Question

(a) Why are alkanes said to be saturated? (b) Is ${\mathrm{C}}_4{\mathrm{H}}_6$ a saturated hydrocarbon? Explain.

Short answer

(a) Alkanes are said to be saturated because all the carbon atoms in an alkane molecule are bonded to the maximum number of hydrogen atoms possible, forming only single bonds. No carbon atoms are bonded through double or triple bonds to other carbon atoms in an alkane molecule. (b) ${\mathrm{C}}_4{\mathrm{H}}_6$ is not a saturated hydrocarbon since the number of hydrogen atoms in this molecule is less than the number of hydrogen atoms present in an alkane molecule with the same number of carbon atoms, indicating the presence of double or triple bonds in its molecular structure.

Step-by-step solution

Understanding saturated hydrocarbons

A saturated hydrocarbon is a hydrocarbon where all the carbon atoms are bonded to the maximum number of hydrogen atoms, forming only single bonds. Alkanes are an example of saturated hydrocarbons as they have the general formula ${\mathrm{C}}_n{\mathrm{H}}_{2n+2}$. There are no double or triple bonds between any carbon atoms, meaning all the bonds are single.

(a) Why are alkanes said to be saturated?

Alkanes are said to be saturated because all the carbon atoms in an alkane molecule are bonded to the maximum number of hydrogen atoms possible, forming only single bonds. No carbon atoms are bonded through double or triple bonds to other carbon atoms in an alkane molecule.

(b) Is ${\mathrm{C}}_4{\mathrm{H}}_6$ a saturated hydrocarbon?

To determine if ${\mathrm{C}}_4{\mathrm{H}}_6$ is a saturated hydrocarbon, we will compare the number of hydrogen atoms in this molecule with the number of hydrogen atoms that would be present in an alkane molecule with the same number of carbon atoms. In an alkane molecule with four carbon atoms (the same number of carbon atoms as in ${\mathrm{C}}_4{\mathrm{H}}_6$), the number of hydrogen atoms should be: $$H=2\cdot C+2=2\cdot 4+2=10$$ Since there are only 6 hydrogen atoms in ${\mathrm{C}}_4{\mathrm{H}}_6$, it is not a saturated hydrocarbon. This is because it does not follow the general formula of alkanes with single carbon-carbon bonds, indicating the presence of double or triple bonds in its molecular structure.

Key concepts

Alkanes

Alkanes are a type of hydrocarbon that are fundamental in organic chemistry. They are often known as saturated hydrocarbons, because they contain only single bonds between carbon atoms. This means each carbon atom is connected to the maximum possible number of hydrogen atoms. The formula for alkanes is typically given as ${\mathrm{C}}_n{\mathrm{H}}_{2n+2}$. This denotes that for every $n$ carbon atoms, there are $2n+2$ hydrogen atoms.

Some common examples of alkanes include methane (${\mathrm{CH}}_4$), ethane (${\mathrm{C}}_2{\mathrm{H}}_6$), and propane (${\mathrm{C}}_3{\mathrm{H}}_8$). Each of these molecules strictly contains carbon-carbon single bonds, following the general algebraic formula for alkanes. This inherent structural characteristic makes alkanes less reactive compared to unsaturated hydrocarbons that include double or triple bonds. They are useful as fuels due to their energy content, releasing heat upon combustion.

Hydrocarbon Structure

The structure of hydrocarbons is defined by the arrangement of carbon and hydrogen atoms. In the simplest terms, hydrocarbons consist of hydrogen atoms bonded to carbon atoms forming chains or rings.

The structures can be classified into several types:

• Linear (straight-chain): As in n-butane (${\mathrm{C}}_4{\mathrm{H}}_{10}$), where the carbon atoms are connected in a single, unbranched sequence.
• Branched-chain: Such as isobutane, where branching occurs in the sequence of carbon atoms.
• Cyclic: This involves carbon atoms forming ring structures. Cyclohexane is an example of a cyclic alkane.

These structures determine the physical and chemical properties of hydrocarbons. Alkanes tend to have a simple tetrahedral geometry, which results from each carbon forming four sigma bonds, making them stable under a wide range of environmental conditions.

Chemical Bonding

Chemical bonding in alkanes and other hydrocarbons is primarily defined by covalent bonds, where electron pairs are shared between atoms. Alkanes only have single covalent bonds, specifically called sigma bonds, between their carbon atoms.

The sigma bond forms as a result of the head-on overlapping of atomic orbitals. This type of bond allows for free rotation around the bond axis, contributing to the flexibility and variation in three-dimensional conformations seen in alkanes. Additionally:

• Covalent bonds are strong and require a high amount of energy to break, thus contributing to the stability of alkanes.
• In saturated hydrocarbons, like alkanes, each carbon forms four sigma bonds: one with another carbon and the remaining with hydrogen atoms, maximizing the overlap of orbitals.

Understanding these bonding principles helps explain why alkanes are relatively less reactive than their unsaturated counterparts, which contain double (pi bonds) and triple bonds.