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Discussion in 'MCAT Study Question Q&A' started by hansen44, May 7, 2008.
How do you tell if a solvent is polar or non polar. For example, dichloromethane? Thank you
A solvent is polar if it has a total dipole movement, that is if the molecule as a whole has a charge concentrated in one area. In the case of CH2Cl2, since it has 2 Cl groups (ewg) facing two opposite directions, the charge pull cancels out and it a nonpolar solvent.
How are the chlorines facing opposite directions? In the case of dichloromethane, the central carbon is sp3 hybridized. The chlorines definitely are not facing opposite directions, hence their bond dipoles do not cancel out. There is not a particularly strong dipole, because the electronegativity difference between C-H is not that far from C-Cl, but there is some net polarity to the compound.
yea, in the example with CH2Cl2 it is an aprotic polar solvent. The Chlorines are 120 degrees apart horizontally, but 109.5 degrees apart from one of the hydrogens vertically. So if you imagine three ropes in a trigonal pyramidal geometry and only two people pulling, you will notice that there will be a net movement.
I think the chlorines must be opposite and cancel the dipole because the book says that its a non-polar solvent. So just to clear something up, if there is a large dipole meaning a large difference between electronegativity than the solvent will be polar? Can you guys give me some examples of common solvents polar and non polar it would really help clear things up, thanks.
Your book is wrong. Molecular dipoles come about because of two factors:
1) bond dipoles between atoms within the molecule (eg C-Cl)
2) the vector sum of these bond dipoles (because dipoles are vectors)
If you take a look at CH2Cl2, there clearly are bond dipoles, C-H & C-Cl, and they definitely do not cancel out, therefore the molecule is indeed polar. Something like CCl4, on the other hand, is indeed a non-polar solvent because the bond dipoles cancel out in the vector sum.
Good examples of polar solvents are basically anything that can hydrogen bond (eg, EtOH, H20.)
Commonly seen polar-aprotic solvents are DMSO, acetone, & HMPA.
dichloromethane is a solvent used in many reactions needing a polar aprotic solvent
It happens to be my second favorite, just after dimethylsulfoxide
Both of which easily pass through regular gloves.
CH2Cl2 is tetrahedral. The carbon is sp3 hybridized. That means that each H and Cl is 109.5 degrees from every other.
Since the 2 Cls are 109.5 degrees apart, they do not cancel and the solvent is polar. It has no H atoms that can participate in hydrogen bonding so it is a polar aprotic solvent.
You might want to use a different book for your review. Fundamental errors are some of the most deadly on the MCAT. because many questions require that you reason your way through the information using fundamental information.