Arielle Cruz
Ethyl bromide, also known as bromoethane, is a chemical compound of the haloalkanes group. It can be prepared from ethyl alcohol, ethane, or ethene. Using ethyl alcohol, ethyl bromide can be prepared by treating it with phosphorus tribromide. This is a substitution reaction where the hydroxyl group (OH) of the alcohol is substituted by the bromide ion (Br). When using ethane, ethyl bromide can be obtained by reacting ethane with bromine in the presence of AlBr3. Finally, ethene can be converted to ethyl bromide by reacting with hydrogen bromide, which is known as the addition of hydrogen halide to alkene or addition hydrohalogenation reaction.
| Characteristics | Values |
|---|---|
| Ethyl bromide preparation from ethyl alcohol | By treating ethyl alcohol with phosphorus tribromide |
| Chemical equation | \(3{C_2}{H_5}OH + PB{r_3} \to 3{C_2}{H_5}Br + {H_3}P{O_3}\) |
| Type of reaction | Substitution reaction |
| Ethyl bromide preparation from ethane | Ethane reacts with bromine in the presence of AlBr₃ |
| Chemical equation | \({C_2}{H_6} + B{r_2}\xrightarrow{{AlB{r_3}}}C{H_3}C{H_2}Br + HBr\) |
| Ethyl bromide preparation from ethene | Ethene reacts with hydrogen bromide |
| Chemical equation | \({C_2}{H_6} + HBr \to {C_2}{H_5}Br\) |
| Type of reaction | Addition hydrohalogenation reaction |
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What You'll Learn
Ethyl alcohol reacts with phosphorus tribromide
Ethyl bromide can be prepared from ethyl alcohol by treating it with phosphorus tribromide. This is a substitution reaction where the hydroxyl group (OH) of the alcohol is substituted by the bromide ion (Br). The reaction can be represented as:
> 3C₂H₅OH + PBr₃ → C₂H₅Br + H₃PO₃
In this reaction, ethyl alcohol (C₂H₅OH) reacts with phosphorus tribromide (PBr₃) to form ethyl bromide (C₂H₅Br) and phosphorous acid (H₃PO₃). The phosphorus tribromide acts as a reagent, replacing the OH group in ethyl alcohol with a bromine atom to form the alkyl bromide, C₂H₅Br.
Phosphorus tribromide is commonly used for the conversion of primary and secondary alcohols to alkyl bromides. It is particularly useful in this reaction as it typically yields higher than hydrobromic acid and avoids issues with carbocation rearrangement. The SN2 substitution step ensures the reaction is effective for primary and secondary alcohols but not tertiary alcohols. If the reacting carbon centre is chiral, the reaction usually results in an inversion of configuration at the carbon alpha to the alcohol, as is typical in SN2 reactions.
Phosphorus tribromide is a highly reactive chemical that can be dangerous. It reacts violently with water and alcohols, evolving corrosive and toxic HBr. Therefore, it is synthesised in situ to avoid these issues.
Overall, the reaction of ethyl alcohol with phosphorus tribromide is a useful method for preparing ethyl bromide, taking advantage of the substitution reaction to replace the OH group in ethyl alcohol with a bromine atom.
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Ethane reacts with bromine in the presence of AlBr3
Ethyl bromide can be prepared from ethane by reacting ethane with bromine in the presence of aluminium bromide (AlBr3). This reaction is known as halogenation, specifically bromination, and it involves substituting a hydrogen atom in the ethane molecule with a bromine atom. The chemical equation for this process is:
${{C_2}{H_6} + B{r_2}\xrightarrow{{AlB{r_3}}}C{H_3}C{H_2}Br + HBr}
During the reaction, the bromine molecule first absorbs UV light and breaks down into two bromine radicals. One of these bromine radicals then reacts with an ethane molecule, removing a hydrogen atom and forming a methyl radical and hydrogen bromide (HBr). The methyl radical subsequently combines with another bromine molecule to produce bromoethane (CH3CH2Br) and another bromine radical. This reaction mechanism is known as free radical substitution.
The preparation of ethyl bromide from ethane can also be achieved through free radical halogenation of alkanes. However, this method often results in a complex mixture of isomeric mono- and polyhaloalkanes, which are challenging to separate into pure compounds.
Alternatively, ethyl bromide can be prepared from ethyl alcohol or ethene. Starting with ethyl alcohol, ethyl bromide can be obtained by treating it with phosphorus tribromide (PBr3). This reaction is a substitution reaction, where the hydroxyl group (OH) in the alcohol is replaced by the bromide ion (Br). The chemical equation for this process is:
$3{{C_2}{H_5}OH + PB{r_3} \to 3{C_2}{H_5}Br + {H_3}P{O_3}}
When using ethene as a starting material, ethyl bromide can be prepared through an addition hydrohalogenation reaction with hydrogen bromide. In this reaction, the hydrogen bromide acts as an electrophile and attacks the carbon-carbon double bond in ethene. The equation for this reaction is:
${{C_2}{H_6} + HBr \to {C_2}{H_5}Br}
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Ethene reacts with hydrogen bromide
Ethene (C₂H₄), also known as ethylene, is a member of the alkene group. When ethene reacts with hydrogen bromide (HBr), the reaction is called an addition hydrohalogenation reaction. In this reaction, the hydrogen halide acts as an electrophile and attacks the carbon-carbon double bond (C=C) of ethene. This double bond then breaks, and a hydrogen atom becomes attached to one of the carbon atoms, while a bromine atom attaches to the other. The product of this reaction is bromoethane (C₂H₅Br).
The reaction between ethene and hydrogen bromide can be written as the following equation:
${C_2}{H_4} + HBr \to {C_2}{H_5}Br$
This reaction is an example of an electrophilic addition reaction, which occurs between hydrogen halides and alkenes. Hydrogen halides include hydrogen chloride (HCl) and hydrogen bromide (HBr). When ethene reacts with hydrogen chloride, the mechanism is the same as with hydrogen bromide, except that chlorine replaces bromine in the product. For example, the reaction between ethene and hydrogen chloride produces chloroethane:
${C_2}{H_4} + HCl \to {C_2}{H_5}Cl$
The preparation of ethyl bromide from ethene involves the addition of hydrogen bromide to the alkene. This reaction is favoured at lower temperatures.
In summary, the reaction of ethene with hydrogen bromide involves the addition of hydrogen halide to the alkene, resulting in the formation of bromoethane. This reaction is an example of an electrophilic addition reaction, with the hydrogen bromide acting as the electrophile and attacking the carbon-carbon double bond of ethene.
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Constant boiling of ethyl alcohol with HBr
Ethyl bromide, or bromoethane, is a chemical compound of the haloalkanes group. It is usually prepared by the addition of hydrogen bromide to ethene. However, it can also be prepared from ethyl alcohol, ethane, and ethene.
Preparation of Ethyl Bromide from Ethyl Alcohol
Ethyl bromide can be prepared from ethyl alcohol by treating it with phosphorus tribromide. This is a substitution reaction where the hydroxyl group (OH) of the alcohol is substituted by the bromide ion (Br). The chemical equation for this reaction is:
3C_2H_5OH + PBr_3 \rightarrow C_2H_5Br + H_3PO_3
Another method to prepare ethyl bromide from ethyl alcohol involves constant boiling with HBr (48%). This method involves refluxing ethanol with phosphorus and bromine, generating phosphorus tribromide in situ. The reaction equation is as follows:
3C_2H_5OH + 3HBr \rightarrow 3C_2H_5Br + 3H_2O
Preparation of Ethyl Bromide from Ethane
When ethane ($C_2H_6$) reacts with bromine in the presence of $AlBr_3*, ethyl bromide is formed. This reaction is an example of free radical halogenation of alkanes. The chemical equation for this reaction is:
C_2H_6 + Br_2 \xrightarrow{AlBr_3} C_2H_5Br + HBr
Preparation of Ethyl Bromide from Ethene
Ethene, or ethylene, can be used to obtain ethyl bromide through an addition hydrohalogenation reaction with hydrogen bromide solution. In this reaction, the hydrogen bromide solution (an electrophile) attacks the carbon-carbon double bond ($C=C$) of ethene. The chemical equation is as follows:
C_2H_4 + HBr \rightarrow C_2H_5Br
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Refluxing ethanol with phosphorus and bromine
To prepare ethyl bromide from ethyl alcohol, a reagent that can replace the OH group from the alcohol with a bromide ion is required. One way to do this is by refluxing ethanol with phosphorus and bromine.
Refluxing is a technique used in distillation processes to condense compounds in a mixture and prevent them from escaping as vapour. This is particularly useful for ethanol because, as a liquid with a low boiling point, it evaporates fairly quickly. By using a reflux apparatus, the ethanol vapour is condensed back into its liquid form, ensuring that all reactants are used in the reaction and maximising the yield of the desired product.
When refluxing ethanol with phosphorus and bromine, red phosphorus and bromine react to form phosphorus tribromide (PBr3). This is a crucial step, as phosphorus tribromide is the reagent that will react with ethanol to produce ethyl bromide. The reaction between red phosphorus and bromine can be represented by the following equation:
> 2P + 3Br2 → 2PBr3
Subsequently, the phosphorus tribromide reacts with ethanol (C2H5OH) in a substitution reaction. In this reaction, the hydroxyl group (OH) of the ethanol is substituted by the bromide ion (Br), forming ethyl bromide (C2H5Br) and phosphorous acid (H3PO3). This reaction can be represented as follows:
> 3C2H5OH + PBr3 → 3C2H5Br + H3PO3
This reflux process is an effective method for preparing ethyl bromide from ethanol, as it ensures that the ethanol remains in its liquid state throughout the reaction, maximising the yield of ethyl bromide.
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Frequently asked questions
Ethyl bromide can be prepared from ethyl alcohol by treating it with phosphorus tribromide. It is a substitution reaction where the hydroxyl group (OH) of the alcohol is substituted by the bromide ion (Br).
The chemical equation for the preparation of ethyl bromide from ethyl alcohol is: $3{C_2}{H_5}OH + PB{r_3} \to 3{C_2}{H_5}Br + {H_3}P{O_3}reacting ethane with bromine in the presence of $AlB{r_3}$.
Ethene can be converted to ethyl bromide by reaction with hydrogen bromide. This reaction is called the addition of hydrogen halide to alkene or the addition hydrohalogenation reaction.
