Alkanes Lecture

Structure and Classification of Alkanes

• Alkanes are aliphatic hydrocarbons that contain only carbon-carbon ($\text{C}-\text{C}$) and carbon-hydrogen ($\text{C}-\text{H}$) $\sigma$ bonds.

• Alkanes are classified into two main categories based on structural arrangement:

  • Acyclic Alkanes: Hydrocarbons containing only linear or branched chains of carbon atoms. Their general molecular formula is $C_n H_{2n+2}$ (where $n$ is a positive integer). Acyclic alkanes are classified as saturated hydrocarbons because they hold the maximum possible number of hydrogen atoms per carbon atom.

  • Cycloalkanes: Hydrocarbons containing carbon atoms joined together in one or more rings. Their general molecular formula is $C_n H_{2n}$. Because of ring formation, cycloalkanes have two fewer hydrogen atoms than an acyclic alkane with the same carbon count.

Geometry and Representations of Alkanes

• All carbon atoms in an alkane are bonded to four other atoms, rendering each carbon $sp^3$ hybridized with a tetrahedral geometry and bond angles of $109.5^\text{o}$.

• Molecular representations serve distinct visualization purposes:

  • Lewis Structures: Show Connectivity and atom placement but do not convey three-dimensional spatial arrangements.

  • 3-D Perspective Drawings: Depict spatial layout using distinct line conventions:

  • Solid lines represent bonds lying within the plane of the page.

  • Wedges represent bonds projecting forward out of the plane of the page.

  • Dashed lines represent bonds extending backward behind the plane of the page.

  • Ball-and-Stick Models: Express three-dimensional tetrahedral geometry, relative spatial orientation, bond lengths, and bond angles.

• Propane ($CH_3 CH_2 CH_3$, molecular formula $C_3 H_8$) contains three carbon atoms in a zigzag arrangement where each carbon atom possesses tetrahedral geometry.

• Pentane ($CH_3 CH_2 CH_2 CH_2 CH_3$, molecular formula $C_5 H_{12}$) features five carbon atoms in a continuous chain. A continuous chain skeleton can be drawn in various zigzags or bent arrangements without changing the identity of the molecule, provided all five carbons remain connected sequentially in a single row.

Constitutional Isomerism in Alkanes

• Constitutional Isomers (also termed structural isomers) are different compounds that share the same molecular formula but differ in the order in which their atoms are connected.

• For alkanes with molecular formula $C_4 H_{10}$, two distinct constitutional isomers exist:

  • Butane: A straight-chain alkane containing four carbons in a continuous row.

  • 2-Methylpropane (Isobutane): A branched-chain alkane containing three carbons in a continuous row with a one-carbon branch.

• The number of possible constitutional isomers increases exponentially as the carbon count increases:

• Complete Series of Unbranched Alkanes ($n$-Alkanes):

  • Carbon 1: Methane ($CH_4$) — 0 isomers

  • Carbon 2: Ethane ($C_2 H_6$) — 0 isomers

  • Carbon 3: Propane ($C_3 H_8$) — 0 isomers

  • Carbon 4: Butane ($C_4 H_{10}$) — 2 constitutional isomers

  • Carbon 5: Pentane ($C_5 H_{12}$) — 3 constitutional isomers

  • Carbon 6: Hexane ($C_6 H_{14}$) — 5 constitutional isomers

  • Carbon 7: Heptane ($C_7 H_{16}$) — 9 constitutional isomers

  • Carbon 8: Octane ($C_8 H_{18}$) — 18 constitutional isomers

  • Carbon 9: Nonane ($C_9 H_{20}$) — 35 constitutional isomers

  • Carbon 10: Decane ($C_{10} H_{22}$) — 75 constitutional isomers

  • Carbon 11: Undecane ($C_{11} H_{24}$) — 159 constitutional isomers

  • Carbon 12: Dodecane ($C_{12} H_{26}$) — 355 constitutional isomers

  • Carbon 13: Tridecane ($C_{13} H_{28}$) — 802 constitutional isomers

  • Carbon 14: Tetradecane ($C_{14} H_{30}$) — 1,858 constitutional isomers

  • Carbon 15: Pentadecane ($C_{15} H_{32}$) — 4,347 constitutional isomers

  • Carbon 20: Icosane ($C_{20} H_{42}$) — 366,319 constitutional isomers

IUPAC Nomenclature of Acyclic Alkanes

• Systemic organic nomenclature established by the International Union of Pure and Applied Chemistry (IUPAC) divides chemical names into three distinct functional parts:

  • Prefix: Identifies the nature, quantity, and location of substituents attached to the parent carbon chain.

  • Parent Name: Denotes the number of carbon atoms present in the longest continuous carbon chain.

  • Suffix: Specifies the principal functional group present in the molecule (for alkanes, the suffix is -ane).

• Substituents (Alkyl Groups):

  • Carbon substituents bonded to the parent carbon chain are known as alkyl groups.

  • Alkyl groups are formed by removing one hydrogen atom from an alkane.

  • To name an alkyl group, the -ane suffix of the parent alkane is replaced with -yl (e.g., methane $CH_4$ becomes methyl $CH_3-$, ethane $CH_3 CH_3$ becomes ethyl $CH_3 CH_2-$).

  • Three-Carbon Alkyl Groups:

  • Propane contains two types of hydrogen atoms: primary ($1^\text{o}$) hydrogens attached to terminal carbons and secondary ($2^\text{o}$) hydrogens attached to the internal carbon.

  • Removal of a $1^\text{o}$ hydrogen forms a propyl group ($CH_3 CH_2 CH_2-$).

  • Removal of a $2^\text{o}$ hydrogen forms an isopropyl group ($(CH_3)_2 CH-$).

• Four-Carbon Alkyl Groups:

  • Removal of a $1^\text{o}$ hydrogen from butane forms a butyl group ($CH_3 CH_2 CH_2 CH_2-$).

  • Removal of a $2^\text{o}$ hydrogen from butane forms a sec-butyl group ($CH_3 CH_2 CH(CH_3)-$).

  • Removal of a $1^\text{o}$ hydrogen from 2-methylpropane forms an isobutyl group ($(CH_3)_2 CHCH_2-$).

  • Removal of a tertiary ($3^\text{o}$) hydrogen from 2-methylpropane forms a tert-butyl group ($(CH_3)_3 C-$).

• Step-by-Step Systemic Nomenclature Protocol for Alkanes:

  • Step 1: Find the parent carbon chain.

  • Determine the longest continuous chain of carbon atoms and assign the parent name based on carbon count.

  • The longest chain does not need to be written in a straight horizontal line.

- If two different chains have equal length, choose the chain containing the greater number of substituents.

• Step 2: Number the carbon chain.

  • Number the parent chain starting from the end closest to the first substituent so that substituents receive the lowest possible locant numbers.

- If the first substituent is located at an equal distance from both chain ends, number from the end that gives the second substituent the lower locant number.

- If numbering from either end yields identical locant sets, assign the lower number to the substituent that comes first alphabetically.

• Step 3: Name and number the substituents.

  • Assign every substituent an alkyl group name and a locant number.

  • Every carbon atom must be accounted for as part of either the parent chain or a substituent.

  • When two or more identical substituents are attached, use multiplying prefixes (di- for 2, tri- for 3, tetra- for 4, etc.) and list each locant number explicitly separated by commas.

• Step 4: Assemble the complete IUPAC name.

  • Place substituents in alphabetical order before the parent name.

  • Ignore structural prefixes (di-, tri-, sec-, tert-) when alphabetizing, but retain prefixes containing hyphenated structural identifiers integrated into the core name like iso (e.g., isopropyl, isobutyl).

  • Separate numbers from letters using hyphens, and separate numbers from other numbers using commas.

  • Format the complete name as a single continuous word without spaces.

  • Example: 5-ethyl-2,6-dimethyloctane.

IUPAC Nomenclature of Cycloalkanes

• Cycloalkanes are named by prepending the prefix cyclo- directly to the parent alkane name corresponding to the carbon count in the ring.

• Rules for naming substituted cycloalkanes:

  • Step 1: Determine the parent structure.

  • Compare the number of carbon atoms in the ring to the number of carbons in the attached alkyl chain.

  • If the ring carbon count is greater than or equal to the chain carbon count, the compound is named as a substituted cycloalkane.

  • If the acyclic chain carbon count exceeds the ring carbon count, the compound is named as an acyclic alkane with a cycloalkyl substituent.

• Step 2: Number the ring carbon atoms.

  • For rings containing a single substituent, no locant number is required (e.g., ethylcyclobutane).

  • For rings containing two or more substituents, begin numbering at one substituent carbon and proceed around the ring in the direction (clockwise or counterclockwise) that gives the next substituent the lowest possible locant number.

- If two different substituents offer identical numbering choices, assign position C1 to the substituent that appears first alphabetically.

• Representative Cycloalkane Names:

  • ethylcyclobutane (1 substituent, no locant number needed).

  • 1-sec-butyl-3-methylcyclohexane (alphabetical priority: sec-butyl before methyl).

  • 1,2-dimethylcyclohexane (numbered 1,2- rather than 1,6-).

  • 1,2,4-triethylcyclopentane (numbered 1,2,4- rather than 1,3,4- or 1,3,5-).

Common Names and Polycyclic Alkanes

• Non-systematic common names exist for various polycyclic alkanes based on geometric visual appearance:

  • Cubane: A hydrocarbon forming a cubic cage structural layout ($C_8 H_8$).

  • Housane: A polycyclic structure resembling a house geometry ($C_5 H_8$).

  • Churchane: A polycyclic structure shaped like a church building.

  • Basketane: A polycyclic structure resembling a woven basket ($C_{10} H_{12}$).

Naturally Occurring Alkanes and Petroleum Refining

• Alkanes originate primarily from natural deposits of fossil fuels:

  • Natural Gas: Composed predominantly of methane ($CH_4$), with smaller quantities of ethane ($C_2 H_6$), propane ($C_3 H_8$), and butane ($C_4 H_{10}$).

  • Petroleum: A complex hydrocarbon mixture containing alkanes with chain lengths ranging from 1 to 40 carbon atoms.

• Petroleum Refining: Separation of crude oil into distinct functional components via fractional distillation based on boiling point differences:

  • Refinery Yield Breakdown (Standard 42-gallon barrel of crude oil):

  • Petroleum starting materials for chemical synthesis: $1.25\,\text{gal}$

  • Asphalt and road oil: $1.3\,\text{gal}$

  • Boiler oil: $2.9\,\text{gal}$

  • Lubricants, waxes, solvents: $4.2\,\text{gal}$

  • Jet fuel: $4.2\,\text{gal}$

  • Diesel and home heating oil: $8.4\,\text{gal}$

  • Gasoline: $19.7\,\text{gal}$

• Fractional Distillation Temperature Cutoffs:

  • Gas fraction: Boiling points below $20^\text{o}\text{C}$.

  • Gasoline fraction ($C_5 H_{12}$ to $C_{12} H_{26}$): Boiling point range $20^\text{o}\text{C} - 200^\text{o}\text{C}$.

  • Kerosene fraction ($C_{12} H_{26}$ to $C_{16} H_{34}$): Boiling point range $175^\text{o}\text{C} - 275^\text{o}\text{C}$.

  • Fuel oil fraction ($C_{15} H_{32}$ to $C_{18} H_{38}$): Boiling point range $250^\text{o}\text{C} - 400^\text{o}\text{C}$.

  • Lubricating oil fraction: Boiling point range above $350^\text{o}\text{C}$.

  • Residue (asphalt): Non-volatile bottom liquid residue.

Physical Properties of Alkanes

• Alkanes consist exclusively of nonpolar $\text{C}-\text{C}$ and $\text{C}-\text{H}$ bonds, resulting in weak intermolecular London dispersion (van der Waals) forces.

• Boiling Point (bp) and Melting Point (mp) Trends:

  • Alkanes possess significantly lower boiling points and melting points compared to polar compounds of comparable molecular weight.

  • Example: Propane ($\text{VDW}$ only, $\text{bp} = -42^\text{o}\text{C}$, $\text{mp} = -190^\text{o}\text{C}$) vs. Acetaldehyde ($\text{VDW} + \text{Dipole-Dipole}$, $\text{bp} = 21^\text{o}\text{C}$, $\text{mp} = -121^\text{o}\text{C}$).

  • Boiling points and melting points increase consistently as carbon chain length increases due to expanded molecular surface area.

  • Butane ($C_4 H_{10}$): $\text{bp} = 0^\text{o}\text{C}$, $\text{mp} = -138^\text{o}\text{C}$.

  • Hexane ($C_6 H_{14}$): $\text{bp} = 69^\text{o}\text{C}$, $\text{mp} = -95^\text{o}\text{C}$.

  • Increased branching lowers boiling points among constitutional isomers because spherical compacting reduces molecular surface contact area.

  • Pentane (unbranched): $\text{bp} = 36^\text{o}\text{C}$.

  • 2-Methylbutane (singly branched): $\text{bp} = 30^\text{o}\text{C}$.

  • 2,2-Dimethylpropane (doubly branched): $\text{bp} = 10^\text{o}\text{C}$.

  • High structural symmetry increases melting point by facilitating compact crystal lattice packing.

  • 2-Methylbutane (less symmetric): $\text{mp} = -160^\text{o}\text{C}$.

  • 2,2-Dimethylpropane (highly symmetric): $\text{mp} = -17^\text{o}\text{C}$.

• Solubility Profile:

  • Alkanes are soluble in organic, nonpolar solvents.

  • Alkanes are insoluble in polar water ("hydrophobic").

Conformations of Acyclic Alkanes

• Conformations are spatial arrangements of atoms generated by free rotation around single $\sigma$ bonds.

• Dihedral Angle: The angle created by two intersecting planes passing through bonds on adjacent atoms.

• Primary Conformations of Ethane:

  • Eclipsed Conformation: $\text{C}-\text{H}$ bonds on the front carbon line up directly with $\text{C}-\text{H}$ bonds on the back carbon. The dihedral angle between adjacent $\text{C}-\text{H}$ bonds is $0^\text{o}$.

  • Staggered Conformation: $\text{C}-\text{H}$ bonds on the front carbon bisect the $\text{H}-\text{C}-\text{H}$ bond angle on the back carbon. The dihedral angle between adjacent $\text{C}-\text{H}$ bonds is $60^\text{o}$.

• Newman Projections: End-on molecular drawings used to project conformational geometry directly along a specific $\text{C}-\text{C}$ bond axis:

  • Step 1: Represents the front carbon as a point (intersection of three bonds) and the back carbon as a circle.

  • Step 2: Draws front bonds meeting at the center point and back bonds projecting outward from the edge of the circle.

  • Step 3: Places attached atoms or groups explicitly onto bond endpoints.

Energy and Stability of Acyclic Conformations

• Ethane Torsional Energy:

  • Staggered conformations represent energy minima; eclipsed conformations represent energy maxima.

  • Staggered ethane is more stable than eclipsed ethane by $12\,\text{kJ/mol}$ ($3.0\,\text{kcal/mol}$).

  • Torsional Strain: Destabilization caused by electron-electron repulsion between aligned $\sigma$ bonding orbitals in eclipsed arrangements.

  • Each eclipsing $\text{H},\text{H}$ interaction costs $4.0\,\text{kJ/mol}$ ($1.0\,\text{kcal/mol}$).

• Propane Conformations:

  • Propane features one $\text{C}-\text{C}$ bond containing two hydrogens and one methyl group ($CH_3$) on one carbon, and three hydrogens on the second carbon.

  • The energy penalty for an eclipsed propane conformation is $14\,\text{kJ/mol}$ ($3.4\,\text{kcal/mol}$), which incorporates two eclipsing $\text{H},\text{H}$ interactions ($4.0\,\text{kJ/mol}$ each) and one eclipsing $\text{H},\text{CH}_3$ interaction ($6.0\,\text{kJ/mol}$ or $1.4\,\text{kcal/mol}$).

• Butane Conformations (Rotation around the C2–C3 bond):

  • Rotation produces six unique conformers (three staggered, three eclipsed):

  • Conformer 1 (Staggered, anti): Dihedral angle between methyl groups is $180^\text{o}$. Lowest overall energy (global minimum).

  • Conformer 2 (Eclipsed): Dihedral angle $120^\text{o}$. High energy local maximum ($16\,\text{kJ/mol}$ above anti).

  • Conformer 3 (Staggered, gauche): Dihedral angle $60^\text{o}$. Higher in energy than anti by $3.8\,\text{kJ/mol}$ ($0.9\,\text{kcal/mol}$) due to steric strain (non-bonded atomic crowding).

  • Conformer 4 (Eclipsed, syn-periplanar): Dihedral angle $0^\text{o}$. Highest energy conformation (global maximum, $19\,\text{kJ/mol}$ above anti) due to combined torsional strain and extreme $\text{CH}_3,\text{CH}_3$ steric eclipsing strain ($11\,\text{kJ/mol}$ or $2.6\,\text{kcal/mol}$).

  • Conformer 5 (Staggered, gauche): Dihedral angle $60^\text{o}$ (degenerate with Conformer 3).

  • Conformer 6 (Eclipsed): Dihedral angle $120^\text{o}$ (degenerate with Conformer 2).

• Summary of Torsional and Steric Strain Energies:

• Eclipsing $\text{H},\text{H}$ interaction: $4.0\,\text{kJ/mol}$ ($1.0\,\text{kcal/mol}$) [Torsional]

• Eclipsing $\text{H},\text{CH}_3$ interaction: $6.0\,\text{kJ/mol}$ ($1.4\,\text{kcal/mol}$) [Torsional]

• Eclipsing $\text{CH}_3,\text{CH}_3$ interaction: $11\,\text{kJ/mol}$ ($2.6\,\text{kcal/mol}$) [Torsional + Steric]

• Gauche $\text{CH}_3,\text{CH}_3$ interaction: $3.8\,\text{kJ/mol}$ ($0.9\,\text{kcal/mol}$) [Steric]

  • Barrier to Rotation: The total energy differential separating the lowest energy conformer from the highest energy conformer.

  • Zigzag Skeletal Structures: Extended zigzag representations depict acyclic alkanes in their lowest energy conformation (where all $\text{C}-\text{C}$ bonds are staggered and bulky alkyl substituents assume anti orientations).

Conformations of Cycloalkanes and Ring Strain

• Cycloalkane stability is heavily influenced by Angle Strain: destabilization caused when bond angles deviate from the ideal tetrahedral angle of $109.5^\text{o}$.

• Planar geometry predictions vs. actual puckered structures:

  • Cyclopropane: Forced planar geometry with internal bond angles of $60^\text{o}$, creating severe angle strain and persistent torsional strain from fully eclipsed $\text{C}-\text{H}$ bonds.

  • Cyclobutane: Internal angles of $90^\text{o}$ in planar form; puckers out-of-plane to alleviate severe torsional eclipsing at a minor cost to angle strain.

  • Cyclopentane: Internal angles of $108^\text{o}$ in planar form; adopts an non-planar envelope puckered shape to reduce hydrogen eclipsing.

  • Cyclodecane: Internal angles of $144^\text{o}$ in planar form; puckers extensively to avoid large-angle expansion strain and non-bonded crowding.

![Angle Strain Deviations](https://assets.knowt.com/pdf-flow-prod/b2519733-d909-48bb-b4db-80da