Pentane Formula: Structure, Chemical Formula, Properties

Pentane Formula: Pentane is the fifth homologue of the alkane series. It is a gasoline odour, easily liquefied gas with a chemical formula of \({{\rm{C}}_5}{{\rm{H}}_{12}}.\) It is typically used to create a polystyrene foam; a foam used to make insulation materials for refrigerators and heating pipes.

Pentane Chemical Formula

Pentane, also known as n-pentane, is composed of carbon and hydrogen atoms only, so it is classified as a hydrocarbon. It is a five-carbon atom containing saturated hydrocarbons consisting only of sigma bonds. The chemical formula of Pentane is \({{\rm{C}}_5}{{\rm{H}}_{12}}\) and its chemical structure is shown below.

Pentane Molar Mass

The molar mass of Pentane, \({{\rm{C}}_{\rm{5}}}{{\rm{H}}_{{\rm{12}}}} = 5\) (Atomic mass of carbon) \(+ 12\) (Atomic mass of hydrogen)
\( = \;5\;\left( {12.01\;} \right) + 12\left( {1.007} \right) = 72.15\;\,{\rm{g/mol}}\)
Hence, one mole of Pentane weighs \(72.15\) grams.

Pentane Hybridisation

Hybridisation is the process in which new hybrid orbitals are formed by mixing up atomic orbitals. These hybrid orbitals are generally of lower energy and suitable for the pairing of electrons to form chemical bonds. In Pentane, each carbon atom is \(s{p^3}\) hybridised.

The ground state of each carbon atom in a pentane molecule has two electrons in its \(2{\rm{s}}\) orbital and \(1\) electron each in \(2{\rm{Px}}\) and \(2{\rm{Py}}\) orbital. The \(2{\rm{Pz}}\) orbital is empty. In its excited state, one paired electron from the \(2{\rm{s}}\) orbital jumps to occupy the empty \(2{\rm{Pz}}\) orbital.
Hence, there are four orbitals, \({\rm{2s}},{\rm{2Px}},{\rm{2Py}},\) and \({\rm{2Pz}},\) in each carbon atom that overlaps and form singly paired hybrid orbitals. These hybrid orbitals readily accept electrons from other atoms and form a sigma bond.

Hence, in Pentane (five carbon atoms), there are twenty \({\rm{s}}{{\rm{p}}^3}\) hybridised orbitals. Out of these twenty orbitals, twelve orbitals are involved in \(\left( {{\rm{s}}{{\rm{p}}^3} – {\rm{s}}} \right)\) sigma bonding with the hydrogen atom. The remaining eight hybridised orbitals overlap with each other to form four \({\rm{C – C}}\) sigma bonds. This is diagrammatically represented as below-

Pentane Molecular Geometry

In Pentane \({{\rm{C}}_{\rm{5}}}{{\rm{H}}_{12}},\) all the carbon atoms are \({\rm{s}}{{\rm{p}}^3}\) hybridised. Hence, the geometry of the molecule is tetrahedral with a bond angle of approx \({109.5^ \circ }.\)
The five carbons are bonded together through sigma bonds, and each carbon atom is bonded to three hydrogens at the terminal and with two hydrogens in the middle. Each \({\rm{H – C – H}}\) angle and \({\rm{H – C – C}}\) angle is approximately \({109.5^ \circ }.\)

Pentane Dipole Moment

Pentane is a non-polar molecule because there is no electronegativity difference between the four \({\rm{C – C}}\) bonds, and the difference in electronegativity between \({\rm{C}}\) and \({\rm{H}}\) is less than \(0.4.\) There are no polar bonds present in Pentane. Hence, Pentane is non-polar.

Pentane Lewis Structure

Lewis structures or electron dot structures are diagrams that represent the distribution of valence electrons of atoms within a molecule. Each dot represents an electron, and a pair of dots between chemical symbols for atoms represents a bond.
Total number of valence electrons \( = 5\) (Valence electrons of carbon) \( + 12\) (Valence electron of hydrogen) \( = \;5\left( 4 \right) + 12\left( 1 \right) = 32\) valence electrons.
These \(32\) valence electrons are distributed as follows.

In the diagram above, the octet and duplet of carbon and hydrogen atoms are satisfied, respectively. Pentane is a saturated hydrocarbon with five carbon atoms and \(12\) hydrogen atoms connected through single covalent bonds.

Pentane Skeletal Structure

Pentane consists of four \({\rm{C – C}}\) bonds and twelve \({\rm{C – H}}\) bonds. All of the bonds in a pentane molecule are single bonds. For this reason, Pentane is classified as an alkane. An alkane is a chemical compound that consists of hydrogen and carbon atoms and is only made of single bonds.

Pentane Isomerism

Pentane is the second smallest alkane hydrocarbon that has isomers after butane. Isomers are molecules with the same molecular formula but different structural arrangements.
The five carbon atoms of Pentane can arrange in three different manners and result in three structural isomers. These are-
1. \({\rm{n}}\)-pentane- It is a straight-chain and the simplest isomer of Pentane.
2. Iso-pentane- It is a branched-chain structure with the IUPAC name \(2\)-methyl butane. In this isomer of Pentane. one carbon atom is connected to four other carbon atoms.
3. Neo-pentane- It is a branched-chain structure with the IUPAC name \(2,2\)-dimethylpropane. In this isomer of Pentane, the central carbon atom is connected to four other carbon atoms.

Properties of Isomers

Isomers have different structural arrangements; hence, they have different properties. Generally, branched-chain isomers have lower boiling and melting points than straight-chain isomers. The more branching there is, the lower the boiling and melting points are. The boiling point of n-pentane is \({\rm{36}}{\,^ \circ }{\rm{C}},\) whereas the boiling point of iso-pentane is \({\rm{28}}{\,^ \circ }{\rm{C}},\) and the boiling point of neopentane is \({\rm{10}}{\,^ \circ }{\rm{C}}.\)

Three-dimensional Representation of Pentane

The three-dimensional structure of an organic compound is represented by the Wedge-dash method. The \(3 – {\rm{D}}\) structure of Pentane is represented by-
1. A solid wedge: Used for a bond that protrudes out of the plane of paper towards the viewer.
2. A dashed wedge: Used for a bond that projects away from the viewer or into the plane of the paper.
3. A solid line: Used for a bond that lies in the plane of the paper.

Considering the above factors, Pentane can be represented as follows-

In Pentane, the different spatial arrangements of hydrogen atoms are readily interconvertible by rotation about \({\rm{C}} – {\rm{C}}\) single bonds resulting in Conformational isomerism. The different structures, known as conformers, are readily interconvertible and thus nonseparable. Conformers are the exact same molecule that differs only in the rotation of one or more sigma bonds.
To better visualise these different conformations, it is convenient to use a drawing convention called the Newman projection. In a Newman projection, we look lengthwise down a specific bond of interest. We depict the ‘front’ atom as a dot and the ‘back’ atom as a larger circle.

Conformers of Pentane represented through Newman projection are shown below.

In Pentane, there are two rotating carbon-carbon bonds, \({\rm{C2 – C3}}\) and \({\rm{C4 – C5}}\) but we will focus on the middle bond between \({\rm{C2}}\) and \({\rm{C3}}.\)
The rotation around the \({\rm{C2 – C3}}\) bond results in three staggered conformers (\({\rm{B,D}}\) and \({\rm{F}}\)) and three eclipsed conformers (\({\rm{A,C}}\) and \({\rm{E}}\)), shown below-

The angle between the sigma bond on the front carbon and the sigma bond on the back carbon is called the dihedral angle.
At a \({0^ \circ }\) dihedral angle, the \({\rm{C}}{{\rm{H}}_{3\;}}\) groups at \({\rm{C}}1\) and \({{\rm{C}}_{\rm{2}}}{{\rm{H}}_{{\rm{5\;}}}}\)  group at \({\rm{C3}}\) position are in eclipsed position(A).
At \(60^\circ \) dihedral angle (clockwise), the pentane molecule is now in a staggered conformation, Gauche(B).
At \(120^\circ \) dihedral angle (clockwise), the pentane molecule is now in a second eclipsed conformation (C) in which both ethyl and methyl groups are lined up with hydrogen atoms.
At \(180^\circ \) dihedral angle (clockwise), the pentane molecule is now in a second staggered conformation (D) called the anti conformation, where the ethyl and methyl groups are positioned opposite each other.
At \(240^\circ \) dihedral angle (clockwise), the pentane molecule is now in a third eclipsed conformation (E) in which both ethyl and methyl groups are lined up with hydrogen atoms.
At \(300^\circ \) dihedral angle (clockwise), the pentane molecule is now in a third staggered conformation (F) called the gauche conformation.

The staggered conformations of pentane form energy ‘valleys’, and the eclipsed conformations are energy ‘peaks’.
1. The gauche conformation is a higher energy valley than the anti conformation due to steric strain. This is due to the repulsive interaction caused by the two bulky ethyl and methyl groups being forced too close together. However, steric strain is lower in the anti conformation (bulky groups are oppositely placed).
2. Steric strain causes the eclipsed A conformation – where the ethyl and methyl groups are as close together as they can possibly be – to be higher in energy.

The diagram below summarises the relative energies for the various eclipsed, staggered, and gauche conformations.
When the rotation around \({\rm{C3}}\) and \({\rm{C4}}\) takes place, the following conformations are observed. The conformers shown below are the two extreme cases of Staggered and Eclipsed conformations.

Sawhorse Projection

In a sawhorse projection, the backbone carbons are represented by a diagonal line, and the terminal carbons are shown in groups, just as in the Fischer projection. A sawhorse projection can reveal staggered and eclipsed conformations in molecules. Below are the three Sawhorse Projections of Pentane.

Sawhorse projection also helps in explaining the stability of conformations. In the staggered form of Pentane, the electron clouds of ethyl and methyl groups are as far apart as possible. Thus, there are minimum repulsive forces, minimum energy and maximum stability of the molecule.
On the other hand, in the eclipsed form, the electron clouds of ethyl and methyl groups are in close proximity resulting in an increase in electron cloud repulsion.
Hence, the Staggered form is more stable.

Preparation of Pentane

Pentanes are saturated hydrocarbons that have five carbon atoms. n-Pentane and iso-Pentane occur naturally in crude oil and as a by-product of natural gas production. Cyclopentane, a ring-shaped molecule, is found more in naphtha (crude petrol), which is obtained in refineries from so-called cracking processes.
1. By hydrogenation of alkenes in the presence of catalysts such as Pt, Pd.
In addition of hydrogen to an alkene (both \(1\)-pentene and \(2\)-pentene), the double bond of the compound breaks and corresponding alkane is obtained.

Physical Properties of Pentane

Chemical Properties of Pentane

1. Pentane related compounds may be formed by replacing a hydrogen atom with another functional group. For example, replacing a hydrogen atom with a hydroxyl group yields pentanol \(\left( {{\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{OH}}} \right).\)
2. Like other hydrocarbons, Pentane undergoes complete combustion and produces carbon dioxide and water. It is an exothermic reaction \({{\rm{C}}_{\rm{5}}}{{\rm{H}}_{{\rm{12}}}}{\rm{\; + \;8}}{{\rm{O}}_{\rm{2}}}{\rm{\;}} \to {\rm{\;5C}}{{\rm{O}}_{\rm{2}}}{\rm{ + \;6}}{{\rm{H}}_{\rm{2}}}{\rm{O + Energy}}\)
3. Combustion may also occur in a limited supply of oxygen, forming carbon monoxide.
4. Pentane reacts with halogens, especially chlorine and bromine, by radical halogenation. Such reactions are unselective, yielding a mixture of the \(1 – ,{\rm{ }}2 – ,\) and \(3-\)chloropentanes, as well as more highly chlorinated derivatives. \({{\rm{C}}_{\rm{5}}}{{\rm{H}}_{{\rm{12}}}}{\rm{\; + \;C}}{{\rm{l}}_{\rm{2}}}{\rm{\;}} \to \;{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{{\rm{11}}}}{\rm{Cl + \;HCl}}\)

Pentane Uses

1. Pentane is used as a blowing agent in the production of polystyrene foam.
   Cosmetics and care products-
2. Very pure, aromatic-free iso-pentane is used in shaving cream or shower gel. It readily evaporates in body temperature and makes the products foam up very finely and creamy.
   Geothermal plants –
3. They are an essential contribution to renewable energy supply. Iso-pentane can be used as a process medium in heat pumps.
4. Process medium in the production of polyethylene (PE) and polypropylene (PP)
5. Liquid iso-pentane is used as a cooling agent in the production of Polyethene(PE), the most widely used plastic in the world. It can also be used as a process medium for Polypropylene (PP), which is produced in the same way in the gas phase.
   Cooling agent
6. n-Pentane can be used as a cooling agent \(\left( {{\rm{R}}601} \right)\) in air conditioning or refrigeration systems.
7. Extruded Polystyrene (XPS) is produced from polymer granulate and Pentane as blowing agents. This is used in the packaging industry, for example, as the well-known packaging chips.

Summary

Due to their physical properties, the isomers of Pentane have a wide variety of applications. The three isomers can be blended with each other or with other chemical components to increase performance, quality or cost-efficiency. Hence, it is essential to learn about their structure and related properties. In this article, we learned the formula, chemical structure and properties of Pentane. We also learnt about its isomers and conformations due to carbon-carbon bond rotation.

FAQs on Pentane Formula

The most commonly asked questions on Pentane formula are answered here:

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