hydrogen bond

Examples of hydrogen bond in the following topics:

• Hydrogen Bonding

  • A hydrogen bond is a type of dipole-dipole interaction; it is not a true chemical bond.
  • This hydrogen atom is a hydrogen bond donor.
  • Greater electronegativity of the hydrogen bond acceptor will create a stronger hydrogen bond.
  • Hydrogen bonds are shown with dotted lines.
  • Where do hydrogen bonds form?

• Hydrogen Bonding and Van der Waals Forces

  • Hydrogen bonds and van der Waals interactions are two types of weak bonds that are necessary to the basic building blocks of life.
  • Two weak bonds that occur frequently are hydrogen bonds and van der Waals interactions.
  • When polar covalent bonds containing hydrogen form, the hydrogen in that bond has a slightly positive charge because hydrogen’s one electron is pulled more strongly toward the other element and away from the hydrogen.
  • This interaction is called a hydrogen bond.
  • Hydrogen bonds are also responsible for zipping together the DNA double helix.

• Hydrogen Bonding

  • The most powerful intermolecular force influencing neutral (uncharged) molecules is the hydrogen bond.
  • Hydrogen forms polar covalent bonds to more electronegative atoms such as oxygen, and because a hydrogen atom is quite small, the positive end of the bond dipole (the hydrogen) can approach neighboring nucleophilic or basic sites more closely than can other polar bonds.
  • In the following diagram the hydrogen bonds are depicted as magenta dashed lines.
  • The molecule providing a polar hydrogen for a hydrogen bond is called a donor.
  • Also, O–H---O hydrogen bonds are clearly stronger than N–H---N hydrogen bonds, as we see by comparing propanol with the amines.

• Biology: DNA Structure and Replication

  • Hydrogen bonding, brought on by electrostatic interactions, is critical to holding together strands of DNA.
  • Electrostatic interactions, in this case otherwise known as hydrogen bonds, are what hold these bonds together.
  • For adenine and thymine, there are two such hydrogen bonds.
  • These enzymes break the electrostatic hydrogen bonds between the two strands.
  • Describe effect of the DNA replication process on the hydrogen bonds

• Background and Properties

  • The important classes of organic compounds known as alcohols, phenols, ethers, amines and halides consist of alkyl and/or aryl groups bonded to hydroxyl, alkoxyl, amino and halo substituents respectively.
  • As noted earlier, the relatively high boiling point of carboxylic acids is due to extensive hydrogen bonded dimerization.
  • Similar hydrogen bonding occurs between molecules of 1º and 2º-amides (amides having at least one N–H bond), and the first three compounds in the table serve as hydrogen bonding examples.
  • The last nine entries in the above table cannot function as hydrogen bond donors, so hydrogen bonded dimers and aggregates are not possible.
  • Indeed, if hydrogen bonding is not present, the boiling points of comparable sized compounds correlate reasonably well with their dipole moments.

• Covalent Bonds and Other Bonds and Interactions

  • Two types of weak bonds that frequently occur are hydrogen bonds and van der Waals interactions.
  • When polar covalent bonds containing hydrogen are formed, the hydrogen atom in that bond has a slightly positive charge (δ+) because the shared electrons are pulled more strongly toward the other element and away from the hydrogen atom.
  • The weak interaction between the δ+ charge of a hydrogen atom from one molecule and the δ- charge of a more electronegative atom is called a hydrogen bond.
  • For example, hydrogen bonds are responsible for zipping together the DNA double helix.
  • Like hydrogen bonds, van der Waals interactions are weak interactions between molecules.

• Analysis of Molecular Formulas

  • The number of hydrogen atoms that can be bonded to a given number of carbon atoms is limited by the valence of carbon.
  • For compounds of carbon and hydrogen (hydrocarbons) the maximum number of hydrogen atoms that can be bonded to n carbons is 2n + 2 (n is an integer).
  • The number of hydrogen atoms in stable compounds of carbon, hydrogen & oxygen reflects the number of double bonds and rings in their structural formulas.
  • The molecular formula is C4H10 (the maximum number of bonded hydrogens by the 2n + 2 rule).
  • Similarly, the introduction of a double bond entails the loss of two hydrogens, and a triple bond the loss of four hydrogens.

• Bond Energy

  • Bond energies are commonly given in units of kcal/mol or kJ/mol, and are generally called bond dissociation energies when given for specific bonds, or average bond energies when summarized for a given type of bond over many kinds of compounds.
  • First, a single bond between two given atoms is weaker than a double bond, which in turn is weaker than a triple bond.
  • Second, hydrogen forms relatively strong bonds (90 to 110 kcal) to the common elements found in organic compounds (C, N & O).
  • Third, with the exception of carbon and hydrogen, single bonds between atoms of the same element are relatively weak (35 to 64 kcal).
  • Indeed, the fact that carbon forms relatively strong bonds to itself as well as to nitrogen, oxygen and hydrogen is a primary factor accounting for the very large number of stable organic compounds.

• Hydrogenation

  • Addition of hydrogen to a carbon-carbon double bond is called hydrogenation.
  • The overall effect of such an addition is the reductive removal of the double bond functional group.
  • Regioselectivity is not an issue, since the same group (a hydrogen atom) is bonded to each of the double bond carbons.
  • Next, two hydrogens shift from the metal surface to the carbons of the double bond, and the resulting saturated hydrocarbon, which is more weakly adsorbed, leaves the catalyst surface.
  • This is often true, but the hydrogenation catalysts may also cause isomerization of the double bond prior to hydrogen addition, in which case stereoselectivity may be uncertain.

• Hydrogenation

  • First, the unsaturated bond binds to the catalyst, followed by H2 dissociation into atomic hydrogen onto the catalyst.
  • Partial hydrogenation reduces most, but not all, of the carbon-carbon double bonds, making them better for sale and consumption.
  • Incomplete hydrogenation of the double bonds has health implications; some double bonds can isomerize from the cis to the trans state.
  • The process of partial hydrogenation adds hydrogen atoms and reduces the double bonds in the fatty acids, creating a semi-solid vegetable oil at room temperature.
  • Hydrogen can be added across a double bond—such as the olefin in maleic acid shown—by utilizing a catalyst, such as palladium.