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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 sp^3 C-sp^3 C (your basic C- C single bond) or sp^3 C-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 sp^3 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
sp 3 C-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):
Cl
Br I
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
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 H
O O
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.
C C
O
HO C C^ H
O O
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:
H
O H
O H O
Can you tell them apart?
Which of the following is the aldehyde and which is
the ketone?
H
O O
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).
C H
O
C C
O
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:
OH
O HO
O OH
O
Can you tell them apart?
Which of the following is the aldehyde and which is
the carboxylic acid?
H
O HO
O
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).
C H
O
HO C
O
Can you tell them apart?
Identify which of the following is the carboxylic
acid, aldehyde and ketone:
H
HO
O
O O
Can you tell them apart?
Identify what’s on either side of the carbonyls:
C C H^ C
HO C O
O O
ketone aldehyde^ 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
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:
Cl
O
Br
O
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?
Br (^) Br
O
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 sp^3 hybridized carbon (alkyl group) attached to the halide.
Br (^) Br
O
alkyl halide (^) acid halide
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: N CH 3 CH 2 CH 2 CN 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.
N nitrile internal alkyne H terminal 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 NO 2. Note how each of the following has a nitro group:
O 2 N N NO^2
O O
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!
