Ethers Lecture Review

Introduction to Ethers

• Definition: An ether is a class of organic compounds characterized by an oxygen atom bonded to two hydrocarbon groups.

• General Formula: The general formula for an ether is represented as $R-O-R^1$.

  • The $R$ and $R^1$ symbols represent hydrocarbon groups.

  • In the formula, $R^1$ signifies that the second alkyl or aryl group can be identical to the first ($R$), or it can be different.

• Structural Comparison: The structure of ethers is similar to that of alcohols and water.

  • In water ($H-O-H$), both atoms attached to the oxygen are hydrogen.

  • In alcohols ($R-O-H$), one hydrogen atom of a water molecule is replaced by an alkyl group.

  • In ethers ($R-O-R$), both hydrogen atoms of a water molecule are replaced by alkyl or aryl groups.

• Key Distinction: Unlike alcohols and phenols, ethers do not possess a hydroxyl ($-OH$) group.

Nomenclature of Ethers

Generic (Common) Nomenclature

• Step 1: The parent name is always the word "ether," which is placed at the end of the name.

• Step 2: The names of the alkyl groups are listed before the word "ether."

  • If the two alkyl groups are identical, the prefix "di-" is used (e.g., diethyl ether).

  • If the two alkyl groups are different, they are listed in alphabetical order.

• Step 3: Spaces must be left between the names of different alkyl groups and before the word "ether."

IUPAC Nomenclature

• The IUPAC system is particularly useful for naming complex compounds with multiple functional groups as it describes them via a root name.

• Rule 1: Identify the longest carbon chain to serve as the base chain and provide the base name (e.g., butane).

• Rule 2: The shorter hydrocarbon group attached to the oxygen is treated as a substituent. The "-yl" ending of the alkyl group is changed to "-oxy."

  • Example: Methyl becomes a "methoxy" group.

  • Example: Ethyl becomes an "ethoxy" group.

• Rule 3: The alkoxy name is placed in front of the base chain name, preceded by a locator number to indicate its position on the chain.

Examples of IUPAC Naming

• $CH_3-O-CH_2-CH_2-CH_2-CH_3$: 1-Methoxybutane.

• $CH_3-CH(CH_3)-CH_2-O-CH_2-CH_3$: 1-Ethoxy-2-methylpropane.

Classification of Ethers

Ethers are classified into two broad categories based on the nature of the substituent groups attached to the oxygen atom:

• Symmetrical Ethers: These occur when the two groups attached to the oxygen atom are identical.

  • Example: $CH_3-CH_2-O-CH_2-CH_3$ (Diethyl ether).

• Asymmetrical (Unsymmetrical) Ethers: These occur when the two groups attached to the oxygen atom are different.

  • Example: $CH_3-O-CH_2-CH_3$ (Ethyl methyl ether).

Physical Properties of Ethers

• Dipole Moment: The $C-O-C$ bond angle is not $180^\circ$, meaning the dipole moments of the two $C-O$ bonds do not cancel each other out. Consequently, ethers possess a small net dipole moment.

• Boiling Point: The boiling points of ethers are comparable to those of alkanes of similar molecular mass. However, they are significantly lower than the boiling points of alcohols with comparable molecular mass because ethers lack the intermolecular hydrogen bonding found in alcohols.

• Solubility:

  • Ethers exhibit water solubility similar to alcohols of comparable molecular mass because the oxygen atom in ether can form hydrogen bonds with water molecules.

  • Solubility decreases as the number of carbon atoms in the molecule increases. This is due to the relative increase in the hydrophobic hydrocarbon content, which reduces the molecule's tendency to form hydrogen bonds.

• Polarity:

  • Ethers are less polar than esters, alcohols, or amines. This is because the oxygen atom is hindered by bulky alkyl groups on both sides, which limits its participation in hydrogen bonding.

  • However, ethers remain more polar than alkenes.

• Hybridization: The oxygen atom in ethers is $sp^3$ hybridized, resulting in a bond angle of approximately $109.5^\circ$.

Preparation of Ethers

Dehydration of Alcohols

• Alcohols undergo dehydration in the presence of protic acids like sulphuric acid ($H_2SO_4$) or phosphoric acid ($H_3PO_4$).

• The specific product (alkene vs. ether) depends on reaction conditions:

  • Ethanol to Ethene: Occurs at $443\,K$ in the presence of $H_2SO_4$.

  • Ethanol to Ethoxyethane: Occurs at $413\,K$ in the presence of $H_2SO_4$.

• Mechanism: The formation of ethers via dehydration is a nucleophilic bimolecular reaction ($S_N2$). An alcohol molecule acts as a nucleophile and attacks a protonated alcohol molecule.

• Limitations: This method is primarily used for primary alkyl groups. The alkyl group must be unhindered, and the temperature must be kept low to prevent the reaction from favoring alkene formation.

Williamson Synthesis

• This is the preferred laboratory method for preparing both symmetrical and asymmetrical ethers.

• Process: An alkyl halide reacts with sodium alkoxide to form the ether.

• Mechanism: It involves an $S_N2$ attack of an alkoxide ion on an alkyl halide.

• Yield: Productivity is highest when using primary alkyl halides. Because alkoxides are strong bases, they may participate in elimination reactions with more hindered halides.

Reaction with Dry Silver Oxide

• Treating an alkyl halide with dry silver oxide ($Ag_2O$) produces an ether.

• Formula: $2C_2H_5Br + Ag_2O \rightarrow C_2H_5-O-C_2H_5 + 2AgBr$.

Chemical Reactions of Ethers

Ethers are generally the least reactive of all functional groups. The ether bond is stable against bases, reducing agents, and oxidizing agents. However, they do undergo cleavage under specific conditions.

Cleavage of $C-O$ Bonds

• Cleavage occurs when ethers are treated with excess hydrogen halides ($HX$) under extreme conditions (concentrated acids and high temperatures).

• Acid Preference: The reactivity of hydrogen halides follows the order: $HI > HBr > HCl$.

• Mechanism:

  1. The oxygen atom of the ether acts as a base and becomes protonated by the acid.

  2. A nucleophilic attack by the halide ion on the protonated ether cleaves the $C-O$ bond.

  3. Initial products are an alkyl halide and an alcohol.

  4. With excess halide, the alcohol further reacts to form a second mole of alkyl halide and water.

Formation of Peroxides

• When ethers are exposed to air in the presence of sunlight or UV light, they react to form peroxide linkages.

Electrophilic Substitution Reactions in Aromatic Ethers

In aromatic ethers (aryl ethers), the alkoxy group ($-OR$) activates the benzene ring toward electrophilic substitution, similar to the hydroxyl group in phenol.

• Directing Effect: The alkoxy group is ortho and para directing. The lone pairs of the oxygen atom participate in resonance with the benzene ring, increasing electron density at the ortho and para positions.

Specific Reactions

• Halogenation: Phenyl alkyl ethers undergo halogenation in the benzene ring. For example, anisole reacts with bromine in ethanoic acid even without an Iron (III) bromide ($FeBr_3$) catalyst. The para isomer is the major product, obtained in 90% yield.

• Nitration: Anisole reacts with a mixture of concentrated nitric acid ($HNO_3$) and sulphuric acid ($H_2SO_4$). This produces a mixture of ortho-Nitroanisole and para-Nitroanisole (the major product).

• Friedel-Crafts Reactions: Developed by Charles Friedel and James Crafts in 1877, these reactions introduce substituents into the aromatic ring via electrophilic substitution in the presence of a Lewis acid catalyst (anhydrous aluminium chloride, $AlCl_3$).

  • Friedel-Crafts Alkylation: Anisole reacts with an alkyl chloride and anhydrous $AlCl_3$ to introduce an alkyl group at the ortho and para positions.

  • Friedel-Crafts Acylation: Anisole reacts with an acyl chloride and anhydrous $AlCl_3$ to introduce an acyl group at the ortho and para positions.

Practical Uses of Ethers

• Dimethyl Ether: Utilized as a refrigerant and as a solvent suited for low-temperature applications.

• Diethyl Ether: Widely known as a common ingredient in general anesthesia for surgical procedures. It is also used as a common solvent for oils, resins, and gums.

• Motor Fuel: Ether is used as a component of motor fuel in combination with petrol.

• Phenyl Ether: Because of its high boiling point, it is utilized as a heat transfer medium.