Double Bonds

A double bond is a chemical bond in which two pairs of electrons are shared between two atoms. This type of bond involves four bond electrons between atoms instead of the two bond electrons usually involved in single bonds. Double bonds tend to be reactive due to a large number of electrons. Double bonds are physically shorter and comparatively stronger than single bonds. Double bonds are drawn as two parallel lines in a chemical structure diagram. The equal sign is used to represent double bonds in formulas. Russian chemist Alexander Butlerov introduced double bonds into structural formulae in the mid-19th century. This article gives an overview of the double bond.

Definition of a double bond

It’s no secret that the more you share, the stronger you become. At least if you’re an atom and share electrons, that’s true! Atoms can share up to three sets of electrons. A double bond is formed when two atoms share two pairs of electrons.

Electrons are always shared in pairs. Each shared electron pair is called a covalent bond. Two shared electrons correspond to a covalent bond. Two sets of two shared electrons equal one covalent double bond. A total of four electrons are shared in the double bond, represented by two parallel lines.

Since the two atoms in a double bond share four electrons, more energy is required to separate the bonded atoms than that required for atoms bonded by a single bond. The energy required to break a double bond between oxygen atoms is three times the energy required to break a single bond between oxygen atoms. The same goes for nitrogen. The mechanism that makes a double bond stronger also makes it shorter. The average bond length of a double-bonded carbon to another carbon atom is 13% less than the average bond length of two single-bonded carbons.

Double bond formation

A double bond is formed when two atoms must share four electrons to obey the octet rule. According to the octet rule, atoms will lose, gain, or share electrons to have eight valence electrons. Double bonds are often formed in nature and are usually made of non-metals such as carbon, nitrogen, and oxygen. To understand how a double bond is formed, let’s look at what happens to the electrons, the building blocks of the bond.

Electrons move erratically and rapidly around atoms. Depending on the electron’s energy level and proximity to the nucleus, the electron will remain in a certain form. The specific shapes that electrons occupy are called electron orbitals. There are four forms of electron orbitals: s, p, d, and f. Double bonds involve the s and p orbitals.

Some atoms can give up more electrons to form double bonds, while others can give up less. Whatever the atom brings, the atom must move around its available electrons to allow sharing of the four electrons.

Each atom must first merge its outermost s orbital with a p orbital to form the sp orbital. This hybrid orbital will contain two electrons. The sp orbitals of each atom will overlap, so each atom has access to both electrons. Such bonds are called sigma bonds. Each atom will then share two electrons with the other atom using one of its two remaining p orbitals. Electrons that are shared in overlapping p orbitals are called pi bonds. The double bond exhibits one σ bond and one π bond.

Example of a double bond

Ethylene (C2H4) is a hydrocarbon with a double bond between two carbon atoms. Other olefins also contain double bonds. Double bonds are observed in imines (C=N), sulfoxides (S=O), and azo compounds (N=N).

The differences between single, double, and triple bonds are as follows:

1. Definition:

Single Bond: A single bond occurs by sharing a pair of valence electrons.

Double Bond: Double bond occurs by sharing two pairs of valence electrons.

Triple Bond: A triple bond occurs by sharing three pairs of valence electrons.

2. Reactivity:

Single Bond: Single bond is less reactive.

Double Bond: Double bonds are moderately reactive.

Triple Bond: Three keys are very snappy.

3. Binding length:

Single bond: A single bond has a high bond length.

Double Bond: Double bonds have an average bond length.

Triple bond: The bond strength of a triple bond is low.

4. Designator:

Single Bond: Single keys are represented by a single dash (CC).

Double bond: A double bond is represented by two parallel dashes (C=C).

Triple bond: A triple bond is represented by three parallel dashes (C≡C).

Examples:

1. Single bond: e.g., alkanes such as methane, ethane, propane, butane, etc.

2. Double bonds: such as ethylene, propylene, carbonyls (C=O), azo compounds (N=N), imines (C=N), and sulfoxides (S=O).

3. Triple bonds: such as nitrogen (N≡N), cyanide ions (C≡N), acetylene (CH≡CH), and carbon monoxide (C≡O).

Conclusion

A covalent bond in which two electron pairs, rather than the usual electron pair, are shared between two atoms, most commonly between carbon and carbon, oxygen, or nitrogen atoms, but several other forms are known. All double bonds, regardless of the atoms connecting them, will consist of one sigma bond and one pi bond. This orbital view of double bonds is only really important at this level in organic compounds. If you want to learn more about it, click on the first link below, which will take you to a menu with a section dedicated to organic links. You’ll find repeated descriptions of ethylene, but you’ll also find information about bonds in benzene and carbon-oxygen double bonds.