Case Based Questions (CBQ) for Haloalkanes and Haloarenes|Biswajit Das

All About Chemistry6 months ago6 months ago023 mins

Question 1

Nucleophilic substitution reactions are a cornerstone of alkyl halide chemistry. These reactions can proceed via two primary mechanisms: SN1 and SN2. The choice of mechanism depends on several factors, including the structure of the alkyl halide, the nature of the nucleophile, and the solvent. In an SN1 reaction, the rate is determined by the stability of the carbocation intermediate, while in an SN2 reaction, steric hindrance at the reaction site is the key factor. Polar protic solvents favor SN1 reactions, and polar aprotic solvents favor SN2 reactions.

(i) Which of the following alkyl halides will undergo an SN1 reaction most readily?

(a) 1-Bromopropane

(b) 2-Bromopropane

(d) Bromomethane

(ii) In an SN2 reaction, what happens to the configuration of the chiral carbon?

(a) Retention

(b) Inversion

(d) No change

(iii) The reaction of CH3−Br with AgCN predominantly gives isocyanide as the product. What is the nature of the nucleophile in this reaction?

(a) Unidentate

(b) Ambidentate

(d) Bidentate

(iv) What is the rate law for an SN1 reaction?

(a) Rate = k[Substrate][Nucleophile]

(b) Rate = k[Substrate]

(d) Rate = k

(v) Which of the following is a polar aprotic solvent?

(a) Water

(b) Ethanol

(d) Acetic acid

Question 2

Haloalkanes are a class of organic compounds where one or more hydrogen atoms of an alkane are replaced by halogen atoms. The polarity of the C-X bond, where X is a halogen, plays a crucial role in determining their physical and chemical properties. Due to the high electronegativity of halogens, the C-X bond is polar, and the carbon atom is electrophilic. The physical properties, such as boiling point and density, are influenced by the size and mass of the halogen atom and the structure of the alkyl group.

(i) The boiling point of haloalkanes decreases in the order:

(a) RI>RBr>RCl>RF

(b) RF>RCl>RBr>RI

(d) RBr>RCl>RI>RF

(ii) Which of the following haloalkanes has the highest dipole moment?

(a) CH3F

(b) CH3Cl

(d) CH3I

(iii) Why are haloalkanes only slightly soluble in water?

(a) They are non-polar.

(b) They cannot form hydrogen bonds.2

(d) They are less dense than water.

(iv) The density of haloalkanes is generally greater than that of the parent alkanes because:

(a) Halogen atoms are lighter than hydrogen.

(b) Halogen atoms are heavier than hydrogen.

(d) The C-X bond is longer.

(v) What is the effect of branching on the boiling point of haloalkanes?

(a) Boiling point increases with branching.

(b) Boiling point decreases with branching.

(d) It depends on the halogen.

Question 3

Elimination reactions are another important class of reactions for alkyl halides, where a small molecule is removed from the substrate to form an unsaturated compound (an alkene). The most common type is β-elimination, in which a halogen atom is removed from the α-carbon and a hydrogen atom from the adjacent β-carbon. This reaction is favored by strong bases and high temperatures. If there are multiple β-hydrogens, the reaction can yield more than one alkene. The major product is predicted by Zaitsev’s rule, which states that the most substituted (most stable) alkene is the major product.

(i) Which reagent is typically used to carry out a β-elimination reaction?

(a) Aqueous KOH

(b) Alcoholic KOH

(d) H2O

(ii) The reaction of 2-bromobutane with alcoholic KOH gives:

(a) But-1-ene as the major product

(b) But-2-ene as the major product

(d) Butan-1-ol as the major product

(iii) According to Zaitsev’s rule, the major product is the alkene that is:

(a) Least substituted

(b) Most substituted

(d) Saturated

(iv) What is the name of the reaction that occurs when an alkyl halide reacts with a strong base to form an alkene?

(a) Substitution

(b) Addition

(d) Hydrogenation

(v) Which of the following factors does not favor an elimination reaction?

(a) High temperature

(b) Strong base

(d) Bulky base

Question 4

Aryl halides are compounds where a halogen atom is directly bonded to a benzene ring. They show a marked difference in reactivity compared to alkyl halides. The C-X bond in aryl halides is shorter and stronger due to the partial double bond character arising from resonance, and the carbon atom is sp2 hybridized. This makes aryl halides unreactive towards nucleophilic substitution reactions under normal conditions. However, they can undergo electrophilic substitution reactions, where the halogen atom is a deactivating but ortho-para directing group. Aryl halides also participate in named reactions such as the Wurtz-Fittig reaction and the Fittig reaction.

(i) Why are aryl halides less reactive towards nucleophilic substitution?

(a) The C-X bond is longer.

(b) The halogen atom is electron-donating.

(d) The benzene ring is electron-rich.

(ii) What is the function of the halogen atom in an electrophilic substitution reaction on a benzene ring?

(a) Activating and ortho-para directing

(b) Deactivating and meta directing

(d) Activating and meta directing

(iii) The reaction of chlorobenzene with sodium in the presence of dry ether to form biphenyl is known as:

(a) Wurtz reaction

(b) Fittig reaction

(d) Ullmann reaction

(iv) Which of the following is an example of a reaction that can substitute the halogen of an aryl halide?

(a) Reaction with aqueous KOH

(b) Reaction with KCN

(d) Reaction with H2O

(v) The C-Cl bond in chlorobenzene has a hybrid orbital of the carbon atom that is:

(a) sp3

(b) sp2

(d) sp3d

Question 5

Haloarenes can be prepared by direct halogenation of an aromatic ring in the presence of a Lewis acid catalyst like anhydrous FeCl3. However, for certain haloarenes, other methods are more suitable. For example, the Sandmeyer reaction is a valuable method for preparing chloro and bromoarenes by treating a freshly prepared diazonium salt with CuCl/HCl or CuBr/HBr. This reaction is a part of a larger sequence of reactions that start from an aromatic amine. The Sandmeyer reaction, along with the Gattermann reaction, allows for the introduction of a halogen atom to the benzene ring.

(i) Which of the following is the correct order of reagents for the Sandmeyer reaction?

(a) Aniline NaNO2/HCl Diazonium salt CuCl/HCl Chloroarene

(b) Aniline CuCl/HCl Chloroarene NaNO2/HCl Diazonium salt

(d) Benzene NaNO2/HCl Diazonium salt CuCl/HCl Chloroarene

(ii) What is the primary role of anhydrous FeCl3 in the halogenation of benzene?

(a) To act as a base

(b) To generate an electrophile

(d) To act as a reducing agent

(iii) The reaction of a diazonium salt with KI gives iodobenzene. This reaction does not require a cuprous salt catalyst. Why?

(a) The iodide ion is a strong nucleophile.5

(b) The reaction is exothermic.

(d) The reaction is an electrophilic substitution.

(iv) The reaction of chlorobenzene with chlorine in the presence of anhydrous FeCl3 will produce which major product?

(a) 1,2-dichlorobenzene

(b) 1,3-dichlorobenzene

(d) Both (a) and (c) are major products

(v) Benzene is treated with a mixture of conc. HNO3 and conc. H2SO4, followed by a reaction with tin and concentrated HCl. The final product is then treated with sodium nitrite and HCl at 273-278 K. This product is then treated with CuCN/KCN. What is the final product?

(a) Chlorobenzene

(b) Bromobenzene

(d) Nitrobenzene

Question 6

Nucleophilic substitution reactions are a hallmark of alkyl halide chemistry. These reactions can proceed through two main pathways, SN1 and SN2. The choice of mechanism is not random but is governed by a combination of factors, including the structure of the alkyl halide, the nature of the nucleophile, and the solvent. In an SN2 reaction, a single step involves the backside attack of a nucleophile, leading to an inversion of the stereochemical configuration. This mechanism is favored by primary alkyl halides and aprotic solvents. Conversely, the SN1 mechanism is a two-step process that involves the formation of a carbocation intermediate. Its rate is determined by the stability of this intermediate, which is why tertiary alkyl halides react faster. Polar protic solvents, which stabilize the carbocation, favor the SN1 pathway.

(i) Why does an SN2 reaction lead to an inversion of configuration?

(ii) Predict the major product when 2-bromo-2-methylpropane reacts with sodium hydroxide. Justify your answer.

(iii) Explain why polar aprotic solvents favor the SN2 mechanism.

(iv) Which of the following substrates would react fastest in an SN1 reaction: CH3−Br, CH3CH2−Br, or (CH3)3C−Br? Give a reason.

(v) What is the order of reactivity of alkyl halides towards an SN2 reaction?

Question 7

The properties of haloalkanes and haloarenes are heavily influenced by the nature of the carbon-halogen bond. This bond is polar due to the higher electronegativity of halogens, making the carbon atom partially positive and the halogen atom partially negative. However, the reactivity towards nucleophilic substitution differs significantly. The boiling points of haloalkanes generally increase with an increase in the size and mass of the halogen atom. This is attributed to the increased strength of van der Waals forces. Despite their polar nature, haloalkanes are only sparingly soluble in water because they cannot form strong hydrogen bonds with water molecules, and the energy required to break existing hydrogen bonds in water is high.

(i) Arrange the following in increasing order of their boiling points: CH3Cl, CH3F, CH3Br, CH3I.

(ii) Explain why haloalkanes are not very soluble in water despite being polar.

(iii) Why is the C-Cl bond in chlorobenzene shorter than in chloromethane?

(iv) Compare the dipole moments of CH3Cl and CH3F.

(v) What type of intermolecular forces exist between haloalkane molecules?

Question 8

Aryl halides are a unique class of halogen-containing compounds where the halogen atom is directly attached to the benzene ring. Unlike their alkyl halide counterparts, aryl halides are significantly less reactive towards nucleophilic substitution. This is because the lone pair of electrons on the halogen atom participates in resonance with the benzene ring, giving the carbon-halogen bond a partial double-bond character and stabilizing the molecule. The sp2 hybridized carbon atom of the ring holds the electrons more tightly, making it less susceptible to nucleophilic attack. However, aryl halides can undergo electrophilic substitution reactions, where the halogen atom acts as a deactivating but ortho-para directing group. These compounds are also key reactants in several important named reactions.

(i) Draw the resonance structures of chlorobenzene and explain how they contribute to its properties.

(ii) Explain why aryl halides are less reactive towards nucleophilic substitution than alkyl halides.

(iii) What is the overall effect of a halogen atom on the reactivity of the benzene ring towards electrophilic substitution?

(iv) Write the chemical equation for the reaction of chlorobenzene with methyl chloride in the presence of anhydrous AlCl3.

(v) Name the reaction that combines an aryl halide and an alkyl halide using sodium metal in the presence of dry ether.

Question 9

Elimination reactions of alkyl halides are often in competition with substitution reactions, especially when a strong base is used. A common type of elimination is dehydrohalogenation, where a hydrogen atom from the β-carbon and a halogen atom from the α-carbon are removed. The reaction is typically carried out using alcoholic KOH and is favored by higher temperatures. When multiple β-hydrogens are available, the reaction can yield more than one alkene. In such cases, the major product is determined by Zaitsev’s rule, which states that the most substituted alkene (the one with the greater number of alkyl groups attached to the double-bonded carbon atoms) is the most stable and therefore the major product.

(i) What is the role of alcoholic KOH in a dehydrohalogenation reaction?

(ii) Give the major product when 2-bromopentane is treated with alcoholic KOH.

(iii) Explain why an elimination reaction is favored at high temperatures.

(iv) Write a chemical equation for a dehydrohalogenation reaction.

(v) What is the main reason for the stability of the most substituted alkene?

Question 10

The synthesis of haloalkanes and haloarenes can be achieved through various methods. Haloalkanes can be prepared from alcohols using phosphorus halides or thionyl chloride (SOCl2), while the halogenation of alkanes in the presence of ultraviolet light is a free-radical substitution reaction. Haloarenes are often prepared via the Sandmeyer reaction, a multi-step process starting with an aromatic amine. This reaction is highly specific and allows for the introduction of chlorine, bromine, or a cyano group. Other important named reactions involving alkyl and aryl halides include the Wurtz reaction, which couples two alkyl halides, and the Fittig reaction, which couples two aryl halides.

(i) Write the chemical equation for the preparation of chlorobenzene from aniline using the Sandmeyer reaction.

(ii) What is the major difference between the Sandmeyer and Gattermann reactions?

(iii) How can you convert an alcohol into an alkyl halide using Darzen’s method? Write the chemical equation.

(iv) Explain why the Wurtz reaction is not a good method for preparing unsymmetrical alkanes.

(v) Write the balanced chemical equation for the reaction of ethyl bromide with sodium metal in the presence of dry ether.

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