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Questions and Answers
Why is thionyl chloride ($SOCl_2$) preferred for the preparation of alkyl halides from alcohols?
Why is thionyl chloride ($SOCl_2$) preferred for the preparation of alkyl halides from alcohols?
- It produces alkyl halides with higher yields compared to phosphorus halides.
- It generates pure alkyl halides and easily removable gaseous byproducts. (correct)
- It is effective for both primary and tertiary alcohols without catalysts.
- It requires lower reaction temperatures compared to other methods.
Which of the following alcohols will react fastest with a given haloacid (HX)?
Which of the following alcohols will react fastest with a given haloacid (HX)?
- All react at the same rate
- Secondary alcohol ($2^\circ$)
- Primary alcohol ($1^\circ$)
- Tertiary alcohol ($3^\circ$) (correct)
What role does $ZnCl_2$ play in the reaction of primary and secondary alcohols with hydrogen halides (HX)?
What role does $ZnCl_2$ play in the reaction of primary and secondary alcohols with hydrogen halides (HX)?
- It increases the reaction temperature.
- It serves as a catalyst to facilitate the reaction. (correct)
- It prevents the formation of unwanted byproducts.
- It acts as a reducing agent.
Why is the direct reaction of phenols with halogen acids (like HCl or HBr) generally not used to prepare aryl halides?
Why is the direct reaction of phenols with halogen acids (like HCl or HBr) generally not used to prepare aryl halides?
Which of the following reagents is used to prepare alkyl iodide (R-I) in good yields from alcohols?
Which of the following reagents is used to prepare alkyl iodide (R-I) in good yields from alcohols?
What is the purpose of using red phosphorus with bromine or iodine in the preparation of alkyl halides from alcohols?
What is the purpose of using red phosphorus with bromine or iodine in the preparation of alkyl halides from alcohols?
Which of the following conditions are typically used to react tertiary alcohols with concentrated hydrochloric acid (HCl)?
Which of the following conditions are typically used to react tertiary alcohols with concentrated hydrochloric acid (HCl)?
What is the main reason for using concentrated solutions of halogen acids in the preparation of alkyl halides from alcohols?
What is the main reason for using concentrated solutions of halogen acids in the preparation of alkyl halides from alcohols?
What is the main byproduct when alcohols react with $PCl_5$ to form alkyl chlorides?
What is the main byproduct when alcohols react with $PCl_5$ to form alkyl chlorides?
Which method is used for the preparation of alkyl chloride by passing dry hydrogen chloride gas?
Which method is used for the preparation of alkyl chloride by passing dry hydrogen chloride gas?
Flashcards
Alkyl Halide Preparation
Alkyl Halide Preparation
Alkyl halides are prepared by replacing the hydroxyl group of an alcohol with a halogen.
Why use Thionyl Chloride?
Why use Thionyl Chloride?
Thionyl chloride (SOCl₂) is preferred in alcohol reactions because the byproducts are gases, simplifying product isolation.
Alcohol Reaction with HX
Alcohol Reaction with HX
Primary and secondary alcohols with HX require a catalyst (ZnCl₂), while tertiary alcohols react without one at room temperature.
Reactivity Order
Reactivity Order
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Phenols Carbon-Oxygen Bonds
Phenols Carbon-Oxygen Bonds
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Alkyl Chloride Preparation
Alkyl Chloride Preparation
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Study Notes
From Alcohols
- Alkyl halides are best prepared from alcohols because they are easily accessible
- The hydroxyl group of an alcohol is replaced by halogen in reactions with concentrated halogen acids, phosphorous halides, or thionyl chloride
- Thionyl chloride is preferred because the other two products are escapable gasses, thus the reaction gives pure alkyl halides
- Tribromide and triiodide are usually generated in situ by reacting red phosphorus with bromine and iodine respectively
- Alkyl chloride preparation is carried out either by passing dry hydrogen chloride gas through a solution of alcohol or by heating a solution of alcohol in concentrated aqueous acid
Reactions
- R-OH + HCl with Zinc Chloride yields R-Cl + H2O
- R-OH + NaBr + H2SO4 yields R-Br + NaHSO4 + H2O
- 3R-OH + PX3 yields 3R-X + H3PO3 (X = Cl or Br)
- R-OH + PCl5 yields R-Cl + POCl3 + HCl
- R-OH with red P/X2 (X2 = Br2 or I2) yields R-X
- R-OH + SOCl2 yields R-Cl + SO2 + HCl
Reactions of primary, secondary, and tertiary alcohols
- Primary and secondary alcohols with HX require the presence of a catalyst, ZnCl2
- With tertiary alcohols, the reaction is conducted by simply shaking with concentrated HCl at room temperature
- Constant boiling with 48% HBr is used for preparing alkyl bromide
- Good yields of R-I may be obtained by heating alcohols with sodium or potassium iodide in 95% phosphoric acid
- The order of reactivity of alcohols with a given haloacid is 3°>2°>1°
- The above method is not applicable for aryl halides because the carbon-oxygen bond in phenols has a partial double bond character and is difficult to break because is stronger than a single bond
By Free Radical Halogenation
- Free radical chlorination or bromination of alkanes gives a complex
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