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Sunday, 17 November 2019

Alcohols Phenols and Ethers Class 12 Notes

Alcohols Phenols and Ethers Class 12 Notes




Alcohols Phenols and Ethers Class 12 Chemistry Notes
www.ChemistryNotesInfo.com

ALCOHOLS

Alcohols are the hydroxyl derivatives of hydrocarbons in which one or more hydrogen atoms are replaced by –OH group.
e.g.  1) CH3OH    (Methyl Alcohol)
        2) CH3 – CH2OH   (Ethyl Alcohol)

Classification of Alcohols

On the basis of the number of hydroxyl groups (-OH) present in the molecule, alcohols are classified as:

1) Monohydric alcohols:

They have only one hydroxyl (-OH) group.
e.g. CH3OH)

2) Dihydric alcohols/glycols/diols:

They have two hydroxyl groups. eg.


3) Trihydric Alcohols/glycerol’s/triols :

They have three hydroxyl groups.

4) Polyhydric alcohols :

They have more than three hydroxyl groups.


Monohybrid alcohols are further classified according to the type of hybridization of the carbon atom to which the hydroxyl group is attached.
A) Alcohols containing C sp3 – OH bond –
In these alcohols, -OH group is attached to SP3 hybridised carbon atom of an alkyl, allyl or benzylic group. They are further classified as primary, secondary and tertiary alcohols according to number of carbon atom.
e.g. 1)    CH3 – CH2 – OH     (Ethyl Alcohol)


Allylic Alcohols
In these alcohols –OH group is attached to a sphybridized carbon atom next to carbon atom.
Benzylic Alcohols
In these alcohols –OH group is attached to a sphybridized carbon atom i.e. vinylic carbon. These alcohols are also called vinylic alcohols.
e.g. CH2 = CH – OH     (vinyl alcohol)
B) Alcohols containing Csp2 – OH bond
In these alcohols –OH group is attached to sp2 hybridized carbon atom i.e. vinylic carbon. These alcohols are also called vinylic alcohols.
e.g.  CH2 = CH – OH    (vinyl alcohol)

Preparation of Alcohols

1) From alkyl halides (Hydrolysis)
Alkyl halides in heating with dilute aqueous alkali give corresponding alcohols.
    R – X     +    NaOH    →    R – OH   +   NaX
Alkyl halides                       Alcohols
e.g.      CH3 – Cl    +    NaOH   →   CH– OH  +   NaCl
       Methyl Chloride                       Methanol
2) From alkynes (Hydration)
The addition of the water molecule across the double bond in alkenes is called hydration of alkenes. It takes place according to Markownikoff’s rule. Hydration of alkenes is carried out by passing an alkene through cold concentrated Sulphuric acid to give alkyl hydrogen sulphate, which on heating with water gives an alcohol.


3) From carbonyl compound (Reduction)
Hydrogenation of aldehydes and ketones is carried out in the presence of catalysts such as finely divided nickel, platinum or palladium, aldehydes on reduction give corresponding primary alcohol while ketones on reduction give corresponding secondary alcohols.
e.g. i) Ethanol is prepared by hydrogenation at acetaldehyde.

CH3 – CHO    +    H2    —-Raney Ni 413k→    CH3 – CH2 – OH
Acetaldehyde                                               Ethanol
ii) Acetone on hydrogenation gives isopropyl alcohol.


Conversion of carboxylic acids and esters to corresponding primary alcohols reduction by LiAlH4 is preferred.


4) From Grignard reagent
Grignard reagent forms addition compound by nucleophilic attack with aldehydes and ketones, which on hydrolysis with dilute acid yields corresponding alcohol.


e.g. Ethanol is prepared by the action of methyl magnesium bromide on methanol in dry ether followed by the hydrolysis of the addition compound using dilute acid.


Structure of Alcohols
The carbon atom attached to –OH group and oxygen atom of the –OH group are sp3 hybridized. The C – O – H bond angle has an approximate tetrahedral value i.e. around 108.9.


Reactions of Alcohols

Alcohol reacts both as nucleophiles and electrophiles. The C–O and O–H bond in alcohols are polar, hence alcohols are quite reactive.
A) Reaction involving breaking of O – H bond
This section of chapter Alcohols Phenols and Ethers contains reaction of alcohols which involve breaking of O-H bond.
i) Action of Metal
Alcohols react with active metals like sodium, potassium, aluminium to give corresponding alcoxide liberating hydrogen gas. These reactions explain acidic nature of alcohols. As number of alkyl group increases, acidic strength of alcohol decreases.
e.g.  2C2H5 – OH  +  2Na  →  2C2H5 – ONa  +  H2  ↑
ii) Esterification
When carboxylic acids are heated with alcohols, they give corresponding esters. This process is called Fischer esterification. The reaction is reversible and accumulation of water may reverse the reaction. The forward reaction is called esterification and the backward reaction is called hydrolysis of ester.

B) Reaction involving breaking of C – O bond
This section of chapter Alcohols Phenols and Ethers contains reactions of alcohols which involve breaking of C-O bond.
i) Reaction with hydrogen halide (HX)
  • Action of HCl
A catalyst (Lewis base) is required for the reaction of primary and secondary alcohols with HCl. For this purpose, Lucas reagent is used which is composed of conc. HCl and anhydrous zinc Chloride.
e.g.    C2H5OH (Ethanol) + HCl    anhy. ZnCl2Δ→  C2H5Cl (Chloroethane) + H2O                         
Action of HBr
Alcohols when heated with HBr give alkyl Bromides. Since HBr is Stronger than HCl, zncl2 catalyst is not required.
C2H5OH (Ethanol) + HBr — Δ→    C2H5Br  (Bromoethane) +  H2O
  • Action of HI
Alcohols on heating with hydrogen iodide give corresponding alkyl iodides. However, most of the alcohols do not give acceptable yields of alkyl iodides.

ii) Reaction with PX3
Alcohols are converted to alkyl Chlorides by reaction with phosphorus trichloride (PCl3).
e.g.     3C2H5OH (Ethanol) + PCl3 (Phosphorous Trichloride) —Δ→     3C2H5Cl (Ethyl Chloridde)  + H3PO (Phosphorus acid)
iii) Dehydration of formation of alkene
Removal of water molecule from an alcohol molecule is called dehydration of alcohol. When alcohol having a β-hydrogen is heated with dehydrating agent like concentrated H2SO­4 an alkene is formed by the loss of water.
e.g.   A primary alcohol is dehydrated by heating with 95% H2SO4 at 443K.
         C2H5OH    95% H2SO4 (443K)→    CH = CH   +   H2O
Saytzeff’s rule:- In dehydrohalogenation or in dehydration, an alkenes formed by elimination has greater number of alkyl groups attached to doubly bonded carbon atom.
iv) Oxidation of formation of aldehydes and Ketones
Alcohols can be oxidized by various oxidizing agent such as acidified K2Cr2Oand H2SO4.
a) Oxidation of Primary alcohols
A primary alcohol on oxidation using acidified potassium dichromate (K2Cr2O7) first gives an aldehyde which on further oxidation gives carboxylic acid.


b) Oxidation of secondary alcohols
A secondary alcohol on oxidation using K2Cr2O7 and H2SO4 give a ketone containing          same number of carbon atoms as in the alcohol.


PHENOLS

Phenols are organic aromatic, hydroxyl compounds, in which one or more hydroxyl (-OH) groups are directly attached to the aromatic nucleus i.e. benzene like ring. It is represented by Ar-OH. These are Alcohols Phenols and Ethers class 12 chemistry notes by ChemistryNotesInfo.com


Classification of Phenols

Phenols are classified as monohydricdihydric and trihydric depending upon presence of one, two or three hydroxyl groups (-OH) attached to aromatic ring.

Preparation of Phenols

1) From chlorobenzene (Dow process)

In this method, Chlorobenzene is fused with NaOH at 623K and 32O atmospheric pressure to get sodium phenoxide further it is hydrolyzed using dilute hydrochloric acid to get phenol.

2) From Chlorobenzene (Raschig method)

In this method, phenol is obtained by heating chlorobenzene with steam at 698K using Ca3(PO4)2 or Sioas a catalyst.

3) From benzene Sulphonic acid

Benzene is sulphonated with oleum (H2S2O7) and benzene sulphonic acid so formed is concentrated to sodium phenoxide on heating with molten sodium hydroxide. Acidification of sodium phenoxide gives phenol.

4) From Cumene (commercial method)

Cumene (isopropyl benzene) is oxidized in the presence of air to cumene hydroperoxide. It is converted to phenol and acetone by treating it with dilute acid.

Structure of Phenol

In phenols –OH group is attached to sp3 hybridised carbon atom of an benzene ring. The carbon – oxygen (C – O) bond length is 136 Pm and C – O – H bond angle is 109ᵒ. Phenol has smaller dipole moment.

Reaction of Phenol

Acidity of Phenol

  • Reaction with sodium
Phenol reacts with sodium to give sodium phenoxide along with evolution of H2 gas.


  • Reaction with NaOH
Phenol dissolves in NaOH to give sodium phenoxide and water.


The Above two reactions show acidic character of phenol. The acidity of phenol is due to its ability to lose hydrogen ion to form phenoxide ions. In a phenol molecule, the hydroxyl group (-OH) is directly attached to SP2 hybridised carbon atom act as an electron withdrawing group.

Electrophilic aromatic substitution

  • Halogenation
Bromination of phenol is treated with bromine at low temperature in a solvent, such as carbon disulphide (CS2) or chloroform (CHCl3) a mixture of O – bromophenol and p – bromophenol is obtained, P – bromophenol is major product.


When phenol is treated with bromine water 2,4,6 – tribromophenol is formed with white precipitate.


  • Nitration of Phenol
When phenol is treated with dilute nitric acid at room temperature a mixture of O – nitrophenol and P – nitrophenol is obtained and O – nitrophenol is major product.


When phenol is heated with concentrated nitric acid in presence of concentrated sulphuric acid, it gives 2,4,6 trinitrophenol.


  • Sulphonation of Phenol
When phenol is treated with concentrated sulphuric acid at room temperature (298K) it gives O – phenol sulphonic acid as major product.


  • Kolbe’s reaction
The electrophilic substitution reaction between phenoxide ion and carbon dioxide to give Ortho – Hydroxybenzoic acid is known as Kolbe’s reaction.

  • Reimer – Tiemann reaction
On treating phenol with Chloroform in the presence of sodium hydroxide, α – CHO group is introduced at ortho position of benzene ring. This reaction is known as Reimer – Tiemann reaction.


  • Reaction with zinc dust
When phenol is heated with zinc dust, it gives benzene and zinc oxide.


  • Oxidation
Phenol is oxidized by chromic acid to a diketone, benzoquinone.


ETHERS

Ethers are derivatives of hydrocarbons in which a hydrogen atom is replaced by an alkoxy (-OR) or an aryloxy (- OAr) group. They are represented as R – O – R.
e.g.  CH3 – O – CH3
       Dimethyl Ether

Classification of ether

1) Simple or symmetrical ethers

The ether’s in which both the alkyl or aryl groups attached to oxygen atom are same, are called simple ethers.
e.g.    CH3 – O – CH3
          Dimethyl Ether

2) Mixed or unsymmetrical ethers

The ethers, in which the two alkyl or aryl group attached to oxygen atom are different, are called mixed ethers.
e.g.       CH3 – O – C2H5
           ethyl methyl ether

Structure of ether

In ether oxygen atom is SP3 hybridized. C – O – C bond angle (111.7ᵒ) is slightly greater than tetrahedral angle. C – O bond length is 141 Pm.


Metamerism

Ethers having same molecular formula but different alkyl groups attached on either side of the oxygen atom are called metamers of each other. This phenomenon is called metamerism.
e.g. Molecular Formula  –  C4H10O
       CH3 – CH2 – O – CH2 – CH3      Diethyl ether, and
       CH3 – O – CH2 – CH2 – CH3       Methyl n – propyl ether.

Methods of preparation of ethers

1) By dehydration of alcohols

When excess of ethyl alcohol is distilled with concentrated sulphuric acid at 443K it gives dimethyl ether. The formation of ether by dehydration of alcohol is a nucleophilic bimolecular reaction (SN2)


2) From alkyl halides (Williamson synthesis)

Simple as well as mixed ethers can be prepared, when an alkyl halide is heated with alcoholic sodium or potassium alkoxide.


Chemical properties of ethers

1) Cleavage of C – O bond by action of HX

a) Cold simple ether reacts with HX to give one molecule of alkyl halide and one molecule of an alcohol, while when heated gives two molecule of alkyl halide.



b) By reacting with HX cold mixed ether gives generally a lower alkyl iodide and a higher alcohol while when heated it gives two different alkyl halides.


2) Hydrolysis

Simple ethers on heating with dilute sulphuric acid under pressure give alcohol.


3) Electrophilic substitution

In this section of chapter Alcohols Phenols and Ethers, we study about electrophilic substitution reactions of ether.
a) Halogenation
Anisole undergoes bromination with bromine in acetic acid.


b) Friedal – crafts reaction
Alkyl and acyl groups are introduced at ortho and para position in anisole on reaction with alkyl halide and acyl chloride respectively in presence of anhydrous aluminium chloride.


c) Nitration
Anisole reacts with nitrating mixture to give a mixture of ortho and para nitro derivatives.


Uses of diethyl ether

It is used for industrial solvent for oils, fats, gum, and resin and in Grignard reagents. It is also used as refrigerant.
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Wednesday, 2 October 2019

Haloalkanes and Haloarenes Class 12

Haloalkanes and Haloarenes Class 12

These are organic chemistry study material on Haloalkanes and Haloarenes class 12 notes.

Haloalkanes and Haloarenes Class 12 Notes

Haloalkanes and Haloarenes Introduction

Alkanes are aliphatic saturated hydrocarbons having general formula CnH2n+2. While compounds that resemble (or which contain a benzene ring as a structural unit) are called as aromatic compounds or arenes.
When one or more hydrogen atoms at aliphatic hydrocarbon are replaced by corresponding number of halogen atoms (Chlorine, Bromine or Iodine) then compounds are called halogen derivatives of alkanes or haloalkanes.
e.g. Methyl Chloride (CH3Cl).
Similarly, when one or more hydrogen atoms of aromatic hydrocarbon are replaced by corresponding number of halogen atoms (Chlorine, Bromine or Iodine) then compounds are called halogen derivatives of arenes or haloarenes.
e.g. aryl halide.  
Haloalkanes and haloarenes are used as solvent for relatively non polar compounds and are commercially important as dry cleaning agents, refrigerants, propellants, drugs etc.

HALOALKANES – Haloalkanes and Haloarenes class 12

CLASSIFICATION OF HALOGEN DERIVATIVES OF ALKANES

1. Monohalogen Derivatives
When one hydrogen atom of alkane is substituted by one halogen atom, the compounds formed are called as monohydrogen derivatives of alkanes.
e.g.  CH3Cl (methyl chloride)
        CH3CH2Br (ethyl bromide)
2. Polyhalogen Derivatives
When more than one hydrogen atoms of alkanes are substituted by corresponding number of halogen atoms, the compounds formed are called polyhalogen derivatives of alkanes. They are further classified as
A] Dihalogen derivatives:-  e.g. CH2Cl2
B] Trihalogen derivatives :- e.g. CHCl3
C] Tetrahalogen derivatives :- e.g. CCl4

ALKYL HALIDES

Alkyl halides are represented by the general formula CnH2n+1X or R-X where R = alkyl groups and X = halogen group. Alkyl halides are further classified into three categories on the basis of the type of the carbon atom to which the halogen atom is bonded.
1. Primary alkyl halides (1ᵒ) – general formula
R – CH2 – X
2. Secondary alkyl halides (2ᵒ) – general formula

3. Tertiary alkyl halides (3ᵒ) – general formula

PREPARATION OF ALKYL HALIDES

1. Preparation of Alkyl Halides from halogenations of alkanes
Alkyl chlorides are formed by the action of chlorine on alkanes in presence of diffused sunlight or ultraviolet light or at high temperature.
Ethane on chlorination forms ethyl chloride
CH3 – CH2 – H (Ethane) + Cl – Cl  —–sunlight→  CH3 – CH2 – Cl (Ethyl Chloride) + HCl
The general formula for preparation of alkyl chloride is
R – H ( Alkane ) + Cl—–sunlight→ R – Cl ( alkyl Chloride )
Alkyl bromides are formed when alkanes are heated with bromine in the presence of anhydrous aluminium tribromide.
CH3 – H ( Methane) + Br – Br —-AlBr3→ CH3 – Br ( Methyl Bromide ) + HBr
Alkyl iodides are formed when alkanes are treated with iodine in the presence of oxidizing agent like HIO3, HgO, HNO3 etc.
5C2H6 (Ethane) + 2I+ HIO3 → 5C2H5I (Ethyl Iodide) + 3H2O       
Reaction of alkanes with fluorine is explosive and the product form hydrofluoric acid is poisonous and corrosive hence alkyl fluorides are not prepared by halogenations of alkanes.
2. Preparation of Alkyl Halides By addition of hydrogen to alkenes
Alkyl halides are obtained when hydrogen halide is treated with alkenes. In this reactions addition of hydrogen halide takes place across the double bond in alkene.
a) With symmetrical alkenes
CH2 = CH2 (Ethane) + HCl → CH – CH2Cl (Ethyl Chloride)
CH2 = CH2 (Ethane) + HBr → CH3 – CH2Br (Ethyl Bromide) 
CH2 = CH2 (Ethane) + HI → CH3 – CH2I (Ethyl Iodide)             
b) With unsymmetrical alkenes
The addition of hydrogen halides to an unsymmetrical alkene gives two products.
In this reaction major product can be identified using Markownikoff’s rule.
3. Preparation of Alkyl Halides From Alcohols
R – OH → R – X
-OH group of alcohol can be replaced by halogen using three types of reagent.
a)  Reaction with hydrogen Halides
Alcohols react readily with hydrogen halides to yield alkyl halides and water.
1. Alkyl chlorides
C2H5OH ( Ethyl Alcohol ) + HCl ——-ZnCl2 ̶Δ→ C2H5Cl (Ethyl Chloride) + H2O
2. Alkyl Bromides
C2H5OH + HBr —-NaBr+H2SO4→ C2H5Br + H2O
Ethyl Alcohol       reflux        Ethyl bromide
3. Alkyl Iodide
C2H5OH (Ethyl Alcohol) + HI  —Δ→  C2H5I (Ethyl iodide) + H2O
b) Reaction with phosphorous halides
i) With phosphorus trihalides (PX3)
Alkyl Chloride are prepared by refluxing alcohols with phosphorus trichloride
3C2H5OH (Ethyl alcohol) + PCl3 —Δ→  3C2H5Cl (Ethyl Chloride) + H3PO4 (Phosphoric acid)

Alkyl bromide and alkyl iodides are prepared by the action of bromine or iodine in presence of red phosphorus on alcohols in situ.
3C2H5OH (Ethyl Alcohol) + PBr3 →  3C2H5Br (Ethyl Bromide) + H3PO3 (Phosphoric acid)
ii) With phosphorous pentahalides (PX5)   
Ethyl alcohol when refluxed with PCl5 forms ethyl Chloride
3C2H5OH (Ethyl alcohol) + PCl5 —Δ→  C2H5Cl (Ethyl Chloride) + HCl + POCl3 (Phosphorous-Oxychloride)
4. Preparation of Alkyl Halides By halogen exchange
Alkyl chloride or bromide when treated with NaI in presence of dry aceton give alkyl iodides. This reaction is known as FinKelstein reaction.
R – Cl + NaI → RI + NaCl
R – Br + NaI → RI + NaBr
e.g.  C2H5Br + NaI →  C2H5I + NaBr

PHYSICAL PROPERTIES OF ALKYL HALIDES

In this part of haloalkanes and haloarenes class 12 notes, we learn about physical properties of alkyl halides.
1. The lower members are gases at room temperature and higher members are liquid or solid.
2. Volatile halogen compound have sweet smell.
3. Boiling points of alkyl halides are greater than that of corresponding hydrocarbons. Boiling point increases with increase in molecular mass and force of attraction between molecules.
4. In case of isomeric haloalkanes, branching results in decrease in boiling point.

5. Alkyl halides are slightly soluble in water due to intermolecular forces of attraction and readily soluble in organic solvents.

CHEMICAL REACTIONS OF ALKYL HALIDES

In this section we learn about different chemical reactions of alkyl halides.

A) Substitution reaction

The reaction in which an atom or a group of atoms are substituted (replaced) from substrate by same number of other atoms or groups are called substitution reaction.
R – X   (Alkyl Halide)  +  Y  (Nucleophile)      →     R – Y   +      X (Halide Ion)
1) Reaction with aq. KOH or NaOH
R – X (Alkyl Halides) +  KOH (aq.) —boil→  R – OH (Alcohol) +  KX
e.g. CH3 – I (Methyl Iodide) + KOH  —boil→  C2H5OH (Ethanol) +  KI
2) Reaction with ammonia (NH3)                                     
R – X (Alkyl Halide alc.) +  H – NH2       ̶̶̶̶ ̶ ̶ ̶ ̶ ̶ Δ̶̶ ̶ ̶ ̶ under pressure ̶ ̶ ̶ ̶ ̶→    R – NH2 (Primary Amine) +  HX
e.g. CH3 – Cl + H – NH2        ̶̶̶̶ ̶ ̶ ̶ ̶ ̶ Δ̶̶ ̶ ̶ ̶ ̶ under pressure ̶ ̶ ̶ ̶→      CH3 – NH2  +  HCl               
3) Reaction with alc. KCN
R – X (Alkyl Halide)  +   KCN   —boil→   R – CN (Alkyl Cyanide)  +  KX
e.g.    C3H7Cl (2-Chloro-propane) + KCN   —boil→     C3H7CN (2-Methyl-propane-nitrile) + KCl
4) Reaction with moist silver oxide                                                       
R – X (Alkyl Halide) + AgOH     —moist AgO2 boil→    R – OH (Alcohol)  +   AgX

B) Elimination reactions                                                 

The reaction in which two atoms or group are removed from adjacent carbon atoms in a molecule to form an unsaturated compound is called an elimination reaction.
Dehydrohalogenation reaction
When alkyl halides are heated with alcoholic solution alkali hydroxide (KOH or NaOH), halogen atom from α-carbon atom and a hydrogen atom from adjacent β-carbon gets eliminated to form corresponding alkenes.


Saytzeff’s Rule

In a dehydrohalogenation reactions, the preferred product is that alkene which has the greater number of alkyl group attached to doubly bonded carbon atoms.

C) Reaction with metals

1) Reaction with sodium : Wurtz reaction
When an alkyl halide is treated with sodium metal in pure and dry ether, a hydrocarbon is formed containing twice the number of carbon atoms present in the alkyl halide.
R – X (Alkyl Halide) +  2Na  +  X – R (Alkyl Halide)   ̶ dry ether→  R – R (Alkane) +  2NaX (Sodium Halide)

e.g. Methyl bromide reacted with sodium in the presence of dry ether forms ethane.
CH3 – Br (Methyl Bromide) +  2Na  +     Br – CH3  (Methyl Bromide) —-dry ether→ CH3 – CH3 (Ethane) + 2NaBr
2) Reaction with magnesium : Formation of Grignard reagent
Grignard reagent is an organometallic compound in which the divalent magnesium is directly linked to an alkyl group and a halogen atom.
It is represented by general formula R – Mg – X. gringnard reagent can be prepared by reaction of alkyl halide with pure and dry magnesium in the presence of dry water.
R – X (Alkyl Halide) + Mg  —dry ether→    R – Mg – X ( Alkyl magnesium halide) (Grignard reagent)
e.g. CH3 – I (Methyl Halide)  +   Mg  dry ether→  CH3 – Mg – I (Methyl magnesium iodide)

Stereochemical aspects of Nucleophilic substitution reaction

Plan polarized light

When an ordinary light is passed through Nicol’s prism the light emerging out of it, consist of rays vibrating in one plane only. Such a beam of light which consist of the ways of light vibrating in one plane is called plane polarized light.

Optical activity

When plane polarized light is passed through solution of certain organic substances like glucose lactic acid etc. the plane of polarized light gets rotated through a certain angle. This property of phenomenon of certain organic substance of rotate the plane of polarized light towards right (clockwise) or towards left (anticlockwise) is called optical activity.

Optically active molecules

The molecules which can rotate the plane of plane polarized light are known as optically active molecules.
e.g. Glucose, Lactic acid, 2-chlorobutane etc.
This are of two types :
1) Dextro-rotatory molecules
Molecules which rotate the plane at polarization to the right hand side are known as dectro rotator molecules. These are denoted by (+) or d sign.
2) Laevo-rotatory molecules
These molecules rotate the plane of polarization to the left hand side are known as laevo molecules. These are denoted by (-) or l sign.

Optically inactive Molecules

The molecules which do not rotate the plane at plane polarized light are called optically inactive molecules.
Enantiomers
Stereo isomers which are non super imposable mirror images of each other and rotate the plane of plane polarized light through the same angle but in opposite direction are known as enantiomers.
Chirality
Any object which is non – super imposable with its mirror image is said to be chiral and this property is known as chirality.

REACTION MECHANISM

Mechanism of a reaction is a step by step description of exactly how the reactant are transformed into products in as much details as possible.

1) SN2 Mechanism

When the rate of nucleophilic substitution reaction depends upon the concentration of the two molecules, the substrate and the nucleophile (both the species), the reaction is said to be second order Nucleophilic substitution reaction. It is represented by SN2.
The rate of SN2 reaction is directly proportional to concentration of the substrate and nucleophilic.

e.g.       CH– Br (Methyl Bromide) + OH (nucleophilic)  → CH3OH (Methanol) + Br
Kinetic expression
: . Rate α [CH3Br] [OH]
: . Rate = K [CH3Br] [OH]
Where K is the specific constant.

2) SN1 Mechanism

When the rate of nucleophilic substitution reaction depends upon the concentration of only one molecule or species (the substrate) and not the nucleophile, the reaction is said to be first order Nucleophilic substitution reaction. It is represented by SN1.
The rate of SN1 reaction is directly proportional to concentration of only one substrate.

Kinetic expression
: . Rate α [(CH3)3C – Br]
: . Rate = K [(CH3)3C – Br]
Where K is the specific constant.

HALOARENES – Haloalkanes and Haloarenes Class 12 Notes

When one or more hydrogen atoms of an aromatic hydrocarbon are replaced or substituted by corresponding number of halogen atoms (such as chlorine, bromine or iodine), the new compound so obtained are called haloarenes.
Ar – H + Cl → Ar – Cl + H
They are represented as Ar – X, where Ar = Aryl Group (C6H5) and X = F,Cl, Br, I.

Classification of Haloarenes

Haloarenes are classified as mono, di or polyhalogen depending on whether they contain one, two or more halogen atoms in their molecules.
The C – X bond length in haloarenes is 1.70A. It is shorter in length and stronger than in alkyl halides having bond length 1.77A.

PREPARATION OF HALOARENES

1) Preparation of Haloarenes By electrophilic substitution.
Aryl Chlorides i.e. haloarenes can be easily prepared by electrophilic substitution reaction of arene with the chlorine (Cl2) and bromine (Br2) respectively in the presence of lewis acid catalysts like FeCl3 , Fe , BCl3 etc. e.g.

2) By SandMeyer’s reaction
When a primary aromatic amine, dissolved or suspended in cold aqueous mineral acid  is treated with sodium nitrite a diazonium salt is formed. Mixing the solution of freshly prepared diazonium salt with Cuprous Chloride (Cu2Cl2) or Cuprous Bromide (Cu2Br2) result in the replacement of the diazonium group by –Cl or –Br. This reaction is known as Sandmeyer’s reaction. e.g.

CHEMICAL REACTION OF HALOARENES

This chemistry notes section of haloalkanes and haloarenes class 12, contain study material on chemical reactions of haloarenes.

1) Halogenation

Chlorobenzene reacts with chlorine in presence of anhydrous ferric chloride to give a mixture of ortho and para dichlorobenzene.

2) Nitration

When Chlorobenzene is heated with nitrating mixture (conc. Nitric acid + conc. Sulphuric acid) a mixture of 1 – chloro – 4 – nitrobenzene and 1 – chloro – 2 – nitro – benzene is formed.

3) Sulphonation

Chlorobenzene with conc. H2SO4 gives 2 – Chlorobenzene Sulfonic acid and 4- chlorobenzene Sulphonic acid.

4) Friedel Craft’s alkylation reaction


5) Friedel Craft’s acylation reaction


6) Reaction with sodium metal: Wurtz fittig reaction

     Aryl halide reacts with alkyl halides and undergoes coupling reaction when treated with sodium metal in the presence of dry ether to form alkyl benzene. This reaction is known as Wurtz Fittig Reaction.

P,P- dichlorodiphenyltrichloroethane (DDT)
DDT is used as insecticide against malaria and it is used to kill various insects like housefly and  mosquitoes.  

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