Experiment #5: Synthetic Chemistry

This week we’re boiling concentrated H2SO4, HCl, and HBr, so please be extra-prepared for this lab, and be on the lookout for any safety hazards throughout the entire lab period.

The Reagent Table

  • This week our Reaction Risk Assessment forms (RRAF) and Experimental sections will require a little more effort to fill out.
  • On the RRAF we fill our the table based on how much we are supposed to use, while the reagent table, that will go into our Experimental section will be filled out with how much we actually used.
  • Reminder for calculating mole quantities of reagent: tBuOH will be supplied in it’s pure form and is a liquid, so to calculate mol, you convert the volume given to a density, then use its molecular weight to find moles.
  • Compare that with 18 M H2SO4 (aq.); H2SO4 is dissolved in water and we have a concentration. So here we use the concentration and the volume to find moles.
  • On the RRAF, try to fit as many chemicals as you can on the given table.
  • For the experimental section in your lab books, you are expected to make two reagent tables, one each for Part 1 and Part 2.
  • To calculate equivalents, first find the molar quantities. Then find the limiting reagent (the lowest molar quantity), and divide the molar quantities of all the other reagents by the moles of limiting reagent.

Sample reagent table for Part 1 to be put in the Experimental section of lab book.

Here in this example, we know that 1-butanol is the limiting reagent. It has the lowest molar quantity and we can see that it’s the compound that gets transformed in the reaction. We set its equivalency at 1.0 and we calculate the rest of the equivalencies based on the molar quantity of 1-butanol. The molar equivalents are an important value since they allow us to see ratios we need for the reaction at any scale.

In Organic I, the lab manual tells us the mass/volume quantities that we need to use. Outside of teaching labs, this is not a particularly helpful practice, and instead the molar equivalents are given. This allows for easy scaling of reactions, and more importantly it allows for easy communication of reagent quantities since we essentially have the molar ratios to use.

Experimental Set-Up For Part 1

Glassware set-up.

In the round bottom, we have 1-butanol, HBr, and HCl. The H2SO4 goes into the dropping funnel.

When adding reagents into the round bottom, use the same graduated cylinder for all measurements.

Again, we are using a Variac to control the heating mantle. In this reaction, we only add heat after the slow addition of H2SO4 from the dropping funnel has been completed.

For this experiment, we have two different sets of hoses. One set of two for the water for the condenser and another set of two for the acid trap. Consider the hazards of allowing an acid-soaked hose to come in contact with water.

When we open flow from the bottom of a dropping funnel, ensure that there is no stopper at the top to allow for proper air flow from the top allowing the H2SO4 to drop down. If one tries to open the stopcock while there is a stopper, one creates a vacuum, and can seize the stopper in a dropping funnel filled with 20 M H2SO4.

Reaction Mechanism

In both parts of this experiment, we are substituting the -OH group for a nucleophile (Cl- or Br-) and using GC to determine which nucleophile was more active in each experiment. However, before any substitutions can occur, -OH group must be transformed into a better leaving group. In this experiment, the HCl and HBr (along with H2SO4) serve to add protons, and also provide nucleophiles for our reaction to carry forward.

Reaction mechanism. Top: Part 1. Bottom: Part 2.