Chapter 1 : elemental analysis

- At the end of this session you should be able to identify the elements present in an organic compound using qualitative elemental analysis .

objectives

An organic compound is a compound which contains mainly carbon atom(s).

♣ Organic compounds are covalent compounds that usually contain carbon, Oxygen and Hydrogen. They are insoluble in water.

Elemental Analysis

♣Elemental analysis aims to determine the nature of the elements present in an organic compound (qualitative) and how much of atoms of each element is present (quantitative).

Qualitative Elemental analysis

It allows the identification of the elements present in a compound (chemical composition) .These elements are

C , H , N , S , Cl , Br , I

A- To Identify “Carbon and hydrogen” elements in an organic compound we use the following methods:

• Pyrolysis

• Incomplete combustion

• Complete combustion

Pyrolysis:

• Is the decomposition of an organic compound

by heating it in absence of air.

If water droplets are formed, this is an evidence for the presence of “H” and “O ” in the organic compound.

Ex: pyrolysis of sugar : C₆H₁₂O₆ → 6C_(S) + 6H₂O

The black powder formed is carbon “C”.

• Incomplete Combustion:
• The organic compound burns with insufficient amount ofO₂ :

A Black deposit is formed, which is an evidence for the presence

of “C ” in the Organic compound.

The organic compound burns in excess ofO₂ :

C is Converted into CO₂

H is Converted into H₂O

CO₂ identification: If lime water becomes turbid or milky when the gases of combustion are bubbled into it :

lime water Ca(OH)₂ becomes turbid due to the formation of calcium carbonate “CaCO₃”.

The Corresponding equation is: Ca(OH)₂ +CO₂ →CaCO₃ + H₂O

lime water

Organic compound +O₂ CO₂ + H₂O

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H₂O Identification:

H₂O released from combustion transforms CuSO₄ anhydrous (White) into hydrated copper sulfate “CuSO₄. 5H₂O” (blue).

so if the Copper sulfate becomes hydrated ⇒ release of water ⇒”H” is a present in the organic Sample.

The corresponding equation is:

CuSO₄ + 5 H₂O → CuSO₄. 5H₂O (Blue color)

B- Organic Compound may contain other chemical elements such as nitrogen (N), Sulfur (S) and halogens {X→ (I, Br, Cl)}.

How to identify these elements?

By mineralisation: The test involves heating / burning the sample (Organic compound) strongly with hot clean molten sodium metal.

S^2- , CN^-, 0r X^-

a) Sulfur

Sulfur present in an organic compound is converted into S^2- by mineralisation.

To a portion of the filtrate obtained, we add lead (Pb²⁺) solution, if a black precipitate is formed

Pb²⁺ + S^2- → PbS_(s) (black precipitate)

⇒S is a part of the organic sample.

b) nitrogen

To identify the presence of N in an organic compound, it should be converted into CN^- cyanide ion by mineralisation

To a portion of the filtrate , we add a solution containing Fe²⁺ followed by adding solution containing Fe³⁺ ions. It forms a blue color (Prussian blue).

⇒N is a part of the organic sample.

c) halogen

To identify the presence of X in an organic compound, it should be converted into X^- halide ion

i- Chlorine

To a portion of the filtrate, we add a freshly

prepared solution of Silver nitrate, a white precipitate is formed :

⇒Cl is present in the organic sample.

ii- bromine

⇒Br is a part of the organic sample.

iii- iodine

⇒ I (Iodine)is present in the organic

sample.

Construct a table showing : the species detected in the compound, test done and observation in case of positive result in order to study the notes easier + solve exercise 6 p:222 from your chemistry book.