Covalent bonds are chemical bonds formed between atoms in molecules that share electrons. In organic chemistry, covalent bonds take place between carbon atoms (inorganic) and atoms containing at least one oxygen atom (oxygenated). Carbon-based compounds have many different types of covalent bond formations, including double, single, triple, and multiple bonds.
The covalent compounds are volatile, generally insoluble in water but soluble in organic solvents. They possess low melting and boiling points. Their solutions do not conduct electricity.
Types of Covalent Bonds
- The simplest type of covalent bond is the double bond. A double bond occurs when two single bonds connect to each other. Double bonds are classified according to their configuration. One end of the double bond is connected to a hydrogen atom, while the other end is connected to a carbon atom. There are three possible configurations of double bonds, referred to as cis-, trans-, or skew configurations. The prefixes cis- and trans-indicate whether the ends of the double bond lie side by side or opposite side with respect to each other. The skew configuration refers to the position of the double bond relative to the plane of the molecule. Skew configurations are not allowed in hydrocarbon chains.
- Single bonds occur when only one electron pair is shared by two atoms. Single bonds are classified based on the number of lone pairs of electrons they contain. Atomic orbital theory classifies bonding patterns based on the number of shared pairs of electrons. The four major bonding patterns are called sp2, sp3, s3, and d⁰. Sp2 means two lone pairs of electrons in the same spatial arrangement; sp3 means three lone pairs with a maximum overlap; s3 means four lone pairs, and d⁰ means five lone pairs. Bonding patterns are commonly represented by the Greek alphabet. Thus, sp2 represents a pattern where the first letter of the Greek alphabet corresponds to its atomic orbitals.
- Triple bonds are formed when three atoms share electrons. Due to increased electronegativity, triple bonds require a greater degree of ionicity than singles and doubles. Triples are often depicted by using the Latin alphabet and representing each atom’s highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). HOMOs and LUMOs are the two lobes of the valence and conduction bands, respectively, which correspond to the positive and negative poles of an electric potential. Electronegative atoms (such as fluorine) tend to occupy the lower lobe of the band structure. Hydrophilic molecules tend to have a higher percentage of the lower lobe.
- Quadruple bonds are formed when four atoms share electrons. Quadruple bonds are rare because quadruple bonds are more reactive than triples and therefore difficult to stabilize. However, some molecules may have quadruple bonds between nonbonding electrons.
Molecules composed entirely of single bonds are known as hydrocarbons, and those composed of double bonds are called olefins. Olefins are subdivided into monoenes and dienes depending on the location of double bonds, and alkenes if any double bonds alternate between carbons. Alkenes are further divided into monoalkenes and dialkenes depending on the number of carbons separating double bonds. Monoalkenols and monoaldehydes are derived from alkenes by adding alcohol or water, and dialkenes are oxidized to produce ketones and carboxylic acids.
Aromaticity is the tendency for an aromatic ring system to exhibit delocalization of pi electrons over the ring and neighboring groups. The extent of this extension determines the stability of the resulting pi system. The degree of aromatic stabilization increases with the number of adjacent pi bonds. The term “aromatic” is defined by resonance, i.e., the formation of stable structures between two or more rings. Aromatics are sometimes called heterocycles.
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Bonded systems are compounds held together by covalent bonds. The largest bonded system is carbon dioxide, which consists of two oxygen atoms and one carbon atom with four covalent bonding systems. Other examples of polyatomic bonded systems include silicon tetrahedrons, sodium chloride crystals, and the benzene ring.