Download Organics – Naming the Molecules

Organics – Naming
the Molecules
Types
Chain lengths
Add–ons
Specific naming
Author: J R Reid
Naming - Introduction
In organic chemistry the names of each
molecules has three main parts:
The type of chemical e.g. Alkane
The length of its longest carbon chain
The add–ons to the main chain of carbons
When we put them together into the name
we use the order:
Add–on : Length of chain : Type of organic
To make life interesting, we give codes to
help us so that we don’t have to write:
One chlorine is attached to the second carbon in a
chain five carbons long in an alkane
Naming - Types
As we have seen
already there are
a few different
types of organic
chemicals. Each
one is given a
specific code
name that gets
tacked on as a
suffix (end of a
word)
Type
Suffix
Example
Alkane
-ane
propane
Alkene
-ene
propene
Alkyne
-yne
propyne
Alkanol
-ol
propanol
Carboxylic -oic
acid
acid
Ester
-oate
propanoic
acid
methyl
propanoate
Naming – Chain Length
The organic molecules
can come in a number
of lengths e.g. 1
carbon, 2 carbons…
The number of
carbons in the main
chain of the molecule
forms a prefix
(beginning of a word)
for the name
Number Prefix
Example
1
Meth-
Methane
2
Eth-
Ethene
3
Prop-
Propyne
4
But-
Butanol
5
Pent-
Pentane
6
Hex-
Hexene
7
Hept-
Heptyne
8
Oct-
Octanol
9
Non-
Nonane
10
Dec-
Decene
Naming – Add-ons I
The main chain can have
stuff added to it. These
might be extra carbon
branches or halogens. It is
important for us to be able
to know exactly what and
where they are.
This forms the very first
part of the name (before
the prefix). Here is how we
do it:
Count the number of
carbons in from the
closest side (along the
main chain). This forms
the number that goes at
the front of the name
Next write the code for
the add-on
Add-on
1 carbon
2 carbon
chain
Code
methyl
ethyl
3 carbon
chain
Chlorine
propyl
Bromine
bromo
H3C
HC
chloro
CH2
CH2
CH3
e.g. 2 methyl pentane
CH3
Naming – Add-ons II
Some times they add
more than on add-on.
This means that the
name needs to show
us how many there
are and where we can
find them.
To do this we need to:
Add all the numbers
showing where each
add-on is found
Sometimes the add-on
code needs a prefix
telling us how many
there are
Number of
the same
add-ons
2
Prefix
3
4
tritetra-
di-
H3C
H3C
CH
HC
CH3
CH3
e.g. 2,3 dimethyl butane
Practice
Specific Naming Rules–
Alkenes and Alkynes
Some types have extra part to their name
specifically telling us where certain features can
be found:
Alkenes – we need to know where the double bond is. In
this case we count in from the closest side – if the
double bond is on the first bond of a pentene we would
call it pent-1-ene
Alkynes – the same rule applies e.g. oct-3-yne
H2C
CH
CH2
CH2
CH2
CH2
CH2
CH3
H3C
CH2
C
C
CH2
CH2
CH2
CH3
H3C
CH2
CH2
CH2
CH2
C
C
CH3
Specific Naming Rules –
Alkanols (Alcohols)
Naming and alcohol is very
similar to naming alkenes
and alkynes. This is
because the location of the
–OH group on the chain is
very important
In this case we number the
carbon (from the closest
end) and locate which
carbon the –OH is on.
For example, ‘propan-1-ol’
means that the –OH is on
the first (number 1)
carbon. ‘Propan-2-ol’ has
the –OH on the second
carbon
Why is there no such
chemical as ‘propan-3-ol’ ?
H 2C
CH2
CH2
CH3
CH
CH2
CH3
CH
CH3
OH
H 3C
HO
H 3C
CH2
OH
Specific Naming Rules Esters
Esters are made when we
react an alcohol with a
carboxylic acid.
Unfortunately they are
difficult to name because
they have a break in their
carbon chain. We therefore
have to add two parts to
their name:
The part from the alcohol
comes first. Its name comes
from how many carbons it
has e.g. methyl, ethyl,
propyl… etc
The part from the
carboxylic acid comes last.
It ends in ‘-oate’ e.g.
methanoate, ethanoate,
propanoate… etc. You can
always tell which part came
from an acid, because acids
have more oxygen atoms
H3C
CH2
CH2
C
O
CH2
CH3
O
Acid half = butanoate
Alcohol half = ethyl
Ethyl butanoate
Practice II