HALOALKANE & HALOARENE CLASS XII CHEMISTRY STUDY MATERIAL

L-10

HALOALKANES AND HALOARENES

Some basic concepts

Alkyl/ Aryl halides may be classified as mono, di, or polyhalogen (tri-, tetra-, etc.)

Compounds depending on whether they contain one, two or more halogen atoms in their structures.

Nature of C-X Bond 

Since halogen atoms are more electronegative than carbon, the carbon halogen bond of alkyl halide is polarised; the carbon atom bears a partial positive charge whereas the halogen atom bears a partial negative

charge. Since the size of halogen atom increases as we go down the group in the periodic table, fluorine atom is the smallest and iodine atom, the largest.

Preparation of alkyl halides

Alkyl halides are prepared by the free radical halogenations of alkanes, addition of halogen acids to alkenes, replacement of –OH group of alcohols with halogens using phosphorus halides, thionyl chloride or halogen acids.

TYPES OF FORCES

The boiling points of organo halogen compounds are comparatively higher than the corresponding hydrocarbons because of strong dipole-dipole and van der Waals forces of attraction. These are slightly soluble in water but completely soluble in organic solvents.

NUCLEOPHILLIC SUBSTITUTION REACTIONS

The polarity of carbon-halogen bond of alkyl halides is responsible for their nucleophilic substitution, elimination and their reaction with metal atoms to form organometallic compounds.

Nucleophilic substitution reactions are categorized intoSN1 and SN2 on the basis of their kinetic properties.

Chirality has a profound role in understanding the reaction mechanisms of SN1 and SN2 reactions.

SN2 reactions of chiral alkyl halides are characterized by the inversion of configuration while SN1 reactions are characterized by racemisation

Aryl halides are prepared by electrophilic substitution to arenes. Fluorides and iodides are best prepared by halogen exchange method.

A number of polyhalogen compounds e.g., dichloromethane, chloroform, iodoform, Carbon tetrachloride, Freon and DDT have many industrial applications.

SOME  CONVERSIONS

HALOALKANES AND HALOARENES

EXPECTED QUESTIONS

1.      Write the structures of the following:

(I)                2-chloro-3-methylbutane (ii) 3-chloro-4- methylhexane (iii) 1-iodo-2,2-dimethylbutane

(iv) 1-bromo-3,3-dimethyl-1-phenylbutane

2.         Write the chlorination reaction of butane?.

 3.        Convert toluene into O-halo toluene?

4.         Explain (a) wurtz- fitting reaction

                (b) Fittig reaction

5.         Explain friedal- craft reaction?

6.         Why Grignard reagent should be prepared under anhydrous conditions?

    Grignard   reagent react with water to form alkanes, therefore they are prepared under anhydrous conditions.

7.         Why haloarenes are less reactive than the haloalkanes towards nucleophilic substitution?

     (i) In haloarenes, there is double bond character between the carbon and hydrogen due to resonance effect which makes them less reactive.

    (ii)  In benzene, carbon being sp2 hybridized is smaller than the Sp3 hybridized carbon in haloalkanes. So, C-Cl bond in aryl halides is shorter and stronger.

8.         p- dichlorobenzene has highest m.p. than those of ortho and meta –isomers?

           p- Dichlorobenzene is symmetrical, fits into the crystal lattice more readily and has higher melting point.

9.         Complete the following equations:

           

Ans:

What are ambident nucleophiles?

Nucleophiles which can attack through two different sites are called ambident    nucleophiles. Example: - Cyanide ion

                                               

11.       Why is sulphuric acid not used during the reactions of alcohols with KI?

     Sulphuric acid is an oxidizing agent. It oxidizes HI produced during the reaction, to I₂.

                              

Nature of C-X Bond

Since halogen atoms are more electronegative than carbon, the carbon halogen bond of alkyl halide is polarized. the carbon atom bears a partial positive charge whereas the halogen atom bears a partial negative

Charge. Since the size of halogen atom increases as we go down the group in the periodic table, fluorine atom is the smallest and iodine atom, the largest.

NAME REACTIONS

SOME IMPORTANT CONVERSIONS

(I)                PROPENE to PROPAN-1-OL

(II)             ETHANOL to BUT-1-ENE

SANDMAYER REACTION

ATTERMANN REACTION

TWO TYPES OF NUCLEOPHILIC SUBSTITUTION

SN1	SN2
1.    Substitution in which rate of reaction depends on conc. of only one of the reactant 2.    Tertiary halide shows such type  of reaction. 3.    Carbocation is formed as intermediate  4.    Favored by non Polar Solvent. 5.    No inversion of molecule is observed. Racemization occurs.	1.    Substitution reaction in which the rate of reaction depends on conc. of  two reactants. 2.    Primary halide shows such type of reaction. 3.    A trigonal Bipyramidal geometry is obtained as intermediate. 4.    Favored by polar solvent. 5.    Inversion of molecule is observed

EXAMPLES OF NUCLEOPHILIC SUBSTITUTION REACTIONS

S No.	Reagent	Nucleophile	Products	Class of products
1.	NaOH,KOH,AgOH	-OH	R − OH	Alcohols
2.	R’ − ONa	-OR’	R’ – O -R	Ethers
3.	NaCN, KCN AgCN	- CN      - NC	R – CN    R - NC	Cyanides(Nitriles) Iso-cyanides
4.	KNO2 AgNO2	- O - N = O       - NO2	R- O – N = O  R – NO2	Alkyl nitrite Nitroalkanes
5.	R’COOAg	- OOCR’	R – OOR’	Esters
6.	NH3	- NH2	R –NH2	Amines
7.	LiAlH4	- H	R - H	Alkanes

1.      Write IUPAC names of the following:

2.      Convert   (i) Benzene into 4- bromonitrobenzene

         

3.      Arrange in order of boiling points.

(a) Bromobenzene, bromoform, chloromethane, di bromomethane

(b) 1-chloropropane, isopropylchloride, 1-chlorobutane

       4 . Convert   (i) Methyl bromide to acetone.

                            (ii) Benzyl chloride to 2-phenyl acetic acid

     5. How will you distinguish between the following pairs of compounds?

          (i) Chloroform and carbon tetra chloride.

          (ii) Benzyl alcohol and chlorobenzene.

ANSWERS       

1.

2.      CONVERSIONS

3.       (a) chloromethane<Bromobenzene< di bromomethane bromoform

(b) isopropylchloride < 1-chloropropane,< 1-chlorobutane

  4.                       dry ether                                               O Mg Br

                                                                                                        H₃O⁺                       Cu/573k

(i) CH₃Br + Mg  à CH₃MgBr   +  CH₃CHO  à  CH₃CHCH₃   à CH₃CH(OH)CH₃  àCH₃COCH₃                                                                                                                     

(ii) C₆H₅CH₂OH +SOCl₂ —→ C₆H₅CH₂Cl + KCN—→ C₆H₅CH₂CN—→ C₆H₅CH₂COOH

5.(a) On heating chloroform and carbon tetrachloride with aniline and ethanolic potassium hydroxide separately chloroform forms pungent smelling isocyanide but carbon tetrachloride does not form this compound.

(b) On adding sodium hydroxide and silver nitrate to both the compounds benzyl chloride forms white precipitate but chloro benzene does not form white precipitate.

Mind mapDiazonium salts

                                                              H₃PO₂                       C₆H₆

                                                                 CuCl / HCl              C₆H₅Cl                                                    

           C₆H₅N₂Cl                                               

                                                                    Cu / HCl                 C₆H₅Cl

                                                       Cu Br / H Br               C₆H₅Br

              

                                                                                                     

 ^-                                                              Cu CN                                     

                                                                                                       C₆H₅CN        

                                                                                                                                                              

                                                            CH₃COOH                           

                                                                                                    C₆H₅ COOCH₃

                                                    HNO2                     

                                                                                              C₆H₅NO₂                      

Common Nucleophlic Substitution Reactions

S No.	Reagent	Nucleophile	Products	Class of products
1.	NaOH,KOH,AgOH	-OH	R − OH	Alcohols
2.	R’ − ONa	-OR’	R’ – O -R	Ethers
3.	NaCN, KCN AgCN	- CN      - NC	R – CN    R - NC	Cyanides(Nitriles) Iso-cyanides
4.	KNO2 AgNO2	- O - N = O       - NO2	R- O – N = O  R – NO2	Alkyl nitrite Nitroalkanes
5.	R’COOAg	- OOCR’	R – OOR’	Esters
6.	NH3	- NH2	R –NH2	Amines
7.	LiAlH4	- H	R - H	Alkanes

VALUE BASED QUESTIONS

Q1.Antiseptic is antimicrobial applied on living tissues. Tincture of iodine is a common example. Rewati had read that iodoform also possesses an appreciable antiseptic property. How will you explain this? Identify the value associated with it.

Ans: Iodoform is used as an antiseptic, particularly for dressing wounds. Actually, on coming in contact with skin it decomposes and slowly loses iodine which accounts for the antiseptic properties of iodoform. Scientific Knowledge and logical thinking.

Q2. Ravi Prasad a farmer has 25 acres of land. He noticed some infection on the leaves of his crops. He called his friend Raghav, who advised him to use DDT. However, Ravi Prasad preferred to use dry powder of neem leaves as an insecticide.

A) Mention reasons why Ravi Prasad prefer using neem powder?

B) In your opinion, who took right decision?

C) Write values associated with above decision?

Ans: a) because neem powder will not harm soil.

        b) Ravi Prasad.

        c) Application of scientific knowledge. Environmentalconcern.

Q3. Gita regularly cleans her artist father’s metal table with an organic liquid given by her father due to this regular activity she had eye irritation. After few months she eventually losses vision in one of her eye.

a) What is the effect of chlorination in the presence of sunlight on methane?

b) Why did gita lose her eye sight?

c) List values associated with the above problem.

Ans: a) Methane gets converted to _CCL4

             _b) Because of release of chorine vapors in the atmosphere.

c) Appropriate knowledge of organic chemical is essential while handling them.

Q4. A farmer cultivating his land near the village pond was also drawing water from it for irrigation. He used insecticide excessively to protect his crops and improve the harvest, over a period of time his agricultural growth improved vastly. But the pond lost its aquatic life.

a) Give the IUPAC name of the compound used as an insecticide but not a pesticide.

b) Contrast the activity in plants and aquatic life with insecticides.

c) What value got expressed in student’s suggestions?

Ans: a) 2, 2-Bis(4-Chlorophenyl)-1,1,1-trichloroethane

        b) Plant growth improved because it is a very effective insecticide but aquatic life got destroyed as it is highly toxic to fish.

       c) Prompt application of chemistry knowledge, environmental concern.

Q5. A sweet smelling organic compound ‘A’ is slowly oxidized by air in the presence of light to a highly to a poisonous gas. On warming with silver powder, it carbide on water.Gas’B’ is also used to make metallic objects reusable. Identify A  and B. Write the values associated to use the reagent as minimum possible to conduct this chemical reaction.

Ans: Compound A is chloroform, Compound B is ethylene. Chloroform is highly toxic in nature its vapor when inhaled damage the central nervous system. Therefore, it should be handled very carefully.