Post by: vahid on 30/04/2018 06:44:37
Suppose two different molecules approach each other and make a dipole-dipole bond. depending on its polarizabilities, Electron arrangement of each molecule is affected by dipole of the other molecule. So does this arrangement of electrons affect the strength of dipole-dipole bond? a molecule with high or low polarizability makes the bond weaker or stronger?
How about a hydrogen bonding? How does polarizability affect on Donor and acceptor of Hydrogen?
@chiralSPO
Post by: chiralSPO on 30/04/2018 15:11:21
The strength of the dipole-dipole interaction is determined by the dipole moments of each molecule (larger dipole moments → stronger interaction), how close they can get to each other (closer → stronger), and how they are oriented with respect to each other (think of them like dipolar magnets).
Polarizability of one of the molecules will influence the magnitude and orientation of one of the interacting dipole moments, maximizing the interaction between the two molecules. So in that sense it will only serve to increase the strength of the dipole-dipole interaction.
But, a highly polarizable molecule will also minimize its own dipole moment when it is by itself. So, even though the interactions with other molecules are increased, the increased stability of the molecule on its own essentially cancels this out (this will vary slightly on a case by case basis).
For example: cyclohexyl methyl ether has a permanent dipole moment of 1.29 D, while anisole (methyl phenyl ether) has a dipole moment of 1.22 D. These molecules are very similar (see below), but anisole is more polarizable than cyclohexyl methyl ether, so it has a smaller permanent dipole moment. However, the boiling point of anisole (MW 108 g/mol) is 154 °C, somewhat higher than cyclohexyl methyl ether (MW 114 g/mol), which boils at 135 °C, despite having a lower molecular mass and a higher dipole moment. This indicates that the attractive intermolecular interactions between anisole molecules are greater than the attractive intermolecular interactions between cyclohexyl methyl ether molecules.
example.png (18.42 kB . 312x212 - viewed 4394 times)
Post by: chiralSPO on 30/04/2018 16:29:00
Hydrogen bonding is a funny thing.
It is largely related to local dipole moments--so even symmetrical molecules that have no overall dipole moment, like benzoquinone, dihydroquinone, and carbon dioxide can participate in hydrogen bonding, and are reasonably to very soluble in water.
nonpolar and water soluble.png (31.9 kB . 400x443 - viewed 4211 times)Hydrogen bonding is like an intermediate form of acid-base interaction. The proton is not completely transferred from one molecule to another, as would be the case for Brønsted acid-base reactions (it's more like a Lewis acid-base interaction), but the X—H bond length of the donor is longer when participating in H-bonding (X–H vs X—H------Y), and the H------Y distance is much shorter than the van der Waals radius of the Y molecule would predict for an intermolecular contact. The strength of the interaction depends on the acidity and basicity of the groups involved. The strongest individual hydrogen bonds are symmetrical (X---H---Y).
This also brings up an interesting phenomenon that is related to polarizability, which is prototropic tautomerism. Polarizability is typically thought of as rearrangement of electrons in a molecule, while tautomerism involves the change in position of nuclei (in the case of prototropic tautomerism, it is the change in position of a proton or hydrogen nucleus).
Essentially, when a proton can be in multiple places on a molecule, the energy of the different tautomers can be very sensitive to the environment around the molecule (for example, what solvent is it in). One of the best examples of this is the case of 2-hydroxypyridine vs 2-pyridone:
tautomers.png (10.62 kB . 249x147 - viewed 4164 times)In nonpolar aprotic solvents, 2-hydroxypyridine is favored, while in very polar and protic solvents, 2-pyridone is favored. So one can think of this as a kind of polarization...
Even more extreme cases involve the formation of zwitterionic molecules. Amino acids typically exist in the overall neutral form in nonpolar aprotic solvents, but in water the major form contains a cation and an anion (think of it as an internal acid base reaction between the amine and the carboxylic acid):
amino acid.png (8.59 kB . 298x131 - viewed 4128 times)
Post by: Bored chemist on 01/05/2018 19:37:53
The difference is largely due to the higher polarisability of benzene