Name: how alkane chain length changes properties
Uploaded: 2023-04-30T18:28:32.000Z
Duration: 2 min 33 s
Channel: JoVE

A series of unbranched alkanes in which successive members differ by a &#8722;CH2&#8722; group is called a homologous series, and each alkane in this series is a homologue.
Alkanes are nonpolar due to the small electronegativity difference between carbon and hydrogen, implying that only weak dispersion forces exist between their molecules.
The strength of these forces increases proportionally with carbon chain length. The first four alkanes are gases at room temperature and atmospheric pressure; moderate carbon chains, from C5 to C17, are liquids. Homologues beyond C17 are solids.
Dispersion forces also influence the different physical properties of alkanes.
Within a homologous series, as surface area increases, the dispersion forces between molecules also increase. Because more energy is required to separate the&#160;molecules, the boiling points of unbranched alkanes rise with each additional carbon.
Unlike straight-chain alkanes, branched alkanes are compact and more spherical, which reduces the area of interaction and the strength of the intermolecular forces. Therefore, compared to the unbranched form, branched isomers boil at lower temperatures.
While the boiling points of straight-chain alka

physical properties of alkanes

Physical Properties of Alkanes

Alkanes are nonpolar molecules due to the presence of only carbon and hydrogen atoms. The electronegativity difference between carbon and hydrogen is minimal, and hence alkanes have a zero dipole moment. This leads to the presence of only dispersion forces between the molecules. The strength of dispersion forces is dependent on the surface area of the molecules on which they act. Since the surface area increases with the molecular length for straight-chain alkanes, the dispersion forces also increase across the homologue with the increasing carbon chain length.
The dispersion forces affect the physical properties of the alkanes and vary their physical state. Based on the number of carbon atoms, the straight-chain alkanes exist in different physical states at a given temperature and pressure. Thus, the boiling point for straight-chain alkanes is directly proportional to their chain length and, in turn, is proportional to the dispersion forces. In contrast, the melting point shows an odd-even behavior, i.e., the odd and even membered alkane form a different melting point trend with increasing chain length.
Branched-chain isomers of alkanes show significant variations in their properties due to the differences in their shape and size compared to the straight-chain alkanes.&#160; For instance, in pentane, the melting point varies drastically between the straight-chain and branched forms. The straight-chain n-pentane melts at a temperature of &#8722;129.8&#176;C, whereas the branched form iso-pentane melts at &#8722;161.0&#176;C. Neopentane, the symmetrically branched isomer, melts at a much higher temperature of &#8722;16.5&#176;C.

Alkanes are nonpolar molecules due to the presence of only carbon and hydrogen atoms. The electronegativity difference between carbon and hydrogen is ...

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JoVE Core

Organic Chemistry

Unit 1

Alkanes and Cycloalkanes

KEY TERMS AND CONCEPTS

how alkane chain length changes properties

Alkane properties change with chain length and molecular shape. Alkanes contain only carbon and hydrogen, so they are nonpolar molecules. The electronegativity difference between carbon and hydrogen is very small, and alkanes therefore have a zero dipole moment. As a result, the only forces between alkane molecules are dispersion forces.

Dispersion forces depend on surface area. In straight-chain alkanes, surface area increases as the carbon chain gets longer. That means dispersion forces also increase across the homologous series as chain length rises. These forces help explain the physical state of alkanes at a given temperature and pressure.

Boiling point is directly related to chain length in straight-chain alkanes. Longer chains have stronger dispersion forces and therefore higher boiling points. Melting point, however, does not follow the same simple pattern. It shows odd-even behavior, where odd and even members of the alkane series follow different melting point trends as chain length increases.

Branched-chain isomers can have very different properties from straight-chain alkanes because their shape and size are not the same. In pentane, the melting point changes sharply between the straight-chain and branched forms. n-Pentane melts at -129.8°C, while iso-pentane melts at -161.0°C. Neopentane, which is symmetrically branched, melts at a much higher temperature of -16.5°C.

How Alkane Chain Length Changes Properties

how-alkane-chain-length-changes-properties

structure of alkanes

Alkane Bonding and Formula Patterns

Alkanes are the simplest hydrocarbons, and their structure starts with carbon-carbon bond formation. That idea helped shape organic chemistry, as August Kekulé and Archibald Scott Couper independently proposed carbon chain formation. Hydrocarbons are organic compounds made only of carbon and hydrogen. Based on the bond order between carbon atoms, hydrocarbons are classified as alkanes, alkenes, and alkynes. Alkanes contain sp3 hybrid carbon atoms. These carbons form sigma bonds with other sp3...

constitutional isomers of alkanes

Alkane Branching and Isomer Counts

Alkanes can form constitutional isomers, which are compounds with the same molecular formula but different structural formulas. This idea is called constitutional isomerism. It becomes important for alkanes with four or more carbons, because they can have more than one structure with the same formula. A straight-chain alkane is named with the prefix n, so the linear form of pentane is n-pentane. Branched isomers use prefixes such as iso and neo. Pentane has two branched isomers, iso-pentane...

nomenclature of alkanes

Naming Alkane Chains and Substituents

Alkane names follow IUPAC rules, a system used to name organic compounds in a clear and standard way. These rules became important in the late 19th century as new chemical compounds were discovered at a fast pace. IUPAC, the International Union for Pure and Applied Chemistry, was established in 1919 to set these naming rules. The naming process for alkanes depends on the length of the carbon chain, the number of substituents, and where those substituents are placed. The first step is to find...

Alkane properties change with chain length and molecular shape. Alkanes contain only carbon and hydrogen, so they are nonpolar molecules. The electronegativity difference between carbon and hydrogen is very small, and alkanes therefore have a zero dipole moment. As a result, the only forces between alkane molecules are dispersion forces. Dispersion forces depend on surface area. In straight-chain alkanes, surface area increases as the carbon chain gets longer. That means dispersion forces also...

newman projections

Molecule Rotations in Newman Projections

Newman projections show molecule rotations around a single bond in a clear two-dimensional view. Chemists use several notations to show the three-dimensional shape of a molecule on a flat surface. The dash-wedge formula is one common way to do this. In that notation, dashed wedges show groups behind the plane, solid wedges show groups coming out of the plane, and plain lines show groups in the plane. Organic molecules can rotate around single bonds. This rotation creates many temporary...

conformations of ethane and propane

Ethane Rotation: Staggered vs Eclipsed

Ethane and propane show how rotation around a carbon-carbon single bond changes a molecule’s shape and energy. This internal rotation creates conformers, which are different arrangements of the same molecule. For ethane, the main conformations are staggered and eclipsed. Staggered ethane is the lower-energy, more stable form. In this shape, the C-H bonds on the front carbon sit 60° away from the C-H bonds on the back carbon. This spacing reduces torsional strain, which is the repulsion that...

conformations of butane

Butane’s Anti, Gauche, and Eclipsed Forms

Butane has several conformations, or shapes made by rotation around a carbon-carbon bond. Unlike ethane and propane, which have only two major conformations, butane shows more than two conformers. The lowest-energy form is the staggered anti conformer. In this shape, the two bulky methyl groups sit opposite each other at a dihedral angle of 180°. This spacing reduces steric repulsion, so the anti form is the most stable. Butane also has two other staggered conformations. These are degenerate,...

cycloalkanes

Cycloalkane Rings and Naming Rules

Cycloalkanes are saturated hydrocarbons with carbon atoms arranged in rings. Because they have two fewer hydrogen atoms than the matching open-chain alkane, their general formula is CnH2n. These ring structures are a common topic in organic chemistry. Chemists often draw cycloalkanes with line-angle formulas. In this shortcut, regular polygons stand for the ring, and each side shows a carbon-carbon bond. The number of corners in the polygon matches the number of carbons in the ring. Naming...

conformations of cycloalkanes

Cycloalkane Ring Strain and Shapes

Cycloalkane ring strain depends on ring shape, bond angles, and bond rotation. Adolf von Baeyer first tried to explain the stability of cycloalkanes with angle strain, which is the strain caused when bond angles differ from the ideal 109.5° tetrahedral angle for sp3-hybridized carbons. His idea worked well for some small rings, but not for every cycloalkane. Cyclopropane and cyclobutane are strained, as expected from their compressed bond angles. Cyclopentane is more strained than Baeyer...

conformations of cyclohexane

Cyclohexane Chair, Boat, and Twist-Boat Forms

Cyclohexane adopts non-planar shapes because a flat ring would create high angle strain and torsional strain. The molecule instead bends into chair and boat conformations. These shapes lower the strain and make the ring more stable. The chair conformation is the most stable form. It is named for its easy-chair shape. In this form, two carbon atoms sit out of the plane, with one above and one below. This arrangement minimizes torsional strain. The bond angles are also very close to the ideal...

chair conformation of cyclohexane

Cyclohexane Ring Flip and Axial Bonds

Cyclohexane is most stable in the chair conformation. In this shape, the ring avoids angle strain because its bond angles are close to the ideal tetrahedral angle of 109.5°. It also avoids torsional strain because the bonds are arranged in a perfectly staggered way. In the chair form, the hydrogen atoms attached to cyclohexane carbons take two positions: axial and equatorial. Axial bonds point straight up or down and run parallel to the ring axis. Equatorial bonds point outward, roughly along...

stability of substituted cyclohexanes

Cyclohexane Chair Axial vs Equatorial

Cyclohexane chair conformations are shaped by the position of a substituent, and that position can change how stable the molecule is. The two chair forms of cyclohexane interconvert rapidly at room temperature. For unsubstituted cyclohexane, both chairs have the same energy and the same stability, so each appears in equal amount at equilibrium. When a hydrogen atom is replaced by a functional group, the two chair forms are no longer equal. In methylcyclohexane, the CH3 group is axial in one...

disubstituted cyclohexanes: cis-trans isomerism

disubstituted-cyclohexanes-cis-trans-isomerism

Cyclohexane Chair Stability and Bulky Groups

Disubstituted cyclohexanes can differ in both cis-trans arrangement and chair stability. In a cyclohexane ring, the substituents may sit on the same side of the ring or on opposite sides. These cis and trans stereoisomers have different physical properties, and they cannot switch into each other without breaking carbon-carbon bonds. Cyclohexane also has chair conformers, which place substituents in axial or equatorial positions. For 1,4-dimethylcyclohexane, the cis isomer has two conformers...

combustion energy: a measure of stability in alkanes and cycloalkanes

combustion-energy-a-measure-of-stability-in-alkanes-and-cycloalkanes

Why Branched Alkanes Burn More Cleanly

Alkanes are low-reactivity hydrocarbons because their C–C and C–H sigma bonds are non-polar. That is why they were once called paraffins, from the Latin words parum, meaning “too little,” and affinis, meaning “affinity.” In the presence of excess oxygen and high heat, alkanes undergo combustion to form carbon dioxide and water. Combustion is the energy source for natural gas, liquified petroleum gas (LPG), fuel oil, gasoline, diesel fuel, and aviation fuel. The heat released in this reaction,...

key terms and concepts

Explore how alkane chain length and branching affect boiling point, melting point, and physical state through dispersion forces in straight-chain and branched isomers.

How Alkane Chain Length Changes Properties | JoVE K12

organic chemistry

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How Alkane Chain Length Changes Properties

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