One way to determine the number of multiple bonds in a compound, is to measure the volume of dihydrogen required to completely react with the compound. This process is called catalytic hydrogenation since it requires the use of a catalyst to get the reaction to go. Equation 1 illustrates process for structure 1.
Note that this is a balanced equation. It says that 1 mole of compound 1 reacts with 1 mole of dihydrogen. Using this stoichiometric relationship and the ideal gas law, PV = nRT, it is straightforward to calculate the number of moles of dihydrogen present in the experimentally measured volume of gas that is consumed in the reaction. For example, if you carried out reaction 1 on 1 mole of compound 1 at atmospheric pressure and room temperature, approximately 20oC, you could calculate the volume of dihydrogen that would be consumed by rearranging the ideal gas law to V = nRT/P since the values of n, R, T, and P are all known:
Solving this equation indicates that V = 24.1 L. Here's a more realistic example. Hydrogenation of a 125.3 mg sample of compound 1 consumes 21.9 mL of dihydrogen. How many moles of gas is this?
This is 0.9104 mmol of dihydrogen. Remember, the formula of compound 1 is C10H16. Its molecular weight is 136.26 g/mol. Therefore, 125.3 mg is 9.196 x 10-4mol or 0.9196 mmol. Within experimental error the molar ratio of compound 1 to dihydrogen is 1/1.
Exercise 1 How many moles of dihydrogen would be required to completely hydrogenate 1 mole of compound 2?
If you were to completely hydrogenate a sample of compound 2 that weighed 0.7316 g, how many mL of dihydrogen would theoretically be consumed? Assume a temperature of 20oC. Express your answer to 4 significant figures.
Exercise 2
A chemist isolated a natural product. Elemental analysis indicated its empirical formula to be C10H14O.
a. What is the index of hydrogen deficiency of this compound?
b. Catalytic hydrogenation of a 0.1036 g sample of this material at 200C and 1 atm consumed 49.80 mL of dihydrogen.
1. How many mmol of compound are there in 0.1036 g? Express your answer to 4 significant figures.
2. How many mmol of dihydrogen are there in 49.80 mL? Express your answer to 4 significant figures.
3. How many mmol of dihydrogen were consumed per mmol of natural product?
c. How many rings are there in this compound?
d. How many multiple bonds are there in this compound?
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