3.1: Nomenclature

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Sep 16, 2022
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- 3: Simple Hydrocarbons
- 3.2: Conformations of Acyclic Hydrocarbons

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Nomenclature

How do we name organic compounds? There are two ways

common name – colloquial name CHCl₃ = chloroform
IUPAC – “official” international naming system of International Union of Pure and Applied Chemistry CHCl₃ = trichloromethane

For starters, we need to know how to describe the atoms and bonds that are depicted using line-angle notation. We use a specific language to describe how many carbons are in a molecule and what kinds of functional groups are in a molecule. Watch the video below for a crash course on this language, which will be followed up with more complicated nomenclature.

Now that you know simple alkane nomenclature and various functional groups, we can begin to discuss more complicated systems.

1. Branched alkanes C_nH_2n₊₂ = suffix -ane

a) identify the longest contiguous chain
b) number the chain; start with a substituent group occupying the lowest number possible
c) locate the substituents, give them a number
d) alphabetize
e) if more than one of the same substituent, use following prefixes: 2 = di; 3 = tri; 4 = tetra, 5 = penta; 6 = hexa; 7 = hepta; 8 = octa; 9 = nona; 10 = deca

Example:

Screen Shot 2021-05-20 at 8.53.20 AM.png

12 carbons = dodecane

2,3,5,9-tetramethyl

6-propyl

10-ethyl

Name: 10-ethyl-2,3,5,9-tetramethyl-6-propyldodecane

Example:

Screen Shot 2021-05-20 at 8.53.25 AM.png

IUPAC name: 3-chloro-4-(1-methylethyl)-5-methylheptane

Common name: 3-chloro-4-isopropyl-5-methylheptane

Other common groups (IUPAC names):

nBu = normal butyl (butyl)

tBu = tertiary butyl (1,1-dimethylethyl)

Screen Shot 2021-05-20 at 8.55.31 AM.png

iPr = isopropyl (1-methylethyl)

Screen Shot 2021-05-20 at 8.55.36 AM.png

iBu = isobutyl (2-methylpropyl)

Screen Shot 2021-05-20 at 8.55.40 AM.png

sBu = secondary butyl (1-methylpropyl)

Screen Shot 2021-05-20 at 8.55.44 AM.png

2. Cycloalkanes C_nH_2n

a) count number of carbons in the ring

b) add prefix “cyclo” to the alkane chain

Examples:

In cycloalkanes, there are often stereoisomers that arise:

3. Bicycloalkanes – a fused or bridged cycloalkane that requires 2 bonds to be broken to become acyclic

a) count # of carbons in the bridged system (do NOT include substituents)
b) add prefix “bicyclo” to parent chain
c) identify bridgehead carbons (ring fusion centers) and count # of intervening carbons in each bridge
d) insert [#.#.#] in between “bicyclo” and parent chain
e) number around the largest ring first, then next largest ring, etc.
f) when two rings are the same size, number ring with substituent first

Examples:

4. Functional groups

If more than one functional group exists, the base or parent name will depend on its priority rank.

IUPAC name: 4-hydroxypentanoic acid (not 4-carboxy-2-butanol - carboxylic acid has higher priority than alcohol!)

Let’s have some practice. What is the IUPAC name for the following molecules?

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