10. HALOALKANES AND HALOARENES
Q. 1. What happens when KI is added in the reaction chamber where the following reaction is taking place ?
CH3—Cl 
+ CH3 – CN
Ans. Since I– is stronger nucleophile thatn CN– and also I– is a better leaving group than Cl–. Addition of I will increases the rate for the given reaction.
Q. 2. Name the Grignard reagent required for converting formaldehyde to n-Butyl alcohol.
Ans. n-Propyl magnesium iodide.
Q. 3. Write structures of the following compounds :
(i) 2-chloro-3-methyl pentane
(ii) 1-chloro-4-ethyl cyclohexane
(iii) 4-tert-Butyl-3-iodohaptane
(iv) 1, 4.-Dibromo but-2-ene
(v) 1-Bromo-4-sec buty 1-2 methyl benzene.
Ans. (i) CH3CH2CH(CH3)CHClCH3
Q. 4. Write IUPAC name of the following:
Ans. (i) 2-Bromopent-2-ene
(ii) 3-Bromo-2-methylbut-1-ene
(iii) 4-Bromo-3-methylpent-2-ene
(iv) 1-Bromo-2-methylbut-2-ene
(v) 1-Bromobut-2-ene
Q. 5. What do you understand by nucleophilic substitituion reaction ? Why do haloalkanes undergo nucleophilic substitution reaction ?
Ans. Nucleophilic Substitution Reaction : A chemical reaction in which a stronger nucleophilic (electron rich group which loves +ve centre) substitutes a weaker nucleophile is called the nucleophilic substitution reaction. For example,
CH2Br + OH- → CH3OH + Br–
is a nucleophilic substitution reaction.
Nucleophilic Substitution reactions of haloalkanes : In haloalkanes (R-X) the halogen is more electronegative than carbon. Thus, C – X bond is polarized as C5+ – X5-. Therefore, a nucleophilic (electron rich species) attacks the positively charged carbon atom and replaces the negatively charged halogen from R-X. Thus a new product is formed.
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-C5+ – X5- + : Z → C-Z + X–
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Nucleophile Product Halide ion
Q .6. What is meant by dehydrohalogenation ?
Ans. Dehydrohalogenation is the elimination reaction shown by a haloalkane which involves the removal of the halogen atom together with a hydrogen atom bonded to a C atom adjacent to C atom bearing the halogen. As a result of dehydrohalogenation an alkene is formed. For example :
Q. 7. What is Grignard reagent and is it prepared ?
Ans. Grignard reagents : Alkyl magnesium halides (RMgX) are called Grignard reagents.
Preparation of Grignard reagents : When a solution of alkyl halide (R – X) in ether is allowed to stand over magnesium turning for some time then metal gradually dissolves. In this reaction alkyl magnesium halide is formed. This compound is called Grignard reagent.
R-X+Mg
R-Mg-X
Haloalkane Grignard reagent
Q. 8. Write the equation for the following reactions :
(i) Alkyl halide reacts with alcoholic solution of potassium cyanide.
(ii) Bromoethane reacts with alcoholic ammonia.
(iii) Haloalkane reacts with silver cyanide.
(iv) Ethyl chloride reacts with sodium lead alloy.
(v) 1-chloropropane is heated at about 573 K in the presence of AlCl3
(vi) Haloalkane is heated with benzene in the presence of anhydrous aluminium chloride.
Ans.
(i) R-X + KCN → R-C = N + KX
Alkyl Potassium Alkyl cynide Potassium
halide cyanide (alkyl nitrile) iodide
(ii) C2H5Br + NH3 (alc.) → C2H2NH2 + HBr
Bromoethane Ethanamine
(iii) R-X + AgCn → R-N = C + AgX
Haloalkane Silver Cyanoalkane
. cyanide
(iv) 4C2H5Cl + 4Na – Pb → (C2H5)4 Pb + 4NaCl + 3Pb
Ethyl chloride Sodium Tetraethyl
lead alloy lead
(v)
CH3 – CH2 – CH2 – Cl + 
C6H5Br +HX
Haloalkane Benzene Alkyl Benzene
(Friedel crafts reaction)
Q. 9. In contrast to arenes, aliphatic hydrocarbons do not undergo nitration easily.
Ans. Nitration proceeds through the attack of the electrophile NO
at a point of high electron density. In arenes, due to resonance such electrophilic reaction is possible, while similar situation in aliphatic hydrocarbons is not possible under normal conditions.
Class 12th Chemistry Short Type Question English
