Qualitative Elemental Analysis of Organic Compounds of Lassaigne’s Test PDF

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Qualitative Elemental Analysis of Organic Compounds :Lassaigne’s TestObjectives: (3 points) To identify elements using Sodium Fusion Extract. To convert nitrogen, Sulphur, and halogens to ionic forms. To analyze and discern qualitative presence of foreign elements. OBSERVATIONS Put a check (√) if th...

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Qualitative Elemental Analysis

Organic Laboratory Manual

Qualitative Elemental Analysis of Organic Compounds : Lassaigne’s Test Objectives: (3 points) • • •

To identify elements using Sodium Fusion Extract. To convert nitrogen, Sulphur, and halogens to ionic forms. To analyze and discern qualitative presence of foreign elements.

OBSERV OBSERVA ATIONS 1.

Put a check (√) if the element is present and (x) if absent. [2pts each, 24 points total] TEST COMPOUND

STRUCTURAL FORMUL FORMULA A

N

S

Cl

Br

I

X

√

x

x

x

X

X

√

x

Sulfonic acid x

Chlorobenzene

C6H3Cl

Department of Chemistry, MSU-IIT 2015-2016

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Qualitative Elemental Analysis

2.

Organic Laboratory Manual

For the elements present in each compound below, give the result indicating presence of the element. (ex. color of solution or precipitate) [2 pts each] TEST COMPOUND

Sulfanilic acid

N

Prussian Blue colour appearance

S

Cl

Br

I

Violet colour appearanc appearance e

–

–

–

–

White colour appearanc appearance e that is soluble in NH4OH

–

–

– Chlorobenzene

QUESTIONS 1.

Why is it necessary to react the test compound with fused sodium metal before any elemental analysis is done? [3 points]

If we fused directly the sodium metal in the said elemental analysis, the organic compounds like Nitrogen, Sulphur, and Halogens (Cl, Br, I) will stay covalent. But if not directly fused the sodium metal and organic compounds will react or mix with each other to enable Nitrogen, Sulphur, and Halogens convert into ionic form. Also, it’s necessary to convert them because it makes them detectable whenever they are ionic.

2.

Give the chemical equation representing the reaction in the following: a.

sodium fusion [2 points]

Na + C + N→ NaCN∆ b.

test for nitrogen [2 points]

3Na4[Fe(CN)6] + 4FeCl3

Department of Chemistry, MSU-IIT 2015-2016

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Qualitative Elemental Analysis

Organic Laboratory Manual

Fe4[Fe(CN)6]3 + 12NaCl

3.

Give the name and formula of the Prussian blue precipitate in the positive test for nitrogen. [1 point each]

Name: Ferri Ferrocyanide Formula: Fe4[Fe(CN)6]3

4.

Explain how these interferences are avoided in the following tests. [2 points each] a.

FeS in test for nitrogen

The FeS in the test for nitrogen can be avoided if the next test will use a different organic compound mixed with the sodium metal in creating Lassaigne’s Extract. For instance, the sulfuric acid used to mixed with LE, then the ferrous ions oxidation aided. The outcome will not produced ferrous sulfide.

b.

Ag2S/AgCN in test for halogen

The LE mixed with HNO3 to make Ag2S and AgCN presence will avoided.

5.

In the test for presence of halogen, what will happen if S or N is present and the solution is immediately added with AgNO3 without boiling? [2 points]

If S and N present in the test of halogen and react AgNO3, the outcome seems different and will not detect any halogens if and only if they are boiled. But when S is added in AgNO3 solution without boiling; the color of the presence precipitate is as clear to pale yellow like bromine. 6.

Give the equation for the following reactions in the test substances. [2 points each] a.

silver nitrate with the halogen present

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Qualitative Elemental Analysis

Organic Laboratory Manual

• Cl– + AgNO3 → AgCl + NO3- • Br– + AgNO3 → AgBr + NO3• I– + AgNO3 → AgI + NO3-

lead nitrate with the sulfur present

b.

•

Pb (NO3)2 + Na2S → PbS + 2NaNO3

i) ferrous sulfate with the cyanide present

c.

•

FeSO4 + 6NaCN → Na4[Fe(CN)6]3 + NaSO4

d. ii) ferrocyanice with ferric chloride

•

3Na4 [Fe(CN)6] + 4FeCl3 → Fe4 [Fe(CN)6]3 + 12NaCl

Department of Chemistry, MSU-IIT 2015-2016

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