double bond

Examples of double bond in the following topics:

• Stereoselectivity in Addition Reactions to Double Bonds

  • As illustrated in the drawing below, the pi-bond fixes the carbon-carbon double bond in a planar configuration, and does not permit free rotation about the double bond itself.
  • We see then that addition reactions to this function might occur in three different ways, depending on the relative orientation of the atoms or groups that add to the carbons of the double bond: (i) they may bond from the same side, (ii) they may bond from opposite sides, or (iii) they may bond randomly from both sides.
  • Since initial electrophilic attack on the double bond may occur equally well from either side, it is in the second step (or stage) of the reaction (bonding of the nucleophile) that stereoselectivity may be imposed.
  • If the two-step mechanism described above is correct, and if the carbocation intermediate is sufficiently long-lived to freely-rotate about the sigma-bond component of the original double bond, we would expect to find random or non-stereoselective addition in the products.

• Cyclization by Intramolecular Addition Reactions

  • If a radical is joined to a double bond by a chain of three or more carbons intramolecular addition generates a ring.
  • The regioselectivity of such additions is governed more by stereoelectronic factors than by substituents on the double bond.
  • As shown in the following diagram, this is at an angle nearly 20º off the perpendicular to the plane of the double bond.
  • Because of this requirement, many cyclizations to moderately sized rings proceed by radical attack at the nearest carbon of the double bond, regardless of substitution.
  • Of course, if the carbon chain tethering the radical site to the double bond is long enough, bonding to either of the double bond carbons accommodates the stereoelectronic factor, and the product is again determined by substitution.

• Configurational Stereoisomers of Alkenes

  • The carbon-carbon double bond is formed between two sp2 hybridized carbons, and consists of two occupied molecular orbitals, a sigma orbital and a pi orbital.
  • Rotation of the end groups of a double bond relative to each other destroys the p-orbital overlap that creates the pi orbital or bond.
  • The essential requirement for this stereoisomerism is that each carbon of the double bond must have two different substituent groups (one may be hydrogen).
  • In the first example, the left-hand double bond carbon has two identical substituents (A) so stereoisomerism about the double bond is not possible (reversing substituents on the right-hand carbon gives the same configuration).
  • In the next two examples, each double bond carbon atom has two different substituent groups and stereoisomerism exists, regardless of whether the two substituents on one carbon are the same as those on the other.

• Double and Triple Covalent Bonds

  • The double bond between the two carbon atoms consists of a sigma bond and a π bond.
  • Triple bonds are stronger than double bonds due to the the presence of two $\pi$ bonds rather than one.
  • Similar to double bonds, no rotation around the triple bond axis is possible.
  • 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.

• Properties of Alkenes

  • Due to the presence of a double bond in their carbon skeletons, alkenes are more reactive than their related alkanes.
  • Alkenes contain a double bond that is composed of one sigma and one pi bond between two carbon atoms.
  • The carbon atoms in the double bond are sp2 hybridized, forming a planar structure.
  • Rotation around the double bond is disfavored, so alkenes form fairly stable isomers depending on the positioning of substituents on the same (cis) or opposite (trans) sides of the double bond.
  • Alkenes are more reactive than their related alkanes due to the relative instability of the double bond.

• Naming Alkenes and Alkynes

  • Alkenes and alkynes are named similarly to alkanes, based on the longest chain that contains the double or triple bond.
  • Alkenes are hydrocarbons that contain one or more double bonds, while alkynes contain one or more triple bonds.
  • Rotation is restricted around the double bond, so prefixes can be added to differentiate stereoisomers.
  • For cycloalkenes, the carbons in the double bond are numbered as positions 1 and 2.
  • For compounds containing both double and triple bonds, the "-ene" suffix precedes the "-yne," and the compound is numbered to minimize the bond positions.

• 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.
  • In this case, sp hybridization leads to two double bonds.
  • The remaining, non-hybridized p-orbitals overlap for the double and triple pi bonds.
  • Describe the role of hybridization in the formation of double and triple bonds.

• 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

  • 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.

• Covalent Bonds

  • Covalent bonding interactions include sigma-bonding (σ) and pi-bonding (π).
  • Single bonds occur when two electrons are shared and are composed of one sigma bond between the two atoms.
  • Double bonds occur when four electrons are shared between the two atoms and consist of one sigma bond and one pi bond.
  • Triple bonds occur when six electrons are shared between the two atoms and consist of one sigma bond and two pi bonds (see later concept for more info about pi and sigma bonds).
  • Unlike an ionic bond, a covalent bond is stronger between two atoms with similar electronegativity.