Your Questions About Hydrogen Bonding

admin answers:

The bases in 2 strands are paired through the ydrogen bonds forming base pairs.
Adenine forms double hydrogen bond with thymine in opposite strand [ vice-versa]
Gaunine is bonded with cytosine with triple hydrogen bonds.

As a result purine comes opposite to pyrimidine.
And this generates approximately uniform distance between the 2 strands of double helix.

admin answers:

When two liquids mix together we say that they are miscible. Liquids which are miscible must share some similarities. They must be similar in size and in the types of intermolecular bonding exhibited. Oil molecules are large and nonpolar. The only attraction between oil molecules are relatively large London dispersion forces. Water molecules are much smaller, and are polar and thus, in addition to weaker London dispersion forces, exhibit dipole-dipole attractions. Lastly, water also exhibits hydrogen bonding which is the strongest of the three van der Waals attractions exhibited by water. Oil does not exhibit either dipole-dipole attraction or hydrogen bonding.

For oil to be miscible in water it must “fit” into the space occupied by water molecules without disrupting the arrangement of molecules too much. For other relatively small, polar molecules which also exhibit hydrogen bonding, like low molecular weight alcohols, this isn’t a problem, but with large, nonpolar molecules like olive oil it is a huge problem.

Energetically speaking, it would require a lot of energy to force an oil molecule between the water molecules. The water molecules will be at a much lower energy when they are hydrogen bonded to one another, and so from the standpoint that systems will go to the lowest possible energy, water and olive oil will not mix.

admin answers:

I don’t quite understand the question but the hydrogen bonding does make water have ‘special’ properties such as boiling points and densities. Will leave the rest up for you to research but hope that helps.

(hint: compare with other similar formula)

Same as e-mail:
I’ve been reading the question again but I am still unsure what exactly what the proffesor asks but I think the question is asking what special properties arise from the hydrogen bonds.

So if I was answering it myself I would talk about the following:

1. Structure of ice: normally solids have higher densities than liquids. However ice floats on water. This is due to hydrogen bonding.

2. When the boiling point of group VI hydrides are compared water has an anomalous high boiling point (compared to H2S, H2Se, H2Te). Again this is caused by the hydrogen bonds.

Maybe the proffesor wants you to deduce these facts from basic theories?

Anyway hope the above helps and obviously do elaborate on the above points. I don’t know a lot of chemistry so there may be other things to talk about but I would have thought these two points were the main things to talk about.

admin answers:

Hydrogen bonding is absolutely vital to biological systems. It is the way in which enzymes anchor onto their substrates, and it is the way in which the two strands of DNA fit together. So it is the basis of metabolism, and also of heredity.

admin answers:

One of the most distinct and easily recognizable peaks in an IR spectrum is the broad O-H absorption of alcohols and phenols. You should note that any significant quantity of a compound will contain a very large number of individual molecules, and each of these may be hydrogen bonded to a slightly different extent. Thus IR absorptions will occur at varying frequencies for each of these bonds. The end result is that the IR peak appears broadened, as it is an average of all these slightly different absorptions……..

It is possible to acquire IR spectra of hydroxyl-containing compounds without seeing this broad signal. By creating a very dilute solution of the sample, or acquiring the IR spectra in the gas phase, hydrogen bonding is prevented through lack of molecular contact. Even in concentrated solution, larger compounds may sterically hinder hydrogen bonding, preventing exchange. In these situations the broad O-H peak is replaced by a sharp signal around 3600 cm-1……

See this link—
http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/InfraRed/irspec1.htm

admin answers:

When hydrogen is attached to N, O, or F, it forms a bond with an atom of N, O, or F on another molecule, either a molecule of the same compound or another compound that meets the criteria for H-bonding. As for the differences in exothermicity, there is undoubtedly more going on than just H-bonding to cause this.

admin answers:

Hydrophobic exclusion.
Water excludes hydrophobic substances that tend to clump or float above the waters surface tension.
Oils are liquids so float together above the water that is arranging itself so a minimal number of hydrogen bonds face the interface with the oil and a maximum number of water molecules align their polar regions.

The inherent molecular nature of water makes it cohesive creating a fluid. Water molecules are polar, the charge balance is asymmetric, with the midpoint slightly negative (where the electron cloud is thicker around the oxygen nucleus) while either end is slightly positive with a thinner cloud around the two hydrogen nuclei. The molecule’s shape is not linear but a bent vee with the positive ends flexed away from the oxygen’s two filled orbitals. This molecule can then make a ‘hydrogen’ bond with that polarity, also called an electrostatic bond. This intermolecular interaction excludes large nonpolar molecules that do not have induced dipoles to let them dissolve.

All molecules experience some intermolecular attractions because the electrons are in a shifting cloud with the nuclei embedded, they are not not rigid forms. This means the mobile electrons can leave the nuclei relatively exposed, for a moment, permitting a slight attraction to an equally slightly negative cloud of electrons on another nonpolar molecule. They polar molecule induces a dipole in a nonpolar molecule. This lets small symmetric nonpolar O2 & CO2 dissolve into water but not large hydrocarbon molecules.