HOW DO I NAME A MOLECULE?
Table of Contents
Introduction
Suffix
Prefix
Substituents on a Substituent
Alkane
Alkyl Halide
Ether
Nitro
Cyclic Alkane
Alkene
Alkyne
Alcohol
Amine
Stereochemistry
Aromatic
Substituted Aromatic
Disubstituted Aromatic
Three or more substituents on a benzene ring
Carboxylic Acid
Acid Halide
Anhydride
Ester
Amide
Carboxylic Acid Salt
Nitrile
Aldehyde
Ketone
Introduction:
First, examine the molecule for functional groups. There is a short hierarchy of
functional groups on p262 in chapter 6 and a more extensive one on p791 in chapter 17.
These functional groups will determine what the parent molecule will be called. This is
represented in the suffix of the IUPAC name. Remember, the IUPAC has three parts:
prefix – stem (or root)- suffix
The Suffix
The suffix tells what the parent functional group is. The stem tells us how many
carbons are in the longest carbon chain that contains the parent functional group. The
prefix includes everything not mention in the stem and suffix. Here is a table of
functional groups, by hierarchy, highest to lowest priority. You may notice that the more
bonds to oxygen, the higher the functional group in the hierarchy.
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Table I: Functional Group Priorities
Structure
O
R
Suffix name
-oic acid
Prefix name
Carboxy
Ester
-oate
Alkoxycarbonyl
Amide
-amide
Amido
Nitrile
-nitrile
Cyano
Aldehyde
-al
Oxo
Ketone
-one
Oxo
Alcohol
-ol
Hydroxy
Amine
-amine
Amino
Alkene
-yne
-*
Alkyne
-ene
-*
C - O - R1
O
R
Name
Carboxylic acid
C - NH2
R
C
N
O
R
R
R-O-H
R
Alkane
-ane
alkyl
* technically, alkene and alkyne do have prefix names, but we won’t worry about
that for now. If you’re desperate to know, check p791.
R stands for a carbon chain of any length. For alkanes, amines, alcohols and
ketones, R has to have at least one carbon, as does R1 on esters. For all other functional
groups, R can have zero carbons (be just hydrogen).
There are a few more, less common functional groups that may be added later, as
time permits. These would include anhydrides, acyl halides, sulfides, thiols, and
disulfides. See the inside back cover of the book for these functional group structures.
The Root:
Now that we have seen the functional groups that determine the suffix, let us take
a look at the stem. The stem gets a name that delineates how many carbons are in the
carbon chain containing the parent functional group. For linear chains, we have:
2
In addition to linear chains, we can have cyclic chains as the parent. These are
named also by number of carbons in the cyclic. We do this by putting the word “cyclo-”
in front of the number of carbons:
The Prefix
There is only one stem, and there is rarely more than one suffix, but there can be
and usually are many prefixes in a single molecules name. That is because the prefix is
responsible for everything else not covered by the single stem and suffix. Because it is
not part of the parent functional group and chain, it is called a substituent. Remove a
hydrogen from the parent carbon chain, put on something else, and it becomes a
substituent.
Table II: Common Substitiuents
Structure
-OR
Name
Ether
Prefix name
alkoxy*
Alkyl halide
-F
-Cl
-Br
-I
-R
alkyl
flouro
chloro
bromo
iodo
alkyl*
3
-NO2
nitro
nitro
Miscellaneous
Vinyl
Allyl
Phenyl
Benzyl
Formyl
* for alkyl and alkoxy, substitute “alk” for the name indicating the number of carbons.
For instance, an alkyl substituent with three carbons would be propyl. An ether with two
carbons would be ethoxy.
Table III: Branched Alkyl Substituents
Structure
Name
iso
(connected at the second C)
Prefix name
isopropyl
iso
(branched at the second C,
connected at the terminal C)
isobutyl
sec
(straight chain connected at the
second C)
sec butyl
tert
(like isobutyl, but connected at
central C instead of end C)
t-butyl or
tert-butyl or
tertiary butyl
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Putting it all together to name a molecule
We have discussed the three parts that go into an IUPAC nomenclature. Now, let
us combine these parts to actually name molecules.
Linear Alkanes
Start with the simplest functional group, or rather, lack of functional group.
Alkanes have only carbon and hydrogens and sometimes, other things from the table of
substituents above. Follow these steps to name an alkane.
1. find longest carbon chain and name it by # of carbons (I
recommend circling the longest chain to see which is the parent –
circled- and which parts are substituents – everything else
2. if a tie parent chain (two or more possible), parent is one that has
more substituents (more substituents = easier to name)
3. name and number substituents
a. substituent is prefix
b. number from side that gives smaller number
c. common names almost never have number, IUPAC always do
4. if more than one substituent
a. if more than one of same substituent, use di-, tri-, tetra-, penta-,
hexa-, heptai. example: 1,1 dichloroethane
b. for every substituent, there is a number
c. substituents are put into alphabetical order
i.
di, tri, tetra, (etc), sec, and tert ignored for
alphabetizing
ii.
iso and cyclo count for alphabetizing
iii.
cyclopentyl before isobutyl, dibromo before chloro
5. numbering if more than one substituent
a. number from side that gives lowest first substituent number
b. if tie, keep going until tie-breaker reached (1st point of
difference)
6. can use “iso-,” “sec-,” or “tert-,” but IUPAC substituent on a
substituent method usually preferred (see below for how to)
7. moosh entire thing into one word
a. dashes separate words and numbers
b. commas separate numbers
Examples:
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Substituents on Substitutents
What if there is no way to have just simple substituents off of your parent? Well,
there is a way to name these more complicated substituents. They get a number saying
where they are on the parent, and then the entire complicated substituent including the
numbers and names of the substituents on the substituent is in parenthesis.
Examples:
O
Cl
2-methoxy-5-(2-methylpropyl)nonane
5-(1-chloro-1-methylethyl)decane
Cycloalkanes
1. Count the number of carbons in the ring and insert cyclo in front
2. If one substituent, no need to number that one substituent
3. If more than one substituent, number them all. If two substituents, the first in the
alphabet gets #1, then number clockwise or counterclockwise, whichever gets you a
lower number for the next substituent.
4. If three or more substituents, number the first so that the 2nd gets the lowest number
possible. If a tie, then number so the next gets as low as possible.
5. cyclics can be substituents. Change the –ane suffix to -yl
5. Stereochemistry (only name if shown). Substituents can be on the same side of the
ring as each other, or on opposite sides (trans). This stereochemistry is usually shown
with perspective drawings (dash-wedge diagrams). Stereochemistry designations go all
the way in front of the molecule. Consider them “super prefixes.” This is only good
when there are only two substituents on the ring. This will later be superceded by R/S
designations.
Examples:
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OCH2CH3
1-sec-butyl-3-nitrocyclohexane or
1-(1-methylpropyl) -3-nitrocyclohexane
1,2,4-trimethylcyclopentane
cis 1-ethoxy-3-methyl
cyclopentane
Alkenes
1. find longest chain containing alkene and number from the side so that alkene # is the
smallest
a. every double bond needs a number to tell where it is in chain. The number can be
in front of the stem (1-chloro-2-pentene) or in between the stem and the suffix
(1-chloropent-2-ene)
2. if 2 double bonds, diene
a. if 3 double bonds, triene
b. if 4, tetraene, then pentaene, hexaene, heptaene, etc
c. every double bond gets a number stating where it is
3. substituents with number (#) as prefixes
a. funct grp determines which side of molecule to start numbering
b. if a tie, then use first substituent rule to break the tie (1st point of difference)
4. if more than 1 substituent, place in alphabetical order in front
5. alkene always #1 in cyclics (so no need to #), other C in pi bond is #2
a. no need to include number of pi bond in cyclic name unless diene, when both
numbers included
b. if multiple alkenes, count clockwise (cw) or counterclockwise (ccw),
whichever gives the lowest 2nd number for alkene. If tied, go to next alkene.
c. if still tied after all alkenes examined, go to substituents for tiebreaker
d. 1st point of difference, not sum of all subst #
6. Stereochemistry: cis/trans and E/Z (another “super prefix”)
a. look at the two substituents on each sp2 C
b. if either sp2 C has two of the same things (2H’s, etc) on it, no cis/trans or E/Z
c. if different, assign relative priorities by atomic number (# of protons)
d. if tie after first atom, keep moving until first point of difference
e. if dble or triple bonds, consider each bond as connected to another of same
molecule (looking at –CH=CH2, first C is connected to two C’s, 1 H)
f. if isotopes, like H and D, higher neutron # has higher priority (D>H)
g. do same for other sp2 C in double bond
h. now compare (E/Z):
i. if higher priority on same side (closer to each other), than molecule is Z
(zusammen)
ii. if higher priority on opposite side (further from each other), than
molecule is E (entgegen)
i. for cis/trans:
i. look for matching substituent on both sp2 carbons.
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ii. if matching substituents on same side of double bond  cis
iii. if matching substituents on opposite side of double bond  trans
iv. if no matching substituents, then cis/trans does not apply
j. in rings, bond is always cis unless explicitly mentioned otherwise. The
reasoning being that almost all rings have cis bonds. It would take a very big ring system
to be able to loop around a planar trans alkene bond.
Examples:
Cl
3-chlorocyclopentene
(not 2-chlorocyclopentene)
4-isopropyl-1-methylcyclohexene or
1-methyl-4-(1-methylethane)cyclohexane
NO2
H3CH2CO
1-ethoxy-6-nitro-1,4-cyclooctadiene
1,6-dimethylcyclohexane
not 2,3-dimethylcyclohexane
All of the above do not need cis/trans or E/Z because they are rings or because there one
of the sp2 carbons has two of the same thing on it (two methyls in first example and two
hydrogens in second example). Below are some alkenes which do need stereochemistry
designations.
Examples:
trans 2-butene
or (E) 2-butene
cis 2-hexene
or (Z) 2-hexene
(Z) 3-methyl-3-heptene
cis/trans cannot be used because substituents
on C-3 have no match on C-4
8
Cl
trans 3-chloro-3-hexene
or (Z) 3-chloro-3-hexene
just because they are the
same doesn’t mean they
have same priority
trans 1,3-hexadiene
(E) 1,3-hexadiene
first alkene doesn’t have
cis/trans or E/Z
2-cis, 5-trans 2,5-octadiene
(2Z, 5E) 2,5-octadiene
Alkynes
Alkyne rules are virtually identical to alkene rules, except no cis/trans isomers
because only one other thing connected to sp carbon in alkyne. Alkene and alkyne are
neck and neck with each other as far as priority goes, but if push comes to shove, the
alkene wins. What the heck does that mean? It means that if a molecule has both an
alkene and alkyne in it, count in from both ends until you get to the first unsaturated
bond, whether it is alkene or alkyne. The lowest number indicates from which side you
count. Only if there is a tie, do you favor the alkene. If both alkene and alkyne present,
you have a double suffix: “-ene” comes first, but loses 2nd “e,” followed by “-yne.” Both
the shortened “-en” and the “–yne” have numbers immediately preceding them
designating where the double and triple bonds are.
1. find longest chain containing alkyne and number from the side so that alkyne # is the
smallest
a. every triple bond needs a number to tell where it is in chain. The number can be
in front of the stem (1-chloro-2-pentyne) or in between the stem and the suffix
(1-chloropent-2-yne)
2. if 2 triple bonds, diyne
a. if 3 triple bonds, triyne
b. if 4, tetrayne, then pentayne, hexayne, heptayne, etc
c. every triple bond gets a number stating where it is
3. substituents with number (#) as prefixes
c. funct grp determines which side of molecule to start numbering
d. if a tie, then use first substituent rule to break the tie (1st point of difference)
4. if more than 1 substituent, place in alphabetical order in front
5. alkynes rarely in cyclics because of 180 bond angle. May be present in large cyclics,
in which case, alkene cyclic rules apply
Examples:
CH3CH2O
7-ethoxy-3-heptyne
or 7-ethoxyhept-3-yne
(Z) hex-4-en-1-yne
(Z) non-3-en-6-yne
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Alcohols
1. find longest carbon chain containing alcohol
2. suffix: drop final “e” from “-ane” (or –ene or –yne) and add –ol
3. straight chains: number from side that gives lowest number to -OH
a. if tie from both ends, use other substituents to break tie
4. can put number before root or directly before suffix (2-propanol or propan-2-ol)
5. if more than one suffix (ex. but-2-en-1-ol), numbers go immediately before suffix
6. if cyclic, alcohol (highest priority substituent so far) automatically #1, so no need to
include number for alcohol unless more than one alcohol group
7. if more than one alcohol, diol, triol, etc and every alcohol needs a number
Examples:
OH
CH3CH2O
OH
4-ethoxy-2-butanol
HO
4-methylcyclohex-3-enol
HO
OH
2-methyl-1,5-heptanediol
cyclopentanol
OH
hept-6-en-3-yn-2-ol
Amines
1. Is amine primary (1), secondary (2), tertiary (3), or quaternary (4)?
2. if primary, can name amine as “amino” substituent (1-aminoethane)
3. or name amine as parent (suffix: remove final “-e” and add “-amine”)(ethanamine)
4. if 2, 3, or 4, name other alkyl substituents preceded with “N-”
5. when alphabetizing substituents, “N-” not counted
6. if 4 (quaternary ammonium salt) name as ionic cmpd (cation first, then anion)
7. if more than one amine, diamine, triamine, etc.
Examples:
NH2
N
H
aminocyclohexane
N-cyclopropyl-1-butanamine
or cyclohexanamine or N-butylcyclopropanamine
N
+
Cl-
cyclopentylethyldimethylammonium chloride
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Stereochemistry
In addition to cis/trans (for cyclics and alkenes) and E/Z stereoisomers (for
alkenes), there are also R/S stereoisomers for chiral carbons (carbons with four different
substituents).
1. R or S?
a. rank the four subst. on chiral carbon according to priority (1-4)
b. lower on periodic table = higher priority (E/Z system, sect. 3.5)
c. orient molecule so lowest priority (#4) is away from you
d. draw arrow around 1-2-3
i. if clockwise arrow (to right): R (rectus in Latin)
ii. if counterclockwise (to left): S (sinister in Latin)
e. R/S in parenthesis in front (a la E/Z from chapt. 3)
2. Trouble orienting? try...
a. rotate and redraw molecule (very slow: use only as last resort)
b. or shift your imaginary eye on the paper
c. or put #4 towards you, then reverse your conclusion (like looking out
the backside of a mirror)
d. or switch two groups to put #4 behind and reverse your conclusion
e. why reverse conclusion in “c” and “d”?
i. doing either of those gives you enantiomer of original
molecule’s configuration
ii. reverse R and S to get original’s configuration again
f. one switch gets you enantiomer of original, two switches puts it back
to original
3. Fischer projections
a. vertical lines going away, so #4 on vertical is fine, do R/S
configuration as normal
b. horizontal lines coming towards you, so switch R/S conclusion when
#4 on horizontal (a la 2c, above) or do as in 2d, above
c. never rotate Fischer projection 90° or flip it over like a pancake
because inherent stereochemistry in drawing will be changed
(what comes towards you and what comes away from you).
Examples:
4
3
CH2CH3
CH3
2
2 HO
H3CH2C
Cl
H
1
4
This is (R) 2-chlorobutane
OCH3
1
CH2CH2CH3
3
This is (S) 3-methoxy-3-hexanol
11
3
CH3
2
H 4
H3CH2C
Cl
1
Normally, rotating right means R, but #4 coming
towards you, so you reverse your conclusion. R becomes S. This is (S) 2-chlorobutane
Aromatics
Aromatics can be parents. Common parent aromatics:
or
benzene
naphthalene
anthracene
Aromatics can also be substituents. Common substituent aromatics:
Monosubstituted Benzenes:
No number is necessary. Many aromatics are named benzene with a substituent name.
Br
bromobenzene
propylbenzene
isopropylbenzene or
(1-methylethyl)benzene
12
Some monosubstuted aromatics are special. That is, they get their own name that does
not include benzene. They keep their special names even when they get a second
substituent. Here are some names in hierarchical order (benzoic acid has highest priority,
while toluene has least priority). Notice once again that the general rule is more bonds to
oxygen gives a higher priority.
benzoic
acid
benzenebenzaldehyde
sulfonic acid
phenol
aniline
anisole
toluene
Disubstituted benzenes
There are three possible arrangements of the two substituents. These can be designated
with numbers, which is the official IUPAC method, or with ortho, meta, para
designations, which are so common that they have leached into and virtually taken over
the IUPAC designations.
Y
X
X
X
Y
Y
number:
common:
1,2
ortho- (o-)
1,3
meta- (m-)
1,4
para- (p-)
If no special parent benzene is involved, need two numbers or o, m, p system. Number
one determined by alphabet. If one special, that is automatically number one – no need to
write “1.” If two specials, number one and root determined by hierarchy.
1,4-dibromobenzene
3-aminobenzoic acid
2-methoxybenzaldehyde 3-methylphenol
Three or more substituents on a benzene:
For benzene rings with more than two substituents, the ortho, meta, para system cannot
be used. You have to use the number system. The special benzene ring with the highest
priority is number 1 and does not need a number. If no special parent, then the first
substituent alphabetically gets the number one spot and you do need to write number 1.
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NHCH3
NO2
O2N
F
Br
NO2
I
4-bromo-1-iodo-2vinylbenzene
2-benzyl-5-flouro-N-methylaniline
2,4,6-trinitrotoluene
Carboxylic Acids
1.
2.
3.
4.
Find the longest carbon chain containing the carboxylic acid
suffix: subtract “-e” on alkane, add “-oic acid”
Carboxylic acid carbon automatically number 1 in chain
If carboxylic acid at both ends, then –dioic acid suffix (no need for numbers
since acids can only be at the two ends
5. If alkene or alkyne present, include a number stating where present
O
O
OH
Cl
OH
(S) 2-methylbutanoic acid
para-(2-chloropropyl)benzoic acid
Acyl Halides
1.
2.
3.
4.
Find the longest carbon chain containing the acyl halide
suffix: change suffix of carboxylic acid from “-oic acid” to “–oyl halide”
Acyl (carbonyl or C=O) carbon automatically number 1 in chain
If alkene or alkyne present, include a number stating where present
O
Cl
O
3-methylpentanoyl chloride
Br
benzoyl bromide
Anhydride
1. Find the longest carbon chain containing the anydride
2. subtract “-e” on alkane, add “-oic” for each “acid” on
either side of shared oxygen, add anhydride at end
3. Acyl (carbonyl or C=O) carbon automatically number 1 in chain
14
4. If alkene or alkyne present, include a number stating where present
5. very similar to common names for ethers
O
O
O
O
O
O
ethanoic propanoic anhydride
benzoic anhydride
Ester
1.
2.
3.
4.
5.
Find the longest carbon chain containing the ester
suffix: change suffix of carboxylic acid from “-oic acid” to “–oate”
parent is the chain with the carbonyl carbon regardless of length
Acyl (carbonyl or C=O) carbon automatically number 1 in chain
alkyl group on other side of carbonyl is a “super” prefix, coming before all
others except stereochemical ones (R/S, E/Z, cis/trans)
6. If alkene or alkyne present, include a number stating where present
7. If cyclic, called a lactone
a. named as a “2-oxocycloalkanone” where “alk” stands for total number
of ring members including oxygen (which is the “2-oxo” part)
O
O
t-butyl 2-methylpropanoate
O
O
5-methyl-2-oxocyclohexanone
Amide
1.
2.
3.
4.
5.
6.
7.
Find the longest carbon chain containing the amide
suffix: change suffix of carboxylic acid from “-oic acid” to “amide”
parent is the chain with the carbonyl carbon regardless of length
Acyl (carbonyl or C=O) carbon automatically number 1 in chain
If alkene or alkyne present, include a number stating where present
Any substituent on the nitrogen gets “N-” label, like amines
Cyclic amides are called lactams
a. named as a “2-azacycloalkanone” where “alk” stands for total number
of ring members including nitrogen (which is the “2-aza” part)
H
N
O
O
N-cyclopentylethanamide
NCH3
N-methyl-2-azacyclopentanone
15
Carboxylic acid salt
1.
2.
3.
4.
5.
Find the longest carbon chain containing the acid salt
suffix: change suffix of carboxylic acid from “-oic acid” to “–oate”
Acyl (carbonyl or C=O) carbon automatically number 1 in chain
If alkene or alkyne present, include a number stating where present
Metal is super prefix, similar to esters
O
O
ONa
sodium benzoate
K +
O
(E) potassium but-2-enoate
Nitrile
1.
2.
3.
4.
5.
Find the longest carbon chain containing the nitrile
suffix: subract nothing from alkane, but add “nitrile”
nitrile carbon automatically number 1 in chain
If alkene or alkyne present, include a number stating where present
As a substituent
CN
CN
2,4-dimethylheptanenitrile
cyanocyclohexane
Aldehyde
1.
2.
3.
4.
Find the longest carbon chain containing the aldehyde
suffix: subract “-e” from alkane, add “-al”
carbonyl carbon automatically number 1 in chain
If alkene or alkyne present, include a number stating where present
O
Br
O
O2N
Br
2,2-dibromobutanal
para-nitrobenzaldehyde
Ketone
1. Find the longest carbon chain containing the ketone
16
2.
3.
4.
5.
suffix: subract “-e” from alkane, add “-one”
number from side which gives lowest number to ketone
for propanone, butanone, or cyclic ketones, no number necessary
If alkene or alkyne present, include a number stating where present
O
pent-4-en-2-one
O
(S) 3-methylcyclopentanone
For more information, see:
http://www.acdlabs.com/iupac/nomenclature/
http://www.cem.msu.edu/~reusch/VirtualText/nomen1.htm
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