Organic Chemistry: Alkanes

• Organic chemistry pertains to the study of Alkanes.

Alkanes

• Alkanes are saturated hydrocarbons, possessing only single carbon-carbon (C-C) and carbon-hydrogen (C-H) bonds.
• Alkanes can be linear, branched, or cyclic in structure.
  • Linear alkanes are referred to as normal alkanes or n-alkanes.
• Alkenes (olefins) have double bonds.
• Alkynes have triple bonds.
• Carbon in Alkanes always has a sp3 hybridization and only single bonds are present, so the carbon geometry is tetrahedral.
• Alkanes structural empirical formula: CnH2n+2n

Structural Isomers

• Butane and isobutane are structural isomers which share the same composition but exhibit different physical and chemical properties.
• Constitutional isomers are different chemical compound, they can be separated by standard methods.

Alkyl Groups

• In branched alkanes, the main chain serves as the longest continuous carbon chain.
• Substituents attached to the main chain are referred to as alkyl groups.
• The alkyl substitute is generally denoted as -R.
• Alkyl groups are derived from alkanes by removing one hydrogen atom and are named with the suffix "-yl".
  • Methyl (CH3) is derived from methane.
  • Ethyl (C2H5) comes from ethane.
  • Propyl (C3H7) is derived from propane.

Classification of Carbon Atoms

• Carbon atoms are classified based on the number of carbon atoms to which they are directly bonded.
• Hydrogen atoms are classified via analogy.

Nomenclature of Alkanes

• Alkane names are based on the number of carbons in the longest continuous chain, called the parent chain.
• The suffix "-ane" is added to indicate that it is an alkane.
• For example; methane (1 carbon), ethane (2 carbons), propane (3 carbons), butane (4 carbons), etc.
• Alkyl groups are formed by removing a hydrogen from the parent chain.
• Alkyl groups are named by replacing the "-ane" suffix with "-yl".
• Often abbreviated as 'R' (for radical)
• -CH3 is "methyl" (from methane)
• -CH2CH3 is "ethyl" (from ethane)

Alkyl Group Types

• Alkyl groups are named based on where they have a connection on the carbon chain.
• Primary: carbon at the end of a chain
• Secondary: carbon in the middle of a chain
• Tertiary: carbon with three carbons attached to it

IUPAC Nomenclature for Branched Alkanes

• The name of a branched alkane is based on the number of carbons in the longest continuous chain.
• Number the atoms in the chain, beginning at the end nearer the first substituent on the chain.
• If there are substituents occurring at equal distances from both ends of the chain, begin numbering at the end providing the lowest number sequence at the first point of difference.
• When two equal chains compete for selection as base chain, choose the one with the greatest number of substituents.
• Substituents are named as alkyl radicals and numbered using the numbering system. Use multipliers (di-, tri-, tetra-, etc.) when two or more substituents are identical.
• Names are arranged alphabetically, ignoring multiplier prefixes.
• Write the name as one word, using hyphens for prefixes and commas to separate numbers.
• Complex substituents are numbered starting from the point of attachment to the main chain and are included in parentheses.
• Halogens (fluoro, chloro, bromo, iodo) on an alkyl chain are treated as substituents.

Physical Properties

• Physical State:
  • C1-C4 alkanes are gases.
  • C5-C17 alkanes are liquids.
  • Those with more than C17 are solids.
• Alkanes are soluble in organic solvents (apolar) but insoluble in water.
• As surface area increases with carbon atoms, there is an increase in boiling point.
• As surface are decreases with branching, there is a decrease in the boiling point.

Natural Sources

• Natural gas and oil are natural sources for alkanes.
• Natural gas is primarily methane.
• Oil is a complex mixture of hydrocarbons ranging from C1 to C40.

Conformational Isomers

• Conformational isomers result from rotation around single bonds, generating distinct spatial geometries.
• Rotation around single bonds is rapid so conformational isomers cannot typically be separated; they are regarded as identical forms.

Ethane Conformational Isomerism

• Eclipsed conformation: All C-H bonds are aligned.
• Staggered conformation: C-H bonds on each carbon bisect the H-C-H angles on the other carbon.
  • The H-C-C-H angle is known as the dihedral angle (0° in eclipsed, 60° in staggered conformation).

Newman Projection

• A drawing convention used to visualize different conformations of a molecule.
• Focuses on looking lengthwise down a specific bond of interest.
• Depicts the front atom as a dot and the back atom as a larger circle.

Drawing the Newman Projection

• Look along the C-C bond and draw a circle (back C atom) with a dot in the center (front C atom).
• Draw the bonds
• Add the atoms.
• In the eclipsed conformation of ethane, the dihedral angle is 0°.
• In the staggered conformation of ethane, the dihedral angle is 60°.
• Analyzing orientation along a carbon-carbon bond uses Newman projection.
• The first step is to choose a perspective.
• Analyze one carbon-carbon bond at a time, ignoring conformations along other bonds.

Propane Conformations

• In the Newman projection, only analyze the confirmation along one C-C bond, and ignore the conformations along all other bonds.
• The eclipsed form possesses more energy than staggered.
• The methyl group is bulkier than a hydrogen atom.

Butane Conformations

• Six different conformations.

Conformational Stability

• Staggered conformations with two large groups at 180° are called anti.
• Staggered conformations with two large groups at 60° are called gauche.
• Anti conformation: CH3 groups are at 180°.
• Gauche conformation: CH3 groups are at 60° and have steric strain.
• Eclipsed conformation: CH3 groups are at 0° and have steric strain.

Factors Increasing Potential Energy

• Steric interactions: Crowding of alkyl groups or substituents.
• Torsional strain: The tendency of σ-bonds to rotate.
• Angle strain: Increase in potential energy due to the bond angles.
• Rotational barrier is the energy difference between two minima.
• Most stable conformation of linear hydrocarbons is staggered.
• Long chains are usually drawn with a zigzag pattern.

Steric Strain

• A severe interaction as bulky groups (or large molecules) become too close.
• When the dihedral angle is small and bulky groups interact, C2-C3 axis rotation relieves steric interactions between methyl groups.

Torsional Stain in Linear Alkanes

• A rotational barrier is the energy difference between two minima.
• The most stable conformation of linear hydrocarbons is staggered.
• Long chains are usually drawn with a zigzag pattern.

Conformation Stability Summary

• There is no perfectly free rotation, and some conformations are more stable than others.
• Staggered conformations are lower in energy than eclipsed.
• Eclipsed conformations are less stable than staggered due to torsional strain.