Question Regarding low percent yield: In the experiment making ethanol from sucr

Question

Question Regarding low percent yield: In the experiment making ethanol from sucrose I have got a (50 %) percent yield .. what could have caused the low percentage?

During Fractional Distillation my boiling solution stopped boiling for a couple of minutes because I was not paying any attention to it. Could that cause the low percent yield ? The purity of my ethanol was 60% .. what could have caused the low purity?

Also, I did this calculation which I'm not sure why? (1.92 g ethanol x 190ml/50 ml x 0.60 = ) 190 ml is the supernatant liquid after fermentation and I took 50 ml from it for distillation. 1.92 g is the final ethanol solution weighted with Grad.Cylinder.

Explanation / Answer

Atom economy is the conversion efficiency of a chemical reaction. It gives you an idea of the highest possible yield of a reaction. Atom economy is given by the formula:

Atom economy = Molecular weight of product X 100 / Sum of molecular weights of all reactants

C12H22O11 + H2O ------> 4C2H5OH + 4CO2

Atom economy = 4 X Molecular weight of ethanol X 100 / (Molecular weight of sucrose + Molecular weight of water)

= 4 X 46.0684 X 100 / (342.2965 + 18.0153)

= 18427.36 / 360.3118

= 51.1%

Hence maximum yield for synthesis of ethanol from sucrose will be 51.1%. So you getting 50% yield is perfectly fine and the problem for low yield is not the synthetic skills but the synthetic route you selected.

Ethanol has a boiling point of 78.5 deg C while water boils at 100 deg C. Hence, ethanol distils out first. However, obtaining 100% ethanol is not possible for it forms an azeotropic or constant boiling mixture. The maximum you can go is 95% and it requires completely inert conditions and use of drying agents like molecular sieves or CaCl2. Evaporation of ethanol and trapping of moisture from atmosphere can also lower the purity. So 60% purity by only fractional distillation is not low.

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