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Organic Chemistry I · Naming

IUPAC nomenclature practice, with answers

An IUPAC name is a set of directions: the parent chain says how long the backbone is, the suffix says which functional group is in charge, and the numbers say where everything sits. Almost every naming mistake comes from one of three slips: the wrong parent chain, numbering from the wrong end, or substituents out of alphabetical order.

Already know the basics? Jump to the 8 practice questions or the worked examples.

Name any open-chain compound in five steps

  1. Find the principal group. If there is an acid, aldehyde, ketone, or alcohol, the highest-ranked one becomes the suffix (see the table below). Halogens and alkyl groups are always prefixes.
  2. Pick the parent chain. Choose the longest continuous chain that contains the principal group (and the C=C, if there is one). The chain can bend; it doesn't have to run straight across the drawing. If two chains tie in length, use the one with more branches.
  3. Number the chain. Give the principal group the lowest number. Then give the C=C the lowest number. Then give the substituents the lowest numbers at the first point of difference. If it's still a tie, the substituent that comes first alphabetically gets the lower number.
  4. Name and sort the substituents. Alkyl groups (methyl, ethyl, propyl...) and halogens (fluoro, chloro, bromo, iodo). List them alphabetically. Ignore di-, tri-, tetra-, sec-, and tert- when sorting, but count iso- (isopropyl sorts under i).
  5. Assemble. One word, commas between numbers, hyphens between numbers and letters: 4-ethyl-2,2-dimethylhexane. For an alkene, add E or Z (or cis or trans) in front when the double bond can have two arrangements.

Which group gets the suffix?

Common groups, highest priority first
GroupAs the suffixAs a prefixExample
Carboxylic acid, –COOH-oic acid (that carbon is C1)carboxy-CH₃CH₂COOH, propanoic acid
Aldehyde, –CHO-al (that carbon is C1)oxo- or formyl-CH₃CH₂CHO, propanal
Ketone, C=O in the chain-oneoxo-CH₃COCH₂CH₃, 2-butanone
Alcohol, –OH-olhydroxy-CH₃CH(OH)CH₃, 2-propanol
Alkene, C=C-ene (in the parent name)not used as a prefix hereCH₃CH=CH₂, propene
Halogen, alkylneverchloro-, bromo-, methyl-...CH₃CHBrCH₃, 2-bromopropane

Acids and aldehydes sit at the end of a chain, so their carbon is always C1 and doesn't need a number: butanal, not 1-butanal.

2-butanol or butan-2-ol? Both, and here's why

IUPAC's newer recommendations put the locant right before the suffix it describes: butan-2-ol, but-2-ene, hexan-3-one. The older style puts it before the whole parent: 2-butanol, 2-butene, 3-hexanone. OpenStax Organic Chemistry, the textbook used in many college courses, writes alkene names the older way and lists both forms for alcohols and ketones. This page follows that: older style first, newer style in parentheses in the solutions. Most instructors accept either, as long as you're consistent.

E/Z in one minute. On each carbon of the C=C, decide which of the two attached groups has higher priority (higher atomic number at the first point of difference). If the two higher-priority groups are on the same side, the alkene is Z (zusammen, together). Opposite sides: E (entgegen). For a simple alkene with one H on each double-bond carbon, cis matches Z and trans matches E.

Worked examples

Example 1: a branched alkane

Name CH₃CH₂CH(CH₃)CH(CH₂CH₃)CH₂CH₂CH₃.

  1. Parent chain: the main line runs CH₃–CH₂–CH–CH–CH₂–CH₂–CH₃, which is 7 carbons. Detouring into the ethyl branch gives only 6. So the parent is heptane.
  2. Substituents: a methyl and an ethyl.
  3. Numbering: from the left, the methyl is on C3 and the ethyl on C4: {3,4}. From the right, the ethyl is on C4 and the methyl on C5: {4,5}. The first point of difference is 3 versus 4, so number from the left.
  4. Alphabetize: ethyl before methyl.

Answer: 4-ethyl-3-methylheptane.

Example 2: an alcohol with a halogen

Name CH₃CHBrCH₂CH(OH)CH₃.

  1. Principal group: –OH. Bromine is just a prefix (bromo).
  2. Parent chain: five carbons containing the OH carbon, so pentanol.
  3. Numbering: the OH must get the lowest number. Starting from the right end, OH is on C2 and Br on C4. Starting from the left would put OH on C4, so that direction is wrong even though Br would get a 2.
  4. Assemble: prefix, then the OH locant and suffix.

Answer: 4-bromo-2-pentanol (newer style: 4-bromopentan-2-ol).

Example 3: an alkene with E/Z

Name CH₃CH=C(CH₃)CH₂CH₃ when the CH₃ on C2 and the CH₂CH₃ on C3 are on the same side of the double bond.

  1. Parent chain: the longest chain through the C=C is CH₃–CH=C–CH₂–CH₃, 5 carbons: pentene.
  2. Numbering: from the left, the C=C starts at C2. From the right, it would start at C3. Use the left, so it's 2-pentene with a methyl on C3.
  3. Priorities on C2: CH₃ beats H.
  4. Priorities on C3: both groups start with carbon. Next atoms: CH₃ has (H,H,H); CH₂CH₃ has (C,H,H). Ethyl wins.
  5. Compare: the two winners, CH₃ on C2 and ethyl on C3, are on the same side, so it's Z.

Answer: (Z)-3-methyl-2-pentene (newer style: (Z)-3-methylpent-2-ene).

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Practice questions

Pick an answer, then check it. A wrong answer tells you what went wrong, so you can try again. Every question also has a full solution.

Question 1Name CH₃CH₂C(CH₃)₂CH₂CH(CH₃)CH₃.

Show solution

Longest chain: CH₃–CH₂–C–CH₂–CH–CH₃, six carbons (hexane). It carries two methyls on one carbon and one methyl on another. Numbering from the right: C2 (one methyl), C4 (two methyls), giving {2,4,4}. Numbering from the left gives {3,3,5}. Compare at the first difference: 2 < 3. Name: 2,4,4-trimethylhexane.

Question 2Name CH₃CHClCH₂CH(CH₃)CH₂CH₃.

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The longest chain has six carbons, including the CH₂CH₃ at the right end. Halogens are treated like alkyl substituents for numbering. From the left: Cl on C2, CH₃ on C4, {2,4}. From the right: {3,5}. Use {2,4}. Alphabetical order: chloro, then methyl. Name: 2-chloro-4-methylhexane.

Question 3Name CH₃CHBrCH₂CH₂CH(CH₃)₂.

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Six-carbon chain. From the left: Br on C2, CH₃ on C5. From the right: CH₃ on C2, Br on C5. Both are {2,5}, so the tie goes to the substituent first in alphabetical order. Bromo comes before methyl, so Br gets C2: 2-bromo-5-methylhexane. This exact example appears in OpenStax section 10.1.

Question 4Name CH₃CH=CHCH(CH₃)CH₂CH₃ (ignore E/Z for this one).

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The longest chain containing the C=C: CH₃–CH=CH–CH–CH₂–CH₃, six carbons. Number from the end nearer the double bond: the C=C starts at C2 (from the other end it would start at C4). The methyl is on C4. Name: 4-methyl-2-hexene (newer style: 4-methylhex-2-ene).

Question 5Name (CH₃)₂C(OH)CH₂CH₂CH₃.

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The OH carbon carries two CH₃ groups and a propyl chain. The longest chain containing it uses one CH₃ plus the propyl: five carbons, pentanol. Number from the end nearer OH: OH on C2, extra methyl on C2. Name: 2-methyl-2-pentanol (newer style: 2-methylpentan-2-ol). OpenStax uses this compound as its example of the two styles.

Question 6Name HOCH₂CH₂CH₂COCH₃ (the C=O is the second carbon from the right).

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Two groups: a ketone and an alcohol. The ketone ranks higher, so the suffix is -one and the OH becomes hydroxy-. Five-carbon chain. Number from the end nearer the ketone: C=O at C2, OH on C5. Name: 5-hydroxy-2-pentanone (newer style: 5-hydroxypentan-2-one).

Question 7Which condensed formula is 3-methylbutanoic acid?

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Butanoic acid is a four-carbon chain, with the COOH carbon as C1. A methyl on C3 gives HOOC–CH₂–CH(CH₃)–CH₃, written (CH₃)₂CHCH₂COOH. Check: C1 = COOH, C2 = CH₂, C3 = CH with the methyl, C4 = CH₃.

Question 8In CH₃CH=CHCH₂CH₃, the CH₃ group and the CH₂CH₃ group are on opposite sides of the double bond. What is the full name?

Show solution

Chain: five carbons, C=C starting at C2. On C2, CH₃ beats H. On C3, CH₂CH₃ beats H. The two higher-priority groups are on opposite sides, so the alkene is E. Name: (E)-2-pentene (newer style: (E)-pent-2-ene). Since each double-bond carbon has one H, this is also trans-2-pentene.

Answer key

  1. B, 2,4,4-trimethylhexane: six-carbon chain, lowest locants at the first point of difference.
  2. C, 2-chloro-4-methylhexane: lowest locants, then alphabetical order.
  3. A, 2-bromo-5-methylhexane: tied locants, so alphabetical order decides.
  4. D, 4-methyl-2-hexene: the C=C gets the lowest number before substituents.
  5. A, 2-methyl-2-pentanol: OH is the suffix and gets the lowest number.
  6. C, 5-hydroxy-2-pentanone: ketone outranks alcohol.
  7. B, (CH₃)₂CHCH₂COOH: number from the COOH carbon.
  8. D, (E)-2-pentene: higher-priority groups on opposite sides.

Frequently asked questions

Should I write 2-butanol or butan-2-ol?

Both name the same compound. Butan-2-ol follows IUPAC's newer recommendations, which put the locant right before the suffix. 2-Butanol is the older style that many textbooks and instructors still use. OpenStax Organic Chemistry shows both for alcohols. Use whichever your course uses, and be consistent.

How do I alphabetize substituents?

Sort by the substituent name itself. Ignore multiplying prefixes like di- and tri-, and ignore the hyphenated prefixes sec- and tert-. Do count iso-, so isopropyl sorts under i. For example, ethyl comes before dimethyl because you compare e with m.

When do I need E or Z in a name?

Whenever each carbon of the double bond has two different groups on it, the alkene can exist in two forms, so the name needs E or Z. If either carbon has two identical groups, such as two H atoms in a CH₂= end, there is no E or Z.

Is IUPAC naming on the MCAT?

The MCAT expects you to recognize functional groups and read names of common organic compounds, since passages use them. Naming is rarely tested as a separate skill, but quick naming makes passages easier to read. Check the current AAMC content outline for exact wording.

Sources

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