Which of the following reagents CANNOT be used to prepare an alkyl chloride from an alcohol?
Select one:
a. HCI, ZnCl2
b. SOCI2
C. PCl5
d. NaCl
The correct answer and explanation is :
The correct answer is d. NaCl.
Explanation:
An alkyl chloride is typically prepared from an alcohol by replacing the hydroxyl group (-OH) of the alcohol with a chloride ion (-Cl). The reagents listed in the options can either promote substitution or facilitate the formation of an alkyl chloride. Let’s analyze each reagent in detail.
- a. HCl, ZnCl2:
- HCl (Hydrochloric acid) is a strong acid that can protonate the alcohol, converting the hydroxyl group (-OH) into a better leaving group, -OH2+. The alcohol can then undergo a SN1 (unimolecular nucleophilic substitution) or SN2 (bimolecular nucleophilic substitution) reaction, depending on the structure of the alcohol (tertiary alcohols favor SN1, while primary and secondary alcohols can undergo SN2).
- ZnCl2 (Zinc chloride) is a Lewis acid that activates the alcohol, especially in the case of primary alcohols, by coordinating with the oxygen atom, making the leaving group more susceptible to substitution.
- The combination of HCl and ZnCl2 is a classic reagent used for preparing alkyl chlorides from alcohols.
- b. SOCl2 (Thionyl chloride):
- Thionyl chloride is a common reagent used for converting alcohols into alkyl chlorides. It reacts with alcohols in a substitution reaction to replace the hydroxyl group with a chloride ion. The reaction proceeds via an SN2 mechanism (due to the strong nucleophilicity of the chloride ion from SOCl2). Thionyl chloride also produces sulfur dioxide (SO2) and hydrogen chloride (HCl) as by-products.
- c. PCl5 (Phosphorus pentachloride):
- Phosphorus pentachloride is a strong chlorinating agent and can convert alcohols into alkyl chlorides. PCl5 reacts with the alcohol by substituting the hydroxyl group with a chlorine atom, also producing phosphorus oxychloride (POCl3) as a by-product. This reaction is highly efficient and is commonly used in laboratory settings.
- d. NaCl (Sodium chloride):
- NaCl (Sodium chloride) is not an effective reagent for converting alcohols to alkyl chlorides. Although NaCl is a source of chloride ions, it is not reactive enough on its own to replace the hydroxyl group of the alcohol in a substitution reaction. For substitution to occur, the hydroxyl group must be first activated (e.g., through protonation or the use of a Lewis acid like ZnCl2), but NaCl cannot activate the alcohol or promote the substitution reaction effectively.
Therefore, NaCl cannot be used to prepare alkyl chlorides from alcohols because it does not provide the necessary conditions for the substitution to occur.