Transcript File

Organic Chemistry
Structure and Nomenclature
What is Organic Chemistry
• This is the branch of
chemistry in which the
focus is on the
compounds that are
composed (primarily) of
carbon and hydrogen.
• It is possible (and
common) for there to
be other elements in
these substances, but
the majority of the
atoms are C and H.
Are There Classes of Organic Compounds?
• Yes – just as we classify
compounds in general
chemistry based on
what elements and
behaviors are observed
(ionic, covalent, acids,
bases, hydrates, etc)…
• We classify organic
compounds based on
the elements present
and the bonding and
structure.
• Alkanes
• Alkenes
• Amines
• Alcohols
• etc
Where do we Start?
• The easiest of the
organic compounds are
the Alkanes.
• They consist of only
carbon and hydrogen
and only have single
bonds between the
atoms.
• Things to remember:
– Carbon will form 4 bonds
– It will use sp3 hybrids
– The bond angles will be
109.5°
– The hydrogen atoms will
always be on the
“outside”.
Alkanes
• The are the organic
compounds that have
the generic formula
CnH(2n+2)
• Examples are:
CH4
C2H6
C3H8
C4H10
C5H12
Naming Alkanes
• In organic, the name of
the compound typically
starts with a prefix that
represents the number
of Carbon atoms in the
“backbone” of the
structure.
•
•
•
•
•
C =
meth
C2 =
eth
C3 =
prop
C4 =
but
C5 =
pent
In the alkanes, all you
have to do is add the
suffix “ane” to let the
reader know that you
have an alkane.
So……
CH4
C2H6
C3H8
C4H10
C5H12
= Methane
= Ethane
= Propane
= Butane
= Pentane
Consider the Structures
• Cambridge will want
you to be able to
represent a given
organic molecule three
different ways.
• Structural or “Lewis
Dot” (perhaps dot and
cross).
• Displayed
• Skeleton
Examples
Isomers
• The word “isomer”
actually translates to
“same units”.
– Iso means “same”
– mer is from the Greek
for “unit”
• These are different
compounds that have
the same chemical
formula.
• The different
compounds are simply
constructed differently
in nature.
• The will have very
different physical and
chemical properties
because of the
difference in structure.
Examples
If you look carefully, you
can see that the two
compounds have the
same numbers of atoms
of each element, but the
construction is obviously
different.
We will need to develop
a method of naming to
communicate to the
reader how the
molecule is built.
Naming These Structural Isomers
• There are several things
that you have to keep in
mind.
– You need to determine
the fundamental
“backbone” of the
molecule – what is the
longest continuous
carbon chain?
• Next – what type of
modification is attached
– ask yourself what has
replaced one of the
hydrogen atoms.
• Then determine the
location of the
attachment – on which
number carbon is the
modification attached?
Example
This is simply “pentane” . It has 5
carbon atoms in a continuous
chain and nothing but hydrogens
attached.
This is an isomer of pentane. If
you look carefully, the longest
continuous chain has only 4
carbon atoms. But, there is a
CH3 (a “methyl” group)
replacing one of the H atoms on
the 2nd carbon atom in the chain.
So, this compound is “2-methyl
butane”.
You can have more than 1
attachment:
These are actually the same –
just different representations
of the same molecule – name
it …
Going the other way…
Draw the following molecules:
3-methyl – 4 ethyl heptane
2,2 – dimethyl propane
3,5 – dimethyl – 6-ethyl decane
Now draw them using skeleton notation.
Substituted Alkanes
• As stated earlier, it is
possible to have organic
compounds that
contain other elements.
• As in the structural
isomers, these
“substitutions” will
simply replace one or
more of the H atoms
attached to carbons.
• When this happens, we
actually have a totally
new class of organic
compound.
• The names of these
new classes of
compounds will reflect
what has been used to
replace an H atom.
Classes of Substituted Alkanes
• If a halogen atom (F, Cl, • If an H atom is replaced
Br, I) is used to replace a
with an “NH2” group,
Hydrogen, the
the compound is now
compound is now an
an “amine” (related to
ammonia).
“alkyl halide”.
• Replacing an H with an
• If an H atom is replaced
“OH” (not a hydroxide
with a “CN” you will
in this context) will give
have a “nitrile” (note
that the carbon chain
you an “alcohol”
will have become
longer).
Alkyl Halides
This is a displayed formula – it
shows the 3-dimensional
structure – the tetrahedral
geometry.
More important here is that one
of the H atoms has been
replaced by a Cl atom.
There are two ways to name
this:
It can be “methyl chloride”
It can be “chloromethane”
Consider this one:
What are the two ways to name this?
Alcohols
Notice that there is an
“OH” group attached to a
two-carbon chain. The
name will have to
account for both of those
factors.
As with the halides, there are two different ways to
name this molecule.
• it can be “ethyl alcohol”
• it can be simply “ethanol”
Variations within the Substituted
Alkanes
All three of these have
the same chemical
formula, but they have
different physical and
chemical properties
because of the location
of the OH group.
Your naming will have to
account for the location.
Amines
As stated earlier, replacing an H atoms with an
“NH2” group will result in an “amine”. Obviously,
the name will have to reflect this change in chemical
identity.
This one is
“ethylamine”.
And finally, Nitriles.
You end up with a nitrile when an H atom is replaced
with the “CN” group. There are several things to know
about these compounds.
1.The “CN” group contains a triple bond – Cambridge
expects you to know this.
2.Adding the CN group will typically increase the
length of the carbon chain – that will require you to
change the name to correspond to the greater number
of carbon atoms.
Consider these two compounds:
Now replace
a H atom with
the CN group
propane
The CN has been added
to the end of the propane.
The molecule is now
named “butanenitrile”