Abstract: The objective of this experiment is to efficiently perform a fischer esterification of 1-Hexanol to form water and hexyl acetate, and to confirm the esterification with a nuclear magnetic resonance (NMR) spectroscopy. It was found that 0.3963 grams hexyl acetate was formed with a percent yield of 33.2%. The product was confirmed using NMR, IR, and boiling point confirmation.
Backround:
A Fischer Esterification is the formation of an ester and water from alcohol and an acid. More specifically it is a nucleophillic acyl substitution reaction carried out under acidic conditions. Carboxylic acids alone are not reactive enough to be attacked by neutral alcohols, but they can be made much more reactive in the presence of a strong acid, such as sulfuric acid or hydrochloric acid. The mineral acid protonates the carbonyl group oxygen atom and gives the carboxylic acid a positive charge. Now positively charged, the carboxylic acid is much more reactive toward a nucleophillic attack by the alcohol.
NMR (Nuclear Magnetic Resonance) spectroscopy is a technique used to determine the actual 3-D image of a molecule. This spectroscopy uses atoms that have nuclear magnetic moments such as isotope-1 Hydrogen and isotope-13 Carbon atoms. The nuclei of NMR-active atoms have one of two nuclear spin states, either (+1/2 or -1/2).
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These atoms when placed in a magnetic field are either spinning with the +1/2 magnetic field or against it (-1/2 spin). A slight majority of the nuclei are aligned with the magnetic field, because this spin orientation constitutes a slightly lower energy spin state. If radio frequency waves of the appropriate energy are supplied, nuclei aligned with this field can absorb this radiation and reverse their spin and now oppose the applied magnetic field. The frequency of radiation needed to induce spin conversion is a direct function with the strength of the magnetic field. This amount of energy that is needed to induce the change in spin must have a frequency that matches its angular precessional frequency. This is called a resonance condition and this is what is graphically interpreted on a NMR printout.
Objective and Materials:
The materials for this experiment is listed on page 59-64(Organic Chemistry laboratory Manual, Haden/McNeil, 2003-04)
Procedure:
The procedure for this experiment is listed on page 59-64
(Organic Chemistry laboratory Manual, Haden/McNeil, 2003-04)
Data:
All data is in laboratory notebook
Calculations:
The following is a calculation for theoretical yield of grams of a 1:1 mole ratio:
Yield (g) = (starting material (g)) * (molecular weight of starting material (mol/g)) *
(1 mol product / 1 mol of reactant) * (molecular weight of product (g/mol)
The following is a sample calculation for yield of methyl stearate from methyl oleate:
1.195g C6OH14 = (0.8466g C6OH14 * 1 mol C6OH14) * (1mol C8O2H16) *
1mol C6OH14 1 mol C6OH14
(297.48g C8O2H16)
1 mol C8O2H16
The following is a calculation to find percent recovery:
% recovery = weight of the pure sample * 100
Weight of theoretical product
The following is a sample calculation for yield of hexyl acetate:
33.2% recovery = 0.39631 (g) / 1.195(g) * 100
Results:
The following is a chart of boiling points, percent yields, and product weight:
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SubstanceBoiling Point (C)Text Boiling Point (K)% Yield (g)Weight (g)
Hexyl acetate464833.20.39631
-Note that the product smelt like vanilla and bananas
-NMR printout is interpreted and attached to the back of report.
-IR printout is interpreted and attached to the back of report.
Discussion:
The objective of this experiment was to efficiently perform a fischer esterification of 1-Hexanol to form water and hexyl acetate, and to confirm the esterification with a nuclear magnetic resonance (NMR) spectroscopy. It was found that 0.3963 grams hexyl acetate was formed with a percent yield of 33.2%. The product was confirmed using NMR, IR, and boiling point confirmation 46 degrees Celcius.
The mechanism for this specific reaction is as follows:
First the protonation of a carbonyl oxygen activates the carboxylic acid toward nucleophillic attack by alcohol, yielding a tetrahedral intermediate. Transfer of a proton from one oxygen atom to another yields a second tetrahedral intermediate and converts the OH group into a good leaving group. Loss of a proton and expulsion of water regenerates the acid catalyst and gives the ester product.
This reaction is a nucleophillic acyl-substitution carried out under acidic conditions of sulfuric acid. The alcohol used was 1-Hexanol which limits the ester to a side hexyl chain.
A number of errors could have occurred in this experiment which caused it too fail. First, if not enough sulfuric acid was used, protonation of the carbonyl group on the carboxylic acid would be difficult to obtain. This in turn would lead to a low percent yield of the final ester product. Second, if the temperature was too high in heating the mixture, reflux would not occur. That is to say the solvent would not boil and then recondense in the reflux condenser. This would cause more the ethyl acetate to boil out as it forms. Third, if the starting material or finished product were spilled when being weighed the percent yield would be incorrect.
To get better results in this experiment would simply be to do as many trials as it takes until the results show a higher percent yield. By doing this one could lower the error percentage and see what the actual amount of Hexyl acetate that can be produced if one was shooting for high yield.
