Please enable JavaScript to access the full features of the site or access our non-JavaScript page. Issue 1, From the journal: Chemical Society Reviews. Very strong hydrogen bonding. The first page of this article is displayed as the abstract. You have access to this article. Please wait while we load your content In the liquid state they are rapidly being formed and broken as the mobile particles move over each other.
The closeness of the bond length indicates that the intramolecular bond is very strong, and of comparable magnitude to the intramolecular one. On a macroscopic scale this is obvious to anyone who falls on ice frozen water and quickly realizes how hard it is, and how strong the bonds are that hold the water molecules to each other. Note the similarity in length between the intermolecular OH bond, and the intramolecular O-H bond. Water also has two lone pairs and two H atoms attached to the highly electronegative oxygen.
This means each water molecule can participate in up to 4 bonds two where it is the h-bond acceptor, and two where it is the h-bond donor. One interesting consequence of this is that water forms a 3D crystalline structure that is sort of based on a distorted tetrahedron. That is, the oxygen is sp 3 hybridized with a tetrahedral electronic geometry, having two bonding orbitals and two lone pairs.
All of these are involved with hydrogen bonds. The lone pairs are functioning as H-bond acceptors, and the hydrogen on the bonding orbitals are functioning as h-bond donors. So each oxygen is attached to 4 hydrogens, two are 1. The fact that ice floats has great ramifications for life. The ice, with it's void space, acts as an insulator. If the ice sunk to the bottom, lakes would completely freeze and aquatic life like fish would not be able to survive the winters.
Boiling points are an indicator of intermolecular forces, and we will look at the phenomena of boiling in more detail in a later section of this chapter. We also learned that there is a velocity profile with different molecules moving at different speeds, but that heavier molecules tend to move slower than lighter ones remember macroscopic observables like liquids and boiling are the result of the interactions of a huge number of molecules which possess a distribution of energies.
So all things equal, we would anticipate that it is easier to boil a lighter molecule than a heavier one, and we would predict the heavier one to have a higher boiling point. You should also revisit this figure after we have covered the section on boiling. Any molecule which has a hydrogen atom attached directly to an oxygen or a nitrogen is capable of hydrogen bonding.
Hydrogen bonds also occur when hydrogen is bonded to fluorine, but the HF group does not appear in other molecules. Molecules with hydrogen bonds will always have higher boiling points than similarly sized molecules which don't have an an -O-H or an -N-H group. The hydrogen bonding makes the molecules "stickier," such that more heat energy is required to separate them.
This phenomenon can be used to analyze boiling point of different molecules, defined as the temperate at which a phase change from liquid to gas occurs. They have the same number of electrons, and a similar length. The van der Waals attractions both dispersion forces and dipole-dipole attractions in each will be similar. However, ethanol has a hydrogen atom attached directly to an oxygen; here the oxygen still has two lone pairs like a water molecule. Hydrogen bonding can occur between ethanol molecules, although not as effectively as in water.
Except in some rather unusual cases, the hydrogen atom has to be attached directly to the very electronegative element for hydrogen bonding to occur. The boiling points of ethanol and methoxymethane show the dramatic effect that the hydrogen bonding has on the stickiness of the ethanol molecules:. It is important to realize that hydrogen bonding exists in addition to van der Waals attractions.
For example, all the following molecules contain the same number of electrons, and the first two have similar chain lengths. The higher boiling point of the butanol is due to the additional hydrogen bonding. Comparing the two alcohols containing -OH groups , both boiling points are high because of the additional hydrogen bonding; however, the values are not the same.
The boiling point of the 2-methylpropanol isn't as high as the butanol because the branching in the molecule makes the van der Waals attractions less effective than in the longer butanol. Hydrogen bonding also occurs in organic molecules containing N-H groups; recall the hydrogen bonds that occur with ammonia. The two strands of the famous double helix in DNA are held together by hydrogen bonds between hydrogen atoms attached to nitrogen on one strand, and lone pairs on another nitrogen or an oxygen on the other one.
In order for a hydrogen bond to occur there must be both a hydrogen donor and an acceptor present. The donor in a hydrogen bond is usually a strongly electronegative atom such as N, O, or F that is covalently bonded to a hydrogen bond. The hydrogen acceptor is an electronegative atom of a neighboring molecule or ion that contains a lone pair that participates in the hydrogen bond.
Since the hydrogen donor N, O, or F is strongly electronegative, it pulls the covalently bonded electron pair closer to its nucleus, and away from the hydrogen atom. The hydrogen atom is then left with a partial positive charge, creating a dipole-dipole attraction between the hydrogen atom bonded to the donor and the lone electron pair of the acceptor. This results in a hydrogen bond. Although hydrogen bonds are well-known as a type of IMF, these bonds can also occur within a single molecule, between two identical molecules, or between two dissimilar molecules.
Intramolecular hydrogen bonds are those which occur within one single molecule. This occurs when two functional groups of a molecule can form hydrogen bonds with each other. In order for this to happen, both a hydrogen donor a hydrogen acceptor must be present within one molecule, and they must be within close proximity of each other in the molecule.
For example, intramolecular hydrogen bonding occurs in ethylene glycol C 2 H 4 OH 2 between its two hydroxyl groups due to the molecular geometry.
Intermolecular hydrogen bonds occur between separate molecules in a substance. They can occur between any number of like or unlike molecules as long as hydrogen donors and acceptors are present in positions where they can interact with one another. I could be wrong but I remember hearing that, in a Hydrogen bond, the electrons are also attracted much closer to the nucleus due to less orbitals in the atom.
This might apply to something else though so I'm not sure! Post by Khoa Vu 3l » Sun Nov 22, am Hydrogen bonding is so strong among dipole-dipole interactions because it itself is a dipole-dipole interaction with one of the strongest possible electrostatic attractions.
Remember that hydrogen bonding cannot occur unless hydrogen is covalently bonded to either oxygen, nitrogen, or fluorine. This is because by covalently bonding to these elements, hydrogen will then bear the largest possible positive partial charge given that oxygen, nitrogen, and fluorine are the most electronegative and, therefore, become some of the most partially negatively charged atoms.
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