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As in a molecule where a hydrogen is attached to nitrogen, oxygen, or fluorine, the electronegative atom attracts the electron cloud from around the hydrogen nucleus and, by decentralizing the cloud, leaves the hydrogen atom with a positive partial charge.
Hydrogen bonds occur in inorganic molecules, such as water, and organic molecules, such as DNA and proteins. In biology, intramolecular hydrogen bonding is partly responsible for the secondary, tertiary, and quaternary structures of proteins and nucleic acids. The hydrogen bonds help the proteins and nucleic acids form and maintain specific shapes.
Boundless vets and curates high-quality, openly licensed content from around the Internet. This particular resource used the following sources:. Skip to main content. Liquids and Solids. Quick links. Email Link. Why are hydrogen bonds so strong relative to other dipole-dipole bonds? For example, why are dipole-dipole bonds involving H so much stronger compared to dipole-dipole bonds involving an element that is even less electronegative, such as Si, or approximately the same electronegativity, such as P?
Why would hydrogen bonds not form between an H atom and a Cl atom? When an ionic substance dissolves in water, water molecules cluster around the separated ions. This process is called hydration.
Water frequently attaches to positive ions by co-ordinate dative covalent bonds. It bonds to negative ions using hydrogen bonds. If you are interested in the bonding in hydrated positive ions, you could follow this link to co-ordinate dative covalent bonding. The diagram shows the potential hydrogen bonds formed with a chloride ion, Cl-.
Although the lone pairs in the chloride ion are at the 3-level and would not normally be active enough to form hydrogen bonds, they are made more attractive by the full negative charge on the chlorine in this case.
However complicated the negative ion, there will always be lone pairs that the hydrogen atoms from the water molecules can hydrogen bond to.
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.
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