Organic Chemistry — Bonding and Structure: Identifying Constitutional Isomers

Question

Indicate whether each of the following sets are constitutional isomers, the same compound, or different compounds.

  1. There are two line structure drawings. The left one has a 5 line chain with one line connected to two other branched lines off carbon 3 and one line off carbon 4. The right one shows a 5 line chain with one line off carbon 2 and one line connected to two other branched lines off carbon 4.
    Set A
  2. There are two organic structures. The left one is a line structure showing four lines with 2 separate lines off carbon 3. The right one is a condensed molecular formula showing CH3-CH2-C-CH2-CH3 with a line to each of 2 CH3 groups off the middle carbon.
    Set B
  3. There are two line structure drawings. The left one has a 4 line chain with a single line off carbon 2 and a single line off carbon 3. The right one has a 4 line chain with a single line off carbon 2 and a single line off carbon 4.
    Set C
  4. There are two line structure drawings. The left one has 5 connected lines with a single line off carbon 2, a single line off carbon 3, and a single line off carbon 4. The right one has 6 connected lines with a single line off carbon 3, a single line off carbon 4, and a single line off carbon 5.
    Set D

 

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  1. Constitutional isomers
  2. Same compound
  3. Constitutional isomers
  4. Different compounds

Refer to Section 7.4: Alkanes and Alkane Isomers (1).

Strategy Map

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Table 1: Strategy Map
Strategy Map Steps
1. Identify the atoms that make up each compound (their chemical formula).
2. Identify the connectivity of each compound.
3. Compare the 2 compounds and identify what is the same/different between them.

Solution

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a. Set A

Both structures have the same formula but different connectivity, which makes them constitutional isomers.

Answer: Constitutional isomers

b. Set B

Both structures have the same formula and connectivity, which makes them the same compound.

Answer: Same compound

c. Set C

Both structures have the same formula but different connectivity, which makes them constitutional isomers.

Answer: Constitutional isomers

d. Set D

Both structures have different formulas, which makes them different compounds.

Answer: Different compounds

Guided Solution

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The guided solution below will give you the reasoning for each step to get your answer, with reminders and hints.

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Table 2: Guided Solution
Guided Solution Ideas
This question is a theory type problem where you must identify structural isomers based on their chemical formula and connectivity.

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Refer to Section 7.4: Alkanes and Alkane Isomers (1).
Recall what a constitutional isomer is.

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Constitutional isomers are 2 or more molecules that have the same chemical formula (are composed of the same atoms) but different connectivity.

For example, the methane group is attached to carbon 4 on one compound and carbon 6 on another, but both have the same chemical compounds.
What will make the 2 compounds different compounds?

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Two compounds are considered different compounds if they have a different arrangement of atoms.
Table 3: Complete Solution
Complete Solution
a. Set A

Count the C and H atoms:

  • We count 7 C and 16 H for each structure.
  • Therefore, both structures have the molecular formula C10H22.
  • Looking closely, you can see the atoms are connected differently, which makes them constitutional isomers.
    Show/Hide Think About This!

    A line structure drawing that has a 5 line chain with 1 line connected to 2 other branched lines off carbon 3 and 1 line off carbon 4.    &   A line structure drawing that has a 6 carbon chain with 1 line connected to 2 branched lines off carbon 3 and 1 line off carbon 5.

    Notice that the C chains in the 2 structures are different.

Answer: Constitutional isomers
b. Set B

Count the C and H atoms:

  • We count 7 C and 16 H for each structure.
  • Therefore, both structures have the molecular formula C7H16.
  • They have the same connectivity, which makes them the same compound.
    Show/Hide Think About This!

    A line structure that has a 5 carbon chain with 2 methyl groups off carbon 3.    &    A condensed molecular formula showing CH3-CH2-C-CH2-CH3 with a line to each of the 2 CH3 groups off the middle carbon.

    These structures are the same structure represented in different ways.

Answer: Same compound
c. Set C

Count the C and H atoms:

  • We count 7 C and 16 H for each structure.
  • Therefore, both structures have the molecular formula of C7H16.
  • Looking closely, we can see they have different connectivity, which makes them constitutional isomers.
    Show/Hide Think About This!

    A line structure drawing that has a 4 line chain with a single line off carbon 2 and a single line off carbon 3.    &   A four line chain that has a single line off carbon 2 and a single line off carbon 4

    Notice that the different position of one of the C branches makes the connectivity different even though the atoms are the same.

Answer: Constitutional isomers
d. Set D

Count the C and H atoms:

  • We count 9 C and 20 H for the left structure and count 10 C and 33 H for the right structure.
  • The structure on the left has a molecular formula of C9H20 and the structure on the right has a formula of C10H22, which makes them different compounds.
    Show/Hide Think About This!

    A line structure drawing that has 5 connected lines with a single line off carbon 2, a single line off carbon 3, and a single line off carbon 4. & A line structure drawing that has 6 connected lines with a single line off carbon 3, a single line off carbon 4, and a single line off carbon 5.

Answer: Different compounds

Check Your Work

Reviewing your work above checking the connectivity and number of atoms will confirm your answers.

Does your answer make chemical sense?

Show/Hide Answer

The formula of compounds is determined by the atoms that compose them.

Multiple compounds can have the same chemical makeup and formula, but their connectivity may be different. If this is the case, they will be constitutional isomers. This differentiation is made because they will often have different chemical properties.

If the number of atoms and connectivity are the same, then they will be the same compound as in (b) where you are comparing a line structure drawing to a condensed structural formula. If the number of atoms is different as in (d), the molecular formula is different and they are different compounds.

PASS Attribution

Media Attributions

All figures are by Ashlynn Jensen, from LibreTexts PASS Chemistry Book CHEM 1500 (3) by Blackstock et al., and are used/updated under a CC BY-NC 4.0 license.

References

1. Farmer, S.; Kennepohl, D.; Sharett, Z.; Morsch, L.; Cunningham, K.; Soderberg, T. 7.4: Alkanes and Alkane Isomers. In CHEM 1500: Chemical Bonding and Organic Chemistry; LibreTexts, 2023. https://chem.libretexts.org/Courses/Thompson_Rivers_University/CHEM_1500%3A_Chemical_Bonding_and_Organic_Chemistry/07%3A_Organic_Chemistry_I_-_Bonding_and_Structure/7.04%3A_Alkanes_and_Alkane_Isomers.

2. Blackstock, L.; Brewer, S.; Jensen, A. PASS Chemistry Book CHEM 1500; LibreTexts, 2023. https://chem.libretexts.org/Courses/Thompson_Rivers_University/PASS_Chemistry_Book_CHEM_1500.

3. Blackstock, L.; Brewer, S.; Jensen, A. 7.3: Question 7.4.6 PASS – Identifying Constitutional Isomers. In PASS Chemistry Book CHEM 1500; LibreTexts, 2023. https://chem.libretexts.org/Courses/Thompson_Rivers_University/PASS_Chemistry_Book_CHEM_1500/07%3A_Organic_Chemistry_I_-_Bonding_and_Structure/7.03%3A_Question_7.4.6_PASS_-_identifying_constitutional_isomers.

4. Farmer, S.; Kennepohl, D.; Sharrett, Z.; Morsch, L.; Cunningham, K.; Soderburg, T. 3.2: Alkanes and Alkane Isomers. In Organic Chemistry (Morsch et al.); LibreTexts, 2022. https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_(Morsch_et_al.)/03%3A_Organic_Compounds-_Alkanes_and_Their_Stereochemistry/3.02%3A_Alkanes_and_Alkane_Isomers.

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