THE NOMENCLATURE OF COMPOUNDS OF ALKANOLS (ALCOHOLS)

 

 INTRODUCTION

  • Alkanols, commonly called alcohols, are organic compounds with one or more hydroxyl groups (–OH) bonded to saturated carbon atoms (sp³-hybridized).
  • Alcohols are one of the most important functional groups in organic chemistry — found in everything from beverages to antiseptics to fuels.

 

GENERAL FORMULA

  • Monohydric alcohols: CₙH₂ₙ₊₁OH (e.g., CH₃OH, C₂H₅OH).
  • The –OH group gives alcohols distinct physical and chemical properties (hydrogen bonding, solubility, reactivity).

 

 IUPAC RULES FOR NAMING ALKANOLS

 

✅ Rule 1: Identify the Longest Carbon Chain Containing the Hydroxyl Group

  • This becomes the parent chain, named based on the corresponding alkane.
  • The chain must include the carbon bonded to the –OH.

 

✅ Rule 2: Replace the “-e” Ending of the Parent Alkane with “-ol”

  • This indicates the presence of an alcohol functional group.
  • Examples:
    • methane → methanol
    • ethane → ethanol
    • propane → propanol

 

✅ Rule 3: Number the Carbon Chain So That the –OH Gets the Lowest Possible Number

  • Number from the end nearest the –OH.
  • Place the locant number before “-ol” to indicate the carbon bearing the –OH.

 

✅ Rule 4: Indicate Position of the –OH Group

  • For alcohols with more than two carbons, always specify the –OH location.
  • Examples:
    • propan-1-ol (–OH on C-1)
    • propan-2-ol (–OH on C-2)

 

✅ Rule 5: Name and Number Other Substituents

  • If there are alkyl or other substituents (e.g., methyl, chloro), identify their positions and list them as prefixes in alphabetical order.

 

✅ Rule 6: For Multiple Hydroxyl Groups (Polyhydric Alcohols)

  • Keep the “-e” ending of the parent alkane and add:
    • -diol (two –OH groups)
    • -triol (three –OH groups)
  • Indicate all positions.
  • Examples:
    • ethane-1,2-diol (HO–CH₂–CH₂–OH) → glycol
    • propane-1,2,3-triol (HO–CH₂–CH(OH)–CH₂–OH) → glycerol

 

 PROGRESSIVE EXAMPLES

🔹 Example 1: Methanol

  • One carbon, –OH on C-1 → methanol (CH₃OH).

🔹 Example 2: Ethanol

  • Two carbons, –OH on C-1 → ethanol (CH₃CH₂OH).

🔹 Example 3: Propan-2-ol

  • Three carbons, –OH on middle carbon → propan-2-ol (CH₃–CHOH–CH₃).

🔹 Example 4: 2-methylpropan-1-ol

  • Parent chain: propane (3 carbons).
  • Methyl substituent on C-2.
  • –OH on C-1.
  • Structure: (CH₃)₂CH–CH₂OH.

 

🔹 Example 5: Butan-2-ol

  • Four carbons, –OH on C-2 → butan-2-ol (CH₃–CHOH–CH₂–CH₃).

🔹 Example 6: 3-chloropentan-2-ol

  • Five carbons (pentane), –OH on C-2, chloro on C-3 → 3-chloropentan-2-ol.

 

🔹 Example 7: Ethane-1,2-diol

  • Two carbons, –OH groups on C-1 and C-2 → ethane-1,2-diol (HO–CH₂–CH₂–OH).

 

🔹 Example 8: Glycerol (Propane-1,2,3-triol)

  • Three carbons, three –OH groups → propane-1,2,3-triol (HO–CH₂–CHOH–CH₂–OH).

 

 NAMING TIP FOR COMMON ALCOHOLS

Some simple alcohols still commonly use trivial names, e.g.:

  • methanol (wood alcohol),
  • ethanol (grain alcohol),
  • isopropanol (isopropyl alcohol → propan-2-ol),
  • glycol (ethane-1,2-diol),
  • glycerol (propane-1,2,3-triol).

But IUPAC names should always be used in scientific writing.

 

🟢 KEY REMINDERS

✅ Always include the lowest locant for the –OH group.
✅ Parent chain must include the –OH group.
✅ Number substituents and multiple –OH groups precisely.
✅ For multiple –OH, use -diol, -triol with locants for each hydroxyl group.

 

 SUMMARY OF ALKANOL HOMOLOGOUS SERIES

Molecular Formula

Common Name

IUPAC Name

CH₃OH

wood alcohol

methanol

CH₃CH₂OH

grain alcohol

ethanol

CH₃CH₂CH₂OH

propyl alcohol

propan-1-ol

CH₃CHOHCH₃

isopropyl alcohol

propan-2-ol

CH₃(CH₂)₂CH₂OH

butan-1-ol

butan-1-ol

 

DERIVATIVES OF ALKANOLS

These derivatives are the dehydration and oxidation products of alkanols. They are: alkoxyalkanes (ethers), alkanals (aldehydes) and alkanones (ketones).

 

IUPAC Nomenclature of Alkoxyalkanes (Ethers)

 Introduction

  • Ethers are organic compounds with the general structure R–O–R′, where R and R′ are alkyl or aryl groups.
  • The characteristic functional group of ethers is the –O– (ether linkage).
  • In IUPAC nomenclature, simple ethers are systematically named as alkoxyalkanes, treating one alkyl group + oxygen as an “alkoxy” substituent on the longer alkyl chain.

 

General Rules for Naming Ethers (Alkoxyalkanes)

✅ 1. Identify the longer of the two carbon chains attached to the oxygen atom.
✅ 2. Name the shorter chain as an alkoxy substituent (alkyl + oxy → alkoxy group).
✅ 3. The parent name is based on the longer alkyl chain.
✅ 4. Number the parent chain giving the lowest possible number to the carbon attached to the –OR group (alkoxy group).
✅ 5. Combine the position of the alkoxy group with the parent name.

 

 Important Notes

  • For simple symmetrical ethers, trivial names are often used (e.g., diethyl ether), but IUPAC prefers the systematic name.
  • For unsymmetrical ethers, always pick the longer carbon chain as the parent alkane, then name the other side as the alkoxy group.

 

 Examples

🔹 Methoxyethane (ethyl methyl ether)
CH₃–O–CH₂CH₃ → Longer chain: ethane → parent: ethane; shorter chain (meth-) as methoxy → methoxyethane.

🔹 Ethoxypropane (propyl ethyl ether)
CH₃CH₂–O–CH₂CH₂CH₃ → Longer chain: propane → parent: propane; shorter chain (ethyl-) as ethoxy → ethoxypropane.

🔹 1-Methoxybutane
CH₃O–CH₂CH₂CH₂CH₃ → Longest chain: butane; methoxy at C-1 → 1-methoxybutane.

🔹 2-Methoxypropane (isopropyl methyl ether)
(CH₃)₂CH–O–CH₃ → Longest chain: propane → numbering gives methoxy on C-2 → 2-methoxypropane.

🔹 1-Ethoxy-2-methylpropane
CH₃CH₂O–CH(CH₃)₂ → Parent chain: 2-methylpropane; ethoxy group on C-1 → 1-ethoxy-2-methylpropane.

 

Common vs. IUPAC Names

Common Name

IUPAC Name

Diethyl ether

Ethoxyethane

Methyl tert-butyl ether (MTBE)

2-methoxy-2-methylpropane

Ethyl methyl ether

Methoxyethane

 

 Key Naming Steps

🔷 Identify longest chain as parent hydrocarbon.
🔷 Name the other chain as an alkoxy substituent.
🔷 Position number if the parent chain has 3+ carbons → give alkoxy group the lowest possible number.
🔷 Combine: position-alkoxy + parent alkane.

 

RACK YOUR BRAIN

  1. What is the IUPAC name of CH₃–O–CH₂CH₃?

  2. Provide the IUPAC name for CH₃CH₂CH₂–O–CH₃.
  3. Name this compound: CH₃CH₂–O–CH(CH₃)₂

 

 Naming Cyclic Ethers

For cyclic ethers, the ring is often named using prefixes like “oxa-”:

  • Epoxides (three-membered cyclic ethers) → named as oxiranes.
  • Example: Ethylene oxide → oxirane.

 

Priority of Ethers in Compounds

In the presence of higher priority functional groups (e.g., –COOH, –CHO, –C=O, –OH, –NH₂), the ether is named as an alkoxy substituent (like other simple substituents), not as a suffix.

🔷 For example:
HOCH₂CH₂–O–CH₃ → 2-methoxyethanol (–OH takes priority → alcohol suffix; ether as methoxy substituent).

 

🔎 Summary

  • Ethers are named in IUPAC as alkoxyalkanes, with one side of the ether group as an alkoxy substituent on the longer alkyl chain.
  • Parent chain: longest alkyl chain.
  • Numbering gives the alkoxy group the lowest possible locant.
  • Combine: position-alkoxy + parent name.

 

IUPAC Nomenclature of Alkanals (Aldehydes)

Introduction

  • Aldehydes are organic compounds characterized by the presence of the –CHO (formyl) group, where a carbonyl (C=O) is bonded to at least one hydrogen.
  • They are derived from alkanes by replacing a terminal hydrogen atom with the –CHO group.
  • Their systematic IUPAC names are alkanals, derived from the longest carbon chain containing the –CHO group.

 

General Rules for Naming Alkanals

✅ 1. Identify the longest continuous carbon chain containing the –CHO group.
✅ 2. Name the parent hydrocarbon by replacing the –e ending of the corresponding alkane with –al.
✅ 3. The carbon of the –CHO group is always carbon-1 in numbering (it gets priority over all other groups).
✅ 4. Number substituents based on their position relative to the –CHO group (carbon-1), giving the lowest possible numbers to substituents.
✅ 5. List substituents with their positions in alphabetical order before the parent name.
✅ 6. If there are multiple –CHO groups, use prefixes like –dial, –trial, etc., e.g., ethanedial (glyoxal).

 

Important Notes

  • Aldehyde group always takes priority over alkyl, halo, nitro, alcohol, ketone, amine, ether, etc. when numbering and naming.
  • No need to indicate the position of the –CHO group in simple alkanals, since it must always be at carbon-1.

 

 Examples

🔹 Methanal (Formaldehyde)
H–CHO → One carbon; parent: methane → methanal.

🔹 Ethanal (Acetaldehyde)
CH₃–CHO → Two carbons; parent: ethane → ethanal.

🔹 Propanal
CH₃–CH₂–CHO → Three carbons; parent: propane → propanal.

🔹 2-Methylpropanal
(CH₃)₂CH–CHO → Longest chain: 3 carbons; methyl group on C-2 → 2-methylpropanal.

🔹 3-Chlorobutanal
CH₂Cl–CH₂–CH₂–CHO → Longest chain: 4 carbons → butanal; Cl at C-3 → 3-chlorobutanal.

🔹 Pentane-1,5-dial (A dialdehyde)
OHC–(CH₂)₃–CHO → Parent: pentane → pentanedial → since aldehyde groups are on both ends → pentane-1,5-dial.

 

Common vs. IUPAC Names

Common Name

IUPAC Name

Formaldehyde

Methanal

Acetaldehyde

Ethanal

Propionaldehyde

Propanal

Butyraldehyde

Butanal

Valeraldehyde

Pentanal

 

Priority of Aldehyde Group

In compounds with multiple functional groups, aldehydes take higher priority than ketones, alcohols, amines, alkenes, alkynes, halides, etc., in choosing the parent chain and numbering.

Naming Compounds with Other Functional Groups

✅ With double/triple bonds: The –CHO carbon is always C-1; indicate the position of unsaturation relative to the carbon chain:

  • CH₂=CH–CH₂–CHO → But-3-enal (or 3-butenal).
  • HC≡C–CH₂–CHO → But-3-ynal.

✅ With hydroxyl groups (–OH): Indicate as a substituent using the prefix “hydroxy-” since aldehyde takes priority:

  • HO–CH₂–CH₂–CHO → 3-hydroxypropanal.
  •  

 

Key Functional Group Suffix Priority (for reference)

In IUPAC nomenclature, suffix priority (highest → lowest):

  1. –COOH (carboxylic acid)
  2. –SO₃H (sulfonic acid)
  3. –COOR (ester)
  4. –CONH₂ (amide)
  5. –CHO (aldehyde) ← our focus
  6. –C=O (ketone)
  7. –OH (alcohol)
  8. –NH₂ (amine)
  9. –C=C/C≡C (alkene/alkyne)
  10. Halides, nitro, etc.

 

IUPAC Nomenclature of Alkanones (Ketones)

 

 Introduction

  • Ketones are organic compounds containing the carbonyl group (C=O) bonded to two carbon atoms (i.e., the carbonyl is within the chain, not at the end like aldehydes).
  • The general formula of simple ketones is R–CO–R′, where R and R′ are alkyl or aryl groups.
  • Their systematic names end with –one, and they are called alkanones in IUPAC nomenclature.

General Rules for Naming Alkanones

✅ 1. Identify the longest carbon chain containing the carbonyl (C=O) group.
✅ 2. Name the parent hydrocarbon by replacing the –e ending of the corresponding alkane with –one.
✅ **3. Number the chain so that the carbonyl carbon gets the lowest possible number, indicating its position with a number before the –one suffix (e.g., pentan-2-one).
✅ 4. List substituents with their positions in alphabetical order before the parent name.
✅ 5. If there are two or more keto groups, use prefixes like –dione, –trione, etc., e.g., butane-2,3-dione.

Important Notes

  • The carbonyl carbon in ketones must not be at carbon-1, otherwise it’s an aldehyde.
  • Position of the carbonyl is essential and must always be indicated in ketone names except for propanone (acetone), where the position is obvious.

 

Examples

🔹 Propanone (Acetone)
CH₃–CO–CH₃ → Three carbons; the only possible position is carbon-2 → propan-2-one → trivial/common name: acetone.

🔹 Butan-2-one
CH₃–CO–CH₂–CH₃ → Four carbons; carbonyl on C-2 → butan-2-one.

🔹 Pentan-3-one
CH₃–CH₂–CO–CH₂–CH₃ → Five carbons; carbonyl on C-3 → pentan-3-one.

🔹 4-Methylpentan-2-one
CH₃–CO–CH₂–CH(CH₃)–CH₃ → Five carbons; methyl group on C-4; carbonyl on C-2 → 4-methylpentan-2-one.

🔹 Hexane-2,5-dione (a diketone)
CH₃–CO–CH₂–CH₂–CO–CH₃ → Longest chain: 6 carbons → hexane; carbonyls on C-2 & C-5 → hexane-2,5-dione.

Common vs. IUPAC Names

Common Name

IUPAC Name

Acetone

Propan-2-one

Acetylacetone

Pentane-2,4-dione

Diethyl ketone

Pentan-3-one

 Naming Compounds with Other Functional Groups

✅ With alkenes/alkynes: Number the chain giving the lowest locants to the carbonyl and the double/triple bond:

  • CH₃–CH=CH–CO–CH₃ → Pent-3-en-2-one.

✅ With substituents: Name and number them as prefixes:

  • CH₃–CO–CH(CH₃)–CH₃ → 3-methylbutan-2-one.

✅ With multiple functional groups: If the ketone is not the highest priority, it’s named as an oxo- substituent:

  • In carboxylic acids, ketone becomes oxo since –COOH has higher priority:
    • HOOC–CH₂–CO–CH₃ → 3-oxobutanoic acid.

Priority of Ketone Group

When multiple functional groups are present, ketones are lower priority than carboxylic acids, sulfonic acids, esters, acid chlorides, amides, and aldehydes but higher than alcohols, amines, alkenes, alkynes, halides, etc.

 

Key Functional Group Suffix Priority (simplified excerpt)

  1. –COOH (carboxylic acid)
  2. –SO₃H (sulfonic acid)
  3. –COOR (ester)
  4. –CONH₂ (amide)
  5. –CHO (aldehyde)
  6. –C=O (ketone) ← our focus
  7. –OH (alcohol)
  8. –NH₂ (amine)
  9. –C=C/C≡C (alkene/alkyne)