IUPAC Nomenclature

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IUPAC Nomenclature — Key Facts for Makerere University (Uganda) Core concept: The IUPAC (International Union of Pure and Applied Chemistry) system provides a systematic method for naming organic compounds, ensuring consistent and unambiguous identification worldwide High-yield points: Mastering the steps: identify longest chain, number from correct end, name substituents, apply functional group suffixes correctly ⚡ Exam tip: Questions often ask you to name compounds given structural formulas, or draw structures from names. Always identify the longest carbon chain first.

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IUPAC Nomenclature — Makerere University (Uganda) Study Guide

1. Fundamental Naming Rules

Steps for Naming Organic Compounds

1. Identify the principal functional group (highest priority group) → determines the suffix
2. Find the longest carbon chain (parent chain) containing the principal functional group
3. Number the chain from the end that gives the principal functional group the lowest number
4. Identify and name substituents (alkyl groups, halogens, nitro, etc.)
5. Assign locants (numbers) to each substituent
6. Assemble the name in alphabetical order with proper formatting

Functional Group Priority (Highest to Lowest)

1. Carboxylic acids (–COOH): suffix -oic acid
2. Anhydrides: suffix -oic anhydride
3. Esters (–COOR): suffix -oate
4. Acid halides (–COCl): suffix -oyl chloride
5. Amides (–CONH₂): suffix -amide
6. Nitriles (–CN): suffix -nitrile
7. Aldehydes (–CHO): suffix -al
8. Ketones (–CO–): suffix -one
9. Alcohols (–OH): suffix -ol
10. Amines (–NH₂): suffix -amine
11. Alkenes (C=C): suffix -ene
12. Alkynes (C≡C): suffix -yne
13. Ethers (C–O–C): prefix alkoxy-
14. Halides (–X): prefix fluoro-, chloro-, bromo-, iodo-
15. Nitro (–NO₂): prefix nitro-

⚠️ Important: When two functional groups are present and one has higher priority, the lower-priority group is named as a prefix.

2. Alkanes (Saturated Hydrocarbons)

Straight Chain Alkanes: CₙH₂ₙ₊₂

Formula	Name	Boiling Point (°C)
CH₄	Methane	−162
C₂H₆	Ethane	−89
C₃H₈	Propane	−42
C₄H₁₀	Butane	−0.5
C₅H₁₂	Pentane	36
C₆H₁₄	Hexane	69
C₇H₁₆	Heptane	98
C₈H₁₈	Octane	126
C₉H₂₀	Nonane	151
C₁₀H₂₂	Decane	174

Branched Chain Alkanes

Alkyl substituents (named by removing one H from alkane):

Group	Structure	Name
–CH₃		Methyl
–C₂H₅		Ethyl
–C₃H₇		Propyl
–CH(CH₃)₂		Isopropyl
–CH₂CH₂CH₃		Propyl
–C(CH₃)₃		tert-Butyl (t-butyl)
–CH₂CH(CH₃)₂		Isobutyl
–CH(CH₃)CH₂CH₃		sec-Butyl (s-butyl)

Numbering rule: Number from the end that gives substituents the lowest numbers. If there is a tie, use alphabetical order to break ties.

Examples:

• CH₃–CH₂–CH₃: Propane (no substituents)
• CH₃–CH(CH₃)–CH₃: 2-Methylpropane (NOT 1-methylpropane — longest chain is 3 carbons)
• CH₃–CH₂–CH₂–CH(CH₃)–CH₃: 2-Methylpentane (NOT 4-methylpentane — number from closer end)
• CH₃–CH(CH₃)–CH(CH₃)–CH₃: 2,3-Dimethylbutane

⚠️ Common mistake: The longest chain determines the parent name — not the longest continuous chain that includes the functional group if it’s not the longest chain.

3. Alkenes and Alkynes (Unsaturated Hydrocarbons)

Alkenes: CₙH₂ₙ (at least one C=C bond)

Naming:

1. Find longest chain containing the C=C double bond
2. Number from the end that gives C=C the lowest number
3. Suffix: -ene
4. If multiple double bonds: -diene, -triene, etc.

Examples:

• CH₂=CH–CH₃: Prop-1-ene (or propene)
• CH₃–CH=CH–CH₃: But-2-ene
• CH₂=CH–CH₂–CH=CH₂: Penta-1,4-diene
• CH₂=C(CH₃)–CH₂–CH₃: 2-Methylbut-1-ene (the C=C gets position 1)

Alkynes: CₙH₂ₙ₋₂ (at least one C≡C triple bond)

Naming:

1. Find longest chain containing the C≡C triple bond
2. Number from the end that gives C≡C the lowest number
3. Suffix: -yne

Examples:

• HC≡CH: Ethyne (acetylene)
• CH₃–C≡CH: Prop-1-yne (or propyne)
• CH₃–C≡C–CH₃: But-2-yne

Geometric Isomerism (E/Z Notation for Alkenes)

When each carbon of the double bond has two different substituents, E/Z isomerism exists:

• E (entgegen): Higher priority groups on opposite sides
• Z (zusammen): Higher priority groups on same side

Cahn-Ingold-Prelog (CIP) priority rules (for assigning E/Z):

1. Compare atomic numbers of atoms directly attached to the double bond carbon
2. If tied, compare the next set of atoms along each chain
3. Multiple bonds count as duplicate attachments

Example — But-2-ene: CH₃–CH=CH–CH₃ has identical groups on each end → no E/Z isomerism CH₃–CH=CH–Cl has different groups → can be E or Z

Example with priority:

        CH₃              Br
         \              /
          C=========C
         /              \
        H               CH₂CH₃

On left C: CH₃ > H → CH₃ is priority 1 On right C: Br > CH₂CH₃ → Br is priority 1 Since priorities are on opposite sides → E isomer

4. Alcohols (–OH Functional Group)

Suffix: -ol Numbering: Give –OH the lowest possible number

Examples:

• CH₃–CH₂–CH₂OH: Propan-1-ol (1-propanol)
• CH₃–CH(OH)–CH₃: Propan-2-ol (2-propanol; isopropanol)
• CH₃–CH₂–CH(OH)–CH₂–CH₃: Pentan-3-ol
• HO–CH₂–CH₂–OH: Ethane-1,2-diol (ethylene glycol)
• HO–CH₂–CH(OH)–CH₂OH: Propane-1,2,3-triol (glycerol)

5. Aldehydes and Ketones (C=O Carbonyl Group)

Aldehydes: –CHO (at end of chain)

Suffix: -al Note: Position 1 is always the carbonyl carbon, so no number is needed in the name.

• HCHO: Methanal (formaldehyde)
• CH₃CHO: Ethanal (acetaldehyde)
• CH₃–CH₂–CHO: Propanal
• CH₃–CH(CH₃)–CHO: 2-Methylpropanal

Ketones: –CO– (in chain)

Suffix: -one Numbering: Carbonyl carbon gets the lowest possible number

• CH₃–CO–CH₃: Propanone (acetone)
• CH₃–CH₂–CO–CH₂–CH₃: Pentan-3-one
• CH₃–CO–CH₂–CH₂–CH₃: Pentan-2-one

6. Carboxylic Acids (–COOH)

Suffix: -oic acid Numbering: Carboxyl carbon is always C-1

• HCOOH: Methanoic acid (formic acid)
• CH₃COOH: Ethanoic acid (acetic acid)
• CH₃–CH₂–COOH: Propanoic acid
• CH₃–CH(Br)–COOH: 2-Bromopropanoic acid
• HOOC–CH₂–COOH: Propane-1,3-dioic acid (malonic acid)

7. Esters (–COO–)

Naming: Acid part (alkanoate) + alkyl part ( alkoxy) Format: alkyl alkanoate

• CH₃COOCH₃: Methyl ethanoate (NOT methyl acetate)
• CH₃COOCH₂CH₃: Ethyl ethanoate
• HCOOCH₃: Methyl methanoate
• CH₃CH₂COOCH₂CH₃: Ethyl propanoate

⚠️ Common mistake: Reversing the order. The alkyl group (RO–) comes first in the name; the acid part (RCOO–) comes second.

8. Amines (–NH₂)

Primary amines (suffix: -amine, with locant):

• CH₃NH₂: Methanamine (methylamine)
• CH₃CH₂NH₂: Ethanamine
• CH₃–CH(NH₂)–CH₃: Propan-2-amine
• H₂N–CH₂–CH₂–NH₂: Ethane-1,2-diamine

Secondary and tertiary amines:

• (CH₃)₂NH: N-Methylmethanamine (dimethylamine) [or simply dimethylamine]
• (CH₃)₃N: N,N-Dimethylmethanamine (trimethylamine)
• CH₃–CH₂–N(CH₃)₂: N,N-Dimethylethanamine

Nomenclature for poly-substituted amines: The prefix “N-” indicates a substituent on the nitrogen atom (not on the carbon chain).

9. Halides and Nitro Compounds

Halides (–X)

Prefix: halo- (fluoro-, chloro-, bromo-, iodo-) Alphabetical order among halogens: bromo- < chloro- < fluoro- < iodo-

Examples:

• CH₃Cl: Chloromethane
• CH₃CH₂Br: Bromoethane
• CH₂Cl–CH₂Cl: 1,2-Dichloroethane
• CH₃–CH(Br)–CH₂–CH(Cl)–CH₃: 2-Bromo-4-chloro-2-methylpentane (numbered to give lowest set of locants: 2+4=6 vs 1+3=4 from other end)

Nitro Compounds (–NO₂)

Prefix: nitro-

• CH₃NO₂: Nitromethane
• C₆H₅–NO₂: Nitrobenzene
• 1-CH₃–2-NO₂–C₆H₃: 2-Nitrotoluene (or o-nitrotoluene)

10. Cyclic Compounds

Cycloalkanes

• Cyclopropane (C₃H₆), cyclobutane (C₄H₈), cyclopentane (C₅H₁₀), cyclohexane (C₆H₁₂)
• Methylcyclohexane: Cyclohexane with a methyl substituent
• Cyclohexene: Double bond in the ring
• Cyclohexanol: –OH on the ring

Aromatic Compounds (Benzene Derivatives)

Benzene as parent: C₆H₅– as a substituent is called “phenyl”

• C₆H₅–CH₃: Methylbenzene (toluene)
• C₆H₅–OH: Phenol (hydroxybenzene)
• C₆H₅–CHO: Benzaldehyde
• C₆H₅–COCH₃: Acetophenone (phenyl ethanone)
• C₆H₅–COOH: Benzoic acid
• 1,2-C₆H₄(CH₃)₂: 1,2-Dimethylbenzene (o-xylene)
• 1,3-C₆H₄(CH₃)₂: 1,3-Dimethylbenzene (m-xylene)
• 1,4-C₆H₄(CH₃)₂: 1,4-Dimethylbenzene (p-xylene)

Ortho (o-): 1,2 positions Meta (m-): 1,3 positions Para (p-): 1,4 positions

11. Multifunctional Compounds

When a compound has multiple functional groups:

1. Identify the HIGHEST priority group → determines the parent name and suffix
2. Lower priority groups become prefixes
3. Number to give highest priority group lowest number
4. List other prefixes in alphabetical order

Examples:

• HO–CH₂–CH₂–CHO: 3-Hydroxypropanal (aldehyde priority over alcohol)
• HOOC–CH₂–CH₂–NH₂: 4-Aminobutanoic acid (carboxylic acid priority over amine)
• CH₃–CO–CH₂–CH₂OH: 4-Hydroxybutan-2-one (ketone suffix, OH prefix)
• CH₃–CH=CH–COOH: But-2-enoic acid (carboxylic acid suffix; double bond prefix)
• O=CH–C₆H₄–CHO: Benzene-1,4-dicarbaldehyde (two aldehyde groups on benzene ring)

12. Exam-Style Questions & Tips

Common exam question patterns at Makerere:

1. “Name the following compound: [structural formula]”
2. “Draw the structural formula of: [compound name]”
3. “State whether the following name is correct; if not, give the correct IUPAC name: [name]”
4. “Draw all possible isomers of [formula] and name them”
5. “Assign E or Z configuration to [alkene name]”

⚡ Exam tips:

• The longest chain determines the parent name — don’t be distracted by branches
• Alphabetical order for prefixes: ignore di-, tri-, sec-, tert-, n- when alphabetizing (but “iso” is considered)
• If there is a tie in numbering, compare the next set of locants
• A carbonyl group in an aldehyde is always at position 1 (no number needed)
• When numbering a chain with multiple substituents, minimize the sum of locants first, not the first number

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13. Complex Nomenclature Scenarios

Bridged Compounds

Bicyclic compounds: Two rings share two carbon atoms.

• Norbornane: bicyclo[2.2.1]heptane
• Naming: bicyclo[a.b.c]alkane where a ≥ b ≥ c

Spiro Compounds

Two rings share only one carbon atom.

• Spiro[4.5]decane: two rings (5 and 6 carbons) sharing one carbon

Stereochemical Nomenclature

R/S System (for chiral centers):

1. Assign priorities to four groups (1=highest, 4=lowest) using CIP rules
2. With lowest priority group pointing away (dashed), trace 1→2→3
3. Clockwise = R (rectus); Counterclockwise = S (sinister)

Example — Lactic acid (CH₃–CH(OH)–COOH): Priority: OH (1) > COOH (2) > CH₃ (3) > H (4) H is pointing back (dashed) → trace 1→2→3: OH → COOH → CH₃ If this is clockwise → R configuration (R-lactic acid)

R/S for E/Z alkenes: Already covered in Section 3 above.

Retained Names (Trivial/Common Names)

Some compounds have widely used common names that are accepted by IUPAC:

Common Name	IUPAC Name
Acetone	Propanone
Formaldehyde	Methanal
Acetaldehyde	Ethanal
Acetic acid	Ethanoic acid
Toluene	Methylbenzene
Phenol	Hydroxybenzene
Aniline	Aminobenzene
Acetylene	Ethyne
Tartaric acid	2,3-Dihydroxybutanedioic acid

Parent Chains with Identical Substituents

When two identical substituents appear, use prefixes:

• 2,2-Dimethyl… (NOT 2-Dimethyl-2-methyl…)
• 1,1,3-Trimethyl… (three methyls at positions 1,1,3)

Selecting the Longest Chain in Complex Molecules

Rule: Choose the chain with the maximum number of:

1. Multiple bonds (prioritize the chain containing the highest priority functional group and/or most multiple bonds)
2. Functional groups
3. Double bonds > triple bonds
4. Lowest set of locants

Example — Complex molecule:

      CH₃
       |
CH₃–CH–CH₂–CH₂–CH₂–CH₃
       |
      CH₃

Longest chain is 6 carbons (hexane) with a methyl substituent at C-2 → 2-Methylhexane (Not 2-methylhexane if there’s also a double bond — then the chain with the double bond would be chosen even if it’s shorter)

The “n-” vs “iso-” vs “neo-” Prefixes

• n- (normal): Straight chain, e.g., n-butane
• iso- (isopropyl): (CH₃)₂CH–CH₂–, e.g., isopentane = 2-methylbutane
• neo-: Highly branched, e.g., neopentane = 2,2-dimethylpropane

Common Mistakes to Avoid

1. Forgetting to alphabetize substituents: 2-Bromo-1-chloropropane (bromo comes before chloro alphabetically)
2. Incorrect numbering direction: Always number to give the functional group and multiple bonds the lowest numbers
3. Naming esters backwards: It’s methyl ethanoate (alkoxy group first), not ethanoic acid methyl ester
4. Confusing -yl and -oyl endings: -yl = substituent (e.g., methyl = CH₃–); -oyl = part of acid (e.g., ethanoyl = CH₃CO–)
5. Forgetting that phenyl is C₆H₅–: Not C₆H₁₁– (that would be cyclohexyl)
6. Using the wrong suffix for ketones: It’s -one, not -al (aldehyde) or -oic acid (carboxylic acid)
7. Ignoring E/Z when possible: Any alkene with four different substituents on the double bond carbons will have E and Z isomers — state which one is named

Practice Problems

Q1: Name the following compounds: (a) CH₃–CH₂–CH(Br)–CH₂–CH₃ (b) (CH₃)₃C–CH₂–CH(CH₃)–CH₃ (c) CH₃–CH=CH–CH(Cl)–CH₃ (d) OHC–CH₂–CH₂–COOH

Q2: Draw structures for: (a) 3-ethyl-2-methylpentanal (b) 2,3-dimethylbut-2-ene (c) 4-hydroxy-3-methylhexanoic acid (d) N-ethyl-N-methylpropanamine

Q3: State whether each name is correct. If not, give the correct IUPAC name: (a) 3-methylbut-1-yne (b) 2-ethylpropanoic acid (c) 1-chloropropan-1-ol (d) But-3-ene

Q4: Assign R/S configuration to the chiral center in 2-bromobutane. Q5: Assign E/Z configuration to: (a) pent-2-ene (b) 1-bromo-2-chloroethene

Solutions to Selected Problems

Q1 (a): 3-bromopentane (number from left or right to give Br the lower number: 3 vs 3 — same, so either way)

Q1 (c): This is 3-chloropent-2-ene or 1-chloro-2-ethyl-2-butene — check longest chain and numbering carefully

Q3 (b): INCORRECT. 2-ethylpropanoic acid: the longest chain is pentanoic acid (5 carbons), giving 3-methylpentanoic acid. The ethyl group is actually part of the main chain.

Q3 (d): INCORRECT. But-3-ene is the same as but-1-ene (numbering should give the double bond the lowest number, so it should be but-1-ene).

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