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Organic Molecules
Alkanes (saturated hydrocarbons)
The Saturated Hydrocarbons, or Alkanes
Molecular Melting
Boiling
State
Name
Formula
Point (oC) Point (oC) at 25oC
methane CH4
-182.5
-164
gas
ethane
C2H6
-183.3
-88.6
gas
propane C3H8
-189.7
-42.1
gas
butane
C4H10
-138.4
-0.5
gas
pentane
C5H12
-129.7
36.1
liquid
hexane
C6H14
-95
68.9
liquid
heptane
C7H16
-90.6
98.4
liquid
octane
C8H18
-56.8
124.7
liquid
nonane
C9H20
-51
150.8
liquid
decane
C10H22
-29.7
174.1
liquid
The alkanes in the table above are all straight-chain hydrocarbons, in which the carbon atoms
form a chain that runs from one end of the molecule to the other. The generic formula for these
compounds can be understood by assuming that they contain chains of CH2 groups with an
additional hydrogen atom capping either end of the chain. Thus, for every n carbon atoms there
must be 2n + 2 hydrogen atoms: CnH2n+2.
Because two points define a
line, the carbon skeleton of the
ethane molecule is linear, as
shown in the figure below.
Because the bond angle in a
tetrahedron is 109.5, alkanes
molecules that contain three or
four carbon atoms can no
longer be thought of as
"linear," as shown in the
figure below.
Propane
Butane
In addition to the straight-chain examples considered so far, alkanes also form branched
structures. The smallest hydrocarbon in which a branch can occur has four carbon atoms. This
compound has the same formula as butane (C4H10), but a different structure. Compounds with
the same formula and different structures are known as isomers (from the Greek isos, "equal,"
and meros, "parts"). When it was first discovered, the branched isomer with the formula C4H10
was therefore given the name isobutane.
Isobutane
The best way to understand
the difference between the
structures of butane and
isobutane is to compare the
ball-and-stick models of
these compounds shown in
the figure below.
Butane
Isobutane
Butane and isobutane are called constitutional isomers because they literally differ in their
constitution. One contains two CH3 groups and two CH2 groups; the other contains three CH3
groups and one CH group. There are three constitutional isomers of pentane, C5H12. The first is
"normal" pentane, or n-pentane.
A branched isomer is also possible, which was originally named isopentane. When a more highly
branched isomer was discovered, it was named neopentane (the new isomer of pentane).
Ball-and-stick models of the three isomers of pentane are shown in the figure below.
n-pentane
Isopentane
Neopentane
There are two constitutional isomers with the formula C4H10, three isomers of C5H12, and five
isomers of C6H14. The number of isomers of a compound increases rapidly with additional
carbon atoms. There are over 4 billion isomers for C30H62, for example.
The Cycloalkanes
If the carbon chain that forms the backbone of a straight-chain hydrocarbon is long enough, we
can envision the two ends coming together to form a cycloalkane. One hydrogen atom has to be
removed from each end of the hydrocarbon chain to form the C C bond that closes the ring.
Cycloalkanes therefore have two less hydrogen atoms than the parent alkane and a generic
formula of CnH2n.
The smallest alkane that can form a ring is cyclopropane,
C3H6, in which the three carbon atoms lie in the same plane.
The angle between adjacent C C bonds is only 60°, which is
very much smaller than the 109.5° angle in a tetrahedron, as
shown in the figure below.
Cyclopropane is therefore susceptible to chemical reactions
that can open up the three-membered ring.
cyclobutane
The angle between adjacent C C bonds in a
planar cyclopentane molecule would be 108°,
which is close to the ideal angle around a
tetrahedral carbon atom. Cyclopentane is not a
planar molecule, as shown in the figure to the
right, because displacing two of the carbon
atoms from the plane of the other three
produces a puckered structure that relieves
some of the repulsion between the hydrogen
atoms on adjacent carbon atoms in the ring.
Any attempt to force the four carbons that form
a cyclobutane ring into a plane of atoms would
produce the structure shown in the figure
below, in which the angle between adjacent
C C bonds would be 90°.
One of the four carbon atoms in the
cyclobutane ring is therefore displaced from
the plane of the other three to form a
"puckered" structure that is vaguely
reminiscent of the wings of a butterfly.
cyclopentane
By the time we get to the six-membered ring in
cyclohexane, a puckered structure can be
formed by displacing a pair of carbon atoms at
either end of the ring from the plane of the
other four members of the ring. One of these
carbon atoms is tilted up, out of the ring,
whereas the other is tilted down to form the
"chair" structure shown in the figure to the left.
cyclohexane
Functional Groups
We’ll start with water . . .
Alcohols (an alkyl substituted water)
ex. primary alcohol
ex. secondary alcohol
ex. tertiary alcohol
Ethers (a dialkyl substituted water) – NOT COVERED IN CHAPTER 2, see problem #75
ex. diethyl ether (sometimes just called “ether”) . . . seen in movies when a person is “knocked
out” . . . the liquid placed on the cloth that is used to cover the victims nose and mouth
Carboxylic acids (oxidized primary alcohols)
ex. acetic (ethanoic) acid, commonly called vinegar, can be formed when ethanol is oxidized
Esters (alkyl substituted carboxylic acids) – not covered IN Chapter 2
ex. ethyl acetate is made when ethanol reacts with acetic acid
Amines (let’s start with ammonia) – these will be discussed in Chapter 4
ammonia
methylamine
diethylammine
trimethylamine