Alkanes and Cycloalkanes

• Saturated hydrocarbons include alkanes and cycloalkanes.
• Moths communicate using pheromones, which are often alkanes.

Topic Outline

• The following topics will be covered: alkane, structures, nomencalture, properties, uses, and occurences

Objectives

• You should be able to: draw alkane and cycloalkane structures from molecular formula, write IUPAC names, explain properties, discuss petroleum mining and refinery, identify important hydrocarbons and their sources.

Introduction

• Hydrocarbons are widely used organic compounds, and the main source of Volatile Organic Compounds(VOCs).
• They are extracted as fossil fuels, refined, and the final product is used as fuels.
• Hydrocarbons are extracted and refined through thermal cracking and fractional distillation of crude oils.

Alkanes

• Hydrocarbons are composed of hydrogen and carbon and can be saturated or unsaturated.

Structure

• There are many ways to represent molecules, including lewis structures, partially condensed structure, condensed structure and molecular formula structures
• Bond-line structures are benchmark representation for organic compounds.

Nomenclature

• IUPAC system is a systematic naming of compounds.
• IUPAC name includes parent name, substituents names, and substituents locations.

Nomenclature: Parent Chain

• To identify the parent chain, find the longest consecutive chain of carbons.
• If there is more than one chain of equal length, choose the one with the most substituents.
• If the parent chain is cyclic, add the prefix "cyclo".

Nomenclature: Substituents

• Rings can be either a parent chain or substituent based on number of carbons
• Substituents with complex branches: number the longest carbon chain within the substituent, starting with the carbon attached to parent chain. Name the substituent, then name and number the side group
• Substituents end in yl instead of ane.
• Some branched substituents have common names such as propyl, isopropyl, butyl, sec-butyl, isobutyl, and tert-butyl.

Nomenclature: Assembly

• Carbons in parent chain are numbered.
• 2-methylpentane signifies a methyl group on carbon #2 of the pentane chain.
• Guidelines to follow when numbering a parent chain:
  • If one substituent is present, number the chain so substituent has the lowest number possible.
  • With multiple substituents, number the chain to give the first substituent the lowest number possible.
  • If there is a tie, number the chain so the second locant gets the lowest number possible.
  • If there is no other tire-breaker, assign the lowest number alphabetically
  • Follow the same rules for cycloalkanes.
• Put the # and name of each substituent before the parent chain name in alphabetical order.
• Use a prefix (di, tri, tetra, penta, etc.) if multiple substituents are identical.
• "di" or "tri" is ignored when alphabetizing the substituents.

Nomenclature: Summary

• Identify parent chain.
• Identify and name substituents.
• Number the parent chain and assign a locant to each substituent.
• List the numbered substituents before the parent name in alphabetical order.

Properties

• Alkanes contain only nonpolar C-H and C-C bonds, exhibiting weak van der Waals forces.
• Alkanes have low boiling/melting points compared to polar compounds.
• Boiling and melting points increase as number of carbons increases due to increased surface area.
• Boiling point decreases with increased branching due to decreased surface area.
• Melting point increases with increased symmetry.

Uses and Occurrences

• Alkanes are extremely unreactive, stable, low-energy, and derived from petroleum/petroleum by-products.
• Petroleum is a complex mixture of alkanes, cycloalkanes, and aromatic hydrocarbons, formed by decomposition of plant and animal material under the earth's crust.
• Components are separated via distillation.
• Natural gas and petroleum constitute the largest/most important sources of alkanes and cycloalkanes.
• Petroleum and natural gas are recovered by drilling.
• Unprocessed natural gas contains 50-90% methane, 1-10% ethane, and up to 8% higher molecular mass alkanes (propane and butanes).