Chapter 3: Organic Compounds: Alkanes and Their Stereochemistry
Functional groups
Functional group: group of atoms with characteristic
chemical behavior
Carbonyl group: C=O double bond
Many functional groups contain a carbonyl:
Reactions of organic molecules
are governed by their functional
groups, regardless of size and
complexity of the molecule.
Alkene: contains C=C double bond
Alkyne: contains C≡C triple bond
Arene: contains a benzene
(or other aromatic) ring
A few other functional groups:
Many functional groups have an
electronegative atom bonded to carbon
(this makes carbon δ+)
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3.2 Alkanes and alkane isomers
Constitutional isomers
Alkanes contain no functional groups - only C-H and C-C
single bonds (all ____ hybridized carbons)
Constitutional isomers: molecules that differ in how
their atoms are connected
Must have same molecular formula
Methane: CH4
Ethane: C2H6
Propane: C3H8
Butane: C4H10
If an alkane has n carbons, how many hydrogens does it
have?
Alkanes are saturated with hydrogen (no more H's can
be added) - a.k.a. aliphatic compounds.
The alkanes shown above are straight-chain or normal
alkanes (C's connected to no more than 2 other C's)
Starting with butane, alkanes can be branched - one or
more C's connect to 3 or 4 other C's) - pencil test
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Drawing alkanes
Names of straight-chain (n-) alkanes
The molecular formula for butane contains no structural
information (C4H10)
1-4 are historical names. 5 and up are named based on
the number of carbons. Memorize 1-10.
Structures can be drawn out showing all bonds, but this
is tedious and only used when absolutely necessary...
Condensed structures can efficiently show how the
atoms are connected (although no bonds are drawn)...
Line structures are handy too...
Common isomer names:
Isobutane:
Isopentane:
Neopentane:
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3.3 Alkyl groups
Types of alkyl groups
Alkyl group: partial structure that remains after
removing one hydrogen from an alkane molecule
Wildcard R for any alkyl group
Replace -ane with -yl in name
Alkyl groups are classified by their connection site (where
the hydrogen was removed from)
Primary: carbon at end of chain
Methane (CH4) →
Ethane (CH3CH3) →
Propane (CH3CH2CH3) →
Secondary: carbon in the middle of a chain
Tertiary: carbon attached to 3 other C's
Butane →
Isobutane →
Primary, secondary, tertiary, and quaternary can be used
to describe the location of functional groups:
Pentane →
Isopentane →
Identify the primary, secondary, and tertiary carbons and
hydrogens in the compounds below:
Neopentane →
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3.4 Naming alkanes
Complex structures
Even complex branched organic molecules can be named
using the systematic IUPAC naming process.
Branching off sub-structures is handled by naming the
sub-structure as if it were a compound itself:
1. Find the parent hydrocarbon chain (longest
continuous chain of carbons - may turn corners!)
If tied, use the chain with the most branches
2. Number atoms on the main chain (start with the end
nearest to a branch point)
If tied, look for the nearest 2nd branch
3. Identify and number the substituents
4. Molecule is named as a single word
Combine multiple identical substituents with
multiplier prefixes like di-, tri-, tetra-, etc.
Alphabetize with substituent names but not
multiplier prefixes
Use hyphens to separate a number and a letter
Use commas to separate numbers
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3.5 Properties of alkanes
3.6 Conformations of ethane
Alkanes are sometimes called paraffins because they
have little affinity for reactions with other substances.
Stereochemistry: concerned with the 3-dimensional
aspects of molecules
There are 2 reactions that alkanes will readily undergo:
1. Combustion: alkanes react with oxygen to
produce carbon dioxide and water, and release a
large amount of heat
Because σ bonds exist directly between two atoms,
rotation is possible around C-C bonds in open-chain
molecules.
2. Radical halogenation (which will be detailed in a
later chapter): H's replaced with Cl's in the
presence of strong ultraviolet light (hν)
Conformations: different arrangement of molecules as a
result of bond rotations
Alkanes have a predictable trend in boiling and melting
points due to the increase in dispersion forces:
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Conformers: molecules with different conformations
(conformational isomers)
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3.7 Conformations of other alkanes
Torsional strain
The rotation about the C-C bond in ethane is not
completely free rotation.
Torsional strain is the barrier to rotation that causes
some conformers to be more stable than others.
Staggered conformation: the hydrogens on carbons 1
and 2 are as far away from each other as possible
each H on C1 is 180o across from an H on C2
this minimizes torsional strain (most stable)
Eclipsed conformation: the hydrogens on C1 are as close
as possible to the hydrogens on C2
each H is directly in front of another
this maximizes torsional strain (least stable)
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Propane has similar conformations to ethane, except the
torsional strain is slightly larger because one of the H-H
interactions is replaced by an H-CH3 interaction.
Butane is more complicated because not all staggered
and eclipsed conformations have the same energy.
Anti conformation: CH3 groups are 180o apart
Gauche conformation: CH3 groups are 60o apart
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Steric strain
The additional interaction in the butane conformations is
steric strain, the repulsive interaction between atoms
trying to occupy the same space.
Butane totally eclipsed conformation (0o):
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