Two weak bonds that occur frequently are hydrogen bonds and van der Waals interactions. Without these two types of bonds, life as we know it would not exist. Hydrogen bonds provide many of the critical, life-sustaining properties of water and also stabilize the structures of proteins and DNA, the building block of cells. Because the hydrogen is slightly positive, it will be attracted to neighboring negative charges.
This interaction is called a hydrogen bond. The views expressed are those of the author s and are not necessarily those of Scientific American.
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Get smart. Sign up for our email newsletter. We know that before , a number of chemical phenomena were observed that led investigators to postulate the existence of weak or secondary bonds involving the interactions between functional groups, in one of which a hydrogen atom is covalently bonded to oxygen or nitrogen atoms.
Unable to display preview. Download preview PDF. Skip to main content. This service is more advanced with JavaScript available. An alcohol is an organic molecule containing an -OH group.
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.
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