Problem 39 - Basic Principles of Organic Chemistry

Question

For each of the following pairs of compounds give a chemical test, preferably a test-tube reaction, that will distinguish the two compounds. Write a structural formula for each compound and equations for the reactions involved. a. 1 -methyl-3-nitrobenzene and phenylnitromethane b. 1 -methyl-4-nitrobenzene and benzenecarboxamide c. benzenamine and cyclohexanamine d. N-methylbenzenamine and 4-methylbenzenamine e. N-nitroso-N-methylbenzenamine and 4-nitroso-N-methylbenzenamine

Step-by-Step Solution

Answer

Use distinctive reactions, such as NaOH reaction or bromine water test, to differentiate between pairs based on solubility or elemental analysis.

Step 1: Identifying the Compounds

First, understand the structures for the given compounds in each pair: - **1-methyl-3-nitrobenzene** is C6H4(CH3)(NO2) with the nitro group at the meta position. - **Phenylnitromethane** is C6H5CH2NO2. - **1-methyl-4-nitrobenzene** is C6H4(CH3)(NO2) with the nitro group at the para position. - **Benzenecarboxamide** is C6H5CONH2. - **Benzenamine (aniline)** is C6H5NH2. - **Cyclohexanamine** is C6H11NH2. - **N-methylbenzenamine (N-methyl aniline)** is C6H5NHCH3. - **4-methylbenzenamine (para-toluidine)** is C6H4(CH3)NH2. - **N-nitroso-N-methylbenzenamine** is C6H5N(NO)CH3. - **4-nitroso-N-methylbenzenamine** is C6H4(CH3)N(NO).

Step 2: Test for 1-methyl-3-nitrobenzene vs Phenylnitromethane

Perform a sodium hydroxide test. Add a few drops of NaOH to each compound.- **Reaction Observation:** Phenylnitromethane reacts with NaOH forming a sodium salt that dissolves.- **Equation for Phenylnitromethane:** \( ext{C}_6 ext{H}_5 ext{CH}_2 ext{NO}_2 + ext{NaOH} ightarrow ext{C}_6 ext{H}_5 ext{CH}( ext{NO}_2) ext{O}^- ext{Na}^+ + ext{H}_2 ext{O} \) 1-methyl-3-nitrobenzene does not react.

Step 3: Test for 1-methyl-4-nitrobenzene vs Benzenecarboxamide

Perform a hydrolysis test with water and heat.- **Observation:** Benzenecarboxamide will undergo hydrolysis upon heating to form benzoic acid, detectable by its acidic nature.- **Equation for Benzenecarboxamide:** \( ext{C}_6 ext{H}_5 ext{CONH}_2 + ext{H}_2 ext{O} ightarrow ext{C}_6 ext{H}_5 ext{COOH} + ext{NH}_3 \)1-methyl-4-nitrobenzene does not react.

Step 4: Test for Benzenamine vs Cyclohexanamine

Conduct a water solubility test followed by reaction with hydrochloric acid.- **Observation:** Benzenamine is soluble in HCl due to its aromatic nature and will form an amine salt.- **Equation for Benzenamine:** \( ext{C}_6 ext{H}_5 ext{NH}_2 + ext{HCl} ightarrow ext{C}_6 ext{H}_5 ext{NH}_3^+ ext{Cl}^- \)Cyclohexanamine may not readily dissolve or react similarly.

Step 5: Test for N-methylbenzenamine vs 4-methylbenzenamine

Use bromine water test to check the reaction.- **Observation:** 4-methylbenzenamine will decolorize the bromine water because it adds bromine across the benzene ring.- **Equation for 4-methylbenzenamine:** \( ext{C}_6 ext{H}_4( ext{CH}_3) ext{NH}_2 + ext{Br}_2 ightarrow ext{C}_6 ext{H}_3( ext{CH}_3) ext{NH}_2( ext{Br})_3 \)N-methylbenzenamine reacts less readily.

Step 6: Test for N-nitroso-N-methylbenzenamine vs 4-nitroso-N-methylbenzenamine

Perform an acid-base extraction test. - **Observation:** 4-nitroso-N-methylbenzenamine shows acid-base behavior because of the presence of nitroso group on a non-aromatic position. - **Equation:** There is no straightforward simple test; the difference is based on position sensitivity of the decomposition or reaction steps, often identified through advanced analytical techniques rather than simple test-tube reactions. However, their structural difference typically implies one's differing solubility in polar solvents.

Key Concepts

Structural Formulas

Understanding the structure of organic compounds is crucial for predicting their behavior in chemical reactions. Structural formulas represent the molecular arrangement and bond connectivity in a compound.
For instance:

• **1-Methyl-3-nitrobenzene** is structured as C₆H₄(CH₃)(NO₂). The nitro group is attached to the benzene ring at the meta position relative to the methyl group.
• **Phenylnitromethane** is written as C₆H₅CH₂NO₂, where the nitro group is attached to the methylene bridge linked to the phenyl group.

Understanding these differences in structure helps us to determine how these compounds might react chemically. Small differences in structural formulas can significantly alter a compound's physical and chemical properties.

Reactions with NaOH

Sodium hydroxide, NaOH, is a strong base used in reactions to test for the presence of certain functional groups, such as nitro groups on aromatic rings. When added to a compound, NaOH can form salts with certain organic molecules, indicating the presence of a reactive group.
For example, in the case of **phenylnitromethane**, NaOH reacts to produce a sodium salt that is soluble in water:

• Reaction: \( \text{C}_6\text{H}_5\text{CH}_2\text{NO}_2 + \text{NaOH} \rightarrow \text{C}_6\text{H}_5\text{CH}(\text{NO}_2)\text{O}^- \text{Na}^+ + \text{H}_2\text{O} \)
• No reaction occurs with **1-methyl-3-nitrobenzene**, so it remains insoluble.

Using NaOH helps in distinguishing between compounds like 1-methyl-3-nitrobenzene and phenylnitromethane based on solubility and salt formation.

Hydrolysis Reactions

Hydrolysis is a chemical process where water is used to break down compounds, often involving the cleavage of chemical bonds. It's particularly useful in distinguishing compounds that can be hydrolyzed to form acids or bases.
Consider **benzenecarboxamide**; exposure to heat and water leads to the hydrolysis and formation of benzoic acid:

• Reaction: \( \text{C}_6\text{H}_5\text{CONH}_2 + \text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_5\text{COOH} + \text{NH}_3 \)

Benzoic acid, being an acidic compound, can be detected by its acidity. In contrast, **1-methyl-4-nitrobenzene** does not undergo hydrolysis, making this a clear differential test. Hydrolysis reactions help in identifying compounds by the products they form upon reaction with water.

Reactions with HCl

Hydrochloric acid, HCl, is a strong acid that is commonly used to test the solubility and reactivity of organic amines. When an amine reacts with HCl, it forms a corresponding amine hydrochloride salt, proving its basic nature.
Take **benzenamine (aniline)** for instance, which dissolves in HCl to form a soluble amine salt:

• Reaction: \( \text{C}_6\text{H}_5\text{NH}_2 + \text{HCl} \rightarrow \text{C}_6\text{H}_5\text{NH}_3^+\text{Cl}^- \)
• This reaction does not happen readily with **cyclohexanamine**, making it a distinguishing test.

Reactions with HCl can clearly differentiate between aromatic amines like benzenamine and other types of amines based on solubility changes and salt formation.

Bromine Water Test

The bromine water test is a qualitative analysis used to identify unsaturated or reactive aromatic compounds by adding bromine to determine if a reaction—typically a halogenation—occurs, indicative of a double bond or certain reactive groups.
For instance, **4-methylbenzenamine** decolorizes bromine water due to bromination:

• Reaction: \( \text{C}_6\text{H}_4(\text{CH}_3)\text{NH}_2 + \text{Br}_2 \rightarrow \text{C}_6\text{H}_3(\text{CH}_3)\text{NH}_2(\text{Br})_3 \)
• Conversely, **N-methylbenzenamine** reacts less readily, maintaining the bromine's color.

This test helps in the identification and differentiation of compounds with specific reactive groups like unsubstituted phenyl rings capable of undergoing bromination.