Solved Organic Chemistry 1 - Worksheet 5-Fall 2020 1. | Chegg.com - Free Printable

Let's go through each pair of structures in the worksheet and categorize them using the appropriate terms: identical, isomers, constitutional isomers, conformational isomers, stereoisomers, enantiomers, and diastereomers.

We'll analyze each pair step by step.

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(a)

Structure 1: Hexane (straight-chain alkane: CH₃CH₂CH₂CH₂CH₂CH₃)
Structure 2: Cyclohexane (cyclic alkane with 6 carbons)

- Molecular formulas:
- Hexane: C₆H₁₄
- Cyclohexane: C₆H₁₂
- They have different molecular formulas, so they are not isomers.
- But both are C₆ hydrocarbons, but different types.

✔ Answer:
- Not identical
- Not isomers (because same molecular formula required for isomerism)
- Different compounds → Not isomers at all

✘ So, none of the listed categories apply except "not identical" — but since we must use the given terms, they are not isomers.
So:
> Neither identical nor isomers — different compounds with different formulas.

But wait — isomers require same molecular formula. Since these have different formulas, they are not isomers.

✔ Final answer: Not isomers (so none of the subtypes apply).

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(b)

Structure 1: 2-Methylbutane (CH₃CH(CH₃)CH₂CH₃)
Structure 2: 3-Methylbutane? Wait — let’s draw it:

Actually, both look like branched alkanes with 5 carbons.

- Structure 1: CH₃–CH(CH₃)–CH₂–CH₃ → 2-methylbutane
- Structure 2: CH₃–CH₂–CH(CH₃)–CH₃ → 2-methylbutane again?

Wait: Is that the same?

Let’s count:
- Both have a 4-carbon chain with a methyl group on carbon 2 → same structure.

But wait — are they drawn differently?

Yes!
Structure 1:
```
CH₃
|
CH₃–CH–CH₂–CH₃
```

Structure 2:
```
CH₃–CH₂–CH–CH₃
|
CH₃
```

These are the same molecule — just rotated. The carbon chain is four carbons long, with a methyl group on the second carbon.

✔ This is 2-methylbutane in both cases.

So, identical molecules.

> ✔ Identical

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(c)

Structure 1: CH₃CH₂CH(CH₃)CH₃ → 2-methylbutane
Structure 2: CH₃CH(CH₃)CH₂CH₃ → also 2-methylbutane?

Wait — no! Let’s write them clearly.

Structure 1:
```
CH₃–CH₂–CH–CH₃
|
CH₃
```
This is 2-methylbutane? No — this is 2-methylbutane only if the branch is on carbon 2.

Wait — actually, this is 2-methylbutane.

But Structure 2:
```
CH₃
|
CH₃–CH–CH₂–CH₃
```
That’s also 2-methylbutane.

Wait — but the names are the same? Are they really the same?

No — wait: both are 2-methylbutane — just drawn differently.

But let’s check the carbon skeleton.

Actually, both are the same compound — 2-methylbutane.

But wait — is that correct?

Wait — no! Let's re-express:

Structure 1:
- Chain: CH₃–CH₂–CH–CH₃
- Methyl on the third carbon? That would be 3-methylbutane?

Wait — naming: longest chain is 4 carbons.

In Structure 1:
- CH₃–CH₂–CH(CH₃)–CH₃ → the carbon with methyl is carbon 3 of a 4-carbon chain → 3-methylbutane

But 3-methylbutane is the same as 2-methylbutane because numbering goes from the end to give lowest numbers.

So, 3-methylbutane = 2-methylbutane — same molecule.

So both are identical.

Wait — but are they?

Wait — actually, no — both are 2-methylbutane — just drawn differently.

But let’s double-check:

Structure 1:
- CH₃–CH₂–CH(CH₃)–CH₃ → carbon 1: CH₃–, carbon 2: –CH₂–, carbon 3: –CH(CH₃)–, carbon 4: –CH₃ → so methyl on carbon 3

But when we number the chain from the other end:
- CH₃–CH(CH₃)–CH₂–CH₃ → now methyl is on carbon 2 → so 2-methylbutane

So yes — same molecule.

✔ So identical

Wait — but the drawings are different — one has the methyl on the right, one on the left — but they're the same molecule.

So identical

But wait — the question says:
(c)
First:
```
CH₃
|
CH₃–CH₂–CH–CH₃
```
Second:
```
CH₃
|
CH₃–CH–CH₂–CH₃
```

These are the same molecule — 2-methylbutane.

So ✔ Identical

---

Wait — hold on! Let me check the actual structures carefully.

Looking back:

(c)
First:
- CH₃–CH₂–CH–CH₃
|
CH₃
→ This is 2-methylbutane? No — the carbon with the methyl is the third carbon of a 4-carbon chain → 3-methylbutane, which is same as 2-methylbutane — yes, identical.

Second:
- CH₃–CH–CH₂–CH₃
|
CH₃
→ This is 2-methylbutane — same.

So yes — identical.

✔ Identical

---

But wait — maybe I'm missing something. Let's move on.

---

(d)

Two cyclohexane derivatives:

- First: axial methyl on cyclohexane ring (chair form)
- Second: equatorial methyl on cyclohexane ring

Both are methylcyclohexane — same connectivity.

But different spatial arrangements due to chair flip.

They are conformational isomers — rapidly interconvertible via ring flipping.

They are not stereoisomers because they are interconvertible without bond breaking.

✔ So:
- Constitutional isomers? No — same connectivity
- Stereoisomers? Not really — they are conformers
- Enantiomers? No — not chiral centers
- Diastereomers? No — not stereoisomers

So:
> ✔ Conformational isomers

Also, not identical (different conformations), but same molecule.

So:
> Conformational isomers — and not identical

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(e)

Two complex molecules — look like phthalimide-based diketopiperazines.

Look closely:

- Both have the same ring system: benzene fused to imide, connected to a six-membered diketopiperazine ring.
- The key difference: stereochemistry at the chiral center.

In the first molecule: the H is up (wedge), in the second: down (dash).

So, one is the mirror image of the other.

Are they enantiomers?

Yes — if they are non-superimposable mirror images.

But do they have a chiral center?

Yes — the carbon with H and N is attached to four different groups:
- H, N, O (from carbonyl), and the ring.

So yes — chiral center.

And the two structures are mirror images.

So:
✔ Enantiomers

Also:
- Stereoisomers? Yes — enantiomers are a type of stereoisomer
- Identical? No
- Constitutional isomers? No — same connectivity
- Diastereomers? No — they are enantiomers, not diastereomers

So:
> ✔ Stereoisomers, enantiomers

---

(f)

Two steroid-like molecules — look like testosterone or similar.

Compare stereochemistry:

- Both have OH and CH₃ groups
- Look at the stereochemistry of the OH group: in one it's up (wedge), in the other down (dash)
- Also, the hydrogen positions differ

But more importantly: are they mirror images?

No — they are not mirror images — they differ in multiple chiral centers.

But they are not identical.

Are they enantiomers? No — not mirror images.

Are they diastereomers? Yes — if they are stereoisomers that are not mirror images.

Check:
- Multiple chiral centers
- Different configurations at some centers
- Not mirror images

So:
✔ Diastereomers
Also: stereoisomers, not identical, not constitutional isomers

So:
> ✔ Stereoisomers, diastereomers

---

(g)

Two cyclohexane derivatives:

- Both have:
- OCH₃ group
- OH group
- On adjacent carbons

Now, stereochemistry:

- First: OCH₃ is wedge (up), OH is dash (down) → trans configuration
- Second: OCH₃ is dash (down), OH is wedge (up) → also trans

But are they the same?

Let’s see:
In the first:
- OCH₃ up, OH down → relative configuration: trans

Second:
- OCH₃ down, OH up → also trans

But are they enantiomers?

No — because the molecule has two chiral centers, and both are trans.

But are they identical?

Let’s think:
If you rotate the molecule 180°, can you superimpose?

Actually, yes — because the ring is symmetric.

Wait — no — because the substituents are on adjacent carbons.

But in cyclohexane, cis and trans isomers are diastereomers.

But here, both are trans — so they might be identical.

But wait:
- In first: OCH₃ up, OH down → trans
- In second: OCH₃ down, OH up → also trans

But are they the same?

Yes — because trans-1,2-disubstituted cyclohexane has two enantiomers if the substituents are different.

Wait — OCH₃ and OH are different → so trans-1,2-disubstituted cyclohexane has two enantiomers.

So:

- The first structure: OCH₃ up, OH down → one enantiomer
- The second: OCH₃ down, OH up → the other enantiomer

But are they mirror images?

Yes — because flipping the molecule gives the mirror image.

So:
✔ Enantiomers

Also:
- Stereoisomers — yes
- Diastereomers? No — they are enantiomers

So:
> ✔ Stereoisomers, enantiomers

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Now, summarize all answers:

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#### (a)
- Structure 1: hexane (C₆H₁₄)
- Structure 2: cyclohexane (C₆H₁₂)
→ Different molecular formulas → not isomers
✔ Not identical, not isomers

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#### (b)
- Both are 2-methylbutane — same structure
✔ Identical

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#### (c)
- Both are 2-methylbutane — same molecule
✔ Identical

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#### (d)
- Same molecule, different conformations (axial vs equatorial methyl)
✔ Conformational isomers
Also: not identical (but interconvertible)

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#### (e)
- Mirror images, chiral center, non-superimposable
✔ Stereoisomers, enantiomers

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#### (f)
- Different stereochemistry at chiral centers, not mirror images
✔ Stereoisomers, diastereomers

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#### (g)
- Two enantiomeric forms of trans-1-(methoxy)-2-hydroxycyclohexane
✔ Stereoisomers, enantiomers

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✔ Final Answers:

(a) Neither identical nor isomers (different molecular formulas)
(b) Identical
(c) Identical
(d) Conformational isomers
(e) Stereoisomers, enantiomers
(f) Stereoisomers, diastereomers
(g) Stereoisomers, enantiomers

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Let me know if you want the reasoning for any specific one clarified!