Ethers, Epoxides, Alkoxylates & Polyurethane Chemistry

IUPAC NOMENCLATURE OF ETHERS

• Two alternative systems are formally accepted by IUPAC.

1 “Straight-Chain” (Dialkyl Ether) Naming

• Name each alkyl (or aryl / cycloalkyl) group as a separate word, follow with the word “ether.”
• Groups are listed alphabetically; identical groups get Greek prefixes (di-, tri-, …).
• Example (shown in class): a cyclopentyl group bonded to a methyl group → “cyclopentyl methyl ether.”

2 Alkoxy (-OR) Substituent Naming

• Treat the entire \ce{R-O-} fragment as a substituent on the longest carbon chain.
• Root name = longest chain; prefix = size of the OR fragment + “-oxy.”
– 1 C = methoxy; 2 C = ethoxy; 3 C = propoxy; 4 C = butoxy …
• Number the parent chain so the alkoxy gets the lowest possible locant.
• Examples discussed:
– \ce{CH3-O-(CH2)6-CH3} → 2-methoxy-hexane.
– Alkoxy on benzene → 3-propoxy-benzene.
• Guideline: Use the alkoxy system whenever one “arm” is complicated (contains alcohols, alkenes, aromatics, etc.) because it keeps the name shorter and clearer.

CYCLIC ETHERS & EPOXIDES

• Many common laboratory solvents (tetrahydrofuran, dioxane, etc.) are 5- or 6-membered cyclic ethers – generally inert.
• Three-membered cyclic ethers (oxiranes / epoxides) are the dramatic exception – highly strained and highly reactive.

Ring-Strain Rationale

• Ideal tetrahedral $\mathrm{sp^3}$ angle: $109.5^\circ$ .
• Oxirane internal angle: $60^\circ$ .
• Deviation $=109.5^\circ-60^\circ \approx 50^\circ$ → enormous angle strain + torsional strain.
• Result: C–O and C–C bonds are “bent,” higher in energy, eager to open.

Common Industrial Epoxides

• Ethylene oxide (EO) – \ce{C2H4O}.
• Propylene oxide (PO) – \ce{C3H6O} (extra methyl lowers polarity, important later).

GENERAL MECHANISM: NUCLEOPHILIC RING OPENING OF EPOXIDES

• The two carbons are electrophilic (partial $+$ ).
• A nucleophile (Nu: = O, N, S lone-pair donor) attacks the less substituted carbon to minimize steric hindrance.
• C–O bond breaks; ring opens; oxygen ends up protonated (neutral) in acidic work-ups.

Nu:⁻     O
   \    /
    C–C        →    Nu–C–C–OH
   /    \                \\
 O      H                 O

Representative Equations

Alcohol addition (lecturer’s full mechanism):
\ce{R'O}H\; +\; \ce{CH2-CH2O} \;→\; R'O-CH2-CH2-OH
Water addition (student exercise):
\ce{H2O} + \ce{CH2-CH2O} \;→\; HO-CH2-CH2-OH (ethylene glycol).
General formula (for ethoxylation):
\ce{R-OH} + n\,\ce{EO} \;→\; \ce{R-(O-CH2-CH2)_n-OH}

• Typical nucleophiles: water, alcohols, phenols, amines → yields glycols, alcohol ethoxylates, phenoxyethanols, β-amino alcohols, etc.

ALCOHOL ETHOXYLATES (AE) & SURFACTANT CHEMISTRY

• Reacting a fatty alcohol (C₁₂–C₁₈) with 1–3 moles of EO gives SLES precursor; final product (after sulfation & neutralisation) = Sodium Lauryl Ether Sulfate.
• General surfactant structure: \ce{RO-(CH2CH2O)_n-SO3^- Na^+} where $n = 1\text{–}3$ .
• High EO count ⇒ higher polarity, higher water solubility, stronger foam.
• Consumer perception: “more foam = cleaner,” hence SLES popularity in shampoos, dish soaps.

PROPOXYLATES – LOW-FOAM ALTERNATIVE

• Substituting some or all EO with PO: \ce{R-(O-CH2-CH(CH3))_m-OH}.
• Extra methyl (from PO) increases hydrophobicity, lowers water solubility, suppresses foam.
• Preferred where foam is undesirable (industrial floor cleaners, wastewater treatment, paint & coatings, food-plant CIP systems).

INDUSTRIAL PRODUCTION NOTES

• Large “alkoxidation” plants (e.g., Solvay Singapore) integrate EO/PO production with downstream alkoxylation to supply surfactants & polyols.

POLYETHER POLYOLS

• Name analysis: “poly-ether” (many \ce{-O-} linkages) + “poly-ol” (many \ce{-OH} groups).
• Synthesised by adding EO and/or PO onto a polyol initiator with multiple hydroxyls.

Common Initiators & Their Functionality (f = number of OH)

• Glycerol (f = 3)
• Trimethylolpropane, TMP (f = 3)
• Pentaerythritol (f = 4)
• Sucrose (f ≈ 8)

Generic Growth Reaction

\ce{Initiator{-}(OH)f} + n\,\ce{EO/PO} \;→\; \ce{(EO/PO)n!{-}Initiator{-}(OH)_f}
• Each arm length (n) and arm count (f) are design variables – directly tuned in the reactor.

POLYURETHANE FORMATION & TAILORING PROPERTIES

• Polyurethane results from \ce{-OH} of polyether polyol + \ce{-NCO} of di- or poly-isocyanate:
\ce{HO-!R-OH} + \ce{OCN-!R'-NCO} \;→\; \ce{-R-O-CO-NH-R'-NH-CO-O-R-}_{n}

Structure–Property Relationships

• Flexibility / soft foams:
– Long EO/PO chains → high segmental mobility.
– Low functionality (3-arm glycerol) → low cross-link density.
• Hard, rigid products (e.g., bowling balls):
– Short arms (low molecular weight polyol).
– High functionality (sucrose, pentaerythritol) → many cross-links.

KEY TAKE-AWAYS & EXAM TIPS

• When faced with “weird” substituents (rings, unsaturation, alcohols), switch to the alkoxy naming system.
• Epoxides are electrophilic because of ring strain; always expect nucleophilic ring opening.
• Attack the less substituted carbon unless strong directing factors (acidic conditions) override.
• Be able to draw and name products for EO/PO + water, alcohols, phenols, amines.
• Count the number of EO units (n) in surfactant names; correlate with foaming and solubility.
• For polyurethane questions, reason qualitatively:
– Long + low-f ⇒ soft & flexible.
– Short + high-f ⇒ hard & brittle.