CH 3: Organic Compounds: Alkanes and Their Stereochemistry

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CH 3: Organic Compounds: Alkanes
and Their Stereochemistry
Renee Y. Becker
CHM 2210
Valencia Community College
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Why this Chapter
• Alkanes are unreactive, but provide useful
vehicle to introduce important ideas about
organic compounds
• Alkanes will be used to discuss basic
approaches to naming organic compounds
• We will take an initial look at 3-D aspects
of molecules
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Functional Groups
• Functional group collection of atoms at a
site that have a
characteristic behavior in
all molecules where it
occurs
• 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
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Functional Groups with Multiple Carbon–Carbon Bonds
• Alkenes have a C-C
double bond
• Alkynes have a C-C
triple bond
• Arenes have special
bonds that are
represented as
alternating single and
double C-C bonds in
a six-membered ring
 &  bonds?
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Functional Groups with Carbon Singly Bonded to an Electronegative
Atom
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Groups with a Carbon–Oxygen Double Bond (Carbonyl Groups)
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7
8
9
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Alkanes
• Alkanes: Compounds with C-C single bonds and C-H
bonds only (no functional groups)
• Connecting carbons can lead to large or small
molecules
• The formula for an alkane with no rings in it 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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Naming Alkanes
Memorize 1-10 for
next class
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Alkane Isomers
• 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, and 2,2dimethylpropane
• 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 Isomers
• Isomers that differ in how their atoms are arranged in
chains are called constitutional 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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Condensed Structures of Alkanes
• 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
– CH3CH2CH2CH3 (butane)
– CH3(CH2)2CH3 (butane)
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Alkyl Groups
• 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
– CH3 is “methyl” (from methane)
– CH2CH3 is “ethyl” from ethane
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Types of Alkyl groups
• Classified by the connection site (See Figure 3.3)
– 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 Hydrogens
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Substituents
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Common Names
• Isobutane, “isomer of butane”
• Isopentane, isohexane, etc., methyl
branch on next-to-last carbon in chain.
• Neopentane, most highly branched
• Five possible isomers of hexane,
18 isomers of octane and 75 for
decane!
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Pentanes
H
H C H
H H H H H
H C C C C C H
H H H H H
n-pentane, C
5H12
H H
H
H C C C C H
H H H H
isopentane, C
5H12
CH3
H3C C CH3
CH3
neopentane, C
5H12
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IUPAC Names
• Find the longest continuous carbon chain.
• Number the carbons, starting closest to the
first branch.
• Name the groups attached to the chain, using
the carbon number as the locator.
• Alphabetize substituents.
• Use di-, tri-, etc., for multiples of same
substituent.
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Longest Chain
• The number of carbons in the longest chain
determines the base name: ethane, hexane.
• If there are two possible chains with the same
number of carbons, use the chain with the
most substituents.
H3C
CH CH2
CH3
CH3
H3C CH2
C
CH
CH2
CH2
CH3
CH3
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Number the Carbons
• Start at the end closest to the first attached
group.
• If two substituents are equidistant, look for the
next closest group.
1
CH3
3
4
H3C CH CH CH2
2
CH2CH3
5
CH2
CH3
CH CH3
6
7
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Name Alkyl Groups
• CH3-, methyl
• CH3CH2-, ethyl
CH3 CH
• CH3CH2CH2-, npropyl
• CH3CH2CH2CH2-, nbutyl
CH3
CH3
isopropyl
CH3
CH CH2
isobutyl
CH3
CH CH2
CH3
sec- butyl
CH3
H3C C CH3
tert-butyl
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Propyl Groups
H H H
H C C C
H H H
H
H C C C H
H H H
H H H
n-propyl
isopropyl
A primary carbon
A secondary carbon
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Butyl Groups
H H H H
H C C C C
H H H H
n-butyl
A primary carbon
H H H H
H
H C C C C H
H
H H H
sec-butyl
A secondary carbon
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Isobutyl Groups
H
H
C H
H
H
C H
H
H
H C
C
H
H
C
H
H H H
isobutyl
A primary carbon
H C
H
C
H
C H
H
tert-butyl
A tertiary carbon
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Alphabetize
• Alphabetize substituents by name.
• Ignore di-, tri-, etc. for alphabetizing.
CH3
CH3
H3C CH CH CH2
CH2
CH CH3
CH2CH3
3-ethyl-2,6-dimethylheptane
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Example 1
Write structures for the following:
a) 3-ethyl-3-methylpentane
b) 4-t-butyl-2-methylheptane
c) 5-isopropyl-3,3,4-trimethyloctane
d) 3-ethyl-2,4,5-trimethylheptane
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Example 2
Provide IUPAC & common names (1-3)
1
4
2
5
3
6
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Name the following
a)
b)
c)
d)
Example 3
3,3-dimethyl-4-isobutylheptane
3,3-dimethyl-4-tert-butylheptane
4-tert-butyl-3,3-dimethylheptane
4-isobutyl-3,3-dimethylheptane
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Complex Substituents
• If the branch has a branch, number the
carbons from the point of attachment.
• Name the branch off the branch using a
locator number.
• Parentheses are used around the complex
branch name.
• For alphabetizing use the first letter of the
complex sub. Even if it is a numerical (di, tri,
etc)
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Complex Examples
a
c
b
d
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Example 4
Draw the structures and give their more
common names
a) (1-methylethyl) group
b) (2-methylpropyl) group
c) (1-methylpropyl) group
d) (1,1-dimethylethyl) group
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Example 5
Draw the structures
a) 4-(1-methylethyl)heptane
b) 5-(1,2,2-trimethylpropyl)nonane
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Assignment
• For next class draw the 9 isomers of
heptane and name them
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Properties of Alkanes
• Called paraffins (low affinity compounds)
because they do not react as most chemicals
• They will burn in a flame, producing carbon
dioxide, water, and heat
• They react with Cl2 in the presence of light to
replace H’s with Cl’s (not controlled)
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Physical Properties: Alkanes
• Solubility: hydrophobic
• Density: less than 1 g/mL
• Boiling points increase with increasing
carbons (little less for branched chains).
• Melting points increase with increasing
carbons (less for odd-number of carbons).
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Boiling Points of Alkanes
Branched alkanes have less surface area contact,
so weaker intermolecular forces.
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Melting Points of Alkanes
Branched alkanes pack more efficiently into a
crystalline structure, so have higher m.p.
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Branched Alkanes
• Lower b.p. with increased branching
• Higher m.p. with increased branching
CH3
CH3
CH3
CH3 C CH2 CH3
CH CH2 CH2 CH3
bp 60°C
mp -154°C
CH3
CH3
CH
CH
CH3
CH3
bp 58°C
mp -135°C
CH3
bp 50°C
mp -98°C
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Example 6
List each set of compounds in order of
increasing boiling point and melting point
a
b
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Example 7
Which of the following has the highest boiling
point?
a) 3-methylpentane
b) 2,2-dimethylbutane
c) Hexane
d) Methane
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Conformers
• Conformation- Different arrangement of atoms
resulting from bond rotation
• Conformations can be represented in 2 ways:
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Torsional Strain
• 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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Conformations of Ethane
• Stereochemistry concerned with the 3-D aspects
of molecules
• Rotation is possible around C-C bonds in openchain molecules (not cyclic)
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Ethane Conformers
• Eclipsed conformer has highest energy
• Dihedral angle = 0 degrees
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Conformational Analysis
• Torsional strain: resistance to rotation.
• For ethane, only 3.0 kcal/mol
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Propane Conformers
Note slight increase in torsional strain due to the
more bulky methyl group.
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Butane Conformers C2-C3
• Highest energy has methyl groups eclipsed.
• Steric hindrance
• Dihedral angle = 0 degrees
totally eclipsed
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Butane Conformers (2)
• Lowest energy has methyl groups anti.
• Dihedral angle = 180 degrees
anti
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Butane Conformers (3)
• Methyl groups eclipsed with hydrogens
• Higher energy than staggered conformer
• Dihedral angle = 120 degrees
eclipsed
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Butane Conformers (4)
• Gauche, staggered conformer
• Methyls closer than in anti conformer
• Dihedral angle = 60 degrees
gauche
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Conformational Analysis
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Higher Alkanes
• Anti conformation is lowest in energy.
• “Straight chain” actually is zigzag.
CH3CH2CH2CH2CH3
H H H H H
C
C
C
C
C
H
H
H H H H
H
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