Hydrocarbons
• Compounds that contain only carbon and hydrogen
• Two classes: Aliphatic and aromatic
2

Unbranched Alkanes
• Referred to as normal or n-alkanes
• Possess a linear carbon chain
5

9

Molecular
Formula
Formulas
10

Problems
1) How many hydrogens does n-Octadecane, an alkane containing 18 carbons, have?
2) Give the molecular, structural, condensed, and skeletal formulas for n-Octadecane
3) Estimate the boiling point and density of n-
Octadecane

• Compounds with the same molecular formula, but different structural formula
• Constitutional Isomers/Structural Isomers: compounds with the same molecular formula but different atom connectivity
C
4
H
10 12

• Standardized by International Union of Pure and Applied Chemistry (IUPAC)
• The current system is called substitutive nomenclature
• Rules for alkane nomenclature extend to most other compound classes
• Apply the following rules:
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Substitutive Nomenclature of Alkanes
1. Unbranched alkanes are named according to number of carbons
2. If branched, find the longest continuous carbon chain and identify this as the principle/parent chain
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Substitutive Nomenclature of Alkanes
3. If two chains are equal in length, select the one with the most substituents
4. Number the principle chain, giving the lower number to the first branching point
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Substitutive Nomenclature of Alkanes
5. Identify the name of each branch and to which carbon on the parent chain it is attached
• Branching groups are called substituents
• Those derived from alkanes are alkyl groups
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• Methyl group
• Attached to C3
3-methyl

6. Construct the name a. Location of branch (3) b. Name of branch (methyl)
• Note: a hyphen goes between the location and branch c. Name of parent chain (hexane)
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Problems
• Name the following molecules

7. When more than one of the same substituent is present:
– Indicate which carbon each substituent is on
– Use Greek prefixes (di-, tri-, tetra-) to indicate how many of each substituent you have
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Substitutive Nomenclature of Alkanes
8. For multiple substituents, select the numbering scheme that gives the smaller number at first point of difference
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9. Cite substituents in alphabetical order regardless of location
• Di-, tri-, tetra-, and hyphenated prefixes tert- and sec- are ignored
• Iso-, neo-, cyclo- are not ignored
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• Name the following molecules
:

• Draw 2-bromo-3-chloro-4-fluoro-2,3,4-trimethylheptane
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10. If the numbering of different groups is not resolved, the first-cited group gets the lowest number
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Highly Condensed Structures
• Highly condensed structures are commonly used
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Classification of Carbon Substitution
• Primary (1 ° ) carbon: A carbon bonded to 1 other carbon
• Secondary (2 ° ) carbon: A carbon bonded to 2 other carbons
• Tertiary (3 ° ) carbon: A carbon bonded to 3 other carbons
• Quaternary (4 ° ) carbon: A carbon bonded to 4 other carbons
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Classification of Carbon Substitution
• Similarly, hydrogens may also be classified as primary, secondary, tertiary, or quaternary
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Problems
• Locate the primary, secondary, tertiary, and quaternary carbons in the following molecule

Cycloalkanes
• Alkanes with closed loops or rings
• Add the prefix cyclo
• Note that cyclohexane has 2 fewer hydrogens than hexane
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Nomenclature of Cyloalkanes
• The same nomenclature rules are followed
• Do not forget the cyclo part of the name
• If the noncyclic carbon chain contains more carbons than the ring, it is named as the parent chain
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Problems
• Name the following compounds
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Problems
• Draw the following compounds
1) 1-chloro-4-ethylcyclohexane
1) 2-bromo-1,1-dimethylcyclobutane
1) 1,1,2,2-tetramethylcyclopropane

Conformations of Alkanes
• Conformational isomers. Rotation about a single bond leads to a series of conformers
• A Newman projection is a visual tool to inspect conformers as viewed down a bond
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Newman Projections
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Staggered and Eclipsed Conformers
• Two energetic extremes are found for ethane
• Other dihedral angles are possible
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Energy vs Dihedral Angle
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Butane
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Conformations of Butane
• Additional conformers are possible for butane
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Problem
1) Draw the Newman projections for the different eclipsed conformers of butane
2) Are there any conformers that are energetically equivalent?

Energy vs Dihedral Angle
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Space-Filling Models of Butane Conformers
• van der Waals repulsion creates a torsional
strain encouraging rotation towards a more stable conformer
• The most stable conformer dominates
2.3 Conformations of Alkanes
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Conformational Analysis
• Staggered conformers are preferred
• van der Waals repulsion influences conformer populations
• Rotation about single bonds is rapid except at
very low temperatures
2.3 Conformations of Alkanes
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Boiling Points
• Boiling point: Temperature at which vapor pressure of substance = atmospheric pressure
• B.P. of unbranched alkanes increases by 20 –
30 ° C per carbon
• Homologous series: differs by CH
2 groups
2.6 Physical Properties of Alkanes
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Intermolecular Interactions for Alkanes
• Electron clouds can be temporarily distorted
2.6 Physical Properties of Alkanes
49

Intermolecular Interactions for Alkanes
• Induced dipole
• van der Waals attraction (or a dispersion interaction)
• Greater intermolecular forces = higher b.p.
2.6 Physical Properties of Alkanes
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Molecular Shape and Boiling Point
• Greater branching = lower b.p.
• Molecules that are spherical have less surface area
2.6 Physical Properties of Alkanes
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Melting Points
• Melting point: Temperature at which a substance transforms from solid to liquid
• A narrow m.p. is an indicator of purity
• Branching interferes with crystal packing leading to lower m.p. values
• Symmetric molecules tend to have unusually high m.p
’ s
2.6 Physical Properties of Alkanes
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Melting Points
• M.P. increases with number of carbons
2.6 Physical Properties of Alkanes
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Other Physical Properties
• Dipole moment: A measure of polarity
• Solubility: Important for determining which solvents can be used (e.g., for reactions)
• Density: Determines whether a compound will be the upper of lower layer if mixed with an immiscible liquid (e.g., alkane + water)
2.6 Physical Properties of Alkanes
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Combustion
• Alkanes are the least reactive organic compounds
• Alkanes react rapidly with O
2 in combustion
• A commercially important reaction with increasingly significant global impact
• Analytically useful for determining molecular formula
2.7 Combustion
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The Petroleum Feedstock
• Most alkanes come from petroleum
• Composed mostly of alkanes and aromatic hydrocarbons
• Purified via fractional distillation
2.8 Occurrence and Use of Alkanes
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Industrial Fractionation of Petroleum
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Functional Groups
• A functional group is a characteristically bonded group of atoms
• Each functional group exhibits its own particular chemical reactivity
• Alkanes may be viewed as the blank template upon which functional groups are placed
2.9 Functional Groups, Compound Classes, and the “ R ” Notation
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Compound Classes
• Compounds with the same functional group comprise a compound class
• Some compounds may contain more than one functional group
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The “ R ” and “ Ph ” Notation
• A convenient way to represent a generic structure or portion of a molecule
• R Notation: R represents all alkyl groups
• A Benzene ring may be called a “ phenyl group ” and can be represented by Ph-
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The “ Ar ” Notation
• Use Ar- for more highly substituted rings
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