One may not be able to judge how far apart they are just by eyeballing two

One may not be able to judge how far apart they are

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Rf values. One may not be able to judge how far apart they are just by “eyeballing” two different lanes, and slight differences in the TLC plate in different lanes could skew results. With two spots in the same lane, it’s easier to visualize the distance between the two spots and the risk of such an error is minimized.
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5) Figure 11.0: From top to bottom, the line structures for benzyl acetate, benzaldehyde, and benzyl alcohol. The least polar molecule is benzyl acetate. It has two oxygens, which indicate potential for a polar bond. The presence of multiple dipole bonds could cause polarity, but since there are so many of them, the polarity “cancels out” and renders the molecule relatively nonpolar. The next-most polar molecule is the benzaldehyde. The carbonyl group generates a polarity, as the oxygen is more electronegative than the carbon. However, the lack of a hydroxyl group means that they cannot form intermolecular hydrogen bonds; this makes them less polar than benzyl alcohol. The most polar molecule is benzyl alcohol. The single hydroxyl group is very electronegative, and allows the molecule to form hydrogen bonds with both other alcohol molecules and water.
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Figure 12.0 : From top to bottom, line drawings are of naphthalene, N,N-dimethylaniline, and aniline respectively. Naphthalene is the least polar of the three compounds. It has no oxygen groups, and the carbon- hydrogen bonds are arranged in such a way that their polarity is negligible. N,N-dimethylaniline is the next-most polar compound. The polarity of the nitrogen group is tempered by the fact that there are two opposed carbon bonds. These distribute the electronegativity difference and make the molecule less polar. Aniline is the most polar molecule of the three. The dipole on N-H, though weaker than the dipole on O-H, is sufficient to allow for polarity. As such, the molecule can form hydrogen bonds with water.
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Figure 13.0 : From top to bottom, line drawings are of Benzophenone, Biphenyl , and Benzoic acid respectively. iii. Benzophenone, Biphenyl, Benzoic acid Biphenyl is the least polar of the three compounds. It’s roughly symmetrical, and has no oxygen or nitrogen groups to induce significant dipole-dipole reactions. Benzophenone is the next-most polar of the compounds. It’s similar in structure to biphenyl, but the addition of a double-bonded oxygen means there is a potential for a polar bond. Benzoic acid is the most polar of the three compounds. The two oxygen groups have a strong electronegativity, which influences the strength of the dipole interactions. However, benzoic acid is still not extremely polar, as the majority of the molecule (the benzene ring) is nonpolar.
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  • Spring '08
  • Chemical polarity, Benzophenone

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