks hydrocarbon. The bond between carbon atoms breaks down and it attaches to the neutrophile. The structure remains the same but the plane gets reversed. This process is called configuration inversion.
SN1 reactions are carried out in a protic environment such as water, alcohol, or acetic acid. It occurs in two step.
The polarised C—Br bond in the step I process undergoes slow cleavage in order to form a carbocation and a bromide ion. In Step II, this carbocation formed gets attacked by the nucleophile to complete the substitution reaction.
Optical Activity: The plane-by-plane polarization generated by passing ordinary light through a Nicol prism rotates as it passes through a solution of a particular compound. These compounds are termed optically active compounds. If the compound has rotated in the plane of linear polarization to the right, i. H. Talk about clockwise and clockwise rotation. It is called dextrorotary (+). If you point the light to the left (counterclockwise), you are said to be left-handed. it is laevorotatory(-). these + and – signs are the optical isomers.
Chirality: Louis Pasteur in 1848 observed that crystals of certain compounds tend to exist in the form of mirror images and this remarkable finding laid the foundation of modern stereochemistry. The term chirality is defined as the mirror images of object that can not be superimposed on each other. A tetrahedral arrangement of four groups (valencies) around a central carbon is tetrahedral, and if all of the substituents attached to that carbon are different, the mirror image of the molecule is not superimposed (overlapped) on the molecule; such carbon is referred to as an asymmetric carbon or stereocentre.
The property of chirality is defined as things that are not superimposable on their mirror counterpart (such as a pair of hands). Chiral molecules are optically active, while achiral objects are those that may be superimposed on mirror images. Achiral objects are optically inactive. Enantiomers are non-superimposable stereoisomers. The optical rotation of a combination containing two enantiomers in equal amounts will be zero because the rotation of one isomer will be canceled by the rotation of the other isomer. The term “racemic mixture” or “racemic modification” refers to a mixture like this.
When a haloalkane containing a β-hydrogen atom is heated with an alcoholic potassium hydroxide solution, the hydrogen atom from the β-carbon atom is removed, and a halogen atom from the α-carbon atom is formed resulting in the formation of alkene.
Because of elimination of β-hydrogen atom, it is often called β-elimination.
IIn 1875 Alexender Zaitsev proposed a rule summarised as “in dehydrohalogenation reactions, the preferred product is that alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms.”
- According to Grignard’s rule, Halides react with some metals to give compounds containing carbon-metal bonds known as organometallic compounds.
- Victor Grignard, in 1900, discovered one important group of organometallic compounds alkyl magnesium halide, RMgX, often known as Grignard Reagents.
- These reagents can be produced by reacting haloalkanes with magnesium metal in the presence of dry ether.
Alkyl halides react with sodium catalysed by Dry Ether to produce hydrocarbons having twice the number of carbon atoms present in the halide.
Reactions of Haloarenes
- The electron pair of the halogen atom in haloarenes is conjugate with the ring π electron, and the following resonance structure is possible.
- As a result, the bond cleavage in haloarene is more difficult than in haloalkane, and therefore, they are less reactive toward nucleophilic substitution reaction.
An example of replacement by hydroxyl group is chlorobenzene that can be transformed into phenol by heating it in an aqueous sodium hydroxide solution at 623K temperature and a pressure of 300 atm.
Electrophilic Substitution Reaction
There are four different types of reaction under this heading.
Friedel- Crafts Reaction
Reaction with Metals
Wurtz -Fittig Reaction
Name reactions are of utmost importance in haloalkanes and haloarenes class 12 notes. Here are two more reactions to pay attention to.
Wurtz-Fittig reaction mixture which results in the formation of an alkyl arene when aryl halide reacts with sodium in dry ether.
with this being the last topic of discussion in haloalkanes and haloarenes class 12 notes there are four sub-parts in this.
- Dichloromethane is used as an aerosol propellant, as a paint remover, and as a process solvent in pharmaceutical manufacturing.
- Methylene chloride damages the human central nervous system.
- Often used as a cleaning and finishing solvent for metals.
- Commonly known as Chloroform.
- It is used as a solvent for alkaloids, fats, iodine, and other substances.
- It is used as a solvent for alkaloids, fats, iodine, and other substances.
- Earlier it was used as a general anesthetic in surgery now being replaced by ether.
- Chloroform is also used in the production of Freon refrigerant R22.
- Triiodo- methane Known as Iodoform.
- It was used as a preservative because of the release of free iodine, not the iodoform itself.
- Due to its unpleasant odor, it was replaced by another iodine-containing formulation.
- Chemically known as Carbon Tetrachloride.
- It was synsthesies in quantity for its use in making coolants and propellants for aerosol cans.
- It is one of the raw materials for chlorofluorocarbons and other chemicals, general solvents, and pharmaceutical manufacturing applications.
- In the mid-1960s, industrially it was employed as a stain remover, a degreasing agent, and a fire extinguisher at home.
- It is produced from carbon tetrachloride by the Swarts reaction.
- Usually made for freezing, aerosol propellant, and air conditioning purposes.
- They are non-toxic, very stable, non-corrosive, non-reactive, and easily liquefiable gases.
- Paul Muller was given a Nobel Prize in Physiology or Medicine in 1948 for this piece of discovery.
- After World War II, the use of DDT had increased significantly across the world against malaria-causing mosquitoes and typhus-causing lice.
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