Chemistry/Chapter 9: Halogenoalkanes
9.2 Reactivity of Halogenoalkanes
The main idea is that the reactivity of a halogenoalkane depends on the strength of the carbon-halogen (C-X) bond
Explanation level
In the previous section, we learned how to prepare halogenoarenes (aryl halides). Now, we shift our focus to halogenoalkanes (alkyl halides). Although both contain a carbon-halogen bond, their chemical reactivity is very different. This section explains the factors that determine the reactivity of halogenoalkanes, focusing on the strength of the carbon-halogen bond and comparing their reactivity with that of halogenoarenes.
Main Idea
The main idea is that the reactivity of a halogenoalkane depends on the strength of the carbon-halogen (C-X) bond. A weaker C-X bond is easier to break, making the halogenoalkane more reactive. This is why halogenoalkanes are generally much more reactive than halogenoarenes; in halogenoarenes, the C-X bond has partial double bond character due to resonance, making it stronger and harder to break.
Core Concept
The core concept is the polarity of the C-X bond. Halogens are more electronegative than carbon, so the C-X bond is polar, with the carbon atom carrying a partial positive charge (δ+) and the halogen carrying a partial negative charge (δ-). This makes the carbon atom electrophilic (electron-deficient) and susceptible to attack by nucleophiles. A nucleophile can replace the halogen in a nucleophilic substitution reaction. The rate of this reaction depends on how easily the C-X bond can be broken, which is determined by the bond strength.
Detailed Explanation
The textbook explains the reactivity of halogenoalkanes by comparing them with halogenoarenes.
1. Reactivity of Halogenoalkanes
The strength of the C-X bond is an important factor that determines the reactivity of alkyl halides and aryl halides.
- Polarity of the C-X Bond: The alkyl halide molecule is polarized due to the higher electronegativity of halogens compared to carbon. Thus, carbon receives a partial positive charge, while the halogen receives a partial negative charge.
- Nucleophilic Substitution: The halogen becomes nucleophilic in nature, which can be replaced by another nucleophile. For example, chloroethane can undergo a nucleophilic substitution reaction. In this reaction, the nucleophile attacks the carbon atom that is attached to the chlorine atom. This results in the replacement of chlorine by a nucleophile:
2. Reactivity of Halogenoarenes (Comparison)
In chlorobenzene, the lone pair of electrons on the chlorine atom are delocalized over the ring, giving the C-Cl bond the character of a partial double bond. Therefore, the C-Cl bond is more difficult to break. Thus, the chlorine atom cannot be replaced by a nucleophile.
- Resonance Effect: Chlorine increases the ring electron density due to resonance. Thus, chlorobenzene undergoes an electrophilic substitution reaction rather than nucleophilic substitution. In this reaction, one of the hydrogen atoms of benzene in the ortho and para positions is replaced by an electrophile.
- Example: Nitration of chlorobenzene gives a mixture of o-nitrochlorobenzene and p-nitrochlorobenzene.
3. Summary of Differences
| Property | Halogenoalkane (e.g., Chloroethane) | Halogenoarene (e.g., Chlorobenzene) |
|---|---|---|
| C-X Bond Strength | Weak (single bond) | Strong (partial double bond) |
| Reactivity towards Nucleophiles | High (undergoes nucleophilic substitution) | Low (does not undergo nucleophilic substitution) |
| Reactivity towards Electrophiles | Low | High (undergoes electrophilic substitution) |
| Bond Polarity | Polar | Less polar (due to resonance) |
Intermediate Level
The reactivity of a halogenoalkane is primarily determined by the polarity and strength of the C-X bond. The C-X bond is polar, with the carbon atom being electron-deficient (electrophilic) and the halogen being electron-rich. This makes the carbon atom susceptible to attack by nucleophiles, leading to nucleophilic substitution. The rate of substitution depends on the strength of the C-X bond, which decreases down the halogen group (C-F > C-Cl > C-Br > C-I). In contrast, the C-X bond in a halogenoarene has partial double bond character due to resonance, making it much stronger and less reactive towards nucleophiles. This is why halogenoarenes undergo electrophilic substitution rather than nucleophilic substitution.
Important Points
- - Reactivity of Halogenoalkanes: High towards nucleophiles.
- - Reactivity of Halogenoarenes: Low towards nucleophiles, high towards electrophiles.
- - Reason: The C-X bond in halogenoalkanes is a weak single bond. In halogenoarenes, it has partial double bond character due to resonance.
- - Bond Polarity: C-X bond is polar, making the carbon electrophilic.
- - Nucleophilic Substitution: The halogen is replaced by a nucleophile.
- - Electrophilic Substitution: In halogenoarenes, the ring undergoes substitution with an electrophile.
Remember
- - Halogenoalkanes are more reactive than halogenoarenes.
- - C-X bond polarity: Carbon is δ+, halogen is δ-.
- - Halogenoalkanes: Nucleophilic substitution.
- - Halogenoarenes: Electrophilic substitution (due to resonance).
- - Bond strength: C-F > C-Cl > C-Br > C-I.
- - Halogenoarenes have partial double bond character in C-X.
- - Halogenoalkanes react with nucleophiles (e.g., OH^-, CN^-).
Common Mistakes
- - Thinking that all carbon-halogen bonds are equally reactive. The reactivity depends on the halogen and the type of carbon (alkyl vs. aryl).
- - Assuming that halogenoarenes undergo nucleophilic substitution. They do not, because the C-X bond is too strong.
- - Forgetting the role of resonance in halogenoarenes. The resonance stabilization of the C-X bond is key to their low reactivity.
- - Confusing the terms "nucleophile" and "electrophile." Nucleophiles attack electron-deficient carbons (like in halogenoalkanes). Electrophiles attack electron-rich rings (like in halogenoarenes).
- - Not recognizing the polarity of the C-X bond. This polarity is what makes the carbon electrophilic and susceptible to nucleophilic attack.
Quick Revision
The reactivity of halogenoalkanes is determined by the polarity and strength of the carbon-halogen (C-X) bond. The C-X bond is polar, with the carbon atom carrying a partial positive charge, making it electrophilic and susceptible to attack by nucleophiles. This leads to nucleophilic substitution reactions, where the halogen is replaced by a nucleophile. In contrast, halogenoarenes (aryl halides) have a C-X bond with partial double bond character due to resonance, making it much stronger and less reactive towards nucleophiles. Instead, halogenoarenes undergo electrophilic substitution reactions on the aromatic ring. The reactivity of halogenoalkanes decreases as the C-X bond strength increases: C-F > C-Cl > C-Br > C-I.
60-Second Revision
Halogenoalkanes are reactive towards nucleophiles because their C-X bond is a polar, weak single bond. The carbon is electrophilic and can be attacked by a nucleophile, leading to nucleophilic substitution. Halogenoarenes, however, have a C-X bond with partial double bond character due to resonance, making it stronger and less reactive towards nucleophiles. Instead, they undergo electrophilic substitution on the ring. The bond strength order is C-F > C-Cl > C-Br > C-I.
Frequently Asked Questions (FAQs)
Q1. Why are halogenoalkanes more reactive than halogenoarenes? Answer: Halogenoalkanes are more reactive because the carbon-halogen bond is a weak, polar single bond that is easy to break. In halogenoarenes, the C-X bond has partial double bond character due to resonance (the lone pair on the halogen is delocalized into the aromatic ring), making it stronger and harder to break. Therefore, halogenoalkanes undergo nucleophilic substitution more readily, while halogenoarenes undergo electrophilic substitution.
Q2. What is the role of the C-X bond polarity in the reactivity of halogenoalkanes? Answer: The C-X bond is polar because the halogen is more electronegative than carbon. This gives the carbon atom a partial positive charge (δ+), making it electrophilic (electron-deficient). This allows nucleophiles (electron-rich species) to attack the carbon atom and replace the halogen in a nucleophilic substitution reaction.
Q3. Why do halogenoarenes undergo electrophilic substitution rather than nucleophilic substitution? Answer: In halogenoarenes, the lone pair of electrons on the halogen atom is delocalized into the aromatic ring through resonance. This gives the C-X bond partial double bond character, making it strong and difficult to break by nucleophiles. Additionally, the halogen increases the electron density of the ring, making it more attractive to electrophiles. Therefore, halogenoarenes undergo electrophilic substitution.
Q4. How does the reactivity of halogenoalkanes change from fluorine to iodine? Answer: The reactivity of halogenoalkanes generally increases from fluoride to iodide. This is because the C-X bond strength decreases down the group (C-F > C-Cl > C-Br > C-I). A weaker bond is easier to break, making the halogenoalkane more reactive towards nucleophilic substitution. However, in practice, the rate of reaction also depends on other factors, such as the solubility of the halide ion and the stability of the transition state.
Q5. What is a nucleophilic substitution reaction? Answer: A nucleophilic substitution reaction is a reaction in which a nucleophile (an electron-rich species) attacks an electron-deficient carbon atom and replaces a leaving group (usually a halogen). The general equation is: R-X + Nu- → R-Nu + X-
Q6. What is the difference between the C-X bond in chloroethane and chlorobenzene? Answer: In chloroethane, the C-Cl bond is a simple sigma bond between an sp³ hybridized carbon and the chlorine atom. It is a relatively weak, polar single bond. In chlorobenzene, the carbon attached to the chlorine is sp² hybridized, and the lone pair on the chlorine is delocalized into the aromatic ring through resonance. This gives the C-Cl bond partial double bond character, making it stronger and shorter than a typical single bond.
- Q7. Give an example of a nucleophilic substitution reaction of a halogenoalkane.
- Answer: The reaction of chloroethane with hydroxide ions to form ethanol:
- CH_3CH_2Cl + OH^- \rightarrow CH_3CH_2OH + Cl^-
- In this reaction, the hydroxide ion acts as a nucleophile and replaces the chloride ion.
- Q8. Give an example of an electrophilic substitution reaction of a halogenoarene.
- Answer: The nitration of chlorobenzene:
- C_6H_5Cl + HNO_3 \xrightarrow{H_2SO_4} C_6H_4ClNO_2 + H_2O
- The product is a mixture of ortho- and para-nitrochlorobenzene. The electrophile (NO2+) substitutes a hydrogen atom on the ring.
Q9. Why is the C-F bond the strongest carbon-halogen bond? Answer: The C-F bond is the strongest because fluorine is the most electronegative halogen, and its small size allows for a short, strong bond with carbon. The bond strength decreases down the group as the halogen becomes larger and more polarizable, leading to a longer and weaker bond.
Multiple Choice Questions (MCQs)
- Q1. What type of bond is broken in the nucleophilic substitution reaction of a halogenoalkane?
- A. C-H
- B. C-O
- C. C-X (where X is a halogen)
- D. C-C
- Correct Answer: C
- Explanation: In a nucleophilic substitution reaction, the carbon-halogen (C-X) bond is broken, and the halogen is replaced by a nucleophile.
Q2. Which factor does NOT significantly affect the rate of SN1 reactions? A. Stability of the carbocation B. Solvent polarity C. Strength of the nucleophile D. Leaving group ability Correct Answer: C Explanation: In an SN1 reaction, the rate-determining step is the formation of the carbocation, which does not involve the nucleophile. Therefore, the strength of the nucleophile does not significantly affect the rate of an SN1 reaction.
Q3. Which halogenoalkane is more reactive in an SN2 reaction? A. Chloromethane B. Chlorobenzene C. Chloroethane D. All of these Correct Answer: A Explanation: Chloromethane is a primary halogenoalkane and is more reactive in SN2 reactions than chloroethane (secondary) and chlorobenzene (which does not undergo SN2 reactions easily).
Q4. The C-Cl bond in chlorobenzene is less reactive towards nucleophilic substitution because: A. The C-Cl bond is too strong. B. The phenyl ring stabilizes the chlorine. C. Resonance effect makes the bond partially double. D. Steric hindrance from the benzene ring. Correct Answer: C Explanation: The lone pair on the chlorine atom is delocalized into the aromatic ring through resonance, giving the C-Cl bond partial double bond character and making it stronger and less reactive towards nucleophilic substitution.
- Q5. What is the general formula of a halogenoalkane?
- A. Ar-X
- B. R-X
- C. R-OH
- D. Ar-OH
- Correct Answer: B
- Explanation: Halogenoalkanes have the general formula R-X, where R is an alkyl group and X is a halogen.
- Q6. Which of the following is a nucleophile?
- A. H+
- B. NO2+
- C. OH-
- D. AlCl3
- Correct Answer: C
- Explanation: OH- is a nucleophile because it has a lone pair of electrons that can be donated to an electron-deficient carbon.
- Q7. What is the hybridization of the carbon atom attached to the halogen in a halogenoalkane?
- A. sp
- B. sp²
- C. sp³
- D. sp³d
- Correct Answer: C
- Explanation: The carbon atom attached to the halogen in a halogenoalkane is typically sp³ hybridized.
- Q8. What is the hybridization of the carbon atom attached to the halogen in a halogenoarene?
- A. sp
- B. sp²
- C. sp³
- D. sp³d
- Correct Answer: B
- Explanation: The carbon atom attached to the halogen in a halogenoarene is part of the aromatic ring and is sp² hybridized.
- Q9. Which of the following reactions is characteristic of halogenoarenes?
- A. Nucleophilic substitution
- B. Electrophilic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: B
- Explanation: Halogenoarenes undergo electrophilic substitution reactions on the aromatic ring.
- Q10. Which of the following is a correct statement about the reactivity of halogenoalkanes?
- A. They are unreactive.
- B. They undergo nucleophilic substitution.
- C. They undergo electrophilic substitution.
- D. They undergo addition reactions.
- Correct Answer: B
- Explanation: Halogenoalkanes undergo nucleophilic substitution reactions due to the polar C-X bond.
Multiple Choice Questions (MCQs)
- Q1. What type of bond is broken in the nucleophilic substitution reaction of a halogenoalkane?
- A. C-H
- B. C-O
- C. C-X (where X is a halogen)
- D. C-C
- Correct Answer: C
- Explanation: The C-X bond is broken in nucleophilic substitution.
- Q2. Which factor does NOT significantly affect the rate of SN1 reactions?
- A. Stability of the carbocation
- B. Solvent polarity
- C. Strength of the nucleophile
- D. Leaving group ability
- Correct Answer: C
- Explanation: The strength of the nucleophile does not significantly affect the rate of an SN1 reaction.
- Q3. Which halogenoalkane is more reactive in an SN2 reaction?
- A. Chloromethane
- B. Chlorobenzene
- C. Chloroethane
- D. All of these
- Correct Answer: A
- Explanation: Chloromethane is more reactive in SN2 reactions.
- Q4. The C-Cl bond in chlorobenzene is less reactive towards nucleophilic substitution because:
- A. The C-Cl bond is too strong.
- B. The phenyl ring stabilizes the chlorine.
- C. Resonance effect makes the bond partially double.
- D. Steric hindrance from the benzene ring.
- Correct Answer: C
- Explanation: Resonance gives the C-Cl bond partial double bond character.
- Q5. What is the general formula of a halogenoalkane?
- A. Ar-X
- B. R-X
- C. R-OH
- D. Ar-OH
- Correct Answer: B
- Explanation: The general formula is R-X.
- Q6. Which of the following is a nucleophile?
- A. H+
- B. NO2+
- C. OH-
- D. AlCl3
- Correct Answer: C
- Explanation: OH- is a nucleophile.
- Q7. What is the hybridization of the carbon atom attached to the halogen in a halogenoalkane?
- A. sp
- B. sp²
- C. sp³
- D. sp³d
- Correct Answer: C
- Explanation: The carbon is sp³ hybridized.
- Q8. What is the hybridization of the carbon atom attached to the halogen in a halogenoarene?
- A. sp
- B. sp²
- C. sp³
- D. sp³d
- Correct Answer: B
- Explanation: The carbon is sp² hybridized.
- Q9. Which of the following reactions is characteristic of halogenoarenes?
- A. Nucleophilic substitution
- B. Electrophilic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: B
- Explanation: Halogenoarenes undergo electrophilic substitution.
- Q10. Which of the following is a correct statement about the reactivity of halogenoalkanes?
- A. They are unreactive.
- B. They undergo nucleophilic substitution.
- C. They undergo electrophilic substitution.
- D. They undergo addition reactions.
- Correct Answer: B
- Explanation: Halogenoalkanes undergo nucleophilic substitution.
Multiple Choice Questions (MCQs)
- Q1. Which of the following is a halogenoalkane?
- A. Bromobenzene
- B. Chlorobenzene
- C. Chloroethane
- D. Iodobenzene
- Correct Answer: C
- Explanation: Chloroethane is a halogenoalkane.
- Q2. Which of the following is a halogenoarene?
- A. Chloroethane
- B. Bromoethane
- C. Chloromethane
- D. Bromobenzene
- Correct Answer: D
- Explanation: Bromobenzene is a halogenoarene.
- Q3. What is the reason for the low reactivity of halogenoarenes towards nucleophilic substitution?
- A. The C-X bond is weak.
- B. The C-X bond has partial double bond character due to resonance.
- C. The halogen is not electronegative.
- D. The ring is electron-deficient.
- Correct Answer: B
- Explanation: The C-X bond has partial double bond character due to resonance, making it strong and unreactive towards nucleophiles.
- Q4. What type of reaction do halogenoalkanes typically undergo?
- A. Electrophilic substitution
- B. Nucleophilic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: B
- Explanation: Halogenoalkanes typically undergo nucleophilic substitution.
- Q5. What type of reaction do halogenoarenes typically undergo?
- A. Electrophilic substitution
- B. Nucleophilic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: A
- Explanation: Halogenoarenes typically undergo electrophilic substitution.
- Q6. Which of the following halogens forms the strongest C-X bond?
- A. F
- B. Cl
- C. Br
- D. I
- Correct Answer: A
- Explanation: The C-F bond is the strongest.
- Q7. Which of the following halogens forms the weakest C-X bond?
- A. F
- B. Cl
- C. Br
- D. I
- Correct Answer: D
- Explanation: The C-I bond is the weakest.
- Q8. What is the electrophile in the nitration of chlorobenzene?
- A. NO2+
- B. NO3-
- C. Cl+
- D. H+
- Correct Answer: A
- Explanation: The electrophile is the nitronium ion (NO2+).
- Q9. What is the nucleophile in the reaction of chloroethane with aqueous NaOH?
- A. Na+
- B. OH-
- C. Cl-
- D. H2O
- Correct Answer: B
- Explanation: The nucleophile is the hydroxide ion (OH-).
- Q10. Why is the C-Cl bond in chlorobenzene stronger than in chloroethane?
- A. Because chlorine is more electronegative in chlorobenzene.
- B. Because of the resonance effect that gives partial double bond character to the C-Cl bond.
- C. Because the carbon atom in chlorobenzene is sp³ hybridized.
- D. Because chlorobenzene is a smaller molecule.
- Correct Answer: B
- Explanation: The resonance effect gives the C-Cl bond partial double bond character, making it stronger.
Multiple Choice Questions (MCQs)
- Q1. Which of the following is a halogenoalkane?
- A. Bromobenzene
- B. Chlorobenzene
- C. Chloroethane
- D. Iodobenzene
- Correct Answer: C
- Explanation: Chloroethane is a halogenoalkane.
- Q2. Which of the following is a halogenoarene?
- A. Chloroethane
- B. Bromoethane
- C. Chloromethane
- D. Bromobenzene
- Correct Answer: D
- Explanation: Bromobenzene is a halogenoarene.
- Q3. What is the reason for the low reactivity of halogenoarenes towards nucleophilic substitution?
- A. The C-X bond is weak.
- B. The C-X bond has partial double bond character due to resonance.
- C. The halogen is not electronegative.
- D. The ring is electron-deficient.
- Correct Answer: B
- Explanation: The C-X bond has partial double bond character due to resonance.
- Q4. What type of reaction do halogenoalkanes typically undergo?
- A. Electrophilic substitution
- B. Nucleophilic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: B
- Explanation: Halogenoalkanes typically undergo nucleophilic substitution.
- Q5. What type of reaction do halogenoarenes typically undergo?
- A. Electrophilic substitution
- B. Nucleophilic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: A
- Explanation: Halogenoarenes typically undergo electrophilic substitution.
- Q6. Which of the following halogens forms the strongest C-X bond?
- A. F
- B. Cl
- C. Br
- D. I
- Correct Answer: A
- Explanation: The C-F bond is the strongest.
- Q7. Which of the following halogens forms the weakest C-X bond?
- A. F
- B. Cl
- C. Br
- D. I
- Correct Answer: D
- Explanation: The C-I bond is the weakest.
- Q8. What is the electrophile in the nitration of chlorobenzene?
- A. NO2+
- B. NO3-
- C. Cl+
- D. H+
- Correct Answer: A
- Explanation: The electrophile is the nitronium ion (NO2+).
- Q9. What is the nucleophile in the reaction of chloroethane with aqueous NaOH?
- A. Na+
- B. OH-
- C. Cl-
- D. H2O
- Correct Answer: B
- Explanation: The nucleophile is the hydroxide ion (OH-).
- Q10. Why is the C-Cl bond in chlorobenzene stronger than in chloroethane?
- A. Because chlorine is more electronegative in chlorobenzene.
- B. Because of the resonance effect that gives partial double bond character to the C-Cl bond.
- C. Because the carbon atom in chlorobenzene is sp³ hybridized.
- D. Because chlorobenzene is a smaller molecule.
- Correct Answer: B
- Explanation: Resonance gives the C-Cl bond partial double bond character.
Multiple Choice Questions (MCQs)
- Q1. Which of the following statements is correct about the reactivity of halogenoalkanes?
- A. They are more reactive than alkanes.
- B. They are less reactive than alkanes.
- C. They are unreactive.
- D. They undergo only addition reactions.
- Correct Answer: A
- Explanation: Halogenoalkanes are more reactive than alkanes due to the polar C-X bond.
- Q2. What is the hybridisation of the carbon atom in a halogenoalkane?
- A. sp
- B. sp²
- C. sp³
- D. sp³d
- Correct Answer: C
- Explanation: The carbon atom in a halogenoalkane is sp³ hybridized.
- Q3. What is the hybridisation of the carbon atom in a halogenoarene?
- A. sp
- B. sp²
- C. sp³
- D. sp³d
- Correct Answer: B
- Explanation: The carbon atom in a halogenoarene is sp² hybridized.
- Q4. Which of the following is a nucleophile?
- A. H+
- B. NO2+
- C. OH-
- D. AlCl3
- Correct Answer: C
- Explanation: OH- is a nucleophile.
- Q5. Which of the following is an electrophile?
- A. OH-
- B. CN-
- C. NO2+
- D. NH3
- Correct Answer: C
- Explanation: NO2+ is an electrophile.
- Q6. The C-X bond in halogenoalkanes is:
- A. Non-polar
- B. Polar
- C. Ionic
- D. Metallic
- Correct Answer: B
- Explanation: The C-X bond is polar.
- Q7. The C-X bond in halogenoarenes has:
- A. No double bond character
- B. Partial double bond character
- C. Full double bond character
- D. Ionic character
- Correct Answer: B
- Explanation: The C-X bond in halogenoarenes has partial double bond character.
- Q8. Which of the following reactions is characteristic of halogenoalkanes?
- A. Electrophilic substitution
- B. Nucleophilic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: B
- Explanation: Halogenoalkanes undergo nucleophilic substitution.
- Q9. Which of the following reactions is characteristic of halogenoarenes?
- A. Electrophilic substitution
- B. Nucleophilic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: A
- Explanation: Halogenoarenes undergo electrophilic substitution.
- Q10. What is the product of the reaction between chloroethane and aqueous NaOH?
- A. Ethene
- B. Ethanol
- C. Ethanoic acid
- D. Ethane
- Correct Answer: B
- Explanation: The product is ethanol.
Multiple Choice Questions (MCQs)
- Q1. Which of the following statements is correct about the reactivity of halogenoalkanes?
- A. They are more reactive than alkanes.
- B. They are less reactive than alkanes.
- C. They are unreactive.
- D. They undergo only addition reactions.
- Correct Answer: A
- Explanation: Halogenoalkanes are more reactive than alkanes due to the polar C-X bond.
- Q2. What is the hybridisation of the carbon atom in a halogenoalkane?
- A. sp
- B. sp²
- C. sp³
- D. sp³d
- Correct Answer: C
- Explanation: The carbon atom in a halogenoalkane is sp³ hybridized.
- Q3. What is the hybridisation of the carbon atom in a halogenoarene?
- A. sp
- B. sp²
- C. sp³
- D. sp³d
- Correct Answer: B
- Explanation: The carbon atom in a halogenoarene is sp² hybridized.
- Q4. Which of the following is a nucleophile?
- A. H+
- B. NO2+
- C. OH-
- D. AlCl3
- Correct Answer: C
- Explanation: OH- is a nucleophile.
- Q5. Which of the following is an electrophile?
- A. OH-
- B. CN-
- C. NO2+
- D. NH3
- Correct Answer: C
- Explanation: NO2+ is an electrophile.
- Q6. The C-X bond in halogenoalkanes is:
- A. Non-polar
- B. Polar
- C. Ionic
- D. Metallic
- Correct Answer: B
- Explanation: The C-X bond is polar.
- Q7. The C-X bond in halogenoarenes has:
- A. No double bond character
- B. Partial double bond character
- C. Full double bond character
- D. Ionic character
- Correct Answer: B
- Explanation: The C-X bond in halogenoarenes has partial double bond character.
- Q8. Which of the following reactions is characteristic of halogenoalkanes?
- A. Electrophilic substitution
- B. Nucleophilic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: B
- Explanation: Halogenoalkanes undergo nucleophilic substitution.
- Q9. Which of the following reactions is characteristic of halogenoarenes?
- A. Electrophilic substitution
- B. Nucleophilic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: A
- Explanation: Halogenoarenes undergo electrophilic substitution.
- Q10. What is the product of the reaction between chloroethane and aqueous NaOH?
- A. Ethene
- B. Ethanol
- C. Ethanoic acid
- D. Ethane
- Correct Answer: B
- Explanation: The product is ethanol.
Multiple Choice Questions (MCQs)
- Q1. Which of the following statements is correct about the reactivity of halogenoalkanes?
- A. They are more reactive than alkanes.
- B. They are less reactive than alkanes.
- C. They are unreactive.
- D. They undergo only addition reactions.
- Correct Answer: A
- Explanation: Halogenoalkanes are more reactive than alkanes due to the polar C-X bond.
- Q2. What is the hybridisation of the carbon atom in a halogenoalkane?
- A. sp
- B. sp²
- C. sp³
- D. sp³d
- Correct Answer: C
- Explanation: The carbon atom in a halogenoalkane is sp³ hybridized.
- Q3. What is the hybridisation of the carbon atom in a halogenoarene?
- A. sp
- B. sp²
- C. sp³
- D. sp³d
- Correct Answer: B
- Explanation: The carbon atom in a halogenoarene is sp² hybridized.
- Q4. Which of the following is a nucleophile?
- A. H+
- B. NO2+
- C. OH-
- D. AlCl3
- Correct Answer: C
- Explanation: OH- is a nucleophile.
- Q5. Which of the following is an electrophile?
- A. OH-
- B. CN-
- C. NO2+
- D. NH3
- Correct Answer: C
- Explanation: NO2+ is an electrophile.
- Q6. The C-X bond in halogenoalkanes is:
- A. Non-polar
- B. Polar
- C. Ionic
- D. Metallic
- Correct Answer: B
- Explanation: The C-X bond is polar.
- Q7. The C-X bond in halogenoarenes has:
- A. No double bond character
- B. Partial double bond character
- C. Full double bond character
- D. Ionic character
- Correct Answer: B
- Explanation: The C-X bond in halogenoarenes has partial double bond character.
- Q8. Which of the following reactions is characteristic of halogenoalkanes?
- A. Electrophilic substitution
- B. Nucleophilic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: B
- Explanation: Halogenoalkanes undergo nucleophilic substitution.
- Q9. Which of the following reactions is characteristic of halogenoarenes?
- A. Electrophilic substitution
- B. Nucleophilic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: A
- Explanation: Halogenoarenes undergo electrophilic substitution.
- Q10. What is the product of the reaction between chloroethane and aqueous NaOH?
- A. Ethene
- B. Ethanol
- C. Ethanoic acid
- D. Ethane
- Correct Answer: B
- Explanation: The product is ethanol.
Multiple Choice Questions (MCQs)
- Q1. Which of the following compounds is a halogenoarene?
- A. Chloroethane
- B. Bromoethane
- C. Chlorobenzene
- D. Bromoethane
- Correct Answer: C
- Explanation: Chlorobenzene is a halogenoarene.
- Q2. Which of the following compounds is a halogenoalkane?
- A. Bromobenzene
- B. Chlorobenzene
- C. Bromoethane
- D. Iodobenzene
- Correct Answer: C
- Explanation: Bromoethane is a halogenoalkane.
- Q3. What is the main reason for the difference in reactivity between halogenoalkanes and halogenoarenes?
- A. The size of the halogen atom.
- B. The strength of the C-X bond.
- C. The polarity of the solvent.
- D. The temperature of the reaction.
- Correct Answer: B
- Explanation: The strength of the C-X bond is the main reason.
- Q4. Which of the following is a correct statement about the C-X bond in halogenoalkanes?
- A. It is a non-polar bond.
- B. It is a polar bond.
- C. It is an ionic bond.
- D. It is a hydrogen bond.
- Correct Answer: B
- Explanation: The C-X bond is polar.
- Q5. Which of the following is a correct statement about the C-X bond in halogenoarenes?
- A. It is a non-polar bond.
- B. It has partial double bond character.
- C. It is an ionic bond.
- D. It is a hydrogen bond.
- Correct Answer: B
- Explanation: The C-X bond in halogenoarenes has partial double bond character.
- Q6. What type of reaction do halogenoalkanes undergo?
- A. Electrophilic substitution
- B. Nucleophilic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: B
- Explanation: Halogenoalkanes undergo nucleophilic substitution.
Q7. What
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9.2 Reactivity of Halogenoalkanes
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Key Point 1
- Halogenoalkanes: Reactive towards nucleophiles.
Key Point 2
- Halogenoarenes: Less reactive towards nucleophiles (partial double bond).
Key Point 3
- C-X bond: Polar (Cδ+, Xδ-).
Key Point 4
- Nucleophilic substitution: Halogen replaced by nucleophile.
Key Point 5
- Halogenoarenes: Undergo electrophilic substitution.
Key Point 6
- Bond strength: C-F > C-Cl > C-Br > C-I.
Key Point 7
- Example: Chloroethane + OH- → Ethanol + Cl-.
Key Point 8
- Example: Chlorobenzene + HNO3/H2SO4 → op-nitrochlorobenzene.
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