ALKANES
L. Definition
Alkanes are a family of organic compounds whose molecules consist only of C and H
and SINGLE BONDS.
You can spot an alkane if its condensed structure has this ratio of C to H
CnH2n+2
The name of an alkane ALWAYS ends in -ANE. The prefix of the name will
reveal the number of carbon atoms.
II. Prefixes
The table below gives the names and condensed structures for the first 20
straight chain (unbranched) alkanes.
meth- = 1
eth- = 2
prop- = 3
and so on
ALWAYS, regardless
of the compound or
family it is in.
First 20 !? How many are there? How high can you count?
You can chain togehter THOUSANDS UPON THOUSANDS of C atoms.
After about 100 (if not fewer), we stop thinking about them as alkanes
and start thinking about them like a plastic known as polyethylene....no need to
worry about that now.
Let's draw the line-bond structures for a couple...
n-propane
3 carbons
n-octane
n-octadecane
8 carbons
18 carbons
The n (italized) before the name means "normal" or that the chain is "straight"
This "crooked" alkane is also n-octadecane.
It is identical to the zig-zag structure above. To get from the zig zag
structure to the crooked structure merely requires that the structure ROTATE
about its C-C single bonds. THIS CAN AND DOES HAPPEN.
More about that later...just remember a "kink" in the chain does not make
a new molecule.
III. Bonding in Alkanes: sp3 Hybridization
Each C atom in an alkane is TETRAHEDRAL. This REQUIRES HYBRIDIZATION of the
2s and 2p orbitals of C (recall the bonding lesson).
H
H
109.5o
H
EACH C ATOM HAS 4
SIGMA (!) BONDS.
H
MO of each bond made from an sp3 hybrid AO on C
and the 1s AO on H.
Let's look at our zig zag and crooked alkane again...
bond angles
are 109.5o
Every single one of those C atoms are sp3 hybridized. Every single one is
tetrahedral in shape. That's why we draw the zig zag structure. It acknowledges the
bond angles and shape in a simple way.
C-C single bonds are formed by an sp3 hybrid AO from each overlapping.
2 overlapped sp3 hybrid AOs
C
C
IV. Branched Alkanes
It is possible for alkanes to be branched. This means that the longest continuous
chain of C atoms has a chain of 1 or more C atoms attached to it.
longest chain is 8 C atoms
1 methyl branch
longest chain is 8 C atoms
2 methyl branches
longest chain is 8 C atoms
1 ethyl branch
3
1
2
Is this a 6 C chain with 1 butyl
branch? Ha. Fooled you. It is
just like the alkane above.
The longest continuous chain is
8 C atoms. There is 1 ethyl branch.
4
5
6
7
8
Branches can have branches.
V. Cyclic Alkanes
The C:H ratio in
cyclic alkanes is
CnH2n
Rings can be as small as 3 carbons. You might wonder how small rings (3,4,5 C atoms)
can form if the C atom wants to be tetrahedral. EXCELLENT OBSERVATION.
Save it for later.
They can also be huge, with lots of
C atoms.
"
"chair" cyclohexane
Just like the open chain alkanes, the cyclic structures want to have 109.5o bond
angles if possible. The rings bigger than 4 C atoms, therefore, are not FLAT in
the plane of this paper, but "bent" or "puckered" to conform to the sp3 orbital
directive. A MOST IMPORTANT EXAMPLE IS SHOWN ABOVE FOR THE 6 C
atom ring called cyclohexane.
And yes, the cyclic alkanes can have branches:
VI. ISOMERS
ISOMERS have the same number of C and H atoms, but arranged in a different
order.
If I asked you to draw the structure of C4H10 what would you draw?
both of these are C4H10
THESE ARE KNOWN AS structural or CONSTITUTIONAL ISOMERS.
Here's a few constitutional isomers C-8. C-8 has 18 constitutional isomers.
this one has the nickname "isooctane".
It's used as a standard to grade the
octane rating of gasoline.
VII. Naming
There are naming rules that direct how to name alkanes or any other organic
compound. Starting with naming alkanes is good because the rules you learn for
these simple structures will apply to other families of compounds.
The rules are set down by an IUPAC (international union of pure and applied chemistry)
comittee of chemists and have been adopted world wide.
The IUPAC systematic naming rules are important for avoiding confusion in
identifying structures. Older names, before IUPAC, were based on the compound's
origin or some property, and are therefore rather arbitrary and random.
Some compounds are so well known by their common, older name, that we still use
them (acetic acid is an example).
I am not going to repeat all the rules here. We'll look at some examples and then you
can practice and read the rules in our text.
First of all, the table at the beginning of the notes shows you how to name
straight chain alkanes. It is common practice to add an n- to the beginning of
the name to indicate that the alkane is linear (n means normal). The exception
is methane, ethane, and propane, which cannot have branched isomers.
2-methylbutane
1. Count the C atoms in the LONGEST CONTINUOUS CHAIN
2. Name that long chain
2. Identify each branch point
3. The LOWEST BRANCH NUMBER is used in the name
4. Name the branch point by adding an "-yl" ending to the prefix.
5. Dashes, not spaces, go between the number and the
branch name
3
1
4
2
4
2
3
1
WRONG NUMBERING OF LONGEST CHAIN
The branch is NOT at C3
CORRECT NUMBERING OF LONGEST CHAIN
The branch is at C2
6
8
2
4
3
3-ethyloctane
1
5
7
Longest straight chain is 8 C atoms, octane
The branch is on C3, ethyl
6
4
8
5
3
3-methyloctane
7
2
1
Be careful. This is NOT a 7 C straight chain with an ethyl branch !!!
2
4
3
1
8
6
7
5
10
6-ethyl-3-methyldecane
9
When naming two or more branches, alphabetize them in the name.
Let's name "isooctane"
3
5
1
2
2,2,4-trimethylpentane
4
No matter how you count it, the longest continuous chain is 5 C atoms, pentane
There are 3 methyl branches, 2 of them are on C2, 1 of them on C4
Put commas between the numbers. When the branches have the same name,
use di, tri, tetra, etc prefixes
4
2
5
1
3
5,6-diethyl-2-methylnonane
8
9
6
7
Longest chain is 9 C atoms
3 branches, 2 ethyl (diethyl), 1 methyl
alphabetize the branch names, ignore the di- prefix when alphabetizing
A couple of cyclic examples:
cyclopentane
Count the carbons in the ring, 5, pentane
add a cyclo- prefix
methylcylcopentane
With one branch, no number is needed
2
1
3
4
1,3-dimethylcyclopentane
5
start numbering at a branchpoint and give each branch the lowest number possible.
2
3
4
1
1-ethyl-3-methylcyclopentane
5
Give the branch points numbers based on the alphabet
ethyl gets 1
methyl gets 3
3-cyclopropyldecane
Don't try to name this with the ring as the "parent" name. Make the
ring an alkyl branch name, cyclopropyl.
VIII. Physical Properties
Non-polar
insoluble in water and other polar solvents
less dense than water
Look at the table below and note what happens to the phase state of an alkane as
more C atoms are added.
Name
methane
ethane
propane
butane
pentane
hexane
heptane
octane
nonane
decane
!mp (C) !bp (C)
!-182!
!-183!
!-190!
!-138!
!-130!
!-95 !
!-90 !
!-57 !
!-51 !
!-30 !
!-164!
!-88 !
!-42 !
!-0 !
!36 !
!69 !
!98 !
!126 !
!151 !
!174 !
!
!
!
!
!
!
!
!
!
!
!density at 0 C (g/ml)
!gas !!
!gas !!
!gas !!
!gas !!
!0.626
!0.659
!0.684
!0.703
!0.718
!0.730
!!
!!
!!
!!
!!
!!
at about 20 C atoms,
alkanes are solid
at room temperature
BRANCHING LOWERS THE MELTING AND BOILING POINTS. For an alkane with
n C atoms, the straight chain will have the higher m.p. and b.p.
THIS IS DUE TO VAN DER WAALS INTERMOLECULAR FORCES OF ATTRACTION.
The linear chains have a larger surface on which a temporary dipole can be created.
more "ball" shaped...less surface contact
Elongated...more surface contact
IX. REACTIONS
Well, alkanes make good SOLVENTS (medium to carry out a reaction in) because
they are INERT. There's not a functional group on them. Mixtures of them and
other hydrocarbons are burned in engines for energy.
However, there are a couple of reactions I want to point out.
Oxidation by Combustion
CnH2n+2
nCO2 +
+ (3n+1)/2 O2
CnH2n
nCO2 +
+ 3n/2 O2
(n + 1) H2O + heat
for open chain
alkanes
for unbranched
cyclic alkanes
n H2O + heat
Heat is a product of these reactions. Measuring the heat can give you information
about C-C and C-H bond strength.
Measuring the heat of combustion of cyclic alkanes gives information about ring
strain (how strained the bonds must be to make a ring structure). More on that
later.
Halogenation
Cl
H2
C
H3C
h#
+
CH3
Cl2
H
H
H3C
H2
C
H
+
CH3
CH3
Cl
55%
45%
Though there are 3x more 1o H positions then 2o, the amount of substitution of
Cl at the 2o position is greater. For bromination...
Br
H2
C
H3C
H
h#
CH3
+
Br2
H3C
H
+
H
H2
C
CH3
CH3
Br
97%
The product is almost entirely Br at the 2o position
3%
X
H3C
H3C
CH3
H
C
+
CH3
C
X2
H3C
+
CH3
CH3
H
C
CH3
H2C
X
X2 = Cl2
x2 = Br2
65%
80%
35%
20%
Despite a 9:1 1o to 3o H position, the 3o substitution product is preferred. WHY?
These reactions form what are called free radicals and the results show that
the formation of 3o and 2o radicals is preferred over 1o and that Br radicals are more position
sensitive than Cl radicals.
Here's part of the mechanism below. The species with a dot on them are free
radicals. That means there is an electron that is unpaired. They are VERY reactive.
h#
2X
o
$H = +243 kJ/mol X2 = Cl2
$Ho = +192 kJ/mol X2 = Br2
X
X
R
R
H
$Ho = +4 kJ/mol X2 = Cl2
$Ho = +25 kJ/mol X2 = Br2
+ HX
R
most stable when all
R groups are C
R
R
X
R
R
R
X
R
R
$Ho values when
methane is the
alkane
X
$Ho = -123 kJ/mol X2 = Cl2
$Ho = -101 kJ/mol X2 = Br2
R
X
+ X
R
According to our product mix data above, the X radical seems to select the 2o or
3o position over 1o. What happens is the the radical fomed on C is more stable
when it is at the 3o > 2o>1o position. The reaction of a halogen radical at a
3o position is most productive. The Br radical is less reactive than the Cl
radical, thus making it even more selective. In other words, its interaction at the
3o position is more productive than at either the 2o or 1o positions.
We'll revisit this reaction when we discuss the properties of free radicals later.
X. Synthesis of Alkanes by Alkyl halide Coupling: Wurtz Reaction
The oldest form of the Wurtz reaction involves the use of Na metal to mediate the coupling
of two alkyl halides. Other metals can be be used to make the alkyl groups couple. The reaction
can be used to make larger alkanes from smaller alkyl groups. Because the reaction proceeds
through both a free radical and nucleophilic substitution mechanism, it has some severe limitations
that you will understand later when we study radicals and substitution reactions.
For now, just realize that this is a classic reaction for producing a new C-C bond between two alkyl
halides.
new C-C bond
Na
R-X + R'X
R-R' + 2NaX
Usually, R = R' (can you think of a reason why these R groups should be the
same? Or, maybe a better question to ask is: what would you get for a product if
R % R' ?) The R goups are saturated hydrocarbons.
For reasons you will understand later, the reaction prefers 1o or 2o alkyl halides.
This reaction is of limited synthetic value since alkanes are readily availble from other
abundant resources (like petroleum for example). Some niche, cool, specialized
sytheses can be accomplished with the Wurtz reaction, such as the intramolecular
reaction below:
+ NaCl + NaBr
1-bromo-3-chlorocyclobutane converted to the bicyclic compound
bicyclo[1.1.0]butane via the Wurtz reaction
Another thing you can appreciate about the Wurtz reaction is this: it is a simple example
of a gigantic and important sub-discipline of organic synthesis known as organometallic
chemistry. Organometallic chemistry is responsible for modern plastics and the plastics
industry in general.