Chapter 2 Alkanes

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

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Formulas Molecular Formula

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

Isomers Compounds with the same molecular formula, but different structural formula Constitutional Isomers/Structural Isomers: compounds with the same molecular formula but different atom connectivity C4H10 12

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 rules: 13

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 14

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 15

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 16

Methyl group Attached to C3 3-methyl

Construct the name Location of branch (3) Name of branch (methyl) Note: a hyphen goes between the location and branch Name of parent chain (hexane) 19

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 21

Substitutive Nomenclature of Alkanes 8. For multiple substituents, select the numbering scheme that gives the smaller number at first point of difference 22

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 23

Problems Name the following molecules: Draw 2-bromo-3-chloro-4-fluoro-2,3,4-trimethylheptane

10. If the numbering of different groups is not resolved, the first-cited group gets the lowest number 25

Highly Condensed Structures Highly condensed structures are commonly used 26

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 27

Classification of Carbon Substitution Similarly, hydrogens may also be classified as primary, secondary, tertiary, or quaternary 28

Problems Locate the primary, secondary, tertiary, and quaternary carbons in the following molecule

Alkanes with closed loops or rings Add the prefix cyclo Cycloalkanes Alkanes with closed loops or rings Add the prefix cyclo Note that cyclohexane has 2 less hydrogens than hexane 30

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 31

Problems Name the following compounds 32

Problems Draw the following compounds 1-chloro-4-ethylcyclohexane 2-bromo-1,1-dimethylcyclobutane 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 35

Newman Projections 36

Staggered and Eclipsed Conformers Two energetic extremes are found for ethane Other dihedral angles are possible 37

Energy vs Dihedral Angle 38

Butane 39

Conformations of Butane Additional conformers are possible for butane 40

Problem Draw the Newman projections for the different eclipsed conformers of butane Are there any conformers that are energetically equivalent?

Energy vs Dihedral Angle 42

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 44

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 45

B.P. of unbranched alkanes increases by 20 – 30 °C per carbon 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 2.6 Physical Properties of Alkanes

Intermolecular Interactions for Alkanes Electron clouds can be temporarily distorted 2.6 Physical Properties of Alkanes

Intermolecular Interactions for Alkanes Induced dipole van der Waals attraction (or a dispersion interaction) Greater intermolecular forces = higher b.p. 2.6 Physical Properties of Alkanes

Molecular Shape and Boiling Point Greater branching = lower b.p. Molecules that are spherical have less surface area 2.6 Physical Properties of Alkanes

A narrow m.p. is an indicator of purity 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

M.P. increases with number of carbons Melting Points M.P. increases with number of carbons 2.6 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) 2.6 Physical Properties of Alkanes

Alkanes are the least reactive organic compounds 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 2.7 Combustion

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

Industrial Fractionation of Petroleum

A functional group is a characteristically bonded group of atoms 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

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 A Benzene ring may be called a “phenyl group” and can be represented by Ph-

The “Ar” Notation Use Ar- for more highly substituted rings