Lecture 2 - Organic Chemistry Rev CME 2024

Alkanes and Their Properties

Introduction to Alkanes

  • Definition of Alkanes

    • Compounds with only C-C single bonds and C-H bonds (no functional groups).

    • Can form large or small molecules depending on carbon connections.

    • General formula: CnH2n+2 (no rings).

    • Saturated with hydrogen (no additional hydrogen can be added).

    • Also referred to as aliphatic compounds.

Basic Structures of Alkanes

  • Examples of Alkanes

    • Methane (CH4)

    • Ethane (C2H6)

    • Propane (C3H8)

    • Butane (C4H10)

Naming Alkanes

  • List of Straight-Chain Alkanes

    • Methane: CH4

    • Ethane: C2H6

    • Propane: C3H8

    • Butane: C4H10

    • Pentane: C5H12

    • Hexane: C6H14

    • Heptane: C7H16

    • Octane: C8H18

    • Nonane: C9H20

    • Decane: C10H22

Properties of Alkanes

  • Types of Alkanes

    • Straight-chain (normal) alkanes: C's connected to no more than 2 other C's.

    • Branched-chain alkanes: C's connected to 3 or 4 other C's.

Types of Carbons in Alkanes

  • Degree of Substitution

    • Primary (1°): 1 carbon connected.

    • Secondary (2°): 2 carbons connected.

    • Tertiary (3°): 3 carbons connected.

    • Quaternary (4°): 4 carbons connected.

Types of Hydrogens in Alkanes

  • Classification of Hydrogens

    • Primary hydrogens (CH3).

    • Secondary hydrogens (CH2).

    • Tertiary hydrogens (CH).

Constitutional Isomers

  • Definition

    • Isomers that differ in the arrangement of atoms in chains.

    • Must have the same molecular formula to be considered isomers.

Isomers

  • Constitutional Isomers

    • Isomers that differ in arrangements of chains but the same molecular formula

    • Compounds other than alkanes can be constitutional isomers of one another

    • Usually have different physical properties

  • Types of Isomers

    • Straight-chain and branched-chain forms (each with 4 carbon atoms) of butane (C4H10).

Physical Properties of Alkanes

  • Trends in Physical Properties

    • Boiling and melting points increase with the size of the alkane.

    • Dispersion forces increase with molecular size, leading to higher melting and boiling points.

Reactions of Alkanes

  • Combustion

    • Called paraffins (low affinity compounds) because they do not react as most chemicals.

    • Alkanes burn in a flame, producing CO2, H2O, and heat.

    • Non-polar and insoluble in water but soluble in non-polar solvents.

  • Radical Halogenation

    • They react with Halogens like Cl2 in the presence of light replaces H’s with Cl’s (not controlled)

Sources of Alkanes

  • Primary Source

    • Petroleum, a complex mixture of alkanes and aromatic hydrocarbons.

Distillation of Petroleum

  • Distillation

    • First step in refining petroleum.

  • Separation Process

    • Components are separated based on volatility.

    • More than 500 different compounds can be obtained from petroleum distillates.

  • Boiling Points

    • <30°C: Natural gas; (C1-C4)

    • 30°C-200°C: Gasoline; (C5-C10)

    • 200°C-250°C: Kerosene, jet fuel; (C12-C18)

    • 250°C-350°C: Diesel fuel, heating oil; (C12<)

    • 350°C-450°C: Lubricating oil; Nonvolatile liquids; (C20<)

Gasoline and Catalytic Cracking

  • Gasoline Production

    • The demand of gasoline is much greater than that supplied by the gasoline fraction of petroleum.

  • Catalytic Cracking

    • Converting hydrocarbons from other fractions of petroleum into gasoline.

Naming Alkanes

  • Nomenclature Rules

    • Prefix-Locant-Parent-Suffix structure.

    • Identify the longest carbon chain and number the carbons.

    • Substituents are identified numbered

    • Write compound name (single word)

    • Name a complex substituents as though it was a compound itself.

IUPAC Names for Simple Alkanes

  • Naming Conventions

    • All alkane names end with -ane.

    • Derived from Greek and Latin roots.

      • one - meth

      • two - eth

      • three - prop

      • four - but

      • five - pent

Alkyl Groups

  • Definition

    • Alkyl group: remove one H from an alkane.

    • General abbreviation “R”.

Nomenclature of Branched-Chain Alkanes

  • Naming Process

    • Use the longest continuous carbon chain as the parent name.

    • Start numbering from the end nearest the first substituent.

    • Use the number obtained by Rule 2 to designate the location of the substituent.

    • For two or more substituents, use the lowest possible individual numbers of the parent chain. The substituents should be listed alphabetically. In deciding alphabetical order, disregard multiplying prefix, such as “di”, “tri” etc.

    • When two substituents are present on the same carbon, use that number twice.

    • For identical substituents, use prefixes di-, tri-, tetra-, so on.

    • When two chains of equal length compete for selection as parent chain, choose the chain with the greater number of substituents.

    • When branching first occurs at an equal distance from either end of the longest chain, choose the name that gives the lower number at the first point of difference.

Conformations of Ethane

  • Stereochemistry

    • Concerned with 3-D aspects of molecules.

    • Sigma bonds are cylindrically symmetrical

    • Rotation around C-C bonds is possible in open-chain molecules.

  • Conformers

    • Conformation

      • Different arrangement of atoms resulting from bond rotation.

      • Sawhorse representation

      • Newman projection.

Torsional Strain

  • Stability of Conformers

    • Staggered conformations are most stable; all 6 C-H bonds are as far away as possible.

    • Eclipsed conformations are least stable; all 6 C-H bonds are as close as possible.

Conformations of Butane

  • Types of Conformations

    • Anti conformation: methyl groups are 180˚ apart.

    • Gauche conformation: methyl groups are 60˚ apart.

Energy Costs for Interactions in Alkane Conformers

  • Interaction Energies

    • Torsional strain and steric strain contribute to energy costs in conformers.

Steric Strain

  • Steric Strain

    • Repulsive interaction occurring between atoms forced closer than their atomic radii allow.