Chapter 2€¦ · Chapter 2 Overview" 2.1 Hydrocarbons" 2.2 Unbranched Alkanes" 2.3 Conformations...

Chapter 2Alkanes

Organic Chemistry, 5th ed.Marc Loudon

Eric J. KantorowskiCalifornia Polytechnic State UniversitySan Luis Obispo, CA

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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



Staggered and Eclipsed Conformers

• Two energetic extremes are found for ethane

• Other dihedral angles are possible



Energy vs Dihedral Angle



Butane



Conformations of Butane

• Additional conformers are possible for butane



Energy vs Dihedral Angle



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



Conformational Analysis

• Staggered conformers are preferred

• van der Waals repulsion influences conformer populations

• Rotation about single bonds is rapid except at very low temperatures



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



Substitutive Nomenclature of Alkanes

1. Unbranched alkanes are named according to number of carbons

2. If branched, determine the principle chain




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




5. Name each branch and identify its location on the principle chain

• Branching groups are called substituents

• Those derived from alkanes are alkyl groups




• Some short branched-chain alkyl groups:




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




7. For multiple groups, each gets its own number

• Include prefixes di-, tri-, tetra-




8. For multiple substituents, select the numbering scheme that gives the smaller number at first point of difference




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




10. If the numbering of different groups is not resolved, the first-cited group gets the lowest number



Highly Condensed Structures

• Highly condensed structures are commonly used



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



Classification of Carbon Substitution

• Similarly, hydrogens may also be classified as primary, secondary, tertiary, or quaternary



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



Skeletal Structures

• Also useful for open-chain alkanes

• The terminus of a line is a CH3



Nomenclature of Cyloalkanes

• The same nomenclature rules are followed

• Do not forget the cyclo part of the name



Cycloalkanes as a Substituent

• If the noncyclic carbon chain contains more carbons than the ring



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



Intermolecular Interactions for Alkanes

• Induced dipole

• van der Waals attraction (or a dispersion interaction)

• Greater intermolecular forces = higher b.p.



Molecular Shape and Boiling Point

• Greater branching = lower b.p.

• Molecules that are spherical have less surface area



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



Melting Points

• M.p. increases with number of carbons



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)



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



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-



The “Ar” Notation

• Use Ar- for more highly substituted rings



Chapter 2€¦ · Chapter 2 Overview" 2.1 Hydrocarbons" 2.2 Unbranched Alkanes" 2.3 Conformations...

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