Transcript Chapter 1 Chemical Bonding and Chemical Structure

Chapter 2
Alkanes
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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Formulas
Molecular
Formula
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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 nOctadecane
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
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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
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)
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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 tertand 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
• 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
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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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Problem
1) 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
2.3 Conformations of Alkanes
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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
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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
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Intermolecular Interactions for Alkanes
• Electron clouds can be temporarily distorted
2.6 Physical Properties of Alkanes
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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
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Molecular Shape and Boiling Point
• Greater branching = lower b.p.
• Molecules that are spherical have less surface
area
2.6 Physical Properties of Alkanes
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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
2.6 Physical Properties of Alkanes
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Melting Points
• M.P. increases with number of carbons
2.6 Physical Properties of Alkanes
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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
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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
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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
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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
2.9 Functional Groups, Compound Classes, and the “R” Notation
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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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