Download Chapter 2: Alkanes Alkanes are molecules comprised of hydrogen

Chapter 2: Alkanes
Alkanes are molecules comprised of hydrogen
and carbon. All bonds are single (σ) bonds.
Alkanes have the general molecular formula
of CnH2n + 2
Today we learn how to name alkanes
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Compounds that have the same molecular formula but different
Connectivity are called constitutional isomers.
C4H10 has two possible structures
There are three constitutional isomers with the
molecular formula of C5H12
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C6H14
C7H16
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Removal of an “H” from an alkane leads to the formation of
An ”alkyl” substituent. The “ane” is dropped from the end of the
name and replaced with "yl".
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Primary carbons are carbons attached to only one other carbon
Secondary carbons are carbons attached to two other carbons
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Tertiary carbons are carbons attached to three other carbons
When naming alkyl substituents the prefixes sec and tert can be used
substituents are not formed from primary carbons, only used
when unambiguous.
Primary hydrogens are attached to primary carbons
Secondary hydrogens are attached to secondary carbons
Tertiary hydrogens are attached to tertiary carbons
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Which one of these is sec-pentyl chloride?
both and neither
Sec-pentyl chloride is an ambiguous name
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Common names for substituents mat be used as long as they
are not ambiguous
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Cycloalkanes have the general molecular formula
of CnH2n. Relative to the chain alkanes the cycloalkanes have
1 one more C-C bond and 2 less C-H bonds
Skeletal structures
each vertex in these polygon shapes represents a "CH2" group
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Skeletal structures-each vertex represents a carbon and carbons
are assumed to be present where a line begins or ends
Monosubstituted cycloalkanes-the number 1 is implied for the substituent
therefore the number 1 is omitted.
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When more than one substituent is present, numbers must be
assigned to the substituents, by the rules learned previously
i.e. the second substituent must get the lowest possible number
NOT
NOT
NOT
1-methyl-5-propylcyclopentane
1-ethyl-4-methylcyclopentane
1,5-dimethylcyclopentane
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In the IUPAC system alcohol names are used as parent names
instead of using naming chains as alkyl substituents. The "ane"
is dropped from the end of the name and "anol" is added.
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The parent alcohol is named for the longest continuous chain
containing the alcohol
The alcohol functional group always receives the lowest number
when numbering the chain.
Not
Not
1-bromo-3-propanol
1-chloro-3-butanol
Not
2,2-dimethyl-4-pentanol
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Cycloakanols-the number 1 is omitted because it is implied
NOT
5-ethyl-3-methylcyclohexanol
2<5
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Amines are classified according to the number of substituents
bonded to nitrogen.
Other functional groups are classified by the type of carbon they
are attached to.
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Physical properties of organic compounds: boiling points
boiling point: temperature at which a liquid becomes a gas
In order to do that the the attractive interactions between molecules
must be overcome
For alkanes the most significant attractive force between molecules
are induced-dipole induced dipole interactions
What is an induced-dipole? although alkanes are essentially neutral
with regards to electronic distribution, since electrons are continuously
moving around the molecule, at any one time a molecule may have
a small dipole moment (small polarization) causing nearby molecules
to acquire small dipole moments. The resultant attractive force
is dependent on the shared surface are between the molecules.
Larger surface area=Larger attractive force.
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linear alkanes have higher boiling points than branched alkanes
of the same molecular formula due to larger surface area
shaped like
a "cigar"
CH3CH2 CH2CH2 CH3
CH3CH2 CH2CH2 CH3
shaped like
a ball
CH 3
CH 3CCH 3
CH 3
CH 3
CH 3CCH 3
CH 3
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What about molecules that have significant dipole moments?
Larger halides have higher boiling points due to higher surface
area and larger Van der Waals contact
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Hydrogen bonding is a strong attractive force between molecules
strong hydrogen bonds pKa=15
weak hydrogen bonds
pKa = 36
strong hydrogen bonds
pKa = 4
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more possible hydrogen bonds lead to higher boiling points
all other things being equal
alkanes with an even number of carbons have more contact area
than those with an even number of carbons
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Solvation of organic compounds
General rule-like dissolves like
nonpolar compounds will dissolve easily in nonpolar media
polar compounds prefer polar media
some compounds have both a nonpolar group and a polar group
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Of the alkyl halides, only alkyl fluorides can form hydrogen
bonds with the hydrogens of water
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The lone pairs of ethers can also form hydrogen bonds with water
however as the alkyl chains increase in length hydrogen bonding
becomes more difficult to achieve
Low molecular weight alcohols and amines can be soluble in water
because they are hydrogen bond acceptors and donors, however as
alkyl chains increase size, solubility in water decreases
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