Question

Give each of the following pairs of compounds a chemical test, preferably a test-tube reaction, which would serve to distinguish one from the other. (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{SH}\) and \(\mathrm{CH}_{3} \mathrm{SCH}_{3}\) (b) \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{~S}\right)_{2}\) and \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2}\right)_{2} \mathrm{~S}\) (c) \(\mathrm{CH}_{3} \mathrm{~S}(\mathrm{O}) \mathrm{OCH}_{3}\) and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{SO}_{3} \mathrm{H}\) (d) \(\mathrm{CH}_{3} \mathrm{~S}(\mathrm{O}) \mathrm{OCH}_{3}\) and \(\mathrm{CH}_{3} \mathrm{~S}(\mathrm{O})_{2} \mathrm{CH}_{3}\) (e) \(\mathrm{CH}_{3} \mathrm{SCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) and \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{CH}_{2} \mathrm{SH}\)

Short answer

Perform the following chemical tests: (a) Lead acetate test: CH3CH2SH + Pb(CH3COO)2 --> CH3CH2SPb + 2CH₃COOH (b) Reaction with NaOH: (CH3CH2S)2 + 2NaOH --> 2CH3CH2SNa + H2O (c) pH test: CH₃CH₂SO₃H is acidic, while CH₃S(O)OCH₃ is neutral. (d) Reaction with alkali solution and silver nitrate: 2CH₃S(O)OCH₃ + 2AgNO₃ + 4NH₄OH → Ag₂O + 2CH₃OH + (NH4)2SO4 + 4H2O (e) Reaction with iodine, ethanol, and NaOH: CH₃OCH₂CH₂SH reacts, producing a reddish-brown color.

Step-by-step solution

(a) Test for Compounds CH3CH2SH and CH3SCH3

Perform a Lead acetate test. For this test, mix each compound in a test tube with a solution of lead(II) acetate. Ethyl mercaptan (CH3CH2SH) reacts with lead (II) acetate to form a black precipitate, while dimethyl sulfide (CH3SCH3) does not react. Reaction involved: CH3CH2SH + Pb(CH3COO)2 --> CH3CH2SPb + 2CH₃COOH

(b) Test for Compounds (CH3CH2S)2 and (CH3CH2)2S

Perform a reaction with sodium hydroxide (NaOH). Mix each compound in a test tube with a solution of NaOH. The compound (CH3CH2S)2 reacts with NaOH to form an insoluble sodium salt, while (CH3CH2)2S does not react. Reaction involved: (CH3CH2S)2 + 2NaOH --> 2CH3CH2SNa + H2O

(c) Test for Compounds CH3S(O)OCH3 and CH3CH2SO3H

Perform a pH test to distinguish between these two compounds. CH₃S(O)OCH₃ has a neutral overall pH, while CH₃CH₂SO₃H is acidic due to the hydrogen atom bound to the sulfonate group. Dissolve both compounds in water and test the pH with a pH indicator. The compound with the lower pH is CH₃CH₂SO₃H.

(d) Test for Compounds CH3S(O)OCH3 and CH3S(O)2CH3

Perform a reaction with alkali solution. Dissolve both the compounds in dimethylformamide (DMF) and add aqueous ammonia dropwise. CH₃S(O)OCH₃ will give a precipitate of silver(I) oxide (Ag₂O) on adding silver nitrate solution while CH₃S(O)₂CH₃ will not. Reaction involved: 2CH₃S(O)OCH₃ + 2AgNO₃ + 4NH₄OH → Ag₂O + 2CH₃OH + (NH4)2SO4 + 4H2O

(e) Test for Compounds CH3SCH2CH2OH and CH3OCH2CH2SH

Perform a reaction with iodine, ethanol, and sodium hydroxide solution (reacts with thiols). In the case of CH₃SCH₂CH₂OH, the thiol group is not present and no reaction occurs. In the case of CH₃OCH₂CH₂SH, the thiol group reacts with iodine producing a reddish-brown color along with sodium iodide and sodium salt of ethanethiol.

Key concepts

Test-Tube Reactions

Test-tube reactions play a crucial role in analytical chemistry, especially when it comes to the qualitative analysis of organic compounds. These experiments, often conducted within the confines of a small glass tube, are designed to reveal the identity of substances or to differentiate between closely related compounds.

For instance, in the context of sulfur compounds, a distinct reaction might involve the formation of a black precipitate when an ethyl mercaptan solution is mixed with lead acetate, distinguishing it from dimethyl sulfide, which shows no such reaction. This simplicity and clarity are inherent to test-tube reactions, making them a fundamental tool for quick and effective qualitative analysis.

The beauty of test-tube experiments lies in their minimal requirements – a test tube, a few reagents, and a potential heat source are often enough to carry out these revealing tests. Students and chemists alike use these reactions for fast and often unambiguous insights into the nature of the substances they are working with. For educational purposes, understanding these reactions provides a solid foundation in laboratory techniques and the behavior of organic compounds.

Qualitative Analysis in Organic Chemistry

In the realm of organic chemistry, qualitative analysis serves as the detective work of identifying substances and understanding their composition. Test-tube reactions are just the beginning; they set the stage for delving into the properties of organic molecules and discerning how they differ from one another.

Qualitative analyses often employ specific chemical reagents that react in a predictable manner with particular functional groups. For example, the use of sodium hydroxide to distinguish diethyl sulfide from its counterpart, diethyl disulfide, is based on the latter's ability to react and form a sodium salt. This type of targeted reaction highlights the power of qualitative analysis in directly pinpointing key structural differences.

Another aspect of qualitative analysis is pH testing, where the acidity or basicity of a compound can be revealed through simple pH indicators. These tests are straightforward yet valuable in differentiating molecules like methoxymethyl sulfide and ethanesulfonic acid, which have distinct pH profiles due to their structural differences.

Sulfur Compounds Identification

Sulfur compounds in organic chemistry present a wide array of functional groups, each with unique reactivity characteristics that can be leveraged in identification tests. Insight into such tests allows students to confidently distinguish among thioethers, thiols, sulfones, and sulfonic acids, a skill that's vital in both academic and practical laboratory settings.

To identify these compounds, chemists use selective reactions that target their specific chemical behaviors. For example, a solution of iodine and sodium hydroxide reacts notably with thiols but not with thioethers, allowing differentiation based on whether a reddish-brown color appears.

Another distinguishing reaction is the use of silver nitrate in the presence of aqueous ammonia, which differentiates compounds like methyl sulfide and dimethyl sulfoxide through the formation (or lack) of a silver oxide precipitate. Such tests not only identify sulfur compounds but also provide a deeper understanding of their underlying chemistry, making sulfur compounds identification a fascinating chapter in the study of organic molecules.