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

  • 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.

• 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.

• Properties of Hydrogen

  • However, monoatomic hydrogen is rare on Earth is rare due to its propensity to form covalent bonds with most elements.
  • Hydrogen is available in different forms, such as compressed gaseous hydrogen, liquid hydrogen, and slush hydrogen (composed of liquid and solid), as well as solid and metallic forms.
  • When hydrogen bonds with fluorine, oxygen, or nitrogen, it can participate in a form of medium-strength noncovalent (intermolecular) bonding called hydrogen bonding, which is critical to the stability of many biological molecules.
  • Compounds that have hydrogen bonding with metals and metalloids are known as hydrides.
  • Many of the hydrogen atom's chemical properties arise from its small size, such as its propensity to form covalent bonds, flammability, and spontaneous reaction with oxidizing elements.

• 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.

• Hybridization in Molecules Containing Double and Triple Bonds

  • The orbitals are directed toward the four hydrogen atoms, which are located at the vertices of a regular tetrahedron.
  • In ethylene (ethene), the two carbon atoms form a sigma bond by overlapping two sp2 orbitals; each carbon atom forms two covalent bonds with hydrogen by s–sp2 overlapping all with 120° angles.
  • The hydrogen-carbon bonds are all of equal strength and length, which agrees with experimental data.
  • Each carbon also bonds to hydrogen in a sigma s-sp overlap at 180° angles.
  • The sp hybridized orbitals are used to overlap with the 1s hydrogen orbitals and the other carbon atom.

• Binary Hydrides

  • In such hydrides, hydrogen is bonded to a more electropositive element or group.
  • Hydride compounds often do not conform to classical electron-counting rules, but are described as multi-centered bonds with metallic bonding.
  • In these substances, the hydride bond, formally, is a covalent bond much like the bond that is made by a proton in a weak acid.
  • Classical transition metal hydrides feature a single bond between the hydrogen center and the transition metal.
  • Their bonding is generally considered metallic.

• 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.