Arielle Cruz
Sec-butyl bromide is a type of haloalkane, which is a derivative of an aliphatic hydrocarbon. Haloalkanes are formed by the replacement of a hydro group with a halogen atom, such as bromine. One way to prepare sec-butyl bromide from an alcohol is by forming a sulfonate of the alcohol (tosylate, mesylate, triflate, etc.) and then reacting that with a source of bromide anions. This can be done through phase-transfer catalysis in dichloromethane/aqueous potassium bromide with tetrabutylammonium bromide catalysis.
| Characteristics | Values |
|---|---|
| Standard way of forming a tosylate | with p-toluenesulfonyl chloride in the presence of a base |
| Base used | pyridine |
| Preferred way of reacting the tosylate with a source of bromide | phase-transfer catalysis in dichloromethane/aqeous potassium bromide with tetrabutylammonium bromide catalysis |
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What You'll Learn
Haloalkanes and Haloarenes
Haloalkanes are represented as RX, where R is the alkyl group and X is the halogen atom attached to the sp3 hybridised carbon atom. Haloarenes are represented as ArX, where Ar is the aryl group and X is the halogen atom attached to the sp2 hybridised carbon atom.
One example of a haloalkane is sec-butyl bromide, which can be prepared from an alcohol through several methods. One approach is to form a sulfonate of the alcohol (tosylate, mesylate, or triflate), creating a good leaving group. This sulfonate can then be reacted with a source of bromide anions. Specifically, the reaction of p-toluenesulfonyl chloride with the alcohol in the presence of a base, such as pyridine, catalyses the formation of the tosylate. This tosylate can then be reacted with a bromide source like aqueous potassium bromide using phase-transfer catalysis with tetrabutylammonium bromide.
Another method to synthesise sec-butyl bromide involves the reaction of but-2-ol with PBr3 to form 2-bromobutane and H3PO3. This reaction utilises phosphorus tribromide as a reagent to directly introduce the bromine atom. However, it's important to note that this method may not be suitable for preparing 2° and 3° alkyl bromides, as the use of concentrated H2SO4 can lead to the formation of alkenes instead.
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Using alkyl bromide and alkyl iodide
To prepare sec-butyl bromide from an alcohol, you can use alkyl bromide and alkyl iodide. However, it is important to note that 2° and 3° alkyl bromide and iodide cannot be formed with Conc H2SO4 as it will convert into an alkene. Instead, you can use phosphoric(V) acid, which is a better option than concentrated sulfuric acid because it does not oxidize iodide ions to iodine and produces very little hydrogen iodide.
A suggested reaction mechanism involves first forming a sulfonate of the alcohol (tosylate, mesylate, triflate, etc.), creating a good leaving group. This can be done using p-toluenesulfonyl chloride in the presence of a base like pyridine, which catalyzes the reaction. The second step involves reacting the tosylate with a source of bromide anions. One way to do this is by using phase-transfer catalysis in a dichloromethane/aqueous potassium bromide solution with tetrabutylammonium bromide catalysis.
Another approach is to use phosphorus(III) bromide or iodide. The alcohol is heated under reflux with a mixture of red phosphorus and either bromine or iodine. However, this method has some drawbacks, including side reactions involving POCl3 reacting with the alcohol.
Additionally, it is worth noting that primary alcohols and methanol react to form alkyl halides under acidic conditions by an SN2 mechanism. In these reactions, the acid protonates the alcohol, and the halide ion displaces a molecule of water (a good leaving group) from carbon, resulting in the formation of an alkyl halide.
Overall, there are multiple methods to prepare sec-butyl bromide from an alcohol using alkyl bromide and alkyl iodide, each with its own advantages and considerations.
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Forming a sulfonate of the alcohol
To form a sulfonate of an alcohol, you can convert the alcohol into a sulfonate ester. This can be done by treating the alcohol with a sulfonyl chloride and a base. The hydroxyl group of the alcohol will nucleophilically attack the sulfur of the sulfonyl chloride, displacing the chloride and forming a sulfonate ester. This is sometimes called a pseudohalide because its reactivity becomes like that of an alkyl halide.
The most commonly used sulfonate esters are tosylates (p-toluenesulfonate esters), mesylates (methanesulfonate esters), and triflates (trifluoromethanesulfonate esters). To form a tosylate, for example, you would use p-toluenesulfonyl chloride (or tosyl chloride). A base, such as pyridine, can then gather up the acidic proton that is released, preventing the production of HCl as a by-product.
The importance of sulfonate esters as intermediates in many substitution reactions cannot be overstated. Once the carbon-oxygen bond is broken in these mechanisms, a negatively charged sulfonate is produced. The negative charge is delocalized between the three oxygens on the sulfur, which means there is resonance stabilization and that this is a very weak base.
It is not favourable for alcohols to undergo elimination and nucleophilic substitution reactions because hydroxyl groups are poor leaving groups. However, by converting them into sulfonate esters, which are much better leaving groups, they can be made to undergo these reactions.
Reduction of sulfonate esters with LiAlH4 often gives yields of less than 50%. Formation of an alcohol (via SO bond cleavage) is often observed when the CO bond is sterically hindered, accounting for the low yields.
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Using p-toluenesulfonyl chloride
Sec-butyl bromide can be prepared from an alcohol through a nucleophilic substitution reaction. This involves making the alcohol more reactive using acidic conditions and then using the Br- ion as the substitution nucleophile. One way to do this is by using p-toluenesulfonyl chloride (TsCl).
Firstly, it's important to understand why we need to use a reagent like TsCl. Alcohols are poor leaving groups because they have a negative charge and high basicity. To make the OH- group a better leaving group, we can react the alcohol with TsCl, which will convert the OH- group into a sulfonic acid ester. This makes it a much better leaving group.
The reaction with TsCl breaks the O-H bond of the alcohol, not the C-O bond. This means that the absolute configuration of the carbon atom attached to the hydroxyl group remains unchanged throughout the reaction. This is useful for controlling stereochemistry in organic synthesis.
Now, here is a step-by-step guide on how to prepare sec-butyl bromide from an alcohol using TsCl:
- Set up your experiment: Obtain a clean 250-mL round-bottom flask with the appropriate ground glass stopper if there is a side arm. Use a cork ring to hold the flask while adding chemicals.
- Prepare an ice-water bath in a 600 mL beaker or use an ice bucket to keep your flask cool when adding concentrated acid.
- Add 20.0 g of sodium bromide to your flask.
- Add 15.0 mL of DI water to the flask and swirl until most of the sodium bromide dissolves. Some crystals will not dissolve until heated.
- Add 15.0 mL of 1-butanol to the reaction flask.
- Place the flask in the ice-water bath or bucket, ensuring it cannot tip over.
- Slowly add 15 mL of concentrated sulfuric acid to the flask, about 1 mL at a time, letting it run down the inside wall of the flask. Swirl the contents after each addition to cool them.
- Assemble the flask into a reflux setup with a condenser column attached. Do not use a drying tube. Add a few boiling stones if possible.
- Reflux the mixture for 45 minutes, ensuring the reflux vapour ring does not rise above the halfway point of the condenser.
- After refluxing, perform a simple distillation to collect the organic compound, 1-bromobutane.
- Perform a Separatory Funnel extraction to isolate your product.
The preparation of sec-butyl bromide from an alcohol using TsCl involves several steps, including reflux and distillation. It is important to carefully follow laboratory procedures and safety precautions when conducting these experiments.
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Phase-transfer catalysis
In the context of preparing sec-butyl bromide from an alcohol, PTC can be employed to promote the reaction between specific reactants. For instance, the synthesis of butyl salicylate, which is an organic compound, can be achieved through PTC using a dual-site phase-transfer catalyst and an ionic liquid in a tri-liquid system.
The dual-site phase-transfer catalyst, such as 1,4-bis(trihexylammoniomethyl)benzene dibromide (BTHAMBB), is crucial in this process. BTHAMBB is synthesised from p-xylylene dibromide and trihexylamine. It plays a role in forming a third liquid phase, which contains a highly concentrated catalyst, thereby facilitating the intrinsic reaction. This third liquid phase is influenced by the physical and chemical properties of the catalytic intermediate produced from the reaction of the aqueous reactant and the catalyst.
Additionally, the ionic liquid trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethyl pentyl)phoshinate (THTDPBP) is introduced to further enhance the product yield. The combination of BTHAMBB and THTDPBP under ultrasonic irradiation in a tri-liquid system results in a significantly higher yield of butyl salicylate compared to not using a catalyst.
The choice of solvent and salt can also impact the formation of the third liquid phase, distribution of the catalyst, and reaction rate. For example, in the reaction between n-butyl bromide and sodium phenolate, the type of solvent and the amount of sodium hydroxide (NaOH) added are critical factors. The addition of sodium bromide (NaBr) and NaOH can drive the catalyst between the aqueous and organic phases or contribute to the formation of a third liquid phase, depending on the polarity of the organic solvent.
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Frequently asked questions
One way is to form a sulfonate of the alcohol (tosylate, mesylate, triflate, etc.) to create a good leaving group, then react that with a source of bromide anions.
The standard way of forming a tosylate is with p-toluenesulfonyl chloride in the presence of a base. Pyridine catalyses the reaction and is often used as the base.
There are many ways to react the tosylate with a source of bromide. One preferred method is using phase-transfer catalysis in dichloromethane/aqueous potassium bromide with tetrabutylammonium bromide catalysis.
Another method is the reaction of 2-butanol with PBr3, yielding 2-bromobutane and H3PO3. However, it is important to note that 2° and 3° alkyl bromide and alkyl iodide cannot be formed with Conc H2SO4 as it will convert it into an alkene.
Sec-butyl bromide is used in modern electronic or cleaning semiconductors, chips, and other components.
