Theo Nash
Ethyl bromide can be prepared from ethyl alcohol by treating it with phosphorus tribromide. This process involves a substitution reaction where the hydroxyl group (OH) of the alcohol is replaced by the bromide ion (Br). The chemical equation for this reaction is: 3C₂H₅OH + PBr₃ → C₂H₅Br + H₃PO₃. Additionally, ethyl bromide can be prepared from ethene or ethylene by reacting it with a hydrogen bromide solution, which is an addition hydrohalogenation reaction. The specific procedure for preparing ethyl bromide from ethyl alcohol involves adding ethanol, water, and concentrated sulfuric acid to a round bottom flask, followed by sodium bromide, zeolites, and careful heating to obtain the final product.
| Characteristics | Values |
|---|---|
| Ethyl Alcohol Treatment | Phosphorus tribromide |
| Chemical Equation | \(3{C_2}{H_5}OH + PB{r_3} \to 3{C_2}{H_5}Br + {H_3}P{O_3}\) |
| Ethane Treatment | Bromine in the presence of \(AlB{r_3}\) |
| Ethene Treatment | Hydrogen bromide |
| Chemical Equation | \({C_2}{H_6} + HBr \to {C_2}{H_5}Br\) |
| Ethanol Mixture | 95% |
| Water Added | 9 mL |
| Sulfuric Acid Added | 19 mL |
| Sodium Bromide Added | 15 g |
| Hydrobromic Acid | 48% |
| Sulfuric Acid Added to Hydrobromic Acid | 65 mL |
| Ethanol Added to Mixture | 145 mL |
| Additional Sulfuric Acid Added | 109 mL |
| Ethanol Added to Red Phosphorus | 60 g |
| Bromine Added to Ethanol and Red Phosphorus Mixture | 60 g |
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What You'll Learn
Using phosphorus tribromide
Ethyl bromide can be prepared from ethyl alcohol by treating it with phosphorus tribromide (PBr3). This is a substitution reaction where the OH group in the ethyl alcohol is replaced by a bromine atom to form ethyl bromide. The reaction can be represented as:
3C₂H₅OH + PBr₃ → C₂H₅Br + H₃PO₃
Phosphorus tribromide is primarily used for the conversion of primary or secondary alcohols to alkyl bromides. It is often preferred over hydrobromic acid as it typically yields higher quantities of the product and avoids issues with carbocation rearrangement. For example, neopentyl bromide can be synthesised from the corresponding alcohol in 60% yield using PBr3.
The reaction with PBr3 occurs in two steps: "activation" and "substitution". In the activation step, the alcohol is converted into a good leaving group by forming a bond to phosphorus (P) and displacing bromine (Br). This is followed by an SN2 substitution at the alcohol carbon, where the bromine ion that was displaced from phosphorus attacks carbon, forming a new C-Br bond and breaking the C-O bond. This results in the formation of an alkyl bromide and the Br2P-OH leaving group.
PBr3 is also used in the manufacture of pharmaceuticals such as alprazolam, methohexital, and fenoprofen, as well as a fire suppression agent marketed under the name PhostrEx. It is important to note that phosphorus tribromide is highly reactive and can cause explosions when exposed to air, so appropriate safety precautions are necessary when handling this compound.
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Using ethane
Ethyl bromide, also known as bromoethane, is a chemical compound of the haloalkanes group. It is a volatile compound with an ether-like odour.
Ethyl bromide can be prepared from ethane by reacting ethane (C₂H₆) with bromine in the presence of AlBr₃. This reaction produces ethyl bromide and HBr:
${C_2}{H_6} + B{r_2}\xrightarrow{{AlB{r_3}}}C{H_3}C{H_2}Br + HBr$
This reaction is a free radical halogenation of alkanes, which can result in a complex mixture of isomeric mono- and polyhaloalkanes. These compounds are difficult to separate as pure compounds.
An alternative method to prepare ethyl bromide is through a reaction with ethanol. This involves refluxing ethanol with phosphorus and bromine, which generates phosphorus tribromide in situ. This reaction is represented as follows:
3C₂H₅OH + PBr₃ → C₂H₅Br + H₃PO₃
Another option is to react ethanol with a mixture of hydrobromic and sulfuric acids.
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Using ethene
Ethyl bromide can be prepared from ethene (or ethylene) by reaction with hydrogen bromide. This reaction is called the addition of hydrogen halide to an alkene, or an addition hydrohalogenation reaction, where hydrogen halide acts as an electrophile and attacks the carbon-carbon double bond of the alkene. The reaction can be written as follows:
${C_2}{H_4} + HBr \to {C_2}{H_5}Br$
This reaction is an example of the addition of halogen acids to alkenes. In this case, the halogen acid is hydrogen bromide, and the alkene is ethene. The product of this reaction is ethyl bromide.
Another way to think about this reaction is in terms of the reagents involved. Ethyl bromide can be prepared from ethyl alcohol by treating it with a reagent that can replace the hydroxyl group (OH) with a bromide ion. In the case of the reaction with ethene, the reagent is hydrogen bromide, which provides the bromide ion necessary for the substitution reaction to occur.
It is worth noting that ethyl bromide is rarely prepared in the laboratory, as it is inexpensive and readily available. However, the preparation of ethyl bromide can serve as a model for the synthesis of other bromoalkanes, which are important compounds in organic chemistry.
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Using ethanol and potassium bromide
To prepare ethyl bromide from ethyl alcohol, you will need a reagent that can replace the OH from the alcohol with a bromide ion. One method to achieve this is by treating ethyl alcohol with phosphorus tribromide, which results in a substitution reaction:
$$
3{C_2}{H_5}OH + PB{r_3} \to 3{C_2}{H_5}Br + {H_3}P{O_3}
$$
In this reaction, the hydroxyl group (OH) of the alcohol is substituted by the bromide ion (Br), forming ethyl bromide.
Another method to prepare ethyl bromide from ethanol and potassium bromide involves the following steps:
Step 1: Prepare the Initial Mixture
Add 50 grams of concentrated sulfuric acid to 25 grams of ethanol in a flask. Stir continuously and allow the mixture to cool.
Step 2: Add Potassium Bromide
Once the mixture has cooled, introduce 60 grams of powdered potassium bromide. The potassium bromide will react with the ethanol to initiate the formation of ethyl bromide.
Step 3: Rest and Distillation
Let the mixture rest overnight. During this time, the reaction between ethanol and potassium bromide proceeds, forming ethyl bromide. After the resting period, proceed with slow distillation to collect the ethyl bromide in a cold receiver flask.
Step 4: Purification
The crude ethyl bromide obtained from distillation is purified through a series of washing steps. First, wash it with an equal volume of concentrated hydrochloric acid, followed by water, and then a small amount of 5% sodium bicarbonate solution. Finally, wash it again with water to ensure the removal of any impurities.
Step 5: Drying and Yield
Dry the purified ethyl bromide with anhydrous calcium chloride. This process will remove any remaining traces of water. The final yield should be around 205 grams of pure ethyl bromide.
These steps outline a detailed procedure for preparing ethyl bromide from ethanol and potassium bromide, including the necessary reagents, equipment, and purification techniques.
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Using hydrobromic acid
To prepare ethyl bromide from ethyl alcohol using hydrobromic acid, you will need a 1-litre double-neck round-bottom flask with a drop funnel and a wide bend. The bend should be connected to a long and efficient condenser assembly and a receiver for downward distillation.
First, place 281ml (or 415g) of 48% hydrobromic acid in the flask. Then, add 65ml (or 120g) of concentrated sulfuric acid in batches, shaking as you do so. As the mixture cools, slowly add 145ml (or 100g) of 95% ethanol. Next, add 109ml (or 200g) of concentrated sulfuric acid to the drop funnel, and collect the ethyl bromine obtained by slow distillation in a cold receiver flask.
The crude ethyl bromine should then be washed with an equal volume of concentrated hydrochloric acid, followed by water, and a small amount of 5% sodium bicarbonate solution. Finally, wash with water again and dry the ethyl bromine with anhydrous calcium chloride. The crude ethyl bromide can be purified by distillation, yielding 205 grams of pure ethyl bromine.
An alternative method involves adding 50g of concentrated sulfuric acid to 25g of ethanol and allowing it to cool under continuous stirring. Then, add 60g of powdered potassium bromide and let the mixture rest overnight. The following day, the ethyl bromine is distilled and washed, first with a dilute sodium carbonate solution and then with water.
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