RBSE Class 12 Chemistry Notes Chapter 10 Haloalkanes and Haloarenes

These comprehensive RBSE Class 12 Chemistry Notes Chapter 10 Haloalkanes and Haloarenes will give a brief overview of all the concepts.

RBSE Class 12 Chemistry Chapter 10 Notes Haloalkanes and Haloarenes

→ In propargyl halides, the halogen atom is attached to sp³-hybridised carbon atom which is next to carbon- carbon triple bond.

→ Invinylic halides the halogen atom is attached to one of the carbon atoms of the carbon-carbon doule bond.

→ In aryl halides, halogen atom is directly attached to the aromatic ring.

→ In IUPAC system of nomenclature alkyl halides are named as haloalkanes.

→ Dihalogen derivatives of alkanes in which two halogen atoms of the same type are present on the same carbon atom are known as alkylidine dihalides.

→ Dihalogen derivatives of alkanes in which the two halogen atoms of the same type are present on the adjacent carbon atoms of the chain are krwwn as alkylene dihalides or vicinal dihalides or vic-dihalids.

→ Dihalogen derivatives of alkanes in which the two same halogen atoms are attached on the terminal carbon atoms of the chain are known as polymethylene dihalides.

→ Alkyl halides contain C—X bond in which carbon atom is sp³-hybridized.

 

→ All the bonds in haloalkanes are sigma (a) bonds.

→ C—X bond is a polar covalent bond due to difference in electronegativity of carbon and hydrogen atom.

→ Haloalkanes exhibit structural as well as stereoisomerism.

→ Alcohols on treatment with halogen atoms give haloalkanes. The reactivity of alcohols being: 3° > 2° > 1° while that of halogen acids being : HI > HBr > HCl.

→ Phosphorus pentachloride (PCl₅) and trihalides react with alcohols to form the corresponding haloalkanes.

→ Almost pure alkyl chlorides are prepared by the action of SOCl₂ on alcohols in presence of pyridine.

→ Haloalkanes can be obtained from suitable alkanes, alkenes and alkynes.

→ Markownikov's Rule : The negative part of the addendom (molecule to be added) goes to that carbon atom of the double bond which carry lesser number of hydrogen atoms.

→ Peroxide Effect or Kharash Effect: In presence of peroxides such as benzyol peroxide the addition of HBr (but not of HCl or HI) to unsymmetrical alkenes takes place contrary to Markownikov's rule. This is called peroxide or Kharash effect.

→ Bromo alkanes can be easily prepared by refluxing the silver salt of a fatty acid with bromine in CCl₄. This reaction is called Borodine Hunsdeicker reaction or Hunsdiecker reaction.

→ Boiling points:

• RI > RBr > RCl > RF,
• RCH₂X > R₂CHX > R₃CX,
• For isomeric haloalkanes boiling points decrease with branching i.e., CH₃CH₂CH₂CH₂Cl > (CH₃)₂CHCH₂Cl > CH₃CH₂CHClCH₃ > (CH₃)₃CCl.

→ Melting point: Amongst isomeric dichlorobenzene, p- isomer has the highest melting point due to symmetry.

→ Density: RI > RBr > RCl > RF.

→ Stability :RF > RCl > RBr > RI.

→ Dipole moments:

• CH₃Cl (1.86 D) > CH₃F (1.847D) > CH₃Br (1.83 D) > CH₃I (1.636 D).
• CH₃Cl (1.86 D) > CH₂Cl₂ (1.62 D) > CHCl₄ (1.03 D),CCl₄(=D)

→ Alkyl halides undergo nucleophilic substitution reactions. These are of two types S_N1 and S_N2.

→ Plane polarised light has vibrations or oscillations only in one plane.

→ Substances which can rotate plane polarized light are called optically active substances and the property of a substance to rotate the plane of polarized light is called optical activity.

→ The objects which are non-superimposable on their mirror images are said to be chiral and this property is known as chirality while those which are super- imposable on their mirror images are called achiral.

→ Molecules which contain one chiral carbon atom are always optically active.

→ Optical isomers which are non-superimposable mirror images of each other are known as enantiomers and the phenomenon is called enantiomerism. They are identical physical and chemical properties but rotate plane polarized light into opposite directions but to the same extent.

→ Stereoisomers which are not mirror images of each other are known as diasteromers and the phenomenon is called diasteromerism.

→ Compounds which do not show optical activity in spite of the presence of asymmetric or chiral carbon atoms are known as mesocompounds.

→ If the incoming group gets attached to chiral centre on the same face from which the leaving group has departed, retention of configuration occurs.

→ If the incoming group gets attached to the face opposite from which the leaving group has departed inversion of configuration occurs.

→ If in a reaction, 50% inversion and 50% retention occurs, the process is called racemization and the product is called a racemic mixture which is always optically inactive.

→ The S_N2 reactions of optically active halides are accompanied by inversion of configuration.

→ In case of optically active alkyl halides S_N1 reactions are accompanied by racemisation.

→ Reactivity of alkyl halides in S_N1 and S_N2 reactions is RI > RBr > RCl > RF and nucleophilicity of halide ions is: I^- > Br^- > Cl^- > F^-.

→ On hydrolysis with aqueous KOH or Ag₂O in boiling water haloalkanes give alcohols but on heating with an alcoholic solution of KOH, haloalkanes undergo dehydrohalogenation to form alkene. Ease of dehyd- rohalogenation of haloalkanes being: 3° > 2° > 1°.

→ Saytzeff Rule : If an haloalkane can undergo dehydrohalogenation in two different ways, then the more highly substituted alkene is the major product.

→ The ease of formation of alkene is: CH₂ = CH₂ < CH₃ — CH = CH₂ < (CH₃)₂C = CH₂.

→ The ease of dehydrohalogenation of alkyl halide is:
CH₃CH₂Br < (CH₃)₂CHBr < (CH₃)₃CBr.

→ The elimination reactions are of two types : E₂ and E₁ reactions.

→ Grignard reagents are obtained by the reaction of haloalkanes with magnesium metal in dry ether.

→ Wurtz Reaction: Alkyl halides react with sodium in dry ether to give hydrocarbons containing double the number of carbon atoms present in the alkyl halides.

→ Williamson's synthesis : Alkyl halides react with sodium or potassium alkoxides to form ethers.

→ Alkyl halides react with an alcoholic solution of KCN to give cyanides as the major product while with AgCN, isocyanides are the major products.

→ Tetraethyl lead (TEL) is used as an atniknocking agent in gasoline.

→ Haloarenes are the common as well as IUPAC name of aryl halides.

→ Sandmeyer Reaction : Chlorobenzene and bromobenzene can be prepared by treating benzene diazonium chloride with CuCl dissolved in HCl or CuBr dissolved in HBr.

→ Hunsdiecker Reaction : Aryl bromides can be prepared by reflluxing the silver salt of aromatic acids with Br₂ in boiling carbon tetrachloride.

→ Chlorobenzene can be prepared on commercial scale by Rasching prosess by passing vapours of benzene, air and HCl over heated CuCl₂.