3. Organic Compounds: Alkanes
and Their Stereochemistry
Based on
McMurry’s Organic Chemistry, 7th edition, Chapter 3
Families of Organic Compounds
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Organic compounds can be grouped
into families by their common structural
features
We shall survey the nature of the
compounds in a tour of the families in
this course
This chapter deals with alkanes,
compounds that contain only carbons
and hydrogens, all connected
exclusively by single bonds
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3.1 Functional Groups
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Functional group - collection of atoms at
a site within a molecule with a common
bonding pattern
The group reacts in a typical way,
generally independent of the rest of the
molecule
For example, the double bonds in simple
and complex alkenes react with bromine in
the same way (See Figure 3.1)
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Double Bond as Functional Group
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Survey of Functional Groups
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Table 3.1 lists a wide variety of functional
groups that you should recognize
As you learn about them in each chapter it
will be easier to recognize them
The functional groups affect the reactions,
structure, and physical properties of every
compound in which they occur
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Types of Functional Groups: Multiple
Carbon–Carbon Bonds
Alkenes have a C-C double bond
 Alkynes have a C-C triple bond
 Arenes (or aromatic
hydrocarbons) have special
bonds that are represented as
alternating single and double CC bonds in a six-membered ring
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Multiple Carbon–Carbon Bonds
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Functional Groups with Carbon Singly Bonded to
an Electronegative Atom
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Alkyl halide: C bonded to halogen (C-X)
Alcohol: C bonded O of a hydroxyl group (C
OH)
Ether: Two C’s bonded to the same O (C O C)
Amine: C bonded to N (C N)
Thiol: C bonded to SH group (C SH)
Sulfide: Two C’s bonded to same S (C S C)
Bonds are polar, with partial positive charge on
C (+) and partial negative charge () on
electronegative atom
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Groups with a Carbon–Oxygen Double Bond
(Carbonyl Groups)
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Aldehyde: one hydrogen bonded to C=O
Ketone: two C’s bonded to the C=O
Carboxylic acid: OH bonded to the C=O
Ester: C-O bonded to the C=O
Amide: C-N bonded to the C=O
Acid chloride: Cl bonded to the C=O
Carbonyl C has partial positive charge (+)
Carbonyl O has partial negative charge (-).
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Problem 3.1: Identify the Functional Groups
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3.2 Alkanes and Alkane Isomers
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Alkanes: Compounds with C-C single bonds and C-H
bonds only (no functional groups)
The formula for an alkane with no rings (acyclic) must
be CnH2n+2 where the number of C’s is n
Alkanes are saturated with hydrogen (no more can be
added)
They are also called aliphatic compounds
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A “saturated” fat (glyceryl stearate):
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Alkane Isomers
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CH4 = methane, C2H6 = ethane, C3H8=
propane
The molecular formula of an alkane with
more than three carbons can give more than
one structure
C4 (butane) = butane and isobutane
C5 (pentane) = pentane, 2-methylbutane
(isopentane), and 2,2-dimethylpropane
(neopentane)
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Methane, ethane, & propane
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Butanes (C4H10):
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Pentanes (C5H12):
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Alkane
Isomers
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Alkane Isomers
Alkanes with C’s connected to no
more than 2 other C’s are
straight-chain or normal
alkanes
 Alkanes with one or more C’s
connected to 3 or 4 C’s are
branched-chain alkanes
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Constitutional (Structural) Isomers
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Isomers that differ in how their atoms are
arranged in chains are called constitutional
(or structural) isomers
Compounds other than alkanes can be
constitutional isomers of one another
They must have the same molecular formula
to be isomers
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Constitutional (Structural) Isomers
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Condensed Structures of Alkanes
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We can represent an alkane in a brief form or in many types of
extended form
A condensed structure does not show bonds but lists atoms,
such as
 CH3CH2CH3 (propane)
 CH3C(CH3)2CH3 (2,2-dimethylpropane)
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Condensed Structures of Alkanes
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3.3 Alkyl Groups
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Alkyl group – remove one H from an
alkane (a part of a structure)
General abbreviation “R” (for Radical, an
incomplete species or the “rest” of the
molecule)
Name: replace -ane ending of alkane with yl ending
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CH3 is “methyl” (from methane)
CH2CH3 is “ethyl” from ethane
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3.3 Alkyl Groups
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3.3 Alkyl Groups
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Alkyl Groups
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Types of Alkyl groups
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a carbon at the end of a chain (primary alkyl
group)
a carbon in the middle of a chain (secondary
alkyl group)
a carbon with three carbons attached to it
(tertiary alkyl group)
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Types of Alkyl groups
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Types of Alkyl groups
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Types of Hydrogens:
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Problem 3.8: Primary, secondary, tertiary,
or quaternary carbons?
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3.4 Naming Alkanes: IUPAC
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Compounds are given systematic names by a
process that uses:
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3.4 Naming Alkanes: IUPAC
Follows specific rules:
 Named as longest continuous chain of C’s
 Carbons in that chain are numbered in
sequence
 Substituents are numbered at their point of
attachment
 Compound name is one word (German
style)
 Complex substituents are named similarly
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1. Find the Parent: longest continuous
carbon chain
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2. Number the atoms in the chain
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3. Identify &
number the
substituents
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4. Write the name
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Practice prob. 3.2: IUPAC name?
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Solution:
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Practice prob. 3.3: structure?
3-isopropyl-2-methylhexane
 C-C-C-C-C-C
 Two substituents: isopropyl &
methyl
 Add hydrogens to complete the
structure
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Solution:
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Problem 3.11: IUPAC names?
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Problem 3.39: Draw structures
1.
2.
3.
4.
5.
6.
2-methylheptane
4-ethyl-2,2-dimethylhexane
4-ethyl-3,4-dimethyloctane
2,4,4-trimethylheptane
3,3-diethyl-2,5-dimethylnonane
4-isopropyl-3-methylheptane
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3.5 Properties of Alkanes
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Called paraffins (low affinity compounds)
because they are relatively unreactive
They will burn in a flame, producing carbon
dioxide, water, and heat
CH4 + 2 O2 → CO2 + 2 H2O + heat
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3.5 Properties of Alkanes
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They react with Cl2 in the presence of light to
replace H’s with Cl’s (not easily controlled)
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Physical Properties
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Boiling points and melting points increase as
size of alkane increases
Forces between molecules (temporary dipoles,
dispersion) are weak
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Physical Properties
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3.6 Conformations of Ethane
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Stereochemistry concerned with the 3-D
aspects of molecules
 bonds are cylindrically symmetrical
Rotation is possible around C-C bonds in
open-chain molecules
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Conformers
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Conformation- Different arrangement of
atoms resulting from bond rotation
Conformations can be represented in 2 ways:
Conformations of Ethane
staggered conformation
eclipsed conformation
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Conformations of Ethane
staggered conformation
eclipsed conformation
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Representing Conformations
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Sawhorse representations show
molecules at an angle, showing a
molecular model
 C-C bonds are at an angle to
the edge of the page and all CH bonds are shown
Newman projections show how
the C-C bond would project endon onto the paper
 Bonds to front carbon are lines
going to the center
 Bonds to rear carbon are lines
going to the edge of the circle
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Newman Projections
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Ethane’s Conformations
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Torsional Strain
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We do not observe perfectly free rotation
There is a barrier to rotation, and some
conformers are more stable than others
Staggered- most stable: all 6 C-H bonds are
as far away as possible
Eclipsed- least stable: all 6 C-H bonds are
as close as possible to each other
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3.7 Conformations of Other
Alkanes
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The eclipsed conformer of propane has 3
interactions: two ethane-type H-H interactions, and
one H-CH3 interaction
Conformations of Other Alkanes
• Conformational situation is more complex for larger alkanes
• Not all staggered conformations has same energy, and not all
eclipsed conformations have same energy
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Conformations of Butane
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Anti conformation- methyl groups are 180˚ apart
Gauche conformation- methyl groups are 60˚ apart
Which is the most energetically stable?
Conformations of Butane
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Steric Strain
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Steric strain- repulsive interaction occurring between atoms
that are forced closer together than their atomic radii allow
Staggered Conformations of Butane
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Conformations of Butane
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Conformations of Butane
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Eclipsed Conformations of Butane
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Gauche conformation: steric strain
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1-chloropropane
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Hydrocarbon Chains: Staggered
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Gasoline: Octane Ratings
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