bond strength

Examples of bond strength in the following topics:

• Alcohol Reactions

  • Unlike the alkyl halides, this group has two reactive covalent bonds, the C–O bond and the O–H bond.
  • Consequently, the covalent bonds of this functional group are polarized so that oxygen is electron rich and both carbon and hydrogen are electrophilic, as shown in the drawing below.
  • Indeed, the dipolar nature of the O–H bond is such that alcohols are much stronger acids than alkanes (by roughly 1030 times), and nearly that much stronger than ethers (oxygen substituted alkanes that do not have an O–H group).
  • The most reactive site in an alcohol molecule is the hydroxyl group, despite the fact that the O–H bond strength is significantly greater than that of the C–C, C–H and C–O bonds, demonstrating again the difference between thermodynamic and chemical stability.

• Physical Properties of Covalent Molecules

  • The Lewis bonding theory can explain many properties of compounds.
  • Lewis theory also accounts for bond length; the stronger the bond and the more electrons shared, the shorter the bond length is.
  • According to the theory, triple bonds are stronger than double bonds, and double bonds are stronger than single bonds.
  • However, the theory implies that the bond strength of double bonds is twice that of single bonds, which is not true.
  • Discuss the qualitative predictions of covalent bond theory on the boiling and melting points, bond length and strength, and conductivity of molecules

• Bond Energy

  • Bond energy is the measure of bond strength.
  • Bond energy is a measure of a chemical bond's strength, meaning that it tells us how likely a pair of atoms is to remain bonded in the presence of energy perturbations.
  • The higher the bond energy, the 'stronger' we say the bond is between the two atoms, and the distance between them (bond length) is smaller.
  • The bond energy is the average of the bond dissociation energies in a molecule.
  • Identify the relationship between bond energy and strength of chemical bonds

• Alkyl Halide Reactions

  • The functional group of alkyl halides is a carbon-halogen bond, the common halogens being fluorine, chlorine, bromine and iodine.
  • The first of these is covalent bond strength.
  • The strongest of the carbon-halogen covalent bonds is that to fluorine.
  • Remarkably, this is the strongest common single bond to carbon, being roughly 30 kcal/mole stronger than a carbon-carbon bond and about 15 kcal/mole stronger than a carbon-hydrogen bond.
  • The carbon-chlorine covalent bond is slightly weaker than a carbon-carbon bond, and the bonds to the other halogens are weaker still, the bond to iodine being about 33% weaker.

• Bond Enthalpy

  • Bond enthalpy is defined as the enthalpy change when a covalent bond is cleaved by homolysis.
  • The strength of bonds between different atoms varies across the periodic table and is well documented.
  • Each bond in a molecule has its own bond dissociation energy, so a molecule with four bonds will require more energy to break the bonds than a molecule with one bond.
  • As each successive bond is broken, the bond dissociation energy required for the other bonds changes slightly.
  • It is evident that bond strength varies significantly for different combinations of elements in the periodic table.

• Bond Order

  • Bond order is the number of chemical bonds between a pair of atoms.
  • Bond order is the number of chemical bonds between a pair of atoms; in diatomic nitrogen (N≡N) for example, the bond order is 3, while in acetylene (H−C≡C−H), the bond order between the two carbon atoms is 3 and the C−H bond order is 1.
  • Bond order indicates the stability of a bond.
  • Bond order is also an index of bond strength, and it is used extensively in valence bond theory.
  • For a bond to be stable, the bond order must be a positive value.

• Bond Energy

  • Since reactions of organic compounds involve the making and breaking of bonds, the strength of bonds, or their resistance to breaking, becomes an important consideration.
  • Bond energy is the energy required to break a covalent bond homolytically (into neutral fragments).
  • 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.
  • The following table is a collection of average bond energies for a variety of common bonds.
  • First, a single bond between two given atoms is weaker than a double bond, which in turn is weaker than a triple bond.

• Double and Triple Covalent Bonds

  • The double bond between the two carbon atoms consists of a sigma bond and a π bond.
  • A triple bond involves the sharing of six electrons, with a sigma bond and two $\pi$ bonds.
  • Triple bonds are stronger than double bonds due to the the presence of two $\pi$ bonds rather than one.
  • Experiments have shown that double bonds are stronger than single bonds, and triple bonds are stronger than double bonds.
  • Double bonds have shorter distances than single bonds, and triple bonds are shorter than double bonds.

• The Phase of Orbitals

  • A bond involving molecular orbitals that are symmetric with respect to rotation around the bond axis is called a sigma bond (σ-bond).
  • If the phase changes, the bond becomes a pi bond (π-bond).
  • For a π-bond, corresponding bonding and antibonding orbitals would not have such symmetry around the bond axis and would be designated π and π*, respectively.
  • This makes the π-bond a weaker bond than the original σ-bond that connects two neighboring atoms; however the fact that its strength is added to the underlying σ-bond bond makes for a stronger overall linkage.
  • Although the π-bond is not as strong as the original σ-bond, its strength is added to the existing single bond.

• Hybridization in Molecules Containing Double and Triple Bonds

  • sp2, sp hybridizations, and pi-bonding can be used to describe the chemical bonding in molecules with double and triple bonds.
  • Ethene (C2H4) has a double bond between the carbons.
  • The hydrogen-carbon bonds are all of equal strength and length, which agrees with experimental data.
  • The chemical bonding in acetylene (ethyne) (C2H2) consists of sp-sp overlap between the two carbon atoms forming a sigma bond, as well as two additional pi bonds formed by p-p overlap.
  • In ethene, carbon sp2 hybridizes, because one π (pi) bond is required for the double bond between the carbons, and only three σ bonds form per carbon atom.