Problem 43
Question
A compound of molecular formula \(\mathrm{C}_{7} \mathrm{H}_{8} \mathrm{O}\) is insoluble in water and dilute sodium bicarbonate but dissolves in dilute aqueous sodium hydroxide. On treatment with bromine water, it readily gives a precipitate of \(\mathrm{C}_{7} \mathrm{H}_{5} \mathrm{OBr}_{3} .\) Write down the structure of the compound.
Step-by-Step Solution
Verified Answer
The compound is para-cresol (4-methylphenol).
1Step 1: Identify Functional Groups
Given that the compound is soluble in dilute sodium hydroxide but not in water or dilute sodium bicarbonate, it suggests the presence of a phenolic group. Phenolic compounds tend to dissolve in NaOH by forming phenoxides, but not in sodium bicarbonate. This indicates the presence of an aromatic phenol.
2Step 2: Analyze Reaction with Bromine Water
When the compound reacts with bromine water and forms a precipitate of \(\mathrm{C}_{7} \mathrm{H}_{5} \mathrm{OBr}_{3} \), it indicates that the compound undergoes substitution of hydrogen atoms on the aromatic ring. This typically occurs with phenols, suggesting the initial compound is a phenol derivative.
3Step 3: Derive the Structure
Given the molecular formula \(\mathrm{C}_{7} \mathrm{H}_{8} \mathrm{O}\) and the information from the reactions, the compound is most likely a phenol with no additional alkyl groups aside from what contributes to the aromatic ring. Considering the structural isomer possibilities, the compound is likely para-cresol (para-methylphenol, aka 4-methylphenol), which aligns with the possibility of forming tri-brominated products.
Key Concepts
Phenolic GroupBromine ReactionMolecular Structure Analysis
Phenolic Group
The phenolic group is a key functional group in organic chemistry. It refers to hydroxyl groups directly attached to an aromatic hydrocarbon ring. This arrangement is typical in compounds known as phenols. One of the unique traits of phenols is their solubility behavior. Unlike simple alcohols, phenols are generally insoluble in water and do not react with sodium bicarbonate (NaHCO₃).
However, they do dissolve in aqueous sodium hydroxide (NaOH). This solubility difference is due to the ability of phenolic groups to lose a hydrogen ion, forming a phenoxide ion. Phenoxides are more soluble in water compared to phenols because they are ionic. This helps explain the compound's solubility in NaOH rather than in NaHCO₃. The para-cresol, identified in this problem, exemplifies these characteristics as a phenolic compound.
However, they do dissolve in aqueous sodium hydroxide (NaOH). This solubility difference is due to the ability of phenolic groups to lose a hydrogen ion, forming a phenoxide ion. Phenoxides are more soluble in water compared to phenols because they are ionic. This helps explain the compound's solubility in NaOH rather than in NaHCO₃. The para-cresol, identified in this problem, exemplifies these characteristics as a phenolic compound.
Bromine Reaction
Bromine reactions with phenolic compounds are quite distinctive. When a compound containing a phenolic group reacts with bromine water, it usually undergoes electrophilic aromatic substitution. In this reaction, bromine atoms replace hydrogens on the aromatic ring, specifically at positions ortho and para to the hydroxyl group. This hallmark feature of phenols is due to their ability to activate the aromatic ring towards electrophilic substitution.
The resulting compound from the reaction in the exercise is \( ext{C}_7 ext{H}_5 ext{OBr}_3 \), which indicates the substitution of three bromine atoms. Such a transformation suggests that we initially had an aromatic structure like para-cresol, where all available ortho and para positions are substituted with bromine leading to a tribrominated phenolic compound. This kind of chemical behavior is key in confirming the presence of a phenol structure in the initial compound.
The resulting compound from the reaction in the exercise is \( ext{C}_7 ext{H}_5 ext{OBr}_3 \), which indicates the substitution of three bromine atoms. Such a transformation suggests that we initially had an aromatic structure like para-cresol, where all available ortho and para positions are substituted with bromine leading to a tribrominated phenolic compound. This kind of chemical behavior is key in confirming the presence of a phenol structure in the initial compound.
Molecular Structure Analysis
Analyzing the molecular structure of organic compounds involves assessing their chemical behavior and reactivity. In the case of the provided exercise, the molecular formula \( ext{C}_7 ext{H}_8 ext{O} \) provides clues about possible structural arrangements. The total of seven carbon atoms hints at an aromatic ring as a backbone for the compound, likely accounting for six carbons.
With one oxygen atom and the solubility and reactivity data, we infer the presence of a hydroxyl group as part of the phenolic group. The exercise reasoning uses chemical clues, such as solubility in NaOH and reaction with bromine, to narrow down possible structures. This process leads to detecting para-cresol as the specific structure due to its substitution pattern and its consistency with the given tribrominated product.
With one oxygen atom and the solubility and reactivity data, we infer the presence of a hydroxyl group as part of the phenolic group. The exercise reasoning uses chemical clues, such as solubility in NaOH and reaction with bromine, to narrow down possible structures. This process leads to detecting para-cresol as the specific structure due to its substitution pattern and its consistency with the given tribrominated product.
- Recognize key functional groups.
- Predict patterns of reactivity
- Match chemical formulae with possible structures.
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