CBSE Class 12 Chemistry Haloalkanes and Haloarenes Worksheet Set B

ASSERTION-REASON TYPE OF QUESTIONS
The question given below consist of an assertion and a reason use the following key to choose appropriate answer
a) Both assertion and reason are correct and reason is the correct explanation of the assertion
b) Both assertion and reason are correct and reason is not the correct explanation of the assertion
c) Assertion is correct but reason is incorrect
d) Assertion is wrong Reason is correct.

Question. Assertion: Alkyl halides are insoluble in water
Reason: Energy is required to break H bonds in water
Answer. B

Question. Assertion: Para chloro-benzene is having higher melting point than other two isomers
Reason: Due to symmetry molecule is well fit in to the lattice.
Answer. A

Question. Assertion: Tertiary alkyl halides are least reactive in S_N² reactions
Reason: Steric hindrance is minimum in tertiary Alkyl halides
Answer. C

Question. Assertion: Only achiral molecules when undergo substitution, optical rotation changes
Reason: When molecule and its mirror images are non superimposable, they show optical activity.
Answer. D

Question. Assertion: Alkyl iodides are more reactive towards nucleophilic substitution reaction
Reason: C-I bond is stronger than any other carbon halogen bond
Answer. C

Question. Assertion: The reaction of Arenes and iodine is carried out in presence of HIO₄
Reason: HIO₄ Oxidises HI formed during the reaction to I₂ and prevents backward reaction.
Answer. A

Question. Assertion: During Finkelstein reaction alkyl chloride is treated with NaI in presence of Acetone
Reason: Acetone is a very good organic solvent
Answer. B

Question. Assertion: Boiling points of alkyl iodides are lower than that of Alkyl chlorides
Reason: Vander force is more in alkyl iodides due to large size of iodine
Answer. D

Question. Assertion: Among isomeric alkyl halides straight chain alkyl halides have highest boiling point
Reason: Dispersion force is minimum in branched chain alkyl halides.
Answer. B

Question. Assertion: Alkyl halides mainly form alkyl cyanides when treated with AgCN
Reason: Due to covalent nature of AgCN attack takes place through Nitrogen.
Answer. D

Question. Assertion: Alkyl halides forms Nitriles when treated with KCN
Reason: CN- is an ambident nucleophile
Answer. B

Question. Assertion: In Mono haloarenes further electrophilic substitution takes place at ortho and para position.
Reason: The lone pair of electrons on Cl is in conjugation with benzene ring, increasing electron density at ortho and para position...
Answer. A

Question. Assertion: Nitration of chloro benzene leads to the formation of Meta nitro chlorobenzene.
Reason: NO₂ group is Meta directing towards electrophilic substitution reaction.
Answer. D

Question. Assertion: Allyl halides react faster in S_N¹ reaction
Reason: Allyl cations are stabilized by resonance
Answer. A

Question. Assertion: Among different halogens alkyl chlorides are more reactive than iodides towards substitution reaction
Reason: C-Cl bond is stronger than C-I bond
Answer. D

Question. Assertion: Hydrolysis of (-) 2-bromo octane Proceeds with the inversion of configuration
Reason: This reaction proceeds via the formation of carbocation.
Answer. C

Question. Assertion: Benzyl halides react faster in S_N¹ reaction
Reason: Benzyl cations formed is stabilized by resonance.
Answer. A

Question. Assertion: When 2-bromo butane is treated with alcoholic KOH But-1-ene is the major product
Reason: During dehydrohalogenation reactions more alkylated alkene is the major product.
Answer. D

Question. Assertion: During the reaction between Tertiary butyl bromide and C₂H₅ONa Alkene is the major product.
Reason: In presence of strong base Elimination competes over substitution.
Answer. A

Question. Assertion: It is necessary to avoid even traces of water from Grignard reagent.
Reason: Grignard reagent is used in preparing alcohols
Answer. B

Question. Assertion: It is difficult to break C-X bond in alkyl halides
Reason: C-X bond in aryl halides have partial double character
Answer. D

Question. Assertion: Propene when treated with HBr form 1- bromo propane as the major product
Reason: During the addition of HBr a secondary carbocation is formed.
Answer. D

Question. Assertion: The presence of NO₂ group at ortho position of aryl halides increases reactivity of haloarenes towards nucleophilic substitution reaction
Reason: Presence of NO₂ group at ortho and para position of aryl halides reduces the electron density in benzene and hence facilitates the attack.
Answer. A

Question. Assertion: Alkyl halides are more reactive towards nucleophilic substitution reaction compared to aryl halides
Reason: In alkyl halides halogen is connected to sp³ hybridised carbon.
Answer. B

Question. Assertion: S_N¹ reaction are generally carried out in presence polar protic solvents such as water
Reason: The energy needed to break C-X bond is obtained through solvation of halide ion with proton of protic solvent.
Answer. A

SHORT ANSWER TYPE QUESTIONS

Question. Why the boiling points of chlorides, bromides and iodides are considerably higher than those of the hydrocarbons of comparable molecular mass?
Answer. Due to greater polarity as well as higher molecular mass as compared to the parent hydrocarbon, the intermolecular forces of attraction (dipole-dipole and van der Waals) are stronger in the halogen derivatives

Question. The para-isomers of isomeric dihalobenzenes are high melting as compared to their ortho- and meta-isomers.
Answer. It is due to symmetry of para-isomers that fits in crystal lattice better as compared to ortho or meta isomer.

Question. Give chemical tests to distinguish between
(a) Benzyl chloride & chloro benzene
(b) Vinyl iodide and ethyl iodide
Answer. 
(a) On adding NaOH solution & aqueous AgNO₃ , Benzyl chloride gives white precipitate of AgCl, while chlorobenzene does not react at room temperature.
(b) On adding NaOH solution & aqueous AgNO₃ , ethyl iodide gives yellow precipitate of Agl, while vinyl iodide does not react .

Question. What do you mean by retention of configuration and inversion of configuration?
Answer.  Retention of configuration is the preservation of integrity of the spatial arrangement of bonds to an asymmetric centre during a chemical reaction or transformation. Here the optical rotation of the reactant and product are the same
Inversion of configuration is the inversion of the spatial arrangement of bonds to an asymmetric centre during a chemical reaction or transformation. Here the optical rotation of the reactant and product are different

Question. Among the isomeric alkyl halides of molecular formula C₄H₉Cl, identify
(a) The optically active isomer
(b) The most reactive halide towards SN1
(c) The isomer with lowest boiling point
Answer.  draw the structure and find out the isomer
(a) sec- Butyl chloride CH₃CH(Cl)CH₂ CH₃
(b) tert- Butyl chloride (CH₃)₃ C Cl
(c) tert- Butyl chloride (CH₃)₃ C Cl

Question. a) State i) Zaitsev (Saytzeff) rule
ii) Markownikoff’s rule b) What happens when methyl chloride is treated with KCN.
Answer. 
a) i) Zaitsev (Saytzeff) rule. The rule states that “in dehydrohalogenation reactions, if there is possibility of formation of more than one alkene the preferred product is that alkene which contains greater number of alkyl groups attached to the doubly bonded carbon atoms.”
ii)Markownikoff’s rule. The rule states that “when an unsymmetrical reagent is added to an unsymmetrical alkene, the positive part of the addendum (adding molecule) goes and attaches to the double bonded carbon containing more number of hydrogen and the negative part of the addendum gets attached to the double bonded carbon containing lesser number of hydrogen atoms b) When methyl chloride is treated with KCN, it undergoes a substitution reaction to give methyl cyanide. CH₃Cl + KCN → CH₃CN +KCl

CASE BASED QUESTIONS

Read the following passage and answer the questions given below.

Alkyl halides are best prepared from alcohols, on treating with Hydrogen halides, phosphorus halides or thionyl chloride. Thionyl chloride is preferred because other two products are escapable gases. The reaction of primary secondary and tertiary alcohols with HCl need a catalyst ZnCl₂. With tertiary alcohol the reaction is conducted by shaking with Con. HCl at room temperature. Tertiary alcohols are more reactive. The above methods are not applicable for the preparation of Aryl halides because the carbon oxygen bond in phenols has a partial double bond character and is difficult to break being stronger than a single bond
A statement of assertion is followed by a statement of reason. Mark the correct choice from the options given below :
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Assertion is false but reason is true

Question. Assertion: Thionyl chloride is the best reagent to convert alcohols to Alkyl chlorides
Reason: With Thionyl chloride alkyl chloride is the only product formed.
Answer. C

Question. Assertion: Primary secondary and tertiary alcohols can be distinguished using Lucas test
Reason: Tertiary alcohols immediately form turbidity due to the formation of alkyl halides.
Answer. B

Question. Assertion: Primary alcohols react fast with HCl and forms alkyl halide at room temperature.
Reason: Alkyl halides are insoluble in water.
Answer. D

Question. Assertion: Aryl halides can be prepared by treating phenols with halogen acid
Reason: In phenol C-O bond is having partial double bond character.
Answer. D

CBSE Class 12 Chemistry Haloalkanes and Haloarenes Worksheet Set B

Very Short Answer

Question. Define dextrorotatory?
Answer. The compound rotates the plane of plane polarised light to the right i.e. the clockwise direction is called dextrorotatory.

Question. Define enantiomers?
Answer. The stereoisomers related to each other as non-superimposable mirror images are called enantiomers.

Question. What is polarimeter?
Answer. The angle by which the plane polarised light is rotated is measured by an instrument is called polarimeter.

Question. What is freons?
Answer. The chlorofluorocarbon compounds of methane and ethane are collectively is called freons.

Question. What are ambident nucleophiles?
Answer. Groups like cyanides and nitrites possess two nucleophilic centres that are called ambident nucleophiles.

Short Answer

Question. What do you mean by racemic mixture?
Answer. A mixture containing two enantiomers in equal proportions will have zero optical rotation as the rotation due to one isomer will be cancelled by the rotation due to the other isomer; such type of mixture is known as racemic mixture.

Question. How α-carbon different from β-carbon?
Answer. Carbon on which halogen atom is directly attached is called α- carbon whereas the carbon atom adjacent to this carbon is called β- carbon.

Question. What do you understand by Wurtz reaction?
Answer. Alkyl halides react with the sodium in dry ether to give hydrocarbons containing double the number of the carbon atoms present in the halide, this reaction is called Wurtz
reaction.

Question. Define racemisation?
Answer. Racemization is a process in organic chemistry that occurs when a compound undergoes a reaction in which the transformation produces an equal mixture of both possible enantiomers.

Question. Distinguish between Wurtz-fittig reaction and fittig reaction?
Answer. A mixture of an alkyl halide and aryl halide gives an alkylarene when treated with the sodium in dry ether and is called Wurtz-fittig reaction whereas aryl halides also give
analogous compounds when treated with the sodium in dry ether in which two aryl groups are joined together is called fittig reaction.

Long Answer

Question. State the difference between achiral and chiral?
Answer. 
1. Chiral means asymmetric in such a way that the structure and its mirror image are not superimposable whereas achiral means symmetric in such a way that it can be superimposed on its mirror image.
2. In chiral molecules, a particular molecule and its mirror image are two different compounds whereas in achiral molecules, the molecule and its mirror image are the same.
3. Chiral molecules are always asymmetric at one or more centres whereas achiral molecules are always symmetric at every center.
4. Chiral molecules can rotate the plane-polarized light clockwise or anticlockwise whereas achiral molecules cannot rotate plane-polarized light in any direction.
5. The molecule and its mirror image are non-superimposable in chiral molecules whereas the molecule and its mirror image are superimposable in achiral molecules.

Question. On what reasons nucleophilic substitution occurs?
Answer. Aryl halides are extremely less reactive towards nucleophilic substitution reactions due to the following reasons:
1. In halogens the electron pairs on the halogen atom are in conjugation with π electrons of the ring.
2. Difference in hybridisation of the carbon atom in C-X bond, in haloalkane the carbon atom attached to the halogen is sp3 hybridised while in the case of haloarenes, the carbon atom attached to halogen is sp2 hybridised.
3. The sp3 hybridised carbon with a greater s-character is more electronegative and can hold the electron pair C-X bond more tightly than sp³- hybridised carbon in haloalkane with less s-character.
4. Instability of phenyl cation, in case of haloarenes the phenyl cation formed as a result of the self-ionisation will not be stabilised by the resonance and so S_N¹ mechanism is ruled out.
5. Because of the possible repulsion, it is less likely for the electron rich nucleophile to approach electron rich arenes.

Question. Write short note on dichloromethane and trichloromethane?
Answer. 
Dichloromethane: Dichloromethane also commonly known as Methylene Chloride, Methylene Dichloride, Di-clo and DCM is a clear, slightly sweet smelling organic liquid chemical solvent which can be pose significant workplace safety risks if strict handling instructions are not followed. As an active ingredient in organic-based paint strippers. In the manufacturing of pharmaceuticals and other medicines. Aerosols for insecticides and spray paint. The manufacturing techniques for creating decaffeinated coffee beans and tea leaves. Dichloromethane isn't miscible with water; it is able to dissolve a wide range of organic compounds. These properties, combined with its volatility, make DCM a highly effective solvent in many industrial processes. Dichloromethane has been proven to cause damage to the brain and central nervous system (CNS). The Environmental Protection Agency (EPA) has classified it as a probable human carcinogen since high levels of exposure to the chemical has been proven to cause liver and lung cancer in animals.
Trichloromethane: Chloroform, or trichloromethane, is an organic compound with formula CHCl₃. It is a colourless, sweet-smelling, dense liquid that is produced on a large scale as a precursor to PTFE. It is also a precursor to various refrigerants. It is one of the four chloromethane and a trihalomethane. It is a powerful aesthetic, euphoriant, anxiolytic and sedative when inhaled or ingested. Trichloromethane has been used as an anaesthetic during surgery. Today it is used in industry in production of other chemicals and as a solvent as well as in laboratory work. Chloroform is a sweetsmelling liquid with an ether-like smell and a slightly sweet taste. Some people compare that smell to the odour of disinfectants, like the one you get in hospitals and medical facilities.

Question. Explain inversion of configuration?
Answer. A process in which the configuration of an atom is changed. If the atom in question is a stereocenter, inversion of configuration usually but not always changes R absolute configuration into S, and S into R. Inversion of configuration can also convert cis into trans, or trans into cis. Inversion of configuration usually happens when an organic compounds undergoes nucleophilic substitution reaction by S_N² mechanism. The nucleophile attacks the stereocenter from the opposite side of the leaving group. This results in formation of the product having Inversion Of Configuration. In the SN1 reaction, the leaving group leaves a carbon (usually an alkyl halide) to form a carbocation, which is then attacked by a nucleophile. The reaction is stepwise (happens in two steps) and the stereochemistry proceeds with a mixture of retention and inversion of configuration.

Question. Write short note on carbon tetrachloride, freons and DDT?
Answer. 
Carbon tetrachloride: Carbon Tetrachloride is a clear, colourless, volatile and very stable chlorinated hydrocarbon. Carbon Tetrachloride is used as a solvent for oils and fats, as a refrigerant and as a dry-cleaning agent. Inhalation of its vapours can depress central nervous system activity and cause degeneration of the liver and kidneys. Carbon tetrachloride was widely used as a cleaning fluid (in industry and dry cleaning establishments as a degreasing agent, and in households as a spot remover for clothing, furniture, and carpeting). Carbon tetrachloride was also used in fire extinguishers and as a fumigant to kill insects in grain.
Freons: Freon-12 (dichlorodifluoromethane, CF₂Cl₂) is commonly known as CFC. It is used as a refrigerant in refrigerators and air conditioners. It is also used in aerosol spray
propellants such as body sprays, hair sprays, etc., it damages the ozone layer. It is manufactured from tetrachlorometahne by Swarts reaction.
DDT: DDT (dichloro-diphenyl-trichloroethane) was developed as the first of the modern synthetic insecticides in the 1940s. It was initially used with great effect to combat malaria, typhus, and the other insect-borne human diseases among both military and civilian populations. DDT has shown to cause chronic effects on the nervous system, liver, kidneys, and immune system. DDT is very insoluble in water and very persistent in the environment, making it a highly polluting hazard.

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