Chem 51LB ‘W15 Wk 4 Lab Lecture Solution There are two part to this problem that need to be determined and then connected to reach the answer. 1. Order the solvents according to polarity. A) 9:1 Hex/EtOAC B) 3:2 Hex/EtOAc C) 4:1 Hex/EtOAc D) 1:1 Hex/EtOAC There are multiple ways to approach this. I prefer to think in terms of percentages. A) 90% hexanes, 10% ethyl acetate
B) 60% hexanes, 40% ethyl acetate C) 80% hexanes, 20% ethyl acetate
D) 50% hexanes, 50% ethyl acetate Now we can put these solvent mixtures in order of polarity. Most polar D B C Least polar A 2. Now we need to know how the polarity of the eluting solvent mixture effects how compounds on the plate travel. We know from reading (and video) on TLC technique that increasing polarity of the eluting solvent mixture causes all spots to move higher on the plate. That tells us the relative polarity of the eluting solvents used in each case. second most Most polar second least least
3. NOW, we can answer the question! Tying parts one and two together, the answer from left to right is: D, C, A, B 1 Chem 51LB ‘W15 Wk 4 Lab Lecture Solution Common Mistakes 1. Mislabeling the polarity ranking of solvent mixtures:​
Remember that hexanes is significantly less polar than ethyl acetate. As the percentage (or ratio) of ethyl acetate in the mixture increases, the polarity of the solvent mixture increases. 2. Reversing the answer order:​
This is usually the result of confusion regarding how solvent polarity affects movement of compounds on the plate. Review the reading and pre­lab videos regarding how this works. 3. Answering in such a way as to say that more polar solvent mixture moves more polar compounds and less polar solvent mixture moves less polar compounds:​
This also results from confusion about how solvent polarity affects movement of compounds. Review! 2