Estimation of Organic Compounds

Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

The learning objective of this lesson will be how to estimate the relative amounts of carbon, hydrogen, nitrogen, the halogens, sulfur, and phosphorus within an organic compound.

Cooking and Chemistry

Are you the type of person that likes to cook or help cook in the kitchen? Cooking can be almost be a science, and in order to get the food to taste right, you have to have the right combinations of ingredients and flavorings. Take for example a pizza. A pizza has dough, tomato sauce, cheese, meat, and seasonings that make it up. In order to get a good pizza, you have to have all of the ingredients together and in the right proportions.

Just like pizza, organic compounds are composed of different atoms (ingredients) and in order to make the compound, those atoms have to be present in defined amounts. But how do we know how much of each atom is present in a compound? That's actually what we will be trying to understand in this lesson. We will be discussing how the relative amounts of different atoms in an organic compound can be estimated and deduced. Our focus will center around carbon, hydrogen, nitrogen, the halogens, sulfur, and phosphorus, since those atoms tend to be the most common in organic compounds. Let's get cooking!

Carbon and Hydrogen in a Compound

Let's get started by talking about how the amount of carbon and hydrogen in a compound can be estimated. The most common method of determining how much carbon and hydrogen are present in a compound is by what's called combustion analysis. When a compound is subjected to combustion analysis, it is burned in the presence of pure oxygen, which produces only carbon dioxide and water as the products.

The carbon dioxide and water are collected and their masses determined, and the amount of carbon and hydrogen that were present in the original compound can easily be estimated by comparing the mass of the original compound to the masses of carbon dioxide and water produced. The specific piece of laboratory equipment typically used for this type of experiment is called a bomb calorimeter.

A bomb calorimeter is commonly used for combustion analysis which is how amounts of carbon and hydrogen are determined

Estimation of Nitrogen in a Compound

In 1893, a scientist by the name of Johan Kjeldahl successfully pioneered and developed a method known as Kjeldahl's method in which the amount of nitrogen in an organic compound can be determined. In this analytical technique, the organic compound is heated in the presence of sulfuric acid, which leads to decomposition of the compound followed by capture of the nitrogen atoms to form ammonium sulfate.

The ammonium sulfate is then reacted with sodium hydroxide which produces ammonia as the reaction product. Once the ammonia is collected, it is neutralized with a known amount of an acid in an acid/base reaction. Since we know how much acid it takes to react with the ammonia, the amount of nitrogen in the original sample can be back-calculated easily.

Estimation of Halogens and Sulfur in a Compound

What if a halogen or sulfur atoms were present in an organic compound? The Carius method is an excellent technique that allows for the determination of the amount of chlorine, bromine, fluorine, iodine, or sulfur contained in a compound. The compound is first heated in the presence of nitric acid and silver nitrate, which converts any halogens or sulfur present into their corresponding silver salt. This salt can then be collected, weighed, and the amount of halogen or sulfur calculated based on the amount of silver salt recovered.

The Carius method of halogen determination relies on the production of a silver salt which allows the amount of halogen present to be calculated

Estimation of Phosphorous in a Compound

If we suspect that our compound may contain phosphorus, we can actually utilize a modified Carius method to determine how much it may contain. The organic compound is first heated with concentrated nitric acid and magnesium, which converts all of the phosphorous present to a salt called magnesium ammonium phosphate. This solid is then burned to produce Mg2 P2 O7 (magnesium pyrophosphate).

The mass of this compound recovered is then used to calculate the relative amount of phosphorus that was present in the original starting compound. Just like in the previous Carius method, this technique relies on selectively converting the element of interest to a solid, which can easily be isolated and its mass determined.

A modified Carius method can be utilized to determine the amount of phosphorus present in an organic compound

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