Ch 12. Alkanes
Introduction to Organic Chemistry: Simple Hydrocarbons
Diagram of Methane. Member of Alkane family of hydrocarbons.
Alkanes have only single bonds.
When carbon is bonded to four atoms, the bonds are oriented
toward the four corners of a regular tetrahedron with carbon in the center.
This is shown using normal lines for bonds in the plane of the
page, dashed lines for bonds receding behind the page, and wedged lines
for bonds coming out of the page.
1. Organic bonds are covalent, i.e arise from sharing of electrons
between adjacent atoms
2. In Chloromethane, the carbon atom is attached to an
Electronegative chlorine, so the bond has polar character (a polar
covalent bond).
3. The Cl atom has a partial negative charge as it has a higher
Electronegativity.
4. Chloromethane is an ‘alkyl halide’ (alkane with a halogen atom).
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In Propane, each C must be bonded to at least one other carbon forming
the “backbone” of the compound. The hydrogens are on the periphery.
Isomers
Of Alkanes
Constitutional Isomers of Alkanes: same molecular formula but
Different connections between the atoms
Isomers will be covered in more detail in lab.
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Constitutional Isomers
The number of constitutional isomers increase with number of carbon
atoms.
Constitutional isomers could be completely different compounds with
Different structures and physical and chemical properties.
Conformational Isomers
Conformational Isomers or Conformers occurs in alkanes. These
Structures have the same molecular formula and group connectivity
But differ in spatial arrangement by the rotation of a single bond.
Conformers are part of isomers generally classed as ‘Stereoisomers’
(details on this topic covered in lab..)
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Drawing Organic Structures
Condensed structure: A shorthand way of drawing structures
in which C-C and C-H bonds are understood rather than
shown.
Alkane Nomenclature by IUPAC standards
All organic compounds under IUPAC have a 3 part naming system.
For Alkane nomenclature:
1. Prefix: Except a few, they are derived from greek numbers
(memorize table 12.3 of text)
2. Suffix: ane
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Alkyl Groups
Alkyl groups are parts of a hydrocarbon after removal of one
H atom from the parent. The –ane suffix is replaced by –yl.
Higher alkanes will have more branched chains and more
branched alkyl groups than simpler alkanes.
Ex: propyl and isopropyl ( from propane and isopropane)
N-butyl, sec-butyl, tert-butyl etc depending on the branching.
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4 possible substitutions.
Primary (1o) , Secondary or sec- (2o), Tertiary or tert- (3o)
And quaternary (4o)
Naming of branched chain alkanes
1. Name the Main Chain
2. Number the carbon atoms in the main chain.
Begin at the end nearer the first branch point:
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Naming of branched chain alkanes
3. Identify the number of the carbon atom in the main chain where
branching occurs
4. In case of two substituents on the same carbon, the same number
Is assigned to both substituent
4: Write the name as a single word, using hyphens to separate the
numbers from the different prefixes and commas to separate
numbers if necessary. If two or more different substituent groups are
present, cite them in alphabetical order (using di, tri, tetra etc..)
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Physical Properties of Alkanes
Reactions of Alkanes
1. Combustion: An oxidation reaction that produces water and
CO2. Incomplete oxidation produces CO (carbon monoxide).
This is a strongly exothermic reaction.
Alkane + O2 Æ CO2 + H2O + heat
2. Halogenation: This reaction is initiated by heat or light.
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Cycloalkanes
Cycloalkanes contain a ring of carbon atoms.
Cyclopropane and cyclobutane are less stable and more reactive
than other cycloalkanes since their bond angles are
considerably compressed.
Both cyclopentane and cyclohexane rings have nearly
ideal bond angles and are therefore stable.
Cyclic and acyclic alkanes are similar in many of their
properties.
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Naming Cycloalkanes
• cycloalkanes are represented simply by
polygons.
In line structures, a C is located at every intersection,
and the number of H atoms necessary to give each C
four covalent bonds is understood. Methylcyclohexane,
for example, looks like this in a line structure:
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• Cycloalkanes are named by a straightforward
extension of the rules for naming open-chain
alkanes. In most cases, only two steps are needed:
• 1: Use the cycloalkane name as the parent. That
is, compounds are named as alkyl-substituted
cycloalkanes rather than as cycloalkyl-substituted
alkanes. If there is only one substituent on the ring, it
is not even necessary to assign a number because all
ring positions are identical.
2: Identify and number the substituents. Start numbering at
the group that has alphabetical priority, and proceed around
the ring in the direction that gives the second substituent the
lower possible number.
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Topics covered
1.
2.
3.
4.
5.
6.
7.
8.
Introduction to Organic Chemistry: Alkanes
Isomerism in Alkanes
Nomenclature of Alkanes
Trends in b.p of Alkanes
Reactions of Alkanes
Cycloalkanes
Nomenclature of Cycloalkanes
Functional groups (memorization)
Practice
Identifying functional groups
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What is the IUPAC name of this molecule? Is it
a straight or a branched chain molecule?
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