The double covalent bonds are chemical bonds in which two atoms share two pairs of electrons. In other words, they are covalent bonds formed by a total of four valence electrons that revolve around two atoms.
Double bonds are very common in organic compounds . For example, all alkenes have a carbon-carbon double bond. Many other functional groups such as aldehydes, ketones, carboxylic acids, and amides also possess double covalent bonds between carbon and oxygen. Still others, such as imines, have C = N double covalent bonds.
Double bonds have many characteristics that distinguish them from other types of covalent bonds such as single bonds and triple bonds. They differ in the shape of the molecules that contain them, in the length and strength of the bond, and other characteristics related to their chemical properties.
Characteristics of the double covalent bond
They are made up of 4 electrons
The reason they are called double bonds is because they contain twice the number of electrons of a single covalent bond, which is formed when two atoms share two valence electrons. This means that every double bond contains 4 electrons.
Of the 4 electrons that form a double bond, 2 of them are found going back and forth between one atom and another, passing through the center of the two. This pair of electrons is called sigma electrons, since they form a sigma (σ) bond.
The other pair of electrons is floating above and below the two atoms, making a kind of sandwich. These electrons are called pi electrons and they form a pi (π) bond.
Together, the sigma bond and the pi bond form the double covalent bond.
They can be polar or apolar
Depending on the electronegativity of the atoms that are linked by means of a double covalent bond, this can be polar or apolar.
For example, a C = C double covalent bond will be an apolar bond, since both atoms are the same and have the same electronegativity. Instead, a C = O double covalent bond will be polar, since oxygen is much more electronegative than carbon.
They are stronger than simple links
Breaking a double bond is more difficult than breaking a single bond between the same two atoms. This is because, to break a double bond, both a sigma bond (which is the same as the single bond) and a pi bond must be broken. Since two bonds must be broken instead of one, the double bond is stronger than the single bond.
Compared to the triple bond, the double bond is weaker and easier to break. The reason is the same as before, only in this case the double bond has fewer bonds to break compared to the triple bond.
They are shorter than simple links
Being stronger bonds than single bonds, double bonds are able to bring the bonded atoms closer than single bonds. For this reason, if a C = C double bond is compared to a CC single bond, the former is always shorter than the latter.
They are longer than triple bonds
Again, since the triple bond is stronger than the double bond, the triple bond is shorter. This implies that double covalent bonds are longer than triple ones.
They are rigid links.
Simple covalent bonds are quite flexible and can also rotate freely. Instead, double bonds are rigid, not flexible, and cannot be rotated, as the pi bond mentioned above would be broken.
They represent unsaturations
A molecule that has a double bond will necessarily have two less bonded hydrogen atoms than it could have. For this reason, a double bond is said to represent unsaturation (each unsaturation corresponds to 2 hydrogens).
They form flat molecules in the shape of a triangle
A carbon atom that forms a double covalent bond and two other single bonds, always forms a flat molecule in which all three bonds are pointing to the ends of a triangle. This shape is called trigonal plane geometry.
Examples of double covalent bonds
C = C bond in ethylene
Ethylene is one of the simplest examples of a double covalent bond, in this case between two carbon atoms.
The molecule is completely flat and it is not possible to rotate the bond in any direction. This double bond is totally apolar, since you are bonding to two equal atoms.
C = O bond in acetone
All ketones contain a carbonyl group, which consists of a carbon atom and an oxygen atom linked by a double covalent bond.
Each atom contributes two unpaired electrons to form both the pi bond and the sigma bond that make up the double bond. Most of the chemical reactions that occur to ketones are due to the C = O double bond.
The O = O bond in the ozone molecule (O 3 )
In this case, the central oxygen atom is bonded to two other oxygen atoms. One of the two bonds is made up of 4 electrons so it corresponds to a double covalent bond. The three atoms and one unshared pair of electrons in the central oxygen atom are all in the same plane on the screen pointing to the corners of a triangle.
Ozone has the peculiarity that the double bond can “jump” from one of the terminal oxygens to the other. In fact, it does so very quickly through a process called resonance.
C = C bonds in butadiene (C 4 H 6 )
Butadiene is an example of an organic compound that possesses two C = C double covalent bonds.
When double bonds are in this form, separated by a single single bond, they are said to be conjugated double bonds.