Ace Your Next Organic Chemistry
Exam.

With these Downloadable PDF Study Guides

Our Study Guides

Alkanes and Nomenclature

By James Ashenhurst

Table of Functional Group Priorities for Nomenclature

Last updated: August 6th, 2019 |

How To Determine Which Functional Group Has “Priority” For Naming Purposes

Here’s a little nomenclature dilemma.

Let’s say you’re trying to name a molecule. You’re familiar with the familiar naming suffixes like -ol, -ene, -ane, -oic acid and so on. But then you come across a molecule which has multiple functional groups.

What do you do? What suffix do you give the molecule?

1-dilemma

We need some kind of priority system for nomenclature. And so, IUPAC (think of the “Ministry  of Magic”, but for chemists) has developed one. If you have a molecule with, say, a carboxylic acid and a ketone you consult the table. The functional group with the highest priority will be the one which gives its suffix to the name of the molecule. So in example #1 above, the suffix of the molecule will be “-oic acid” , not “-one”, because carboxylic acids are given higher priority. However, if a ketone is present with an alcohol (example 3) then we will use the suffix, “-one” because ketones have a higher priority for nomenclature than alcohols.

[You might ask: what is this based on? It’s an arbitrary agreement by IUPAC [source], although note that there is some correlation between the oxidation state of the carbon and the priority (more oxidized groups tend to be higher priority). However this really is an example of something you have to either look up , memorize, or have a computer do for you. It’s not conceptual. ]

[Note: this takes into account the latest recommendations of the IUPAC Blue Book  (2013 edition)] 

Highest Priority Groups:  Carboxylic Acids, Sulfonic Acids, Esters, Acid Halides, Amides

Note that with the exception of sulfonic acids, these are all carboxylic acid derivatives. IUPAC goes  into way more detail than we need to here. [Note]. Just for the record these “rules for  seniority” can  be found in section P-41 of the Blue Book, page 428 of the 2013 edition]

The “seniority rules” continue in the following order, where we are cherry-picking the most common examples.

Next In Line: Nitrile, Aldehyde, Ketone, Alcohol, Thiol, Amine

Again, this is not a complete list – we’re cherry picking the most commonly encountered functional groups here.

Alkenes And Alkynes

If carbon-carbon multiple bonds are present in the molecule, they are considered as substituents with a priority (or “seniority”, according to IUPAC) lower than that of amines.

So for a molecule with an alkene and an  alcohol, the alcohol has priority and the molecule has the suffix, “-ol”. The presence of the double bond is noted with the locant followed by the prefix, “en-“. For example, pent-4-en-1-ol.

If no higher-priority groups are present, the suffix for a molecule containing an alkene will be “-ene”, such as in pent-1-ene.

For an alkyne, the corresponding prefix is “-yn” and the suffix is “yne”.

At this point the methodology for naming molecules changes slightly. In the absence of one of the above functional groups, the suffix will always be “-ane”, “-ene”, or “-yne”, depending on whether any unsaturation is present in the molecule, and any lower-ranked substituents will be prefixes.

Alkenes vs. Alkynes: Which  Takes “Priority”?

This brings us to a common source of confusion in nomenclature. When an alkene and an alkyne are present in a molecule, which takes priority?

It  depends on what you mean by “priority”.

For the purposes of the name, “-ene” comes before “-yne” alphabetically. So when an alkene and an alkyne are present in  the same molecule, the ending will always be “yne”. 

For the purposes of numbering, if there is a tie between an alkene and an alkyne for determining the lowest locant, the alkene takes priority.

IUPAC says it this way:

Right. Let’s move along to the other functional groups.

Functional Groups That Are Always Prefixes: Halides, Alkoxides, Azides, Nitro

Some functional groups have been deemed unworthy of ever getting their own suffixes. For nomenclature purposes, they are forever out of the limelight, subservient to the -ane, -ene, or -yne ending of the parent hydrocarbon  (or “parent hydride”, as IUPAC calls it).

These groups include the halides (bromo, chloro, fluoro, iodo), ethers (“alkoxy”), azide and nitro functional groups. Source: Table 5.1, Section P-59.1.9 of the 2013 Blue Book (Page 630).

Some Examples With Multiple Functional Groups

Here are some examples of applying the order of functional group priorities to solve nomenclature problems. The highest ranked functional group becomes the suffix – it’s highlighted in red.

3-applying the priority

This covers most of the functional groups you’ll meet in Org1/Org2, if you run into a thioketone or some other bizarre entity, you’ll probably want to see Reusch or Wikipedia.

 

Comments

Comment section

122 thoughts on “Table of Functional Group Priorities for Nomenclature

  1. I think ether should be right after amines and alkane after nitro? some other website seem to say that, which one is correct?

    1. According to my view friend your thought is wrong because you should know that we write fg as prefix only when it has less priority and you can not use alkane as prefix. nitro group always remains in the form of prefix and ether vice versa. So fg having high priority is used in the form of suffix. Ether is not used as suffix so it has less priority than alkane. It will be after alkane and not after amine.

      1. Alkane can be used as a prefix… It’s even stated to be alkyl e.g. Methyl, ethyl, propyl and so on. If ether is supposed to be before amine, then kindly show proof of how a carbon with ether as suffix should be named. If you’re confused between nitro (NO2) and amine (NH2), then please search it up and don’t mislead others.

  2. i think alkyne should come before alkene: it should be alkyne-alkene-alkane, decreasing unsaturation and increasing saturation…

    1. You’re right Christopher. I found this from a book I am using to study for my DAT exam. I hope it helps
      “Note: The group B functional groups (alkene and alkyne” are considered to have equal priority: in a molecule with both double and a triple bond, whichever is closer to the end of the chain determines the direction of numbering. In the case where each would have the same position number, the double bond takes the lower number. In the name, “ene” comes before “yne” because of alphabetization. See examples on the next page”

      1. Ya it’s true. Alkene gets preference due to its alphabetical order. And if alkyne present at lower locant than alkene so alkyne will written with lower no of carbon atom. Or in other way just sum the locant of alkene and alkyne and in whichever the sum is less that will be followed.

    1. It’s just due to alphabetization. For example we would number 2-bromo 3-nitro butane based on the fact that bromine is higher up in (our) alphabet than nitro.

      1. Dude…that is so not how you should apply nomenclature. Everything is not alphabetical order. You should go take classes before you post shit on the internet….totally misleading others.

    2. Because according to IUPAC rule functional group having first letter and which came earlier in alphabetical system will be written first.
      I.e. in bromo ‘ b’ came first in alphabetical system then ‘ N ‘ which is the first letter of nitro.

        1. I believe it is at least partially based on oxidation state, with higher oxidation state having higher priority. But that doesn’t explain why alkenes are higher priority than alkynes. Nomenclature is a human convention.

      1. Carboxy may be used when one of the chains attached to the root carboxylic acid also has the carboxylic acid functional group. E.g. 4-(Carboxymethylene)-2,5-heptadienedioic acid.

  3. Why does alkyne have more priority ovr alkene? Because i have seen compounds where the least no: is given to alkynes ie., they hav been given most priority ovr alkene..for example, 6-chloro-4-ethyl-5-methylhept-5-en-1-yne

    1. The yne might have been given priority in this case because the parent chain could be numbered in such a way to make one of the unsaturations C1, and it happened to be the yne and not the ene.

      1. According to: Principles of
        Chemical Nomenclature
        A GUIDE TO
        IUPAC RECOMMENDATIONS, Edited by G.J. Leigh

        Page 80-81:

        If double and triple bonds are present in a structure, they are considered together
        when assigning lowest locants. Only when this does not allow a resolution do double bonds receive the lowest locants.

        In a name, the ending -ene is cited before -yne, but
        with elision of the final ‘e’.

        Examples
        12. HCC-CH=CH-CH3 pent-3-en-l-yne
        13. HCECCH=CH2 but-l-en-3-yne

  4. Since ethers are “substituent-only” (named only by prefix), are peroxides prefix-only as well? What about epoxides? (I think that’s a little less clear, eg. oxirane.)

    How would a peroxyacid RC(=O)OOH (“peracid”) or a perester RC(=O)OOR’ be handled?

    I’m guessing a carbonate ROC(=O)OR’ takes priority over an ester RC(=O)OR’ ?

    1. I agree, I think that ethers should be higher priority than alkanes. In the case of CH3OCH2CH3Ito should be named as ethyl methyl ether.

  5. i have a doubt.suppose there’s a compound containing both alkene and alkyne functional groups. what gets higher preference?

    1. If the carbon chain contains both the double and triple bond and they are both on the terminal carbons, then prioritization is given to the double bond. But if both double and triple bond are not on the terminal carbons, prioritization is given to the triple bond. These are called enynes.

  6. Respected sir,
    I have a doubt which may be small for you , but it brings more confusion if sulphonic acid and carboxylic acid are given which should be given priority first sir . sir please answer me and please give me a explanation.
    Thanking you sir!

  7. Why NO2 is not taken as principal functional group , since the compound which is more withdrawing is considered to be as more powerful functional group!! So why is it so????

  8. Very informative and well organised….
    However it would be awesome if sulphonic acid and anhydrides could be added too :D
    They’re pretty superior
    Thanx though, this saved me a ton of time

  9. The so-called “Table of Functional Group Priorities For Nomenclature” can be misleading. It is not in accordance with past (1979, 1993) or present (2013) IUPAC recommendations.

  10. This is for a true or false question:

    “Butanal” is another name for isobutanol.
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    I am almost 100% certain this is false, but I just want to be sure that there is no way, or possible arrangement of the alcohol group (-OH) that can occur that would result in it being possible to name it like a aldehyde right?

  11. I think that the priority order of functional group is this :
    1. -COOH
    2. -SO3H
    3. -COOR
    4. -COX
    5. -CONH2
    6. -CN
    7. -CHO
    8. =C=O
    9. -OH
    10. -NH2
    11. =C=C=
    12. -C-=C-

  12. I think it is good to provide this type of chart to the student because this help them in their study
    So Thanks!!!
    ???????????????????????

  13. In the ease of open chain compounds the secondary prefix is added just before the root word in the alphabetical order. why is it so?

  14. In this priority table,Sulphonic Group(Functional Group) is not present.

    I want to know what is the real place of sulphonic group according to IUPAC

  15. alkynes have priority over alkenes. During nomenclature of long chain carbon compounds, numbering done in such a way as to locate double or triple bond by shortest route. Doesn’t matter which functional group arrive first. In any way alkynes are preferred over alkenes.

  16. Your priority table is very very wrong. I think you should go study some more before misleading others. Please correct them our just remove the page

  17. Well everything is fine, but i think sulphonic acid is missing which should be placed just below carboxylic acid. Thanks

  18. When alcohol is on high priority than numbering should begin from alcohol. eg.
    CH3CH(OH)CH3.
    IUPAC NAME – 2-PROPANOL
    BUT why can’t be 1-methyl Ethanol

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.