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Karnataka 2nd PUC Chemistry Question Bank Chapter 10 Haloalkanes and Haloarenes
2nd PUC Chemistry Haloalkanes and Haloarenes NCERT Textbook Questions
Name, the following halides according to IUPAC system and classify as alkyl, allyl, benzyl (primary, secondary, tertiary), vinyl or aryl halides:
Give the IUPAC names of the following compounds:
(iii) ClCH2C = CCH2Br
Write the structures of the following organic halogen compounds.
(vii) l-Bromo-4-sec-butyl-2-methyl- benzene
Which one of the following has the highest dipole moment?
- Tetrachloromethane (CCl4) is a symmetrical molecule and has zero dipole moment.
- In chloroform (CHC13), the resultant dipole moment of two C- Cl bonds is opposed by the resultant dipole moment of C – Cl, and C- H bonds. Since the latter resultant dipole moment is smaller, the molecule as a whole has dipole moment (μ) = 103 D).
- In dichloromethane (CH2Cl2), the resultant dipole moments of two C – Cl and two C – H bonds reinforce one another. The molecule has a maximum dipole moment (μ) = 1.62 D.
CH2Cl2 has the maximum dipole moment value.
A hydrocarbon C5H10 does not react with chlorine in dark but gives a single monochloro compound C5H9Cl in bright sunlight. Identify the hydrocarbon.
(i) The hydrocarbon with molecular formula C5H10 is either an alkene or cycloalkane.
(ii) Since the hydrocarbon does not react with chlorine (Cl2) in the dark, it is not an alkene.
(iii) As it forms only a single monochloroderivative in bright sunlight, it is cyclopentane which is symmetrical. In this, all the ten hydrogen atoms are identical.
Write the isomers of the compound having the formula C4H9Br.
There are four isomers of the compound having the formula C4H9Br. These isomers are given by
Write the equations for the preparation of 1-iodobutane from
What are ambident nucleophiles? Explain with an example.
Ambident nucleophiles are nucleophiles having two nucleophilic sites. Thus ambident nucleophiles have two sites through which they can attack. For example, nitrite ion is an ambident nucleophile.
Nitrite ion can attack through oxygen resulting in the formation of alkyl nitrites. Also, it can attack through nitrogen resulting in the formation of nitroalkanes.
Which compound in each of the following pairs will react faster in SN2 reaction with OH?
1. CH3Br or CH3I
2. (CH3)3CCl or CH3Cl
(1) CH3– I will react faster than CH3– Br because I ion is a better leaving group than Br– ion. Alternatively, the bond dissociation enthalpy of C- I bond is less than that of C – Br bond.
(2) CH3Cl will react faster because of a less steric hindrance as compared to (CH3)3CCl
Predict all the alkenes that would be formed by dehydrohalogenation of the following halides with sodium ethoxide in ethanol and identify the major alkene:
2. 2-Chloro-2-methyl butane,
1. In this compound, all β hydrogens are equivalent thus dehydrohalogenation gives only one alkene.
2. In the given compound, there are two different sets of equivalent P – hydrogen atoms labelled as ‘a’ and ‘b’ thus dehydrohalogenation yields two alkenes.
Sayt zeff’s rule implies that in dehydrohalogenate reactions, the alkene having a greater number of alkyl groups attached to a doubly bonded carbon atoms is preferably produced.
Therefore, 2-methyl but – 2- ene is the major product in the reaction.
3. In the given compound, there are two different sets of equivalent P – hydrogen atoms labelled as ‘a’ and ‘b Thus dehydrogenation of the compound yields two alkenes.
2 -1 thyl – 3, 3 – demethyl but – tene
Again according to Saytzeff’s rule
3,4,4 – trimethylpent – 2- ene is the major product.
How will you bring about the following conversions?
(i) Ethanol to but-1-yne
(ii) Ethane to bromoethene
(iii) Propene to 1-nitropropane
(iv) Toluene to benzyl alcohol
(v) Propene to propyne
(vi) Ethanol to ethyl fluoride
(vii) Bromomethane to propanone
(viii) But-l-ene to but-2-ene
(ix) 1-ChIorobutane to n-octane
(x) Benzene to biphenyl.
1. The dipole moment of chlorobenzene is lower than that of cyclohexyl chloride?
2. Alkyl halides, though polar, are immiscible with water?
3. Grignard reagents should be prepared under anhydrous conditions?
In chlorobenzene, the Cl-atom is linked to an sp2 hybridized carbon atom. In cyclohexyl chloride, the Cl atom is linked to an sp3 hybridized carbon. Now, sp2 hybridized carbon has more s-character than sp3 hybridized carbon atom. Therefore, the former is more electronegative than the latter. Therefore, the density of electrons of the C-Cl bond near the Cl-atom is less in chlorobenzene than in cyclohexyl chloride. Moreover, the the-R effect of the benzene ring of chlorobenzene decreases the electron density of the C-Cl bond near the Cl-atom. As a result, the polarity of the C-Cl bond in chlorobenzene decreases. Hence, the dipole moment of chlorobenzene is lower than that of cyclohexyl chloride.
2. To be miscible with water, the solute-water force of attraction must be stronger than the solute and water-water forces of attraction. Alkyl halides are polar molecules and so held together by dipole-dipole interactions. Similarly, strong H-bonds exist between the water molecules. The new force of attraction between the alkyl halides and water molecules is weaker than the alkyl-halide- alkyl halide and water-water forces of attraction. Hence, alkyl halides (though polar) are immiscible with water.
3. Grignard reagents are very reactive. In the presence of moisture, they react to give alkanes
R-Mg-X + H2O → RH + Mg(OH)X
Therefore, Grignard reagents should be prepared in anhydrous conditions.
Give the uses of freon 12, DDT, carbon tetrachloride and iodoform.
It is an important pesticide.
Freon-12: Since freons have been found to be one of the factors responsible for the depletion of the ozone layer, they are being replaced by other harmless compounds in many countries.
D.D.T.: DDT is highly toxic and has strong insecticidal properties and thus it was widely used as an insecticide and pesticide.
Carbon Tetrachloride: It is produced in large quantities for use in the manufacture of refrigerants and propellants for aerosol cans. It is used as feedstock in the synthesis of chlorofluorocarbons and other chemicals, pharmaceutical manufacturing and general sol¬vents use. Until the mid-1960s, it was widely used as a cleaning fluid both in the industry as a degreasing agent and in the home, as a spot remover, and as a fire extinguisher.
Iodoform: It was earlier used as an antiseptic but the antiseptic properties are due to the liberation of free iodine and not due to iodoform itself. Due to its objectionable smell, it has been replaced by other formulations containing iodine.
Write the structure of the major organic product in each of the following reactions:
Write the mechanism of the following reaction:
The given reaction is
The reaction is an SN2 reaction. In this reaction, CN– acts as the nucleophile and attacks the carbon atom to which Br is attacked. CN– is an ambident nucleophile and can attack through both C and N. In this case, it attacks through the C-atom.
Arrange the compounds of each set in order of reactivity towards SN2 displacement:
1. 2-Bromo-2-methyl butane, 1-Bromopentane, 2-Bromopentane
2. l-Bromo-3-methyl butane, 2-Bromo-2-methyl butane, 3-Bromo-2-methyl butane
3. 1-Bromobutane, l-Bromo-2, 2-dimethylpropane, l-Bromo-2-methyl butane, l-Bromo-3-methyl butane.
An SN2 reaction involves the approaching of the nucleophile to the carbon atom to which the leaving group is attached. When the nucleophile is sterically hindered, then the reactivity towards SN2 displacement decreases. Due to the presence of substituents, hindrance tp the approaching nucleophile increases in the following order.
1- Bromopentane < 2-bromopentane < 2 – Bromo-2-methyl butane
Hence, the increasing order of reactivity towards SN2 displacement is
2- Bromo – 2- methyl butane < 2-bromopentane < 1-Bromopentane
since steric hindrance in alkyl halides increases in the order of 1° <2° < 3°, the increasing order of reactivity towards SN2 displacement is 3° < 2° <1°.
Hence, the given set of compounds can be arranged in the increasing order of their reactivity towards
SN2 displacement as:
2-Bromo – 2- methyl butane < 3-Bromo-2-methyl butane < 1-Bromo -3- methyl butane.
The steric hindrance to the nucleophile in the SN2 mechanism increases with a decrease in the distance of the substituents from the atom containing the leaving group. Further, the steric hindrance increases in increases with an increase in the number of substituents. Therefore, the increasing order of steric hindrances in the given compounds is as below:
1- Bromobutane < 1-Bromo-3-methylbutane < 1 -Bromo-2-methylbutane < 1 -Bromo-2, 2- demethylbutane.
Hence, the increasing order of reactivity of the given compound towards the SN2 mechanism is
l-Bromo-2, 2-dimethylbutane < 1 -Bromo-2-methyl butane < l-Bromo-3-methyl butane < 1-Bromobutane.
Out of C6H5CH2Cl and C6H5CHClC6H5, which is more easily hydrolysed by aqueous KOH?
p-Dichlorobenzene has higher m.p. and solubility than those of o- and m-isomers. Discuss.
The p-isomer being more symmetrical fits closely in the crystal lattice and thus has stronger intermolecular forces of attraction than o- and m-isomers. Since during melting or dissolution, the crystal lattice breaks, therefore, a large amount of energy is needed to melt or dissolve the p-isomer than the corresponding o-and m-isomers. In other words, the melting point of the p-isomer is higher and its solubility lower than the corresponding o-and m-isomers.
How the following conversions can be carried out?
(i) Propene to propan-l-ol
(ii) Ethanol to but-l-yne
(iii) 1-Bromopropane to 2-bromopropane
(iv) Toluene to benzyl alcohol
(v) Benzene to 4-bromonitrobenzene
(vi) Benzyl alcohol to 2-phenylethanoid acid
(vii) Ethanol to propane nitrile
(viii) Aniline to chlorobenzene
(ix) 2-ChIorobutane to 3, 4-dimethyl hexane
(x) 2-Methyl-l-propene to 2-chloro-2-methylpropane
(xi) Ethyl chloride to propanoic acid
(xii) But-l-ene to n-butyl iodide
(xiii) 2-ChIoropropane to 1-propanol
(xiv) Isopropyl alcohol to iodoform
(xv) Chlorobenzene to p-nitrophenol
(xvi) 2-Bromopropane to 1-bromopropane
(xvii) Chloroethane to butane
(xviii) Benzene to diphenyl
(xix) tert-Butyl bromide to isobutyl bromide
(xx) Aniline to phenyl isocyanide
The treatment of alkyl chlorides with aqueous KOH leads to the formation of alcohols but in the presence of alcoholic KOH, alkenes are major products. Explain.
In an aqueous solution, KOH almost completely ionises to give OH– ions. OH– ions is a strong nucleophile, which leads the alkyl chloride to undergo a substitution reaction to form alcohol.
On the other hand, an alcoholic soln of KOH contains an alkoxide (RO–) ion, which is a strong base. Thus, it can abstract hydrogen from the β – carbon of the alkyl chloride and form an alkene by eliminating a molecule of HCl.
OH– ion is a much weaker base than RO– ion. Also, the OH– ion is highly solvated in an aqueous solution and as a result, the basic character of the OH- ion decreases. Therefore, it cannot abstract hydrogen from β – carbon.
Primary alkyl halide C4H9Br (a) reacted with alcoholic KOH to give compound (b). Compound (b) is reacted with HBr to give (c) which is an isomer of (a). When (a) is reacted with sodium metal it gives compound (d), C8H18 which is different from the compound formed when n-butyl bromide is reacted with sodium. Give the structural formula of (a) and write the equations for all the reactions.
There are two primary alkyl halides having the formula C4H9Br. They one n-butyl bromide and isobutyl bromide
Since (a) reacts with Na to give a compound C8H18 which is different from the compound formed when n – butyl bromide is reacted with Na, (a) has to be isobutyl bromide.
What happens when
(i) n-butyl chloride is treated with alcoholic KOH,
(ii) bromobenzene is treated with Mg in the presence of dry ether,
(iii) chlorobenzene is subjected to hydrolysis,
(iv) ethyl chloride is treated with aqueous KOH,
(v) methyl bromide is treated with sodium in the presence of dry ether,
(vi) methyl chloride is treated with KCN?