Hydrocarbons are organic molecules composed exclusively of carbon and hydrogen atoms. These molecules are classified based on the nature of the bonds between their carbon atoms. The arrangement and type of bonds dictate how many hydrogen atoms can be attached, which determines the molecule’s stability and chemical behavior. The fundamental distinction in hydrocarbon classification is based on whether the molecule is considered saturated or unsaturated.
What Defines Saturated and Unsaturated
The terms “saturated” and “unsaturated” refer directly to the bonding capacity of the carbon chain. A saturated hydrocarbon holds the maximum possible number of hydrogen atoms. This arrangement is achieved when every carbon-carbon bond is a single covalent bond.
An unsaturated hydrocarbon is one where the carbon chain is not fully saturated with hydrogen atoms. This occurs because the molecule contains at least one carbon-carbon multiple bond, such as a double or triple bond. These multiple bonds result in fewer hydrogen atoms than its saturated counterpart with the same number of carbon atoms. The presence of multiple bonds allows the molecule to bond with additional atoms, which is the basis for its “unsaturated” designation.
The Characteristics of Alkanes
Alkanes are a class of hydrocarbons where the carbon atoms are connected solely by single covalent bonds, known as sigma bonds. This single-bond framework is the defining structural feature. The shape around each carbon atom in a non-cyclic alkane is tetrahedral, with bond angles of approximately 109.5 degrees.
These molecules adhere to the general chemical formula \(C_n H_{2n+2}\), where ‘n’ represents the number of carbon atoms. Alkanes can exist as straight chains, branched structures (isoalkanes), or in rings (cycloalkanes). However, the core characteristic of only single carbon-carbon bonds remains consistent.
Why Alkanes Are Classified as Saturated
Alkanes are classified as saturated hydrocarbons because their structure meets the chemical definition of saturation. Since every carbon atom is linked by a single bond, there are no double or triple bonds available to accommodate more atoms. This single-bond arrangement ensures the carbon chain is bonded to the maximum possible number of hydrogen atoms.
This structural characteristic results in low chemical reactivity. Alkanes are often referred to by the historical name “paraffins,” meaning “lacking affinity.” They primarily undergo substitution reactions, where a hydrogen atom is replaced by another atom. They do not undergo addition reactions, confirming that the molecule cannot “add” any more hydrogen atoms.
How Unsaturated Hydrocarbons Differ
Unsaturated hydrocarbons are categorized into two main groups based on the type of multiple bond present. Alkenes contain at least one carbon-carbon double bond. Their general formula for non-cyclic structures is \(C_n H_{2n}\), indicating two fewer hydrogen atoms than the corresponding alkane.
Alkynes contain at least one carbon-carbon triple bond. The general formula for non-cyclic alkynes is \(C_n H_{2n-2}\), reflecting the loss of four hydrogen atoms compared to an alkane. The presence of these multiple bonds makes both alkenes and alkynes significantly more reactive than alkanes. They readily undergo addition reactions, where the multiple bond breaks to form new single bonds with incoming atoms.