CHM 121: Ethers and Cyclic Ethers

Introduction to Ethers

• General Formula: Ethers are represented by the general formula $R-O-R'$.
• Definition: Ethers are chemical compounds in which an oxygen atom is bonded to two organic groups, which may be either alkyl or aryl groups.
• University Distinction: These notes are based on the General Chemistry II (CHM 121, 2 units) curriculum at the University of Lagos, as taught by Dr J. u. Izunobi. The institutional motto is "Indeed and in Truth."

Physical and Chemical Properties of Ethers

• Polarity: Ethers are significantly less polar than alcohols.
• Solubility: They are not soluble in water.
• Thermal Properties: Ethers exhibit lower Melting Points (MP) and Boiling Points (BP) compared to alcohols of similar molecular weight.
• Reactivity: Chemically, ethers are generally inert, meaning they do not react easily with most reagents.
• Safety Hazard: All ethers are characterized by being very flammable.
• Molecular Geometry:
  • Bond Angle: Ethers have a nearly tetrahedral bond angle. For example, the bond angle in dimethyl ether is $112^{\circ}$.
  • Hybridization: The oxygen atom in an ether is $sp^3$-hybridized.
  • Dipole Moment: The oxygen atom imparts a slight dipole moment to the ether molecule.

Nomenclature of Ethers

Common Names

• The common naming system remains very popular.
• Method: Each carbon chain attached to the oxygen is treated as a substituent (branch) off the central oxygen atom.
• Suffixes: List each side with a "-yl" ending and add the word "ether" at the end of the name.
• Verbatim Examples:
  • $CH_3-O-CH_3$: dimethyl ether
  • $CH_3-O-CH_2CH_2$: methyl ethyl ether
  • $CH_3CH_2-O-CH_2Cl$: chloromethyl ethyl ether

IUPAC System

• Base Name Identification: Identify the longest carbon chain and use it as the parent/base name.
• Functional Group Priority: One exception occurs if the shorter chain contains a functional group that alters the name; in such cases, the priorities change.
• Substituent Naming: The shortest carbon chain is named as an "alkoxy" substituent, using an "-oxy" ending.
• Numbering: The location of the ether bond on the long chain must be numbered to be as low as possible.
• Standard Rules: Other standard IUPAC rules for naming substituents apply.
• IUPAC Examples:
  • $CH_3-CH_2-CH_2-O-CH_3$: 1-methoxypropane
  • $CH_3-CHCl-O-CH_3$: 1-chloro-1-methoxyethane
  • $CH_3-CH_2-CH(CH_3)-O-CH_2-CH_2-CH_3$: 2-propoxybutane
  • $CH_3-CH(O-CH_2-CH_2Cl)-CH_2-CH_2-OH$: 3-(2-chloroethoxy)-1-butanol

Ether Synthesis and Preparation

Industrial Preparation: Dehydration

• Reagent: Diethyl ether is prepared industrially through the dehydration of ethanol.
• Catalyst: The process is catalyzed by sulphuric acid ($H_2SO_4$).
• Versatility: This method can also be used with other primary alcohols.

Williamson Ether Synthesis

• Importance: This is considered the best method for the preparation of ethers.
• Reaction: It involves the reaction of metal alkoxides with primary alkyl halides or tosylates.
• Alkoxide Preparation: Alkoxides are typically prepared by reacting an alcohol with a strong base, such as sodium hydride ($NaH$).

Silver Oxide ($Ag_2O$) Catalyzed Synthesis

• Reaction: Alcohols can react directly with alkyl halides in the presence of $Ag_2O$ to form an ether in a single step.
• Named Case Study: Glucose reacts with excess iodomethane ($CH_3I$) in the presence of silver oxide ($Ag_2O$) to generate a pentaether with a yield of $85\%$.

Reactions of Ethers

Acidic Cleavage

• Condition: Because ethers are generally unreactive, high temperatures are required.
• Reagents: Strong acids such as Hydroiodic acid ($HI$) or Hydrobromic acid ($HBr$) are used.
• Mechanisms:
  • $S_N2$ Mechanism: Strong acids produce an alkyl halide from the less hindered component of the ether.
  • $S_N1$ Mechanism: Tertiary ethers undergo cleavage via the $S_N1$ pathway.

Cyclic Ethers and Epoxides

Overview

• Behavior: Most cyclic ethers behave like acyclic ethers, but properties change significantly if the ring is 3-membered.
• Solvent Examples: Dioxane and tetrahydrofuran ($THF$) are commonly used as solvents.

Epoxides (Oxiranes)

• Etymology: The term "oxirane" is derived from "ox" (oxygen), "ir" (rooted in "tri" for 3-membered), and "ane" (saturated).
• Industrial Importance: Ethylene oxide (also known as oxirane or 1,2-epoxyethane) is a crucial industrial intermediate.
• Ethylene Oxide Preparation: Prepared by reacting ethylene with oxygen ($O_2$) at $300\,^{\circ}C$ using a silver oxide catalyst.

Preparation of Cyclic Ethers

• Using Peroxyacids: An alkene can be treated with a peroxyacid (such as meta-Chloroperoxybenzoic acid, or $mCPBA$) in a solvent like dichloromethane ($CH_2Cl_2$).
  • Example: Cycloheptene + meta-Chloroperoxybenzoic acid $\rightarrow$ 1,2-Epoxycycloheptane + meta-Chlorobenzoic acid.
• From Halohydrins:
  • Step 1: Addition of $HO-X$ (where $X$ is a halogen) to an alkene produces a halohydrin.
  • Step 2: Treatment of the halohydrin with a base triggers an intramolecular Williamson ether synthesis to yield an epoxide.

Reactions of Cyclic Ethers

• Ring-Opening (Acidic): Water adds to epoxides in the presence of dilute acid at room temperature.
  • Product: A 1,2-diol (vicinal diol, where hydroxyl groups are on adjacent carbons).
  • Mechanism: The acid protonates the oxygen; water then adds to the opposite side, resulting in trans-addition.
• Halohydrin Formation from Epoxides: Anhydrous $HF$, $HBr$, $HCl$, or $HI$ reacts with an epoxide to give a trans-product.
• Addition of Grignard Reagents:
  • Ethylene oxide reacts with Grignard reagents ($RMgX$) to add a $-CH_2CH_2OH$ group to the hydrocarbon chain of the reagent.
  • Scope: Acyclic ethers and larger ring ethers do not typically undergo this reaction.

Crown Ethers

• Description: These are large cyclic ethers consisting of repeating units, typically ($-OCH_2CH_2-$).
• Naming Convention ($x$-crown-$y$):
  • $x$: Represents the total number of atoms in the entire ring.
  • $y$: Represents the total number of oxygen atoms in the ring.
  • Example: 18-crown-6 ether refers to an 18-membered ring containing 6 oxygen atoms.
• Function: The central cavity of a crown ether is electronegative, which allows it to attract and bind cations.