Transcript AICE Chemistry

AICE Chemistry
Organic
Introduction to Alkanes
What is Organic Chemistry?
• This is the area of Chemistry that focuses on
compounds consisting of primarily carbon and
hydrogen.
• The are three primary areas of study:
– Structure
– Properties
– Reactions
Alkanes
• These are the organic compounds that have
the generic formula
CnH(2n+2)
• It is this ratio of Carbon to Hydrogen that
places an organic compound into the Alkanes
category.
• Examples include the following: CH4 C3H8
C8H18
Where do Alkanes come from?
• Alkanes, like most of the hydrocarbons, are
obtained from the fossil fuels – crude oil
production.
• The student should realize that crude oil (raw
material pumped from the ground (or from
under the sea bed) is a mixture of very large
hydrocarbons. (lots of carbon atoms)
• The simpler alkanes are obtained by a process
called “cracking”.
Cracking ??
• This is an industrial process that uses heat to
break carbon-carbon bonds in crude oils.
• The result is a set of “smaller” hydrocarbons that
may be separated by the differences in their
boiling points.
• The AICE student may be asked about “cracking”
and may also be asked to supply a product in an
equation representing a hypothetical cracking
reaction.
• Remember that the Law of Conservation of Mass
still applies in organic chemistry.
A sample question regarding cracking.
• A crude oil has the chemical formula C26H54 . This
crude material is “cracked” and it is observed that
one of the products has the formula C14H28. Write
a balanced equation for the reaction, clearly
indicating the identity of the other product.
• Solution: this is nothing more than subtracting.
There is one reactant and there are two products.
Simply apply Conservation of Mass.
• Note – this is your 1st organic reaction !!!
Let’s move on to structure…
• In alkanes, all of the
carbon atoms will be in
sp3 hybrids and will form
4 sigma bonds.
• That means that all of the
bond angles will be 109.5
degrees.
• The carbon atoms will
form a “backbone” for
the molecule and all of
the hydrogen atoms will
be on the “outside” of the
molecule.
What about names and formulas?
• The key to this part of the
discussion is to realize
that Chemistry has
assigned prefixes to
represent specific
numbers of carbon atoms
in the molecule.
• You will simply have to
memorize these prefixes
and the number of
carbon atoms being
defined.
So what are these prefixes?
•
•
•
•
•
C1
C2
C3
C4
C5
=
=
=
=
=
meth
eth
prop
but
pent
•
•
•
•
•
C6
C7
C8
C9
C10
=
=
=
=
=
hex
hept
oct
non
dec
Note that we do not typically write a “1” for a subscript
– that is only there to establish the numerical pattern.
Think about the carbon-hydrogen ratio in
alkanes to answer the following questions.
• What is the formula of
an alkane that has 5
carbon atoms?
• How many hydrogen
atoms are there in a
molecule of octane?
(notice the naming –
the –ane suffix tells you
that it is an alkane)
• An alkane has 22
hydrogen atoms. What
is the name of this
alkane?
• A cracking process
starts with C16H34. One
of the products has the
formula C10H20. What is
the name of the other
product?
Representing alkanes:
• There are three different methods that may be
used to represent a given alkane.
• The first is the regular chemical formula
method (the Stock system) in which you
simply use symbols and subscripts.
– For example, butane has 4 carbons and 10
hydrogens and would be represented as
C4H10
The second method:
• This is called a structural formula. It is sort of
like a regular chemical formula in that it does
have symbols and subscripts, but the formula
also gives the reader an insight as to how the
molecule is assembled.
• In the case of butane, the structural formula
would be:
CH3CH2CH2CH3
And finally…
• We have a “displayed”
formula. This method of
representing alkanes
will have you actually
drawing the molecule
to illustrate the
bonding. In the case of
alkanes, it is not
typically necessary to
show the actual bond
angles.
A quick check:
H
• Write the chemical
formula for a molecule
of hexane.
• Write the displayed
formula for a molecule
of heptane.
• Write the structural
formula for a molecule
of pentane.
• What is the name of the
compound illustrated
below?
H
H
H
H
H
H
H
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
What if the organic molecule has a
“branched-chain” structure?
• The naming here gets a bit more complicated
because you have to account for the actual structure
of the branch itself and indicate the location of that
new addition to the molecule.
• Now go to one of the computers, copy the following
web address into a browser, and read through the
page presented. Then work through the interactive
exercise on naming branched chain alkanes.
www.sciencegeek.net/APchemistry/organic/ochem.shtml