Chemistry/Chapter 9: Halogenoalkanes
9.3 Reactions of Halogenoalkanes or Alkyl Halides
The main idea is that the carbon-halogen bond in an alkyl halide is polarized because the halogen is more electronegative than carbon.
Explanation level
In the previous section, we learned that halogenoalkanes (alkyl halides) are highly reactive towards nucleophiles due to the polar nature of the carbon-halogen bond. This section introduces the two main types of reactions that halogenoalkanes undergo: nucleophilic substitution and elimination.
Main Idea
- Nucleophilic substitution (SN) reactions: The halogen is replaced by a nucleophile.
- Elimination reactions: A hydrogen halide (HX) is removed, forming a double bond.
Core Concept
The core concept is the inductive effect of the halogen. The halogen pulls electron density towards itself, creating an electrophilic carbon (δ+) that is susceptible to attack by nucleophiles. The halogen acts as a leaving group, departing with its pair of electrons. The relative rates of substitution versus elimination depend on factors such as the structure of the alkyl halide (primary, secondary, tertiary), the strength and concentration of the nucleophile/base, and the reaction conditions (temperature, solvent).
Detailed Explanation
The textbook explains that in an alkyl halide molecule, the carbon-halogen bond is polarized due to the more electronegative nature of the halogen than carbon. So, halogen atoms induce a dipolar character in carbon-halogen bonds. The electron density is drawn more towards the halogen atom. As a result, the halogen atom acquires a partial negative charge and the carbon atom a partial positive charge. This is called the inductive effect. C^{\delta+} - X^{\delta-} This polar character of the C-X bond is responsible for characteristic chemical reactions of alkyl halides.
- Nucleophilic substitution reactions (SN Reactions)
- Elimination reactions
1. Nucleophilic Substitution Reactions (SN)
The textbook introduces this topic, with a detailed mechanism covered in the next section (9.3.1). The general reaction is: R-X + Nu^- \rightarrow R-Nu + X^- A nucleophile (Nu-) attacks the electron-deficient carbon atom and replaces the halogen (X-), which departs as a leaving group.
2. Elimination Reactions
In an elimination reaction, a hydrogen halide (HX) is removed from the alkyl halide, resulting in the formation of an alkene. R-CH_2-CH_2-X \rightarrow R-CH=CH_2 + HX The halogen and a hydrogen atom from an adjacent carbon are removed, and a double bond is formed between the two carbon atoms. Elimination reactions are typically favored by strong bases, higher temperatures, and tertiary alkyl halides.
Summary of Reactions
| Reaction Type | Description | General Equation | Product |
|---|---|---|---|
| Nucleophilic Substitution | Halogen replaced by a nucleophile | R-X + Nu- → R-Nu + X- | Substituted product |
| Elimination | HX removed, forming a double bond | R-CH2-CH2-X → R-CH=CH2 + HX | Alkene |
Intermediate Level
Halogenoalkanes undergo two main types of reactions: nucleophilic substitution and elimination. The polar C-X bond makes the carbon electrophilic, allowing nucleophiles to attack and replace the halogen (substitution). The halogen leaves as a halide ion. Alternatively, a base can remove a proton from a carbon adjacent to the halogen, leading to the elimination of HX and the formation of an alkene. The competition between substitution and elimination depends on the structure of the alkyl halide (primary, secondary, tertiary), the nature of the nucleophile/base, and the reaction conditions. Primary alkyl halides typically favor substitution, while tertiary alkyl halides are more prone to elimination.
Important Points
- - C-X bond polarity: Carbon is δ+, halogen is δ-.
- - Inductive effect: Halogen pulls electron density towards itself.
- - Two main reactions: Nucleophilic substitution and elimination.
- - Nucleophilic substitution: Halogen replaced by a nucleophile.
- - Elimination: HX removed, forming an alkene.
- - Substrate structure: Primary favors substitution; tertiary favors elimination.
- - Nucleophile/base strength: Strong nucleophile favors substitution; strong base favors elimination.
Remember
- - C-X bond is polar (Cδ+, Xδ-).
- - Halogen is a good leaving group.
- - Two reactions: Nucleophilic substitution (SN) and elimination (E).
- - SN: Halogen replaced by a nucleophile.
- - Elimination: Forms a double bond by removing HX.
- - Primary alkyl halides: Favor substitution.
- - Tertiary alkyl halides: Favor elimination.
- - Reaction conditions matter: Temperature, solvent, and reagent affect the outcome.
Common Mistakes
- - Thinking that the C-X bond is non-polar. It is polar due to the electronegativity difference between carbon and the halogen.
- - Forgetting that the halogen is a leaving group. In both substitution and elimination, the halogen departs with its bonding electrons.
- - Confusing substitution and elimination products. Substitution replaces the halogen; elimination removes HX and forms a double bond.
- - Assuming all alkyl halides react the same way. The outcome depends on the substrate (primary, secondary, tertiary) and the reaction conditions.
- - Not recognizing the role of the nucleophile or base. A strong nucleophile promotes substitution; a strong base promotes elimination.
Quick Revision
Halogenoalkanes (alkyl halides) are characterized by a polar carbon-halogen bond, where the carbon atom is electron-deficient (electrophilic) and the halogen is electron-rich. This polarity is due to the inductive effect of the halogen. Alkyl halides undergo two main types of reactions: nucleophilic substitution (SN) and elimination. In nucleophilic substitution, a nucleophile attacks the electron-deficient carbon and replaces the halogen. In elimination, a base removes a proton from an adjacent carbon, leading to the removal of HX and the formation of an alkene. The preferred reaction pathway depends on the structure of the alkyl halide and the reaction conditions.
60-Second Revision
Halogenoalkanes have a polar C-X bond (Cδ+, Xδ-) due to the inductive effect of the halogen. They undergo two main reactions: nucleophilic substitution (SN), where a nucleophile replaces the halogen, and elimination, where HX is removed to form an alkene. Primary alkyl halides favor substitution, while tertiary alkyl halides favor elimination.
Frequently Asked Questions (FAQs)
Q1. What is the inductive effect in halogenoalkanes? Answer: The inductive effect is the polarization of the carbon-halogen bond due to the higher electronegativity of the halogen compared to carbon. The halogen pulls electron density towards itself, creating a partial negative charge on the halogen (δ-) and a partial positive charge on the carbon (δ+). This makes the carbon atom electrophilic and susceptible to nucleophilic attack.
- Nucleophilic substitution reactions (SN): A nucleophile replaces the halogen.
- Elimination reactions: A hydrogen halide (HX) is removed, forming a double bond (alkene).
Q3. Why is the carbon atom in a halogenoalkane electrophilic? Answer: The carbon atom is electrophilic because the electronegative halogen withdraws electron density, leaving the carbon with a partial positive charge. Electrophiles are electron-deficient species that are attracted to nucleophiles.
- Structure of the alkyl halide: Primary alkyl halides favor substitution, while tertiary alkyl halides favor elimination.
- Strength of the nucleophile/base: A strong nucleophile favors substitution; a strong, bulky base favors elimination.
- Reaction conditions: Higher temperatures and less polar solvents often favor elimination.
Q5. What is a leaving group? Answer: A leaving group is an atom or group of atoms that departs with a pair of electrons during a substitution or elimination reaction. In halogenoalkanes, the halogen (e.g., Cl-, Br-, I-) is a good leaving group.
- Q6. Write the general equation for a nucleophilic substitution reaction of a halogenoalkane.
- Answer: The general equation is:
- R-X + Nu- → R-Nu + X-
- where R is an alkyl group, X is a halogen, and Nu- is a nucleophile.
- Q7. Write the general equation for an elimination reaction of a halogenoalkane.
- Answer: The general equation is:
- R-CH2-CH2-X → R-CH=CH2 + HX
- An alkene is formed, and a hydrogen halide (HX) is eliminated.
Q8. Why are halogenoalkanes more reactive than alkanes? Answer: Halogenoalkanes are more reactive than alkanes because the C-X bond is polar, making the carbon atom electrophilic and susceptible to attack by nucleophiles. In alkanes, the C-H and C-C bonds are non-polar and much less reactive.
Q9. What is the difference between SN1 and SN2 reactions? Answer: SN1 is a two-step mechanism involving the formation of a carbocation intermediate, and the rate depends only on the concentration of the alkyl halide. SN2 is a one-step mechanism where the nucleophile attacks simultaneously as the leaving group departs, and the rate depends on both the alkyl halide and the nucleophile. Primary alkyl halides favor SN2, while tertiary alkyl halides favor SN1.
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
- D. C-C
- Correct Answer: C
- Explanation: In nucleophilic substitution, the carbon-halogen (C-X) bond is broken, and the halogen is replaced by a nucleophile.
- Q2. What is the inductive effect in a halogenoalkane?
- A. The halogen donates electron density to carbon.
- B. The halogen withdraws electron density from carbon.
- C. The carbon withdraws electron density from the halogen.
- D. The carbon donates electron density to the halogen.
- Correct Answer: B
- Explanation: The halogen is more electronegative than carbon, so it withdraws electron density, creating a partial positive charge on the carbon.
- Q3. Which of the following is a characteristic reaction of halogenoalkanes?
- A. Electrophilic substitution
- B. Nucleophilic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: B
- Explanation: Halogenoalkanes undergo nucleophilic substitution reactions.
- Q4. Which of the following is another name for an elimination reaction in a halogenoalkane?
- A. Dehydrohalogenation
- B. Hydrogenation
- C. Hydration
- D. Halogenation
- Correct Answer: A
- Explanation: Elimination of HX from a halogenoalkane is called dehydrohalogenation.
- Q5. Which type of alkyl halide is most likely to undergo an SN2 reaction?
- A. Primary
- B. Secondary
- C. Tertiary
- D. Quaternary
- Correct Answer: A
- Explanation: Primary alkyl halides are least sterically hindered and most reactive in SN2 reactions.
- Q6. Which type of alkyl halide is most likely to undergo an SN1 reaction?
- A. Primary
- B. Secondary
- C. Tertiary
- D. Quaternary
- Correct Answer: C
- Explanation: Tertiary alkyl halides form the most stable carbocation intermediates, favoring SN1 reactions.
- Q7. What type of reaction do halogenoalkanes undergo when a strong base is used?
- A. Nucleophilic substitution
- B. Elimination
- C. Addition
- D. Oxidation
- Correct Answer: B
- Explanation: A strong base promotes elimination reactions by removing a proton from the carbon adjacent to the halogen.
- Q8. What is the leaving group in a nucleophilic substitution reaction of a halogenoalkane?
- A. Carbon atom
- B. Halogen atom
- C. Hydrogen atom
- D. Nucleophile
- Correct Answer: B
- Explanation: The halogen atom is the leaving group.
- Q9. Which of the following is a polar bond?
- A. C-H
- B. C-C
- C. C-X
- D. C-O
- Correct Answer: C
- Explanation: The C-X bond is polar due to the electronegativity difference between carbon and the halogen.
- Q10. What is the general formula for an elimination reaction of a halogenoalkane?
- A. R-X + Nu- → R-Nu + X-
- B. R-CH2-CH2-X → R-CH=CH2 + HX
- C. R-X + H2 → R-H + HX
- D. R-X + H2O → R-OH + HX
- Correct Answer: B
- Explanation: In an elimination reaction, HX is removed to form an alkene.
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
- D. C-C
- Correct Answer: C
- Explanation: The C-X bond is broken.
- Q2. What is the inductive effect in a halogenoalkane?
- A. The halogen donates electron density to carbon.
- B. The halogen withdraws electron density from carbon.
- C. The carbon withdraws electron density from the halogen.
- D. The carbon donates electron density to the halogen.
- Correct Answer: B
- Explanation: The halogen withdraws electron density.
- Q3. Which of the following is a characteristic reaction of halogenoalkanes?
- A. Electrophilic substitution
- B. Nucleophilic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: B
- Explanation: Halogenoalkanes undergo nucleophilic substitution.
- Q4. Which of the following is another name for an elimination reaction in a halogenoalkane?
- A. Dehydrohalogenation
- B. Hydrogenation
- C. Hydration
- D. Halogenation
- Correct Answer: A
- Explanation: Elimination of HX is called dehydrohalogenation.
- Q5. Which type of alkyl halide is most likely to undergo an SN2 reaction?
- A. Primary
- B. Secondary
- C. Tertiary
- D. Quaternary
- Correct Answer: A
- Explanation: Primary alkyl halides favor SN2.
- Q6. Which type of alkyl halide is most likely to undergo an SN1 reaction?
- A. Primary
- B. Secondary
- C. Tertiary
- D. Quaternary
- Correct Answer: C
- Explanation: Tertiary alkyl halides favor SN1.
- Q7. What type of reaction do halogenoalkanes undergo when a strong base is used?
- A. Nucleophilic substitution
- B. Elimination
- C. Addition
- D. Oxidation
- Correct Answer: B
- Explanation: A strong base promotes elimination.
- Q8. What is the leaving group in a nucleophilic substitution reaction of a halogenoalkane?
- A. Carbon atom
- B. Halogen atom
- C. Hydrogen atom
- D. Nucleophile
- Correct Answer: B
- Explanation: The halogen atom is the leaving group.
- Q9. Which of the following is a polar bond?
- A. C-H
- B. C-C
- C. C-X
- D. C-O
- Correct Answer: C
- Explanation: The C-X bond is polar.
- Q10. What is the general formula for an elimination reaction of a halogenoalkane?
- A. R-X + Nu- → R-Nu + X-
- B. R-CH2-CH2-X → R-CH=CH2 + HX
- C. R-X + H2 → R-H + HX
- D. R-X + H2O → R-OH + HX
- Correct Answer: B
- Explanation: The elimination of HX forms an alkene.
Multiple Choice Questions (MCQs)
- Q1. Which of the following is a nucleophile?
- A. H+
- B. NO2+
- C. OH-
- D. AlCl3
- Correct Answer: C
- Explanation: OH- is a nucleophile.
- Q2. Which of the following is an electrophile?
- A. OH-
- B. CN-
- C. NO2+
- D. NH3
- Correct Answer: C
- Explanation: NO2+ is an electrophile.
- Q3. What is the hybridization of the carbon atom in a halogenoalkane?
- A. sp
- B. sp²
- C. sp³
- D. sp³d
- Correct Answer: C
- Explanation: The carbon atom is sp³ hybridized.
- Q4. What is the hybridization of the carbon atom in a halogenoarene?
- A. sp
- B. sp²
- C. sp³
- D. sp³d
- Correct Answer: B
- Explanation: The carbon atom is sp² hybridized.
- Q5. 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.
- Q6. 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.
- Q7. 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.
- Q8. What type of reaction do halogenoarenes undergo?
- A. Electrophilic substitution
- B. Nucleophilic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: A
- Explanation: Halogenoarenes undergo electrophilic substitution.
- Q9. 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.
- Q10. 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.
Multiple Choice Questions (MCQs)
- Q1. Which of the following is a nucleophile?
- A. H+
- B. NO2+
- C. OH-
- D. AlCl3
- Correct Answer: C
- Explanation: OH- is a nucleophile.
- Q2. Which of the following is an electrophile?
- A. OH-
- B. CN-
- C. NO2+
- D. NH3
- Correct Answer: C
- Explanation: NO2+ is an electrophile.
- Q3. What is the hybridization of the carbon atom in a halogenoalkane?
- A. sp
- B. sp²
- C. sp³
- D. sp³d
- Correct Answer: C
- Explanation: The carbon atom is sp³ hybridized.
- Q4. What is the hybridization of the carbon atom in a halogenoarene?
- A. sp
- B. sp²
- C. sp³
- D. sp³d
- Correct Answer: B
- Explanation: The carbon atom is sp² hybridized.
- Q5. 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.
- Q6. 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 has partial double bond character.
- Q7. 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.
- Q8. What type of reaction do halogenoarenes undergo?
- A. Electrophilic substitution
- B. Nucleophilic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: A
- Explanation: Halogenoarenes undergo electrophilic substitution.
- Q9. 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.
- Q10. 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.
Multiple Choice Questions (MCQs)
- Q1. What is the main reason for the reactivity of halogenoalkanes?
- A. The C-H bond is polar.
- B. The C-X bond is polar.
- C. The C-C bond is polar.
- D. The C-X bond is non-polar.
- Correct Answer: B
- Explanation: The C-X bond is polar.
- Q2. What type of reaction occurs when a halogenoalkane is treated with a strong base?
- A. Nucleophilic substitution
- B. Elimination
- C. Addition
- D. Oxidation
- Correct Answer: B
- Explanation: A strong base promotes elimination.
- Q3. What type of reaction occurs when a halogenoalkane is treated with a strong nucleophile?
- A. Nucleophilic substitution
- B. Elimination
- C. Addition
- D. Oxidation
- Correct Answer: A
- Explanation: A strong nucleophile promotes substitution.
- Q4. What is the product of an elimination reaction of a halogenoalkane?
- A. Alkane
- B. Alkene
- C. Alkyne
- D. Alcohol
- Correct Answer: B
- Explanation: The product is an alkene.
- Q5. What is the product of a nucleophilic substitution reaction of a halogenoalkane?
- A. Alkane
- B. Alkene
- C. Substituted alkane
- D. Alkyne
- Correct Answer: C
- Explanation: The product is a substituted alkane.
- Q6. Which of the following is a good leaving group?
- A. OH-
- B. NH2-
- C. Cl-
- D. CH3-
- Correct Answer: C
- Explanation: Cl- is a good leaving group.
- Q7. Which of the following is a poor leaving group?
- A. I-
- B. Br-
- C. Cl-
- D. OH-
- Correct Answer: D
- Explanation: OH- is a poor leaving group.
- Q8. What is the inductive effect?
- A. The withdrawal or donation of electron density through sigma bonds.
- B. The delocalization of pi electrons.
- C. The formation of hydrogen bonds.
- D. The transfer of protons.
- Correct Answer: A
- Explanation: The inductive effect is the withdrawal or donation of electron density through sigma bonds.
- Q9. Which of the following alkyl halides is most reactive towards SN1?
- A. Primary
- B. Secondary
- C. Tertiary
- D. Methyl
- Correct Answer: C
- Explanation: Tertiary alkyl halides are most reactive towards SN1.
- Q10. Which of the following alkyl halides is most reactive towards SN2?
- A. Primary
- B. Secondary
- C. Tertiary
- D. Methyl
- Correct Answer: D
- Explanation: Methyl halides are most reactive towards SN2.
Multiple Choice Questions (MCQs)
- Q1. What is the main reason for the reactivity of halogenoalkanes?
- A. The C-H bond is polar.
- B. The C-X bond is polar.
- C. The C-C bond is polar.
- D. The C-X bond is non-polar.
- Correct Answer: B
- Explanation: The C-X bond is polar.
- Q2. What type of reaction occurs when a halogenoalkane is treated with a strong base?
- A. Nucleophilic substitution
- B. Elimination
- C. Addition
- D. Oxidation
- Correct Answer: B
- Explanation: A strong base promotes elimination.
- Q3. What type of reaction occurs when a halogenoalkane is treated with a strong nucleophile?
- A. Nucleophilic substitution
- B. Elimination
- C. Addition
- D. Oxidation
- Correct Answer: A
- Explanation: A strong nucleophile promotes substitution.
- Q4. What is the product of an elimination reaction of a halogenoalkane?
- A. Alkane
- B. Alkene
- C. Alkyne
- D. Alcohol
- Correct Answer: B
- Explanation: The product is an alkene.
- Q5. What is the product of a nucleophilic substitution reaction of a halogenoalkane?
- A. Alkane
- B. Alkene
- C. Substituted alkane
- D. Alkyne
- Correct Answer: C
- Explanation: The product is a substituted alkane.
- Q6. Which of the following is a good leaving group?
- A. OH-
- B. NH2-
- C. Cl-
- D. CH3-
- Correct Answer: C
- Explanation: Cl- is a good leaving group.
- Q7. Which of the following is a poor leaving group?
- A. I-
- B. Br-
- C. Cl-
- D. OH-
- Correct Answer: D
- Explanation: OH- is a poor leaving group.
- Q8. What is the inductive effect?
- A. The withdrawal or donation of electron density through sigma bonds.
- B. The delocalization of pi electrons.
- C. The formation of hydrogen bonds.
- D. The transfer of protons.
- Correct Answer: A
- Explanation: The inductive effect is the withdrawal or donation of electron density through sigma bonds.
- Q9. Which of the following alkyl halides is most reactive towards SN1?
- A. Primary
- B. Secondary
- C. Tertiary
- D. Methyl
- Correct Answer: C
- Explanation: Tertiary alkyl halides are most reactive towards SN1.
- Q10. Which of the following alkyl halides is most reactive towards SN2?
- A. Primary
- B. Secondary
- C. Tertiary
- D. Methyl
- Correct Answer: D
- Explanation: Methyl halides are most reactive towards SN2.
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Quick revision
Revision Zone
9.3 Reactions of Halogenoalkanes or Alkyl Halides
Sticky Notes · Quick Revision
Key Point 1
- C-X bond: Polar (Cδ+, Xδ-).
Key Point 2
- Inductive effect: Halogen withdraws electrons.
Key Point 3
- Two reactions: SN (substitution) and E (elimination).
Key Point 4
- SN: R-X + Nu- → R-Nu + X-.
Key Point 5
- Elimination: R-CH2-CH2-X → R-CH=CH2 + HX.
Key Point 6
- Primary: Substitution.
Key Point 7
- Tertiary: Elimination.
Key Point 8
- Strong base: Favors elimination.
Key Point 9
- Strong nucleophile: Favors substitution.
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