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Chapter 2Alkanes
Organic Chemistry, 5th ed.Marc Loudon
Eric J. KantorowskiCalifornia Polytechnic State UniversitySan Luis Obispo, CA
Chapter 2 Overview
• 2.1 Hydrocarbons
• 2.2 Unbranched Alkanes
• 2.3 Conformations of Alkanes
• 2.4 Constitutional Isomers and Nomenclature
• 2.5 Cycloalkanes and Skeletal Structures
• 2.6 Physical Properties of Alkanes
• 2.7 Combustion
• 2.8 Occurrence and Use of Alkanes
• 2.9 Functional Groups, Compound Classes, and the “R” Notation
2
Hydrocarbons
• Compounds that contain only carbon and hydrogen
• Two classes: Aliphatic and aromatic
32.1 Hydrocarbons
Unbranched Alkanes
• Referred to as normal or n-alkanes
• Possess a linear carbon chain
• Lewis structure:
• Condensed formulas:
42.2 Unbranched Alkanes
Nomenclature of n-Alkanes
52.2 Unbranched Alkanes
Conformational Isomers
62.3 Conformations of Alkanes
• 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
Newman Projections
72.3 Conformations of Alkanes
Staggered and Eclipsed Conformers
• Two energetic extremes are found for ethane
• Other dihedral angles are possible
82.3 Conformations of Alkanes
Energy vs Dihedral Angle
92.3 Conformations of Alkanes
Butane
102.3 Conformations of Alkanes
Conformations of Butane
• Additional conformers are possible for butane
112.3 Conformations of Alkanes
Energy vs Dihedral Angle
122.3 Conformations of Alkanes
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
132.3 Conformations of Alkanes
Conformational Analysis
• Staggered conformers are preferred
• van der Waals repulsion influences conformer populations
• Rotation about single bonds is rapid except at very low temperatures
142.3 Conformations of Alkanes
Isomers
• Different compounds with the same molecular formula
• Isomers that differ in atom connectivity are constitutional isomers or structural isomers
152.4 Constitutional Isomers and Nomenclature
Organic Nomenclature
• 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 10 rules in order
162.4 Constitutional Isomers and Nomenclature
Substitutive Nomenclature of Alkanes
1. Unbranched alkanes are named according to number of carbons
2. If branched, determine the principle chain
172.4 Constitutional Isomers and Nomenclature
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
182.4 Constitutional Isomers and Nomenclature
Substitutive Nomenclature of Alkanes
5. Name each branch and identify its location on the principle chain
• Branching groups are called substituents
• Those derived from alkanes are alkyl groups
192.4 Constitutional Isomers and Nomenclature
Substitutive Nomenclature of Alkanes
• Some short branched-chain alkyl groups:
202.4 Constitutional Isomers and Nomenclature
Substitutive Nomenclature of Alkanes
6. Construct the name with number, substituent, and principle chain (note the hyphen)
• This is a constitutional isomer of heptane
• It has 7 carbons, but is named as a derivative of hexane
212.4 Constitutional Isomers and Nomenclature
Substitutive Nomenclature of Alkanes
7. For multiple groups, each gets its own number
• Include prefixes di-, tri-, tetra-
222.4 Constitutional Isomers and Nomenclature
Substitutive Nomenclature of Alkanes
8. For multiple substituents, select the numbering scheme that gives the smaller number at first point of difference
232.4 Constitutional Isomers and Nomenclature
Substitutive Nomenclature of Alkanes
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
242.4 Constitutional Isomers and Nomenclature
Substitutive Nomenclature of Alkanes
10. If the numbering of different groups is not resolved, the first-cited group gets the lowest number
252.4 Constitutional Isomers and Nomenclature
Highly Condensed Structures
• Highly condensed structures are commonly used
262.4 Constitutional Isomers and Nomenclature
Classification of Carbon Substitution
• Primary carbon: Bonded to 1 other carbon
• Secondary carbon: Bonded to 2 other carbons
• Tertiary carbon: Bonded to 3 other carbons
• Quaternary carbon: Bonded to 4 other carbons
272.4 Constitutional Isomers and Nomenclature
Classification of Carbon Substitution
• Similarly, hydrogens may also be classified as primary, secondary, tertiary, or quaternary
282.4 Constitutional Isomers and Nomenclature
Cycloalkanes
• Alkanes with closed loops or rings
• Add the prefix cyclo
• Note that cyclohexane has 2 less hydrogens than hexane
292.5 Cycloalkanes and Skeletal Structures
Skeletal Structures
• An important (and time-saving!) structure-drawing convention
• Each vertex represents a carbon
• Enough hydrogens are understood to be present to complete carbon’s tetravalency
302.5 Cycloalkanes and Skeletal Structures
Skeletal Structures
• Also useful for open-chain alkanes
• The terminus of a line is a CH3
312.5 Cycloalkanes and Skeletal Structures
Nomenclature of Cyloalkanes
• The same nomenclature rules are followed
• Do not forget the cyclo part of the name
322.5 Cycloalkanes and Skeletal Structures
Cycloalkanes as a Substituent
• If the noncyclic carbon chain contains more carbons than the ring
332.5 Cycloalkanes and Skeletal Structures
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 CH2 groups
342.6 Physical Properties of Alkanes
Intermolecular Interactions for Alkanes
• Electron clouds can be temporarily distorted
352.6 Physical Properties of Alkanes
Intermolecular Interactions for Alkanes
• Induced dipole
• van der Waals attraction (or a dispersion interaction)
• Greater intermolecular forces = higher b.p.
362.6 Physical Properties of Alkanes
Molecular Shape and Boiling Point
• Greater branching = lower b.p.
• Molecules that are spherical have less surface area
372.6 Physical Properties of Alkanes
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
382.6 Physical Properties of Alkanes
Melting Points
• M.p. increases with number of carbons
392.6 Physical Properties of Alkanes
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)
402.6 Physical Properties of Alkanes
Combustion
• Alkanes are the least reactive organic compounds
• Alkanes react rapidly with O2 in combustion
• A commercially important reaction with increasingly significant global impact
• Analytically useful for determining molecular formula
412.7 Combustion
The Petroleum Feedstock
• Most alkanes come from petroleum
• Composed mostly of alkanes and aromatic hydrocarbons
• Purified via fractional
distillation
422.8 Occurrence and Use of Alkanes
Industrial Fractionation of Petroleum
432.8 Occurrence and Use of Alkanes
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 templateupon which functional groups are placed
442.9 Functional Groups, Compound Classes, and the “R” Notation
Compound Classes
• Compounds with the same functional group comprise a compound class
• Some compounds may contain more than one functional group
452.9 Functional Groups, Compound Classes, and the “R” Notation
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
• The phenyl group may be represented by Ph-
462.9 Functional Groups, Compound Classes, and the “R” Notation
The “Ar” Notation
• Use Ar- for more highly substituted rings
472.9 Functional Groups, Compound Classes, and the “R” Notation