CBSE Class 12 Chemistry Haloalkanes and Haloarenes Worksheet Set A

Question. Select the correct order of dipole moment of alkyl halides.
(a) RI > RBr > RCl > RF
(b) RCl > RF > RBr > RI
(c) RF > RCl > RBr > RI
(d) None of the above
Answer. B

Question. Which among the following properties makes tertiary alkyl halides practically inert to SN 2mechanism?
(a) Insolubility
(b) Instability
(c) Steric hindrance
(d) Inductive effect
Answer. C

Question. Product obtained on reacting ethyl chloride with aq. KOH is
(a) CH₃ CH₂ OH
(b) CH₃ -- CH₃ 
(c) CH₂ == CH₂
(d) CH₃ CH₂ OK  
Answer. A

Question. Which of the following is responsible for iodoform reaction?
(a) Formalin
(b) Acetic acid
(c) Ethanal
(d) Methanol
Answer. C

Question. 2-bromobutane on dehydrobromination gives CH₃ CH == CHCH₃ which is also called
(a) Hoffmann product
(b) Saytzeff product
(c) Hoffmann-Saytzeff product
(d) Markownikoff product
Answer. B

Question. Which of the following is incorrect with respect to chloroform?
(a) Colourless, sweet smelling liquid
(b) Insoluble in water
(c) Highly inflammable
(d) Can be used as inhalational anaesthetic agent
Answer. C

Question. 1-chlorobutane on reaction with alc. KOH forms
(a) 1-butene
(b) 2-butane
(c) 1-butanol
(d) 2-butanol
Answer. A

Question. Which of the following is known as freon that makes its use as a refrigerant?
(a) CCl₂ F₂ 
(b) CHCl₃
(c) CH₂ F₂ 
(d) CF₄
Answer. A

Question. Alkyl halides react with dialkyl lithium cuprate to give
(a) alkanes
(b) alkenes
(c) alkyl copper halide
(d) alkenyl halide
Answer. A

Question. Reactivity of chlorobenzene is
(a) less than benzyl chloride
(b) more than ethyl bromide
(c) equal to methyl chloride
(d) more than isopropyl chloride
Answer. A

Question. Aryl halide is less reactive than alkyl halide towards nucleophilic substitution
(a) because it is a less stable carbonium ion
(b) due to large C¾Cl bond energy
(c) due to inductive effect
(d) because of resonance stabilisation and sp² -hybridisation of C-attached to halide
Answer. D

Question. The formula for gem-dibromide is
(a) CH₃ CH(Br)OH(Br)CH₃
(b) CH₂ (Br)CH₂ CH₂
(c) CH₃ CBr₂ CH₃
(d) CH₂BrCH₃ Br
Answer. C

Question. Study the following reaction.
C₂ H₅ OH +SOCl₂   → Pyridine C₂ H₅ Cl + SO₂ + HCl
The above reaction is also known as
(a) Kharasch effect
(b) Darzen’s process
(c) Williamson’s synthesis
(d) Hunsdieker synthesis reaction
Answer. B

Question. The reaction shown below undergoes formation of carbocation. The major product of this will be
(a) trans-1, 3-dibromocyclohexane
(b) cis-1, 2-dibromocyclohexane
(c) cis-1, 3-dibromocyclohexane
(d) trans-1, 2-dibromocyclohexane
Answer. A

Question. Decreasing order of reactivity of HX in the following reaction ROH +HX→ RX +H₂O
(a) HI > HBr > HCl > HF
(b) HBr > HCl > HI > HF
(c) HCl > HBr > HI > HF
(d) HF > HBr > HCl > HI
Answer. A

Very Short Answer

Question. What are benzylic halides?
Answer. These are the compounds in which the halogen atom is bonded to a sp3 hybridised carbon atom attached to an aromatic ring.

Question. What is the use of synthetic halogen compounds?
Answer. Synthetic halogen compound i.e. chloroquine is used for the treatment of malaria.

Question. What are halogenated compounds?
Answer. Halogenated compounds persist in the environment due to their resistance to breakdown by soil bacteria.

Question. What is the use of halothane?
Answer. Halothane is used as an anaesthetic during surgery.

Question. What is Finkelstein reaction?
Answer. The Finkelstein reaction is an organic reaction where an alkyl halide is converted into another alkyl halide by reacting with a metal halide salt.

Short Answer

Question. What do you mean by the allylic halides?
Answer. An allylic halide is the one where the carbon atom next to a double bonded carbon atom carries one or more halogen atoms. Carbon next to the double bonded carbon is known as an allylic carbon. Such halides are reactive in both Sn1 and Sn2 mechanisms.

Question. How vinylic halides are different from aryl halides?
Answer. An aryl halide is a compound in which a halogen is bound to the carbon of a benzene ring or another aromatic ring whereas in a vinylic halide, a halogen is bound to a carbon of a double bond.

Question. How aryl chlorides and bromides can be prepared?
Answer. Aryl chlorides and bromides can be easily prepared by electrophilic substitution of arenes with the chlorine and bromine respectively in the presence of the Lewis acid catalysts like iron or iron(III) chloride.

Question. What is the nature of C-X bond?
Answer. Nature of C-X bond. Haloarenes are the chemical compounds containing arenes, where one or more hydrogen atoms bonded to an aromatic ring are replaced with halogens.The nature of C-X bond depends on both the nature of carbon in the aromatic ring and the halogen attached.

Question. Define alkyl halides?
Answer. Alkyl halides is also known as haloalkanes are compounds in which one or more hydrogen atoms in an alkane have been replaced by halogen atoms like fluorine, chlorine, bromine or iodine. Alkyl halides are classified into primary, secondary and tertiary alkyl halides.

Long Answer

Question. What are the physical properties of the haloalkanes?
Answer. Physical properties of the haloalkanes are:
1. Physical State: Alkyl halides are colourless in nature in its pure state. So, bromides and iodides develop colour on exposure to light. Many of the halogen compounds having volatile nature have a sweet smell. Haloarenes are also colourless liquids or crystalline solids that have a characteristic smell.
2. Boiling Point: The boiling point of haloalkanes and haloarenes depends upon the intermolecular forces of attraction. The boiling points of derivatives of chlorides, bromides, and iodides are comparatively higher the hydrocarbons of the similar molecular mass. The boiling point also increases for isomeric haloalkanes. The boiling point decreases with the branching of the compound.
This is because branching of haloalkanes results in the lesser surface area, thus decreasing the van der Waal’s forces interaction.
3. Melting point: The melting point is based on the strength of the lattice structure of a compound. Isomeric dihalobenzenes have almost similar boiling points but the difference can be seen in the melting points. Para-isomer has a higher melting point in comparison to ortho-isomer and meta-isomer of the same compound. It is because para-isomers have highly compact crystal lattice in comparison to ortho-isomer and meta- isomer.
4. Density: Density is directly proportional to the mass of any compound.
Therefore, as the mass increases down the homologous series, the density increases. Density increases with the increase in the number of the carbon and halogen atoms.
5. Solubility: Alkyl halides are slightly soluble in water. Even though haloalkanes and haloarenes are polar compounds, they are immiscible with water. The strength of the intermolecular forces between organic solvents and carbonhalogen compounds is similar to carbon-halogen molecules and solvent molecules.

Question. State the difference between SN1 reaction and SN2 reaction?
Answer. 
1. SN1 reaction is unimolecular and a first order reaction. So substrate affects the reaction rate whereas SN2 reaction is bimolecular or a second order reaction. So,
both substrate and nucleophile affect the reaction rate.
2. SN1 reaction has only 1 step whereas SN2 reaction has 2 steps.
3. In SN1 reaction polar protic such as alcohol is a favourable solvent whereas In SN2 reaction polar aprotic such as DMSO and acetone are favourable solvents.
4. In SN1 reaction weak or neutral nucleophile is required whereas in SN2 reaction strong nucleophile is required.
5. SN1 reaction generally has two intermediate states whereas SN2 reaction generally has one intermediate state.
6. In SN1 reaction carbocation stability is the key factor of the reaction whereas SN2 reaction steric hindrance is the key factor of the reaction.

Question. State the difference between haloalkanes and Haloarenes?
Answer. 
1. Haloalkanes are a group of organic compounds containing alkanes combined with halogens whereas haloarenes are a group of organic compounds containing aromatic compounds which has one or more halogen atoms.
2. Haloalkanes are odourless whereas haloarenes has a sweet odour.
3. Haloalkanes participate in SN2 reactions whereas haloarenes do not participate in SN2 reactions.
4. Haloalkanes are aliphatic whereas haloarenes are aromatic.
5. Haloalkanes can be primary, secondary or tertiary but no aromatic rings whereas haloarenes has aromatic rings which have one or more halogen atoms attached to the aromatic ring.

Question. What are the methods of preparation of haloalkanes, explain?
Answer. The methods of preparation of haloalkanes are:
1. From Alcohols: The most convenient method of preparation of haloalkane is from alcohols. R-OH when reacts with suitable reagents, the reaction results in the formation of R − X. The suitable reagents that help in the reaction are concentrated halogen acids, phosphorous halides, thinoyl chloride.
2. From hydrocarbons: Preparation of Haloalkanes from hydrocarbons is possible by 3 different methods. They are: free radical halogenation, from alkane’s addition of hydrogen halides, addition of halogens. Alkyl bromides and alkyl chloride formation are possible by the free radical halogenation reaction.
However, radicals are very non-selective in nature. Moreover, radicals are nonspecific and highly reactive intermediates that result in the formation of the mixture of products. An alkene is converted to the corresponding alkyl halide
by the reaction with the hydrogen chloride hydrogen bromide or hydrogen iodide.
3. Halogen exchange: An alkyl chloride or alkyl bromide reacts with sodium iodide in acetone to form alkyl iodides, this reaction is known as Finkelstein reaction.
The reaction is an equilibrium reaction so there is a possibility of forming other products. The solubility difference of alkyl halides in acetone is used for driving the reaction in the forward direction.

Question. What are the chemical properties of the haloalkanes?
Answer. The chemical properties of the haloalkanes are:
1. Nucleophilic substitution reactions: The halogen atom in haloalkanes is more electronegative than the carbon atom attached to it. As a result, haloalkanes are polar i.e., the carbon atom acquires a partial positive charge and the halogen atom acquires a partial negative charge because of the presence of this partial positive charge on the carbon atom, the carbon atom is prone to getting attacked by nucleophilic reagents (electron rich species). When a strong nucleophile attacks this carbon atom, a new bond between the carbon atom and the incoming nucleophile is formed and the halide ion gets removed.
2. Elimination Reaction: In elimination reaction, some molecules leave the compound leading to the formation of a double or a triple bond. When a haloalkane with β- hydrogen atom is heated with the alcoholic solution of potassium hydroxide, so there is the elimination of the hydrogen atom.
3. Reaction with metals: Most of the organic chlorides, bromides and iodides react with certain metals to form compounds containing carbon metal bond. Such compounds are called organometallic compounds. Haloalkanes react with magnesium in dry ether to form alkyl-magnesium halides, also known as Grignard's reagents.

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