What is Ring Expansion?

Ring expansion is a rearrangement reaction where a cyclic compound increases its ring size (e.g., 5-membered → 6-membered ring).
It usually happens during carbocation rearrangements or nitrene/carbene insertions.

🔹 Why Does It Happen?

Rings expand mainly for stability reasons:

1. Carbocation stability – a less stable carbocation shifts to form a more stable one.
2. Ring strain relief – small rings (3, 4, 5-membered) are strained; expansion to 6-membered reduces strain.
3. Formation of more stable rings – 6-membered rings are especially stable due to minimal angle and torsional strain.

🔹 General Mechanism

Typical situation: a carbocation is adjacent to a ring.

• A bond adjacent to the carbocation migrates (1,2-shift) into the positive center.
• This migration enlarges the ring size by one carbon.

Stepwise:

1. Generate a carbocation (often via protonation, halonium ion, or leaving group loss).
2. Adjacent C–C bond migrates toward the carbocation center.
3. New carbocation forms inside the enlarged ring.

Do ring expansion always to cyclohexane?

Ring expansion does not always stop at cyclohexane, but cyclohexane is the most common and stable product when smaller rings expand, because:

• Cyclohexane is strain-free (ideal bond angles ~109.5°, no angle strain).
• Rings smaller than 6 (3-, 4-, 5-membered) often expand to reach 6.
• Rings larger than 6 (like 7 or 8) may rearrange in different ways but they don’t necessarily “shrink” back to 6.

General trends:

1. Cyclopropane (3-membered) → expands to cyclobutane, cyclopentane, or cyclohexane (depending on the mechanism).
2. Cyclobutane (4-membered) → prefers expansion to cyclopentane or cyclohexane (to reduce strain).
3. Cyclopentane (5-membered) → expands most often to cyclohexane (since 6 is especially stable).
4. Cyclohexane (6-membered) → usually does not expand, because it’s already the most stable.
5. Cycloheptane or larger → can undergo rearrangements, but not necessarily to cyclohexane; instead, they may give different products depending on the reaction condition

In SN1 (Substitution Nucleophilic Unimolecular) reactions involving alcohols (R-OH), the mechanism requires the departure of a leaving group to form a carbocation intermediate in the rate-determining step. The hydroxyl group (OH) in alcohols is a poor leaving group because it is a strong base (OH⁻) and holds onto the substrate too tightly, making ionization difficult.

To facilitate the reaction, the alcohol is first protonated under acidic conditions (e.g., using H₂SO₄ or HCl) to form an alkyloxonium ion (R-OH₂⁺). This protonation weakens the C-O bond and converts the leaving group to neutral water (H₂O), which is a much better leaving group due to its weaker basicity and greater stability. The water then departs, generating the carbocation (R⁺), which can be attacked by the nucleophile in the subsequent fast step.

Without this protonation, the SN1 pathway would be inefficient or impossible for alcohols, as the energy barrier for OH⁻ departure is too high. This is why SN1 reactions on alcohols are typically conducted in acidic media.

The order of reactivity of alcohols with hydrogen halides (HX, like HCl, HBr, HI) depends mainly on how easily the C–OH bond can be broken and replaced by halogen.

Key points:

• The reaction proceeds either via carbocation formation (SN1) or direct displacement (SN2) depending on the substrate.
• Carbocation stability (tertiary > secondary > primary) governs the rate in SN1 cases.
• Resonance-stabilized carbocations (benzylic, allylic) are even more reactive than tertiary.

Reactivity order:

Benzylic alcohol > Allylic alcohol > Tertiary alcohol > Secondary alcohol > Primary alcohol

Explanation:

1. Benzylic alcohols → form benzylic carbocations stabilized by resonance with aromatic ring → very fast.
2. Allylic alcohols → form allylic carbocations stabilized by resonance with adjacent double bond → also very fast.
3. Tertiary alcohols → form stable 3° carbocations → faster than 2° or 1°.
4. Secondary alcohols → less stable carbocation, slower.
5. Primary alcohols → do not form stable carbocations easily, react only by SN2 mechanism with HX, so they are slowest.

Final order:
Benzyl > Allyl > 3° > 2° > 1°

Ethylidene dibromide
Structure: CH₃–CHBr₂
IUPAC name: 1,1-dibromoethane

Isopropylidene dichloride
Structure: (CH₃)₂CCl₂
IUPAC name: 2,2-dichloropropane

Ethylene dibromide
Structure: Br–CH₂–CH₂–Br
IUPAC name: 1,2-dibromoethane

Isobutene dibromide (addition product of Br₂ across 2-methylprop-1-ene)
Structure: CH₂Br–CBr(CH₃)₂ (derived from CH₂=C(CH₃)₂ + Br₂)
IUPAC name: 1,2-dibromo-2-methylpropane

Tetramethylene dibromide
Structure: Br–(CH₂)₄–Br
IUPAC name: 1,4-dibromobutane

Which is a better nucleophile in aqueous solution?

• In water (a protic solvent), small, highly basic ions like RO⁻ are strongly solvated by hydrogen bonding.
• Larger, less basic ions like RS⁻ are less solvated and therefore more available to attack electrophiles.

Answer: RS⁻ is the better nucleophile in aqueous solution.

Which is a better nucleophile in DMSO?

• DMSO is a polar aprotic solvent — it does not strongly solvate anions.
• Hence, nucleophilicity parallels basicity in aprotic solvents.
• Since RO⁻ is the stronger base, it is also the stronger nucleophile in DMSO.

Answer: RO⁻ is the better nucleophile in DMSO.

a. Br⁻ or Cl⁻ in H₂O → Br⁻

• In protic solvents larger, more polarizable anions are less strongly solvated and therefore better nucleophiles: I⁻ > Br⁻ > Cl⁻ > F⁻.

b. Br⁻ or Cl⁻ in DMSO → Cl⁻

• In polar aprotic solvents nucleophilicity follows basicity (smaller, harder = better): Cl⁻ is a better nucleophile than Br⁻ in DMSO.

c. CH₃O⁻ or CH₃OH in H₂O → CH₃O⁻

• The anion (methoxide) is far more nucleophilic than neutral methanol, even though it is somewhat solvated in water.

d. CH₃O⁻ or CH₃OH in DMSO → CH₃O⁻

• In a polar aprotic solvent the anionic nucleophile is even more reactive (less solvated), so methoxide wins by a larger margin.

e. HO⁻ or ⁻NH₂ in H₂O → HO⁻

• Practical point: ⁻NH₂ is so basic that it is protonated by water (it doesn’t exist appreciably in aqueous solution), so hydroxide is the available nucleophile.

f. HO⁻ or ⁻NH₂ in DMSO → ⁻NH₂

• In an aprotic medium where ⁻NH₂ can exist, the stronger base (⁻NH₂) is the stronger nucleophile.

g. I⁻ or Br⁻ in H₂O → I⁻

• Same protic-solvent rule: the larger, more polarizable I⁻ is less solvated and therefore the better nucleophile in water.

h. I⁻ or Br⁻ in DMSO → Br⁻

• In polar aprotic solvents nucleophilicity follows basicity/charge density: the smaller Br⁻ is the stronger nucleophile than I⁻.

When you first look at alcohols (ROH) and thiols (RSH), they seem quite similar — both contain a hydrogen attached to a group 16 element (oxygen or sulfur).
Yet, when it comes to acidity, thiols clearly win.
In this post, we’ll explore why thiols (RSH) are stronger acids than alcohols (ROH) — step by step.

Step 1: Acid strength depends on conjugate base stability

The strength of an acid depends on how stable its conjugate base is after losing a proton (H⁺).

• Alcohols (ROH) lose H⁺ to form alkoxide ions (RO⁻)
• Thiols (RSH) lose H⁺ to form thiolate ions (RS⁻)

The more stable the conjugate base, the stronger the acid.

So, we must compare the stability of RO⁻ and RS⁻.

Step 2: The role of atom size and polarizability

Sulfur (S) is larger and more polarizable than oxygen (O).
That means the negative charge on sulfur in RS⁻ is spread over a larger volume, making it more stable.

In contrast, oxygen is smaller and holds the negative charge tightly on a small area, creating stronger charge density and less stability.

Result: RS⁻ is more stable than RO⁻ → RSH is a stronger acid.

Step 3: Bond strength difference

The S–H bond is weaker than the O–H bond.
It takes less energy to break the S–H bond and release a proton.

Weaker bond → easier H⁺ release → stronger acid

Step 4: Electronegativity vs Polarizability

Although oxygen is more electronegative than sulfur, electronegativity isn’t the main factor here.
For acid strength, charge delocalization and bond strength dominate — and sulfur’s large, polarizable nature stabilizes the anion far better than oxygen.

tep 5: pKa values tell the story

A lower pKa means a stronger acid — so thiols are clearly more acidic than alcohols.

✅ Final Summary

Conclusion

Thiols (RSH) are stronger acids than alcohols (ROH) because their conjugate bases (RS⁻) are more stable.
Sulfur’s larger size and greater polarizability allow the negative charge to spread out, and the weaker S–H bond makes proton loss easier.
So, while oxygen wins in electronegativity, sulfur wins in stability, making RSH the stronger acid overall.