Chapter 2Alkanes

Hydrocarbons

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

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Unbranched Alkanes

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

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FormulasMolecular Formula

Problems1) 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

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

12C4H10

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:

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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

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6. Construct the namea. Location of branch (3)b. Name of branch (methyl)

• Note: a hyphen goes between the location and branch

c. Name of parent chain (hexane)

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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Problems• 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

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• 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

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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Problem1) 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

462.3 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

472.3 Conformations of Alkanes

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

482.6 Physical Properties of Alkanes

Intermolecular Interactions for Alkanes

• Electron clouds can be temporarily distorted

492.6 Physical Properties of Alkanes

Intermolecular Interactions for Alkanes

• Induced dipole• van der Waals attraction (or a dispersion

interaction)• Greater intermolecular forces = higher b.p.

502.6 Physical Properties of Alkanes

Molecular Shape and Boiling Point

• Greater branching = lower b.p.• Molecules that are spherical have less surface

area

512.6 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

522.6 Physical Properties of Alkanes

Melting Points

• M.P. increases with number of carbons

532.6 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)

542.6 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

552.7 Combustion

The Petroleum Feedstock

• Most alkanes come from petroleum• Composed mostly of alkanes and aromatic

hydrocarbons• Purified via fractional

distillation

562.8 Occurrence and Use of Alkanes

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

592.9 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

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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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