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Naming Organic Compounds
>> Parts of this equation/concept include:
| A. Finding Names from Structures |
- Find the longest continuous chain of carbons. Determine the
root word from Table 2. If there are more than 12 carbons, combine
the root words, using deca for 10 and the root word (ended with
an a and in front of dec) for the number of carbons beyond 10.
For example, hexadec refers to 16 carbons.
Table 2 Root Names Based on Number of Carbons
| Number of Carbons |
Root Word |
| 1 |
meth |
| 2 |
eth |
| 3 |
prop |
| 4 |
but |
| 5 |
pent |
| 6 |
hex |
| 7 |
hept |
| 8 |
oct |
| 9 |
non |
| 10 |
dec |
| 11 |
undec |
| 12 |
dodec |
- Look at the carbon–carbon bonds. If all are singly bonded,
add an to the end of the root word. If there is a carbon–carbon
double bond, add en to the root word. If there is a carbon–carbon
triple bond, add yn to the root word. If there is an aromatic
ring, your root word becomes benzene. If the carbons are
bonded in a ring, in addition to the suffix added for type of
bonds (single, double, triple), add cyclo as a prefix to
the root word, which should be the number of carbons in the ring.
- Look for oxygens or nitrogens. Identify the type of functional
group. Ethers and amines are different. For everything else, add
the appropriate ending to the suffix as shown in Table 3.
Table 3 Endings for Functional Groups
| Functional Group |
Ending |
| only C and H atoms |
e (alkanes, alkenes, and alkynes) |
| alcohol |
ol |
| aldehyde |
al |
| ketone |
one |
| carboxylic acid |
ic acid |
- Find the end of the longest chain that is closest to the functional
group. Count from that end to the carbon to which the functional
group is attached. Use that number, followed by a hyphen as a
prefix (e.g., 1-).
- Look for chains of carbons that are branches off the main
chain. Starting at the same end as step 4, or (if there is
no functional group) the end closest to the branch, number
the carbons on the main chain. Name the branch with the number
of the main chain carbon, a hyphen, then the root word for
the number of carbons in the branch with the suffix yl
(e.g., 1-ethyl). Use that as a prefix to what you have so
far in the name.
- If there is more than one branch, start numbering carbons
at the end of the main chain that will result in branches
with the lowest set of numbers. If the branches are the same,
separate the numbers with commas and use the naming prefixes
in Chapter 4 to designate how many are present (e.g., 1,2-dibutyl).
If the branches are different, name each branch (as in step
5a) and separate the prefixes with dashes. (e.g., 1-methyl-2-ethyl).
- Look for halogens bonded to carbons. Use the same numbering
system determined in step 4 or 5. If carbons were not numbered
in these steps, number them starting at the end closest to
a halogen. Add a prefix with the number of the carbon the
halogen is attached to, followed by a hyphen, then the root
name of the halogen followed by an o (3-chloro).
- If there is more than one halogen, list each carbon a halogen
is bonded to, separated by commas and a prefix to designate
how many of that type of halogen is present (like step 5b).
For example, 2,3,3-trichloro.
>> Example 1
What are the names of the following compounds?
- (CH3)2CHCH2CH2CH2CHCH2
Solution:
- This consists of a six-membered ring, so the root is cyclohex.
There are no double or triple bonds, nor any elements but carbon
and hydrogen, so the suffix is ane, making cyclohexane.
There is a branch with a one-carbon chain, which adds a methyl
prefix. You would start numbering at the end as close to the
branch as possible. However, since this is a cyclic compound,
there is no end, so you would start at the branch. Since there
is only one branch, the numbering is redundant. The best name
is methylcyclohexane, but 1-methylcyclohexane
is acceptable.
- The longest chain has five carbons, making the root word pent.
There are no carbon–carbon double bonds, so pent
is followed by an to make pentan. The functional
group is a ketone, so the suffix is one—making pentanone.
The location of the functional group needs to be identified.
If you start numbering the compound (at either end), the functional
group is on carbon 3 making the full name 3-pentanone.
- This longest chain is seven carbons long. It branches at the
end. There is a double bond at the other end. The seven-carbon
chain makes it hept. The double bond adds hepten.
Since there is no other functional group, this makes it heptene.
The double bond starts with an end carbon, so that carbon is
carbon number 1, and so 1-heptene. If you continue numbering
the carbons, the branch is on carbon 6. The branch has one carbon,
a methyl. The name is 6-methyl-1-heptene.
-
The longest chain is six carbons long, although there are
three ways to make a six-carbon chain. The root word is hex.
The best choice is the chain that includes the functional
groups. In this example, the only functional group is an alkene.
Thus hexene. The chain is numbered, so that the functional
group will have the lowest value. The longest chain, numbered
in that manner, would be  .
Either system will give the same name. The alkene is at carbon
2. There is one carbon branch (methyl) at carbon 2 and a two-carbon
one (ethyl) at carbon 3. Therefore the name would be 1-methyl-2-ethyl-2-hexene.
>> Naming Ethers
When naming ethers, name either side of the oxygen using the rules
above and a yl suffix. String the two pieces together as
separate words and add the third word, ether, at the end.
If both pieces are the same, use the name only once with the prefix
di. (e.g., dimethyl ether, not methyl methyl ether.)
>> Naming Amines
Name each carbon chain attached to the nitrogen according to the
preceding rules, ending with the suffix yl. String each piece
together as one word ending with amine. If two pieces are
the same, use the prefix di rather than repeating the name.
If three pieces are the same, use the prefix tri rather than
repeating the name (as with ethers).
>> Example 2
Name the following compounds.
- CH3CH2OCH2CH2CH2CH3
- (CH3CH2)2NH
- CH3NH2
- CH3OCH3
Solution:
- This is an ether. One side of the oxygen has two carbons (ethyl)
and the other has four carbons (butyl). Therefore the name of
the compound is ethyl butyl ether.
- This is a secondary amine (it has a nitrogen attached to two
carbons). The carbon chains attached to the nitrogen are the
same and have two carbons (ethyl), so the name of this compound
is diethylamine.
- This is a primary amine. The one-carbon group is a methyl.
Therefore the name of this compound is methylamine.
- This is an ether. The groups on either side of the oxygen
are both methyl groups. Therefore the name of this compound
is dimethyl ether.
>> back
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| B. Finding Structures from Names |
>> Example 3
What are the structures of the following compounds?
- 2,2-dimethylbutane
- 3-propyl-5-hexynal (hexanynal)
- 1-octanol
- tripropylamine
- 3-methylpentanoic acid
- 5-ethyl-2-heptyne
Solution:
-
The butane is a four-carbon chain. There are two methyl (one-carbon)
groups on the second carbon.
-
3-propyl-5-hexynal has a six-carbon chain (hex), with
a triple bond between the 5 and 6 carbons (yn) and
an aldehyde (al) on the end (#1 carbon). Aldehydes
must be on the end; otherwise they would be ketones. A three-carbon
branch (propyl) is on the third carbon.
-
1-octanol. There are eight carbons in the chain (oct),
no double or triple bonds (an), and an alcohol (ol)
on the first carbon (on either end).
CH3CH2CH2CH2CH2CH2CH2CH2OH
-
tripropylamine. The amine is a nitrogen that has three bonds.
There are three propyl groups attached to the amine, so there
is no space for other hydrogens. All the groups are the same
(tri) and have three carbons (propyl). The structure
could be written as
(CH3CH2CH2)3N
or 
- 3-methylpentanoic acid. There are five carbons in the chain
(pent), no double or triple bonds (an), and a
carboxylic acid at the end (oic acid). Carboxylic
acids can only be at the end, and the carbon of that group is
carbon 1. On the third carbon from the carboxylic acid is a
one-carbon group (methyl).
-
5-ethyl-2-heptyne is a seven-carbon chain (hept) with
a triple bond between the second and third carbons (yne)
and a two-carbon group (ethyl) on the fifth carbon.
>> Geometric Isomers
This is only an issue for alkenes. In addition, if the groups attached
to one of the carbons in the double bond are the same, there are
no geometric isomers.
The prefixes cis and trans are used when one group
on the carbons opposite the double bond is the same. If those groups
are on the same side, it is a cis isomer; if they are on
opposite sides, a trans isomer.
>> Example 4
Identify the following as cis and trans isomers. Name each compound.
Solution:
-
The group that is the same is the H group. It is on the same
side (bottom) of the double bond; therefore it is a cis
isomer.
The longest carbon chain is from the upper left to the upper
right, seven carbons (hept). The double bond (en)
is between the 3 and 4 carbons (3). As above, it is the cis
isomer. Consequently, the name is cis-3-heptene.
-
The group that is the same on either side of the double bond
is CH3CH2 (ethyl). The group
is on opposite sides of the double bond; it is therefore a
trans isomer.
The longest chain is again from the upper left to the upper
right, and seven carbons long (hept). The double bond
is between the 3 and 4 carbons (3- and ene). There
is also a two-carbon branch from carbon 4. So the name is
trans-4-ethyl-3-heptene.
-
The group that is the same is the ethyl group on the carbon
to the right. Because those are the same carbon, this compound
does not have a geometric isomer.
The longest chain is from the lower left to the upper or
lower right. It doesn't matter; thelength will be nine carbons
(non). The double bond (en) is closer to the
right end, so the numbering should start from there. That
makes the double bond between carbons 3 and 4. There is also
a two-carbon branch on carbon 3. The name is 3-ethyl-3-nonene.
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