IUPAC Nomenclature: Basic System for Naming Organic Compounds (A.1–A.7 Notes)

Page 1

  • Title of the chapter: THIRD EDITION Organic Chemistry, Joel Karty

  • Topic: PRINCIPLES AND MECHANISMS, Interchapter A – Nomenclature: The Basic System for Naming Organic Compounds

  • Subtopics listed in this section: Alkanes, Haloalkanes, Nitroalkanes, Cycloalkanes, and Ethers

Page 2

  • A.1 THE NEED FOR SYSTEMATIC NOMENCLATURE: AN INTRODUCTION TO THE IUPAC SYSTEM

  • IUPAC provides a standard for naming inorganic and organic compounds.

  • Two directional capabilities emphasized:

    • Given a molecular structure, you can derive its IUPAC name, piece by piece.

    • Given the IUPAC name, you can draw its structure, piece by piece.

Page 3

  • A.2 ALKANES AND SUBSTITUTED ALKANES

  • Alkanes basics:

    • Contain only C–C and C–H single bonds.

    • They contain no functional groups.

  • Straight-chain alkanes are the basic form, also called linear alkanes.

  • Rules for naming straight-chain alkanes:

    • All straight-chain alkanes have the suffix

    • The root of the name depends on the number of carbon atoms; prefixes specify the number of carbons:

    • Common prefixes include meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, dec- for 1–10 carbons.

  • Root and suffix: suffix is

  • The naming convention uses a root corresponding to the number of carbons and the suffix

Page 4

  • Straight-Chain Alkanes (Table A-1)

  • Examples (as listed):

    • Methane (CH₄) – 1 carbon

    • Ethane (C₂H₆) – 2 carbons

    • Propane (C₃H₈) – 3 carbons

    • Butane (C₄H₁₀) – 4 carbons

    • Pentane (C₅H₁₂) – 5 carbons

    • Hexane (C₆H₁₄) – 6 carbons

    • Heptane (C₇H₁₆) – 7 carbons

    • Octane (C₈H₁₈) – 8 carbons

    • Nonane (C₉H₂₀) – 9 carbons

    • Decane (C₁₀H₂₂) – 10 carbons

  • Note: The table reinforces the root naming for straight-chain alkanes with increasing carbon count.

Page 5

  • Substituted Alkanes

  • Key idea: If a hydrogen atom of an alkane is replaced by another atom or group, the molecule is substituted.

  • There can be multiple substituents in a molecule, and the IUPAC name must account for all of them.

  • Rules for naming substituents depend on the type of substituents present in the molecule.

  • Concepts to remember:

    • Substituents affect the root by not changing the base alkane length, but add prefixes to indicate their presence and positions.

    • Alphabetical order is used when listing multiple substituents, ignoring multiplying prefixes (di-, tri-, etc.) for the order.

Page 6

  • A.3 HALOALKANES AND NITROALKANES: ROOTS, PREFIXES, AND LOCATOR NUMBERS

  • Halo substituents and corresponding names:

    • –F becomes fluoro

    • –Cl becomes chloro

    • –Br becomes bromo

    • –I becomes iodo

  • Nitro substituent is –NO₂ and is treated as the prefix "nitro" when naming substituents.

  • Locator numbers (locants) are used to indicate the position of each substituent on the main chain.

  • Key rules:

    • Identify the longest continuous chain to determine the root alkane.

    • Attach substituent prefixes (e.g., fluoro-, chloro-) to the left of the root, with locants preceding the substituent name as needed.

    • If there is more than one identical substituent, use multiplying prefixes (di-, tri-, tetra-, etc.) before the substituent name.

    • Substituent locants are separated from the substituent name by a hyphen; multiple substituents require commas between locants and hyphens before each substituent name.

Page 7

  • How to Name a Straight-Chain Alkane with One Substituent

  • Step-by-step process:
    1) Identify the main chain: the longest continuous chain of carbon atoms; the root is the corresponding alkane from Table A-1.
    2) Add the substituent name as a prefix to the root.
    3) Number the carbon atoms of the chain from the end that gives the substituent the lowest possible locant (C-1 is at the end that places the substituent at the smallest number).
    4) Add the locator number to the left of the substituent name, separated by a hyphen. No spaces in the IUPAC name.

Page 8

  • Examples of Haloalkanes

  • Key ideas illustrated:

    • Numbering should be chosen so chlorine gets the smallest locator number (e.g., 1- for Cl when possible).

    • The longest chain determines the root; for example, in bromomethane, the root is methane because the longest chain has one carbon.

    • For iodo substituents, the locator number should be 1 if possible; the name 2-iodoethane would be incorrect when 1-iodoethane gives a smaller locator.

  • Specific examples:

    • 1-Chloropropane from a three-carbon root (propane) with a chlorine substituent at C-1

    • 2-Chloropropane for a chlorine at the middle carbon

    • Bromomethane for a Br substituent on methane

    • 1-iodoethane vs. 2-iodoethane: the correct locator is 1 for I in this molecule, so the correct name is 1-iodoethane (not 2-iodoethane).

Page 9

  • SOLVED PROBLEM: What is the IUPAC name of this molecule? (A.1) NO₂

  • Interpretation:

    • Nitro group (-NO₂) is a substituent; the root is methane if there is only one carbon (i.e., the molecule is nitromethane, CH₃NO₂).

    • The IUPAC name for this likely simple structure is nitromethane (systematic form: nitromethane). The nitro group is named as a prefix (nitro-) to the root methane.

Page 10

  • SOLVED PROBLEM: Think What is the IUPAC name of this molecule? (A.1) Think

  • Solve tasks to determine:

    • How many carbon atoms are in the longest continuous carbon chain?

    • What is the corresponding root?

    • What are the two options for assigning C-1?

    • Which option gives the nitro group the lower locator number?

    • How should the locator numbers and substituents be indicated in the IUPAC name?

  • This page outlines a structured approach for solving A-1 problems: identify the longest chain, choose the end that minimizes locants, and assign substituent prefixes with proper locants.

Page 11

  • SOLVED PROBLEM: Solve What is the IUPAC name of this molecule? (A.1) Think

  • Solve details:

    • Nitro is added as a prefix to the root.

    • The locator number 2 precedes the substituent name and is separated by a hyphen.

    • The complete IUPAC name given: 2ext−nitropentane2 ext{-nitropentane}

    • This demonstrates using nitro as a prefix and placing the locator before it.

  • Question prompts: how to indicate the locants and substituents in the final name.

Page 12

  • When the Main Chain Has Two or More Substituents

  • Conceptual illustrations (G represents substituents) and correct vs incorrect numbering:

    • CORRECT numbering example shows that numbering from left to right gives the first substituent locator number 2, since left-to-right numbering minimizes the first locant compared to 4 if numbered the other way.

    • INCORRECT numbering example demonstrates that starting from the other end can yield poor locant distribution (e.g., first substituent locant 6).

    • Another approach shows numbering from the right such that first and second substituents each have locator number 1; this is sometimes possible if the substituents are symmetrically placed.

  • Core idea: choose the direction that yields the lowest set of locants when read in the numbering order, prioritizing the first point of difference.

Page 13

  • Adding Prefixes

  • Steps: 1) Write each substituent as a prefix and order alphabetically (e.g., bromo-, fluoro-, nitro-, chloro-). 2) Add multiplying prefixes if necessary:

    • di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-
      3) Add one locator number for each substituent, separated from the substituent name by a hyphen.

  • Multiplying prefixes are not considered in the alphabetical ordering; they precede the substituent name as shown above.

Page 14

  • SOLVED PROBLEM: What is the IUPAC name of this molecule? (A.2)

  • Structure indicated by: O₂N on left, NO₂ near top, and F atoms around (four Fs total).

  • Conceptual takeaway:

    • Two nitro groups and four fluoro substituents are present on the carbon chain.

    • The naming would require identifying the longest chain, applying the correct root, and then listing substituents in alphabetical order with appropriate multiplying prefixes (e.g., tetrafluoro-, dinitro-) and locants.

  • Because the exact positions aren’t explicitly detailed in the transcript image, the lesson emphasizes how to approach such a problem rather than providing a single final name.

Page 15

  • SOLVED PROBLEM: Think and Solve prompts for a new molecule (A.2)

  • Think: Ask the same sequence of questions as in A.1/A.2 problems:

    • How many carbon atoms are in the longest continuous carbon chain?

    • What is the corresponding root?

    • From which end should numbering begin?

    • What are the substituents, and which comes first alphabetically?

    • How do you indicate the number of each kind of substituent, and what locator number is assigned to each substituent?

  • This page reinforces the methodical approach for solving substitution and prefixing problems in A.2.

Page 16

  • SOLVED PROBLEM: Solve (A.2) Think and Solve

  • Example details:

    • The longest continuous carbon chain has six carbon atoms, so the root is hexane.

    • The page reiterates the question about which end to begin numbering from to achieve the lowest locants.

  • Core idea: determine the root from the longest chain and set the direction of numbering to minimize locants for substituents.

Page 17

  • SOLVED PROBLEM: Solve 2

  • Mixed substituents problem: nitro and fluoro substituents

  • Alphabetical order dictates that fluoros come before nitro groups in the name.

  • An example outcome given: 1,1,3,5-tetrafluoro-1,4-dinitrohexane

    • Explanation:

    • Four fluoro substituents: positions 1, 1, 3, 5 (hence tetrafluoro- with locating numbers 1,1,3,5).

    • Two nitro substituents: positions 1 and 4 (hence dinitro- with locants 1,4).

    • Root: hexane (six carbons).

  • This example demonstrates how to combine multiple substituent types with both multiplicity prefixes and locator numbers in the final IUPAC name.

Page 18

  • What If There Is a Locant Number Tie?

  • Alphabetical order helps resolve ties when two substituents could occupy the same locant positions.

  • Example demonstrates: bromo vs fluoro where "bromo" comes before "fluoro" alphabetically.

  • Side-by-side comparison shows the two possible orderings:

    • 1-Bromo-3-fluoropropane

    • 1-Fluoro-3-bromopropane

  • Rule highlighted: choose the arrangement that gives the substituent encountered first a lower locant, applying alphabetical order as a tie-breaker when locants would otherwise be equal.

Page 19

  • A.4 ALKYL SUBSTITUENTS: BRANCHED ALKANES AND SUBSTITUTED BRANCHED ALKANES

  • Naming Straight-Chain Alkyl Groups
    1) Identify the corresponding alkane with the same number of carbons as the alkyl substituent.
    2) Replace the suffix -ane with -yl to obtain the alkyl substituent name.

  • Key takeaway: The process mirrors straight-chain alkane naming but with -yl suffix for substituents derived from the corresponding straight-chain alkane.

Page 20

  • Naming Alkyl Groups (examples)

  • Example 1: The methyl group is on C3 in the depicted chain, yielding a name such as 3-Methylhexane (the main chain is hexane, methyl at C3).

  • Example 2: A molecule showing two chlorine substituents on one fragment with a propyl substituent elsewhere yields a name like 4,4-Dichloro-5-propyloctane (illustrating multiplying prefixes and ordering rules).

  • Important concepts:

    • Longest chain selection for the root remains crucial when multiple possible substituent patterns exist.

    • Multiplying prefixes apply to identical substituents (di-, tri-, tetra-, etc.).

Page 21

  • Apply Caution When Numbering

  • Visual representations can bias perception; always count carbons to locate the true longest chain rather than relying solely on drawn appearance.

  • Practical tip: verify the longest chain length before assigning locants or root.

Page 22

  • Branched Alkyl Substituents Naming Branched Alkyl Groups

  • Steps:
    1) Identify the main chain of the branched alkyl group.
    2) Assign the alkyl group’s root name based on that main chain.
    3) Number the carbon atoms of the branched alkyl group’s main chain; the attachment point is C-1.
    4) Add prefixes and locator numbers.

  • This process helps to name complex branched alkyl groups consistently as substituents in larger molecules.

Page 23

  • Incorporating Branched Alkyl Groups into an IUPAC Name

  • Examples (illustrative descriptions from the page):

    • (a) The "1-methylethyl" group (isopropyl) attached to a larger chain, giving names like 3-(1-methylethyl)-2-nitrohexane.

    • (b) The "1-ethyl-2,2-dimethylbutyl" group as a substituent, illustrating how complex branched substituents are represented with locants.

    • (c) The "1-ethyl-2,2-dimethylbutyl" group attached to a decane backbone resulting in names like 2,3,4-tribromo-5-(1-ethyl-2,2-dimethylbutyl)decane.

    • (d) The "1,1-dimethylethyl" substituent (tert-butyl) example in a longer chain name.

    • (e) A mixed substituent example: 4-(1,1-Dimethylethyl)-3,3-difluoroheptane (note: the original transcript shows a small typographical inconsistency; the idea is illustrating tert-butyl and fluoro substitutions within a heptane backbone).

  • Takeaway: Branched substituents can be quite complex; systematic naming requires identifying the substituent’s root, assigning locants, and integrating into the main chain name with proper alphabetical order and multiplicity prefixes.

Page 24

  • A.5 CYCLIC ALKANES AND CYCLIC ALKYL GROUPS

  • Core idea:

    • If a molecule contains at least one ring made entirely of carbon atoms, the ring can serve as the root (cycloalkane) or can be treated as a substituent (cycloalkyl group).

Page 25

  • Determining If a Ring Is the Root or a Substituent

  • Steps to decide: 1) Distinguish ring carbons from chain carbons; a carbon that is part of a ring is not counted as part of a chain. 2) Compare the number of carbons in the carbon ring to the number in the longest continuous carbon chain.

    • If the ring has as many or more carbons than the longest chain, the ring is the root (cycloalkane).

    • Otherwise, the longest chain establishes the root and the ring is a cycloalkyl substituent.

Page 26

  • Numbering System on Cycloalkanes

  • If the root is a cycloalkane:
    1) C-1 is the carbon with the greatest number of attached substituents.
    2) Numbering around the ring (clockwise or counterclockwise) proceeds so that the next substituent is encountered as early as possible.

Page 27

  • SOLVED PROBLEM: Write the IUPAC name for this molecule.

  • Visual shows a cycloalkane with substituents Br and Cl.

  • Concept: apply cycloalkane naming rules with proper locants to minimize numbers and maintain alphabetical order when applicable.

Page 28

  • SOLVED PROBLEM: Think Write the IUPAC name for this molecule. Think Solve

  • Typical questions:

    • How many carbon atoms are in the longest carbon chain and the largest carbon ring?

    • Which establishes the root?

    • Which carbon atom gives the lowest locator number to the first substituent? to the second substituent?

    • Should numbering increase clockwise or counterclockwise?

  • The emphasis is on choosing the root (ring vs chain), then optimal numbering direction for minimal locants.

Page 29

  • SOLVED PROBLEM: Solve Write the IUPAC name for this molecule. Think Solve

  • Example explanation:

    • The longest carbon chain has one carbon; the largest carbon ring has six carbons, so the root is cyclohexane.

    • The top carbon of the ring is chosen to be C-1 because the locator number is 1 for both the first and second substituents (in the example, both substituents are CH3).

    • If C-1 were the carbon attached to Br or Cl instead, the locator number for the next substituent would be 2.

    • Questions to consider: which carbon gives the lowest locator numbers for the first and second substituents, and whether numbering should proceed clockwise or counterclockwise.

Page 30

  • A.6 ETHERS AND ALKOXY GROUPS

  • Key definitions:

    • An ether functional group can be represented as R–O–R′, where an oxygen connects two alkyl groups.

    • Alkyl groups can be the same or different.

    • Naming rules resemble those for alkanes, but first we must determine which alkyl group has the longer chain to establish the root/parent in the ether name.

Page 31

  • Naming an Ether

  • Procedure: 1) Establish the root: the alkane or cycloalkane that has the same number of carbons as the main chain or ring in R is the root of the ether. 2) Name the alkoxy group: remove the suffix -yl from the name of the corresponding alkyl or cycloalkyl group (R′) and add the suffix -oxy. Examples:

    • –OCH3 is methoxy

    • –OCH2CH3 is ethoxy
      3) Assemble the IUPAC name by combining the root and the alkoxy prefix in the appropriate order with any other substituents following the usual rules.

Page 32

  • Ethers That Require Locator Numbers

  • Examples shown to illustrate when locants are needed for the ether name:

    • 1-Methoxypropane

    • 2-Methoxypropane (isopropyl methyl ether)

    • 3,3-Dichloro-2-methoxypentane

    • 1,4-Diethoxycyclohexane

    • 2-Methoxy-1,1,3,3-tetramethylcyclopentane

Page 33

  • A.7 TRIVIAL NAMES OR COMMON NAMES

  • Historical note: Many organic compounds were named based on properties or origins before systematic nomenclature was established.

  • A sizable number of trivial or common names remain in regular use (e.g., formaldehyde, glucose, adrenaline).

  • Importance: While systematic IUPAC names are preferred in formal contexts, common names persist in education, industry, and historical references.

Summary and Practical Takeaways (Integrated)

  • IUPAC nomenclature is a systematic method to name organic compounds based on their structure, with rules that link the name to the structure and vice versa.

  • Core building blocks of naming:

    • Root: determined by the longest carbon chain or the ring (cycloalkane) that serves as the core of the molecule.

    • Substituents: halogens (fluoro-, chloro-, bromo-, iodo-), nitro (-NO₂), alkyl groups (-yl), and cycloalkyl groups.

    • Locants: numbers that indicate positions of substituents on the main chain or ring; aim to minimize the locant set and apply alphabetical ordering for tie-breaking.

  • Prefix rules:

    • Multiplying prefixes (di-, tri-, tetra-, etc.) indicate multiple identical substituents and come before the substituent name.

    • When alphabetizing substituents, multiplying prefixes are ignored for alphabetical order (e.g., fluoro- comes before nitro-, even if there are four fluoro substituents).

  • Important strategies:

    • Always identify the longest continuous carbon chain or the major ring to establish the root.

    • When you have multiple substituents, determine the numbering direction that gives the lowest set of locants (lexicographic rule).

    • For ethers, name the larger alkyl or cycloalkyl group as the root and treat the other group as an alkoxy substituent (e.g., methoxy, ethoxy) with appropriate locants when needed.

    • Use trivial names sparingly in formal contexts; rely on the systematic IUPAC names for precise communication, while recognizing common usage in practice.

(Note: The examples and problems in Pages 7–18 illustrate the practical application of these rules, including the effect of locants, alphabetical ordering, and multiplying prefixes. Pages 19–33 extend these ideas to branched alkyl groups, cycloalkanes, and ethers, culminating in a comprehensive framework for naming a wide variety of organic compounds.)