Enantiomers - Chemistry Steps - Free Printable

This image presents 8 pairs of molecules (a–h), and the task is to determine whether each pair represents:

- Identical molecules (same compound, same spatial arrangement),
- Enantiomers (non-superimposable mirror images),
- Diastereomers (stereoisomers that are not mirror images), or
- Constitutional isomers (same molecular formula, different connectivity).

We’ll analyze each pair one by one using stereochemistry rules (R/S configuration, chirality centers, symmetry, etc.).

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## 🔍 General Strategy:
1. Identify chiral centers.
2. Assign R/S configuration if possible.
3. Compare configurations:
- If all chiral centers are inverted → enantiomers.
- If some are same, some inverted → diastereomers.
- If identical configuration → identical.
- If connectivity differs → constitutional isomers.

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## ✔ Pair a)
Molecules: Both are 2-butanol with OH on C2.

- Left: Wedge OH → assume R or S? Let’s assign.
- Priority: OH > CH₂CH₃ > CH₃ > H
- If OH is wedge → rotating from OH→ethyl→methyl gives clockwise → R
- Right: Dashed OH → opposite → S

✔ Enantiomers — mirror images, non-superimposable.

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## ✔ Pair b)
Cyclohexyl fluoride — fluorine on a ring carbon.

- Cyclohexane ring is symmetric; the two structures show F in equatorial vs axial? NO — actually, these are chair conformers, but more importantly — they are drawn as flat rings with wedges/dashes.

Wait — look closely: both are cyclohexane with F on one carbon. The left has F wedge (coming out), right has F dash (going back). But since the ring is symmetric and there’s only one substituent, this is just a drawing convention — flipping the ring doesn’t change identity.

Actually — no chiral center! Cyclohexyl fluoride with one substituent is achiral because the molecule has a plane of symmetry through the F and the opposite C.

So both drawings represent the same molecule — just rotated or viewed from different sides.

✔ Identical

*(Note: Some might argue it’s enantiomers if misread as chiral — but no chiral center exists here.)*

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## ✔ Pair c)
2-chloro-3-methylbutane — chiral center at C2 (Cl attached) and possibly C3?

Structure: CH₃–CH(Cl)–CH(CH₃)–CH₃

C2 is chiral: bonded to H, Cl, CH₃, and CH(CH₃)CH₃.

Left: Cl dashed → let’s assign R/S.

Right: Cl wedge → opposite configuration.

But wait — are these mirror images? YES.

Also, note: C3 is also chiral? No — C3 is bonded to H, CH₃, CH(Cl)CH₃, and CH₃ — two methyl groups → not chiral (two identical groups).

Only one chiral center at C2.

So left = R, right = S → mirror images.

✔ Enantiomers

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## ✔ Pair d)
Tetrahydrofuran with ethyl group — 2-ethyltetrahydrofuran.

Chiral center at C2 (carbon adjacent to O with ethyl group).

Left: ethyl group dashed → S or R?

Right: ethyl group wedge → opposite.

Only one chiral center → so mirror images.

✔ Enantiomers

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## ✔ Pair e)
2-bromo-3-chlorobutane — two chiral centers: C2 (Br) and C3 (Cl)

Left molecule: Br dash, Cl wedge → assign R/S

Right molecule: Br wedge, Cl dash → opposite at both centers.

So if left is (2R,3S), right is (2S,3R) — which is the mirror image.

✔ Enantiomers

*(Note: (2R,3S) and (2S,3R) are enantiomers; (2R,3R) and (2S,3S) would also be enantiomers.)*

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## ✔ Pair f)
N-ethylpropanamine? Wait — structure is CH₃CH₂–NH–CH(CH₃)CH₂CH₃

Nitrogen is bonded to H, ethyl, and sec-butyl? Actually — nitrogen is chiral? NO — nitrogen inversion makes it non-chiral unless constrained.

But here — the carbon adjacent to N is chiral: carbon bonded to H, NH, CH₃, CH₂CH₃.

So chiral center at carbon.

Left: H dashed → let’s say R

Right: H wedge → S

Only one chiral center → mirror images.

✔ Enantiomers

*(Note: Even though N can invert, the carbon stereocenter is stable and defines chirality.)*

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## ✔ Pair g)
1,2-dimethylcyclohexane — with methyls on adjacent carbons.

Left: both methyls up (wedge) → cis isomer? Or trans?

Actually — in cyclohexane, if both substituents are on same side → cis.

But here — left: both wedges → both up → cis-1,2-dimethylcyclohexane

Right: one wedge, one dash → trans

Wait — look again:

Left: top methyl wedge, bottom methyl wedge → both up → cis

Right: top methyl dash (down), bottom methyl wedge (up) → trans

So different relative stereochemistry → diastereomers

Also — cis and trans 1,2-disubstituted cyclohexanes are diastereomers.

✔ Diastereomers

*(Note: They are not mirror images — cis has plane of symmetry? No — cis-1,2-dimethylcyclohexane is chiral? Actually, cis is meso-like? No — in chair form, cis-1,2 can exist as racemic pair, but here we’re comparing cis vs trans — definitely diastereomers.)*

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## ✔ Pair h)
3-bromo-2-butanol with vinyl group? Wait — structure is CH₂=CH–CH(OH)–CH(Br)–CH₃

Chiral centers at C2 (OH) and C3 (Br)

Left: OH wedge, Br dash

Right: OH dash, Br wedge

So both centers inverted → mirror image.

Assign: if left is (2R,3S), right is (2S,3R) → enantiomers.

✔ Enantiomers

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# 🧾 Final Answers:

| Pair | Relationship |
|------|--------------------|
| a) | Enantiomers |
| b) | Identical |
| c) | Enantiomers |
| d) | Enantiomers |
| e) | Enantiomers |
| f) | Enantiomers |
| g) | Diastereomers |
| h) | Enantiomers |

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## 💡 Key Takeaways:
- One chiral center → enantiomers if opposite config.
- Multiple chiral centers → enantiomers if ALL inverted; diastereomers if SOME inverted.
- No chiral center → usually identical (unless constitutional isomers).
- Cyclic compounds — check cis/trans relationships for diastereomers.
- Always verify if the molecule is actually chiral (e.g., symmetry, identical groups).

Let me know if you’d like R/S assignments for any specific pair!