Identifying Functional
Groups
The Key to Survival
Why is this so important?
 Over and over again, you will be asked to do reactions,
the details to which you will receive in lecture and via
your textbook. For each reaction, reagents will be used
to convert one functional group into another.
 If you cannot recognize what a functional group looks
like, it will be impossible for you to do this process…
The facts that you need to remember won’t mean
anything… And you won’t be able to draw the right
pieces on your molecules…
What is a “functional group”?
 A functional group is any collection of bonded atoms
that are not simply sp3C-sp3C (your basic C-C single
bond) or sp3C-H.
 The only family in organic chemistry that has only C-C
and C-H single bonds is the alkane family and they are
recognized by their LACK of functional groups (or
functionality, if you will).
Why is this necessary?
 Knowing your functional groups will make your life
much easier when its time for you to identify them in a
lab assignment (Infrared Spectroscopy) or when you
have to remember facts, like the reaction of an alkene
with water and a catalyst forms an alcohol. If you don’t
know what an alkene or alcohol looks like, this class is
impossible.
 In order to identify the functional groups, you have to
know what sorts of bonds are in each kind of functional
group.
 We’ll scroll through the functional groups, one at a time
and compare and contrast, until you (hopefully) get
used to what they look like.
 Just remember, organic professors are devious – and
sometimes the mere change from a zig-zag structure to
a condensed formula can entirely confuse you!
Alkanes
 Just for comparison’s sake, let’s start with something
that has no functional groups. Alkanes have C-C single
bonds and C-H bonds and all the carbons are sp3
hybridized. All of the following showing below are
alkanes. Those in rings are still alkanes, and are
referred to as cycloalkanes but they don’t have
functional groups either.
Alkyl Halides
 The alkyl halide is identified by the presence of an
sp3C-X bond in the structure. Halides can be fluorine,
chlorine, bromine or iodine (X=F, Cl, Br or I). Each of
the following has an alkyl halide (or more specifically,
alkyl bromide, alkyl chloride, alkyl iodide, if you wish):
Br
I
Cl
Alkenes
 An alkene has a C=C in its structure. This C=C might
have carbon groups or hydrogen atoms attached.
Each of the following are alkenes:
Alkynes
 An alkyne has a triple bond between two carbon atoms
in its structure. Triple bond might have carbon groups
on both or perhaps one carbon group and one
hydrogen atom attached (usually not drawn!). If there
are two carbon groups attached, an “internal alkyne”.
Note the linear structure, as should be the case for sp
hybridized carbon atoms.
Alkynes
 An alkyne has a triple bond between two carbon atoms
in its structure. If there is one carbon group attached
(and one hydrogen atom), the functional group is still
called an alkyne and we fine-tune the identification by
calling it a “terminal alkyne”:
is the same as
H
Arenes or Aromatic Rings
 An arene or aromatic ring, for the purpose of this
introductory course, is a six-membered ring of carbon
atoms, with alternating double and single bonds. It can
have up to six groups attached to it. These may also be
called “aryl” groups. The following are examples of
aromatic rings:
Ethers
 The ether is identified by the presence of C-O-C bond
in the structure. The carbon groups on the left and
right side can be alkyl groups or aryl groups but they
cannot be carbonyls (C=O). Each of the following is an
ether:
O
O
O
Epoxides
 The epoxide is a specific type of ether functional group,
and contains the C-O-C bond inside a three-membered
ring. Each of the following is an epoxide:
O
O
O
The Amine
 The amine functional group has a nitrogen atom. The
nitrogen atom can have two hydrogen atoms (NH2),
one hydrogen atom and one carbon group (NHR) or
two carbon groups (NHR2 or NHRR’). The carbon
groups CANNOT be C=O (carbonyl). Note how each
of the following has a nitrogen:
NH2
N
H
N
What is what?
 One of the following is an alkyl halide, one is an ether
and one is an amine. Identify which is which.
H
N
O
Br
What is what?
 One of the following is an alkyl halide, one is an ether
and one is an amine. The ether has the oxygen
surrounded by carbons. The alkyl halide has the
HALIDE and the amine has the nitrogen atom.
O
ether
H
N
Br
alkyl halide
amine
What is what?
 One of the following is an alkene, one is a terminal
alkyne and one is an internal alkyne. Assign the
structures to their type of functional group.
What is what?
 One of the following is an alkene (with a double bond)
and the others have triple bonds. They are alkynes.
The alkyne in the middle of the chain is the internal
alkyne and the other, at the end of the chain, is the
terminal alkyne.
terminal alkyne
alkene
internal alkyne
The Carbonyl Compounds
 There are a whole series of compounds that contain
the carbonyl group, C=O. The classification of the
functional group is dependent upon what is attached to
either side of the carbon atom of the carbon group.
Identify what is attached to the left and right sides of
each of the following:
O
HO
O
O
H
The Carbonyl Compounds
 The first compound has OH on the left and C on the
right.
 The second compound has C on the left and C on the
right.
 The third compound has C on the left and H on the
right.
HO
O
O
O
C
C
C
C
H
The Ketone (NOT KEYTONE)
 The ketone functional group has a carbonyl group,
C=O, surrounded by carbon groups on both sides.
They can be alkyl groups or aromatic rings. The
condensed formula for a ketone group is –C(O)-. Note
how each of the following has carbon groups on both
the left and the right:
O
O
O
The Aldehyde
 The aldehyde functional group has a carbonyl group,
C=O, surrounded by a carbon group on one side and a
H atom on the other. The carbon group can be alkyl or
aromatic. The condensed formula for an aldehyde is –
CHO. Note how each of the following has a carbon
group on one side and a hydrogen atom on the other:
O
O
H
H
H
O
Can you tell them apart?
 Which of the following is the aldehyde and which is the
ketone?
O
O
H
Can you tell them apart?
 The compound on the left is the aldehyde functional
group (see the H on the right of the carbonyl?) and the
compound on the right is the ketone (two carbon
groups attached).
O
C
O
H
C
C
The Carboxylic Acid
 The carboxylic acid functional group has a carbonyl
group, C=O, surrounded by a carbon group on one side
and a hydroxyl group, OH, on the other. The carbon
group can be alkyl or aromatic. Note how each of the
following has a carbon group on one side and an –OH
on the other:
O
O
OH
HO
O
OH
Can you tell them apart?
 Which of the following is the aldehyde and which is the
carboxylic acid?
O
O
H
HO
Can you tell them apart?
 The compound on the left is the aldehyde functional
group (see the H on the right of the carbonyl?) and the
compound on the right is the carboxylic acid (carbon on
right, OH on left).
O
C
O
H
HO
C
Can you tell them apart?
 Identify which of the following is the carboxylic acid,
aldehyde and ketone:
O
O
H
HO
O
Can you tell them apart?
 Identify what’s on either side of the carbonyls:
O
O
C
C
ketone
H
HO
C
aldehyde
C
O
carboxylic acid
The Ester (NOT ESTHER)
 The ester functional group has a carbonyl group, C=O,
surrounded by a carbon group on one side and an OR’
group on the other. Either carbon group, attached to the
C=O or on the O atom, can be alkyl or aromatic. Note
how each of the following has a carbon group on one
side and an OR on the other:
O
O
O
O
O
O
Can you tell them apart?
 Identify which of the following is the ketone and which
is the ester?
O
O
O
Can you tell them apart?
 Identify what’s on either side of the carbonyls:
O
O
C
O
ester
C
C
ketone
Can you tell them apart?
 Note the difference between an ester and an ether.
Esters have an OR group attached to a carbonyl.
Ethers do not.
O
ester
O
O
ether
 These are the two most confused functional groups!
Can you tell them apart?
 Does the following molecule contain an ester or an
ether?
O
O
Can you tell them apart?
 With the OR group on the C=O, this is an ester.
O
O
 What about this molecule? Does it have an ester?
O
O
Can you tell them apart?
 With the OR group NOT on the C=O, there is not an
ester in this molecule. Notice that this has TWO
different functional groups, a ketone on the left and
ether on the right.
ketone
O
C
C
C
O
C
ether
The Acid Halide
 The acid halide functional group has a carbonyl group,
C=O, surrounded by a carbon group on one side and
an –X atom on the other. X is typically a chloride or
bromide. Note how each of the following has a carbon
group on one side and an X on the other:
O
O
Cl
Br
The Acid Anhydride
 The acid anhydride functional group has two carbonyl
groups, C=O, surrounding a central oxygen atom, and
carbon groups on the other sides of the carbonyls. The
carbon groups can be the same or different. Note how
each of the following has the two carbonyl groups,
surrounding an oxygen atom:
O
O
O
O
O
O
Can you tell them apart?
 Which is an ester and which is an acid anhydride?
O
O
O
O
O
Can you tell them apart?
 Note the identifying characteristic of the double
carbonyl in an acid anhydride. An ester only has one
carbonyl (C=O).
O
O
O
O
O
acid anhydride
ester
Can you tell them apart?
 Which is an alkyl halide and which is an acid halide?
Both have halides (in this case, bromides) but what
makes them different?
O
Br
Br
Can you tell them apart?
 The acid halide has its chloride or bromide attached
directly to a carbonyl group (C=O). The alkyl halide
has an sp3 hybridized carbon (alkyl group) attached to
the halide.
O
Br
alkyl halide
acid halide
Br
The Nitrile
 The nitrile functional group has a C-N triple bond, and
the nitrogen has a lone pair. Its commonly mistaken
that the nitrogen atom has an undrawn hydrogen atom
but that would be an error in both structure and
thought. The condensed formula for a nitrile is –CN.
Each of the following contain a nitrile:
CH3CH2CH2CN
N
N
Can you tell them apart?
 Which is an alkyne (terminal, internal) and which is a
nitrile?
N
Can you tell them apart?
 The nitrile has a nitrogen atom in its triple bond. The
other two are alkynes. The alkyne at the end of the
chain is the terminal one (with a typically unseen
hydrogen now shown) while the one in the center of the
chain is the internal alkyne.
terminal alkyne
H
N
nitrile
internal alkyne
The Nitro
 The nitro functional group is typically drawn as one of
two possible equivalent resonance forms (they look
alike), containing one N=O and one N-O.
The
condensed formula for a nitro group is NO2. Note how
each of the following has a nitro group:
O
N
O
O2N
NO2
So there you go…
 You have now seen all of the functional groups
commonly encountered in organic chemistry. There
are others lurking out there but if you know THESE,
you are in really great shape… Now go try that
problem set to double check yourself!