Q.Expand each of the following condensed formulas into their complete structural formulas.
The key idea is to expand each condensed formula by writing out every carbon–carbon and carbon–hydrogen bond explicitly. For (a) CH₃CH₂COCH₂CH₃, the complete structure is pentan-3-one; for (b) CH₃CH=CH(CH₂)₃CH₃, it is hept-2-ene.
Let’s start with the concept. A condensed formula like CH₃CH₂COCH₂CH₃ is a shorthand — it groups atoms to show the carbon skeleton and functional groups without drawing every bond. To get the complete structural formula, you need to “unpack” each group: every CH₃ means a carbon with three hydrogens, every CH₂ means a carbon with two hydrogens, and special groups like CO (carbonyl) or CH=CH (double bond) must be shown with their multiple bonds. The trick is to follow the order of atoms exactly as written, because the condensed formula is read left to right.
A common mistake is to misplace the double bond or carbonyl group. For example, in CH₃CH₂COCH₂CH₃, the CO is a carbonyl (C=O) between two ethyl groups — not at the end. Always check that the carbon count matches.
Now, let’s work through each part step by step.
(a) CH₃CH₂COCH₂CH₃
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Identify the carbon chain and functional group.
The formula has five carbons in total: CH₃ (1), CH₂ (2), CO (3), CH₂ (4), CH₃ (5). The CO group is a carbonyl (C=O), so this is a ketone. The carbon of the carbonyl is carbon number 3.
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Write the carbon skeleton.
Connect the carbons in order: C1–C2–C3–C4–C5. At C3, attach an oxygen with a double bond (C=O). All other bonds are single.
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Add hydrogen atoms to satisfy tetravalency.
- C1 (CH₃): already has 3 H’s, so it’s complete.
- C2 (CH₂): needs 2 H’s.
- C3 (carbonyl carbon): has a double bond to O and single bonds to C2 and C4 — that’s 4 bonds, so no H’s.
- C4 (CH₂): needs 2 H’s.
- C5 (CH₃): needs 3 H’s.
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Draw the complete structural formula.
The structure is:
H H O H H | | || | | H — C — C — C — C — C — H | | | | H H H HThis is pentan-3-one (or diethyl ketone).
You can also write it as CH₃–CH₂–CO–CH₂–CH₃, where the dashes are single bonds and CO shows the double bond to oxygen. That’s already a “semi-expanded” form, but the full structural formula shows every atom and bond.
(b) CH₃CH=CH(CH₂)₃CH₃
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Identify the carbon chain and the double bond.
The formula has: CH₃ (1), CH=CH (carbons 2 and 3 with a double bond), then (CH₂)₃ means three CH₂ groups (carbons 4, 5, 6), and finally CH₃ (7). So total 7 carbons — this is an alkene.
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Write the carbon skeleton with the double bond.
Connect: C1–C2=C3–C4–C5–C6–C7. The double bond is between C2 and C3.
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Add hydrogen atoms.
- C1 (CH₃): 3 H’s.
- C2 (part of C=C): already bonded to C1 and C3 with a double bond — needs 1 H.
- C3 (part of C=C): bonded to C2 (double) and C4 (single) — needs 1 H.
- C4, C5, C6 (each CH₂): each needs 2 H’s.
- C7 (CH₃): 3 H’s.
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Draw the complete structural formula.
H H H H H H H | | | | | | | H — C — C = C — C — C — C — C — H | | | | | | H H H H H HThis is hept-2-ene. Note that the double bond is between carbons 2 and 3, so the name is hept-2-ene (not hept-3-ene).
The (CH₂)₃ in the condensed formula is a compact way to write three consecutive CH₂ groups. When expanding, just “unfold” it into three separate carbons, each with two hydrogens.
The complete structural formulas are: (a) pentan-3-one, shown as CH₃–CH₂–CO–CH₂–CH₃ with all bonds;
(b) hept-2-ene, shown as CH₃–CH=CH–CH₂–CH₂–CH₂–CH₃ with all bonds.
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