10.5 Preparing Alkyl Halides from Alcohols
2 min read•may 7, 2024
Turning into is a key skill in organic synthesis. This process involves replacing the hydroxyl group with a halogen atom, using various reagents like or . The reactivity depends on the alcohol type.
react fastest, while primary ones are slowest. Different strategies exist for making specific halides, including fluorides. The reaction pathway and outcome can be influenced by factors like solvent choice and substrate structure, with sometimes competing.
Synthesis of Alkyl Halides from Alcohols
Alcohol to alkyl halide conversion
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- Alcohols react with hydrogen halides (HX) like , , or to form alkyl halides through of the hydroxyl group followed by by the halide ion (HI > HBr > HCl in reactivity)
- Thionyl chloride (SOCl) converts alcohols to via nucleophilic addition of the alcohol to SOCl, then elimination of HCl and SO byproducts
- (PBr) reacts with alcohols to yield through nucleophilic substitution of the alcohol with PBr followed by hydrolysis of the resulting , producing (HPO) as a byproduct
- The hydroxyl group acts as a leaving group in these reactions, being replaced by the halide
Alcohol reactivity in halogenation
- Tertiary alcohols are most reactive in halogenation due to the stability of the intermediate, followed by , with being the least reactive because of the instability of the primary carbocation intermediate
- Tertiary (most reactive)
- Secondary
- Primary (least reactive)
- Secondary and tertiary alcohols can undergo during halogenation reactions to form a more stable carbocation intermediate, while primary alcohols typically do not rearrange
- The of the product depends on the reaction mechanism and the nature of the carbocation intermediate
Strategies for alkyl fluoride synthesis
- (diethylaminosulfur trifluoride) converts alcohols to through nucleophilic substitution of the alcohol with DAST, then elimination of diethylamine and sulfur dioxide
- () reacts with alcohols to form alkyl fluorides via nucleophilic substitution of the alcohol with the fluoride ion from the reagent
- (PBSF) in the presence of a base forms a with the alcohol, which then undergoes nucleophilic substitution with a fluoride ion to yield the alkyl fluoride
Reaction Considerations
- can influence the rate and outcome of these reactions, with polar protic solvents generally favoring substitution reactions
- vary depending on the substrate and conditions, ranging from SN1 to SN2 pathways
- Competing can occur, especially with secondary and tertiary substrates, leading to alkene formation
Key Terms to Review (33)
Alcohols: Alcohols are organic compounds containing a hydroxyl (-OH) functional group attached to a saturated carbon atom. They are widely used in various chemical reactions and have diverse applications in industry, medicine, and everyday life.
Alkyl Bromides: Alkyl bromides are a class of organic compounds consisting of a bromine atom covalently bonded to an alkyl group, which is a hydrocarbon chain or ring. These compounds are widely used in organic synthesis and have various applications in chemistry and industry.
Alkyl Chlorides: Alkyl chlorides are organic compounds consisting of a chlorine atom bonded to an alkyl group, which is a hydrocarbon chain. These compounds are widely used in various chemical reactions and industrial processes due to their versatility and reactivity.
Alkyl Fluorides: Alkyl fluorides are organic compounds that contain a carbon-fluorine (C-F) bond. They are a type of alkyl halide, where the halogen is fluorine. Alkyl fluorides are important in the context of organic chemistry reactions, particularly in the alkylation of acetylide anions and the preparation of alkyl halides from alcohols.
Alkyl Halides: Alkyl halides are organic compounds that consist of an alkyl group (a hydrocarbon chain) bonded to a halogen atom (fluorine, chlorine, bromine, or iodine). They are widely used in organic synthesis and have various applications in chemistry and biology.
Anti stereochemistry: Anti stereochemistry describes the spatial arrangement in a chemical reaction where two substituents are positioned on opposite sides of a double bond or ring structure after the reaction. It is particularly relevant in the halogenation of alkenes, resulting in products where the added atoms are located across from each other.
Carbocation: A carbocation is a positively charged carbon atom that is part of an organic molecule. These reactive intermediates play a crucial role in various organic reactions, including electrophilic additions, nucleophilic substitutions, and elimination reactions.
Carbocation Rearrangements: Carbocation rearrangements are a type of reaction in organic chemistry where a carbocation (a positively charged carbon atom) undergoes a structural change to form a more stable carbocation. This process is particularly relevant in the context of preparing alkyl halides from alcohols.
DAST: DAST, or Dehydrohalogenation Assisted Substitution Technique, is a method used in organic chemistry to prepare alkyl halides from alcohols. It involves the conversion of an alcohol to an alkyl halide through a two-step process involving dehydration and subsequent halogenation.
Elimination reactions: Elimination reactions are a type of organic reaction where two atoms or groups are removed from a molecule, resulting in the formation of a double bond. These reactions often involve the loss of small molecules like water or hydrogen halides from larger organic molecules.
Elimination Reactions: Elimination reactions are a class of organic reactions where two atoms or groups are removed from a molecule, typically resulting in the formation of a new carbon-carbon double bond. These reactions are an important aspect of organic chemistry, as they allow for the conversion of various functional groups and the synthesis of alkenes and other unsaturated compounds.
HBr: HBr, or hydrobromic acid, is a strong acid composed of hydrogen (H) and bromine (Br). It is an important reagent in organic chemistry, commonly used in various reactions and processes, including the addition of HBr to alkenes, the preparation of alkyl halides from alcohols, and electrophilic additions to conjugated dienes.
HCl: HCl, or hydrochloric acid, is a strong, corrosive acid that plays a crucial role in various chemical processes and reactions. It is composed of hydrogen (H) and chlorine (Cl) atoms, and its unique properties make it an important component in numerous organic chemistry topics, including acid-base chemistry, electrophilic additions, alcohol reactions, ester chemistry, and peptide sequencing.
HI: HI, or hydroiodic acid, is a strong acid composed of hydrogen (H) and iodine (I). It is a key reagent used in the preparation of alkyl halides from alcohols, as described in the topic 10.5 Preparing Alkyl Halides from Alcohols.
Hydrogen Halides: Hydrogen halides are a class of compounds consisting of a hydrogen atom bonded to a halogen atom, such as fluorine, chlorine, bromine, or iodine. These compounds are important in the context of preparing alkyl halides from alcohols, as they serve as the halogenating agents in these reactions.
Leaving Groups: A leaving group is an atom or group of atoms that departs from a molecule during a substitution or elimination reaction. It is the part of the molecule that is replaced or removed, allowing for the formation of a new bond or the creation of a new molecule.
Nucleophilic Substitution: Nucleophilic substitution is a fundamental organic reaction where a nucleophile (a species that donates electrons) replaces a leaving group attached to a carbon atom, resulting in the formation of a new carbon-nucleophile bond. This process is central to many organic transformations and is particularly relevant in the context of alkyl halides, alcohols, carboxylic acids, and amines.
Nucleophilic substitution reactions: Nucleophilic substitution reactions are a class of chemical reactions in organic chemistry where an electron-rich nucleophile selectively bonds with or attacks the positive or partially positive charge of an atom or a group of atoms to replace a leaving group. The reaction is characterized by the substitution of a nucleophile for a leaving group, which can occur via different mechanisms (SN1 or SN2).
Olah's Reagent: Olah's reagent, also known as magic acid, is a strong superacid system used in organic chemistry for the activation and conversion of alcohols into alkyl halides. It plays a crucial role in the preparation of alkyl halides from alcohols, a common transformation in organic synthesis.
Perfluorobutanesulfonate ester: A perfluorobutanesulfonate ester is a chemical compound that consists of a perfluorinated butyl group bonded to a sulfonate ester functional group. These compounds are often used in various industrial and commercial applications due to their unique properties, such as resistance to heat, chemical reactions, and water-repellency.
Perfluorobutanesulfonyl fluoride: Perfluorobutanesulfonyl fluoride is a highly reactive and stable organic compound that is commonly used as a precursor in the synthesis of various fluorinated compounds. It is a key intermediate in the preparation of alkyl halides from alcohols, as described in the context of the 10.5 Preparing Alkyl Halides from Alcohols topic.
Phosphite Ester: A phosphite ester is an organic compound containing a phosphite group (P(OR)3) esterified with an alcohol. These esters are important intermediates in the synthesis of various phosphorus-containing compounds, particularly in the context of preparing alkyl halides from alcohols.
Phosphorous Acid: Phosphorous acid, also known as phosphonic acid, is an inorganic compound with the chemical formula H3PO3. It is a weak acid that plays an important role in the context of preparing alkyl halides from alcohols, as it can be used as a reducing agent in certain reactions.
Phosphorus Tribromide: Phosphorus tribromide is a colorless, fuming liquid compound with the chemical formula PBr3. It is an important reagent in organic chemistry, particularly in the context of preparing alkyl halides from alcohols, the chemistry of acid halides, and the alpha bromination of carboxylic acids.
Primary Alcohols: Primary alcohols are organic compounds with the hydroxyl (-OH) functional group attached to the first (primary) carbon atom of an alkyl chain. These alcohols are important intermediates in various organic reactions, including the preparation of alkyl halides, the oxidation of alcohols, and the synthesis of carboxylic acids.
Protonation: Protonation is the process of adding a proton (H+) to a molecule or atom, resulting in the formation of a positively charged species. This fundamental chemical reaction is central to various organic chemistry topics, as it can significantly influence the reactivity and stability of molecules.
Pyridinium Poly(hydrogen Fluoride): Pyridinium poly(hydrogen fluoride) is a reagent used in organic chemistry to convert alcohols into alkyl halides, specifically alkyl fluorides. It is an important tool for preparing alkyl halides from alcohols, a common transformation in the field of organic synthesis.
Reaction Mechanisms: Reaction mechanisms describe the step-by-step process by which a chemical reaction occurs, including the rearrangement of atoms, the formation and breaking of chemical bonds, and the movement of electrons. Understanding reaction mechanisms is crucial for predicting the products of a reaction, explaining experimental observations, and designing new synthetic routes in organic chemistry.
Secondary Alcohols: Secondary alcohols are organic compounds in which the hydroxyl group (-OH) is attached to a carbon atom that is bonded to two other carbon atoms. This structural feature distinguishes secondary alcohols from primary alcohols, where the hydroxyl group is attached to a terminal carbon, and tertiary alcohols, where the hydroxyl group is attached to a carbon with three other carbon substituents.
Solvent Effects: Solvent effects refer to the influence that the surrounding solvent environment can have on the behavior and properties of chemical reactions, molecules, and spectroscopic measurements. The nature and polarity of the solvent can significantly impact the energetics, kinetics, and outcomes of various organic chemistry processes.
Stereochemistry: Stereochemistry is the study of the three-dimensional arrangement of atoms in molecules and how this arrangement affects the chemical and physical properties of the substance. It examines the spatial orientation of atoms and their relationship to one another, which is crucial in understanding many organic chemistry concepts.
Tertiary Alcohols: Tertiary alcohols are a class of organic compounds where the hydroxyl (-OH) group is attached to a carbon atom that is connected to three other carbon atoms. This unique structural feature gives tertiary alcohols distinct properties and reactivity patterns compared to primary and secondary alcohols.
Thionyl Chloride: Thionyl chloride (SOCl2) is a highly reactive organic compound commonly used as a reagent in various chemical reactions, particularly in the context of preparing alkyl halides from alcohols, as well as in the reactions and chemistry of carboxylic acids and their derivatives.