Chapter 12 Saturated Hydrocarbons - Alkanes

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

The Petroleum Feedstock Most alkanes come from petroleum Composed mostly of alkanes and aromatic hydrocarbons 3

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Purification via fractional distillation 5

Physical Properties of Hydrocarbons Solubility: – Tend to be insoluble in water – Can be used for protective coatings – Good preservative for reactive metals Density: – Tend to have lower densities than water 0.6 g/mL – 0.8 g/mL 7

Boiling Points Boiling point: Temperature at which liquid turns to vapor – Temperature at which vapor pressure of substance = atmospheric pressure – B.P. of unbranched alkanes increases by approx. 20 – 30 °C per carbon 9

Intermolecular Interactions for Alkanes Electron clouds can be temporarily distorted 10

Intermolecular Interactions for Alkanes Induced dipole van der Waals attraction (or a dispersion interaction) Greater intermolecular forces = higher b.p. 11

Molecular Shape and Boiling Point Greater branching = lower b.p. Molecules that are spherical have less surface area 12

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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 Physical Properties of Alkanes

Melting Points M.P. increases with number of carbons 15

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 16

Unbranched Alkanes Referred to as normal or n-alkanes Possess a linear carbon chain 17

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21 Formulas Molecular Formula

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

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 23 C 4 H 10

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: 24

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 25

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 26

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 27

Methyl group Attached to C3 3-methyl

30 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)

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 32

Substitutive Nomenclature of Alkanes 8. For multiple substituents, select the numbering scheme that gives the smaller number at first point of difference 33

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 34

Problems Name the following molecules :

Draw 2-bromo-3-chloro-4-fluoro-2,3,4-trimethylheptane 36

10. If the numbering of different groups is not resolved, the first-cited group gets the lowest number 37

Highly Condensed Structures Highly condensed structures are commonly used 38

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 39

Classification of Carbon Substitution Similarly, hydrogens may also be classified as primary, secondary, tertiary, or quaternary 40

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 42

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 43

Problems Name the following compounds 44

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Problems Draw the following compounds 1)1-chloro-4-ethylcyclohexane 2)2-bromo-1,1-dimethylcyclobutane 3)1,1,2,2-tetramethylcyclopropane

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- 47

The “Ar” Notation Use Ar- for more highly substituted rings 48