Alcohols, Phenols, and Ethers Chemistry Notes

Alcohols, Phenols, and Ethers

Learning Outcomes

• Give the systematic names of alcohols, phenols, and ethers based on their structure.
• Draw the structures of alcohols, phenols, and ethers from their systematic names.
• Correlate the physical properties of alcohols, phenols, and ethers with their molecular structure.
• Describe the chemical reactions of alcohols, phenols, and ethers.

Concept Map

• Alcohols, phenols, and ethers are organic derivatives of water ($H-O-H$).
  • Alcohol: $R-O-H$ (hydroxyl group, $-OH$)
  • Phenol: Benzene ring with an $-OH$ group attached.
  • Ether: $R-O-R'$ where R and R' can be the same or different alkyl or aryl groups.

Reactions:

• Alcohols:
  • Oxidation.
  • Nucleophilic substitution.
  • Elimination.
  • Acid-base reactions.
• Phenols:
  • Acid-base reactions.
  • Electrophilic aromatic substitution (EAS).
• Ethers:
  • Can cyclize to form epoxides (three-membered cyclic ring).
  • Epoxides can undergo cleavage.

Structure and Bonding in Alcohols

• The functional group in alcohol is the hydroxyl group ($-OH$), not hydroxide ($OH^{-}$, which has a negative charge).
• The carbon atom attached to the hydroxyl group in alcohols is typically $sp^3$ hybridized.
• Alcohols are classified based on the number of alkyl groups attached to the carbon bearing the hydroxyl group:
  • Primary ($1^o$) alcohol: The carbon with the $-OH$ is attached to one other carbon.
  • Secondary ($2^o$) alcohol: The carbon with the $-OH$ is attached to two other carbons.
  • Tertiary ($3^o$) alcohol: The carbon with the $-OH$ is attached to three other carbons.

Enols, Phenols, and Ethers

• Enol: An alkene with a hydroxyl group attached to one of the carbons in the double bond. The carbon bearing the $-OH$ group is $sp^2$ hybridized.
• Phenol: A hydroxyl group is attached to a carbon in a benzene ring (cyclic and conjugated).
• Ether: $R-O-R'$. R and R' can be symmetrical or unsymmetrical alkyl or aryl groups.
• Epoxide (oxirane): A cyclic ether with the oxygen atom in a three-membered ring.
  • The $C-O-C$ bond angle is approximately 60 degrees, which is a significant deviation from the tetrahedral bond angle of 109.5 degrees, resulting in angle strain.
  • Due to the angle strain, epoxides are more reactive than acyclic ethers.
  • Oxygen hybridization in epoxides is generally $sp^3$.
  • The shape is bent, similar to that of water.

Hybridization and Geometry

• The oxygen atom in alcohols is $sp^3$ hybridized with a bond angle of 109.5 degrees, resulting in a bent geometry.
• Two pairs of nonbonding electrons occupy the greatest volume in space, pushing the attached substituents inward.
• Ethers also have $sp^3$ hybridized oxygen atoms but may have slightly different bond angles compared to alcohols due to the presence of bulky alkyl groups.

Classification of Alcohols

• Primary ($1^o$): The carbon bearing the $-OH$ is attached to one other carbon.
• Secondary ($2^o$): The carbon bearing the $-OH$ is attached to two other carbons.
• Tertiary ($3^o$): The carbon bearing the $-OH$ is attached to three other carbons.
• Phenol: The $-OH$ group is directly attached to a benzene ring.

IUPAC Nomenclature for Alcohols

• Find the longest carbon chain that contains the $-OH$ group.
• Drop the "e" from the alkane name and add the suffix "ol".
• Number the chain starting from the end closest to the hydroxyl group.
• Number and name all substituents.
• Example:
  • 2-methyl-1-propanol
  • 2-methyl-2-propanol
  • 2-butanol
  • 3-bromo-3-methylcyclohexanol

Unsaturated Alcohols

• The hydroxyl group takes precedence in numbering.
• The carbon bearing the $-OH$ group should have the lowest possible number.
• The names alkene or alkyne are used to indicate the presence of double or triple bonds respectively.
• Example:
  • 4-penten-2-ol (old version)
  • pent-4-en-2-ol (1997 revision).

Hydroxy Substituent

• When the hydroxyl group is part of a higher priority class of compound, it is named as "hydroxy".
• Example: 4-hydroxybutanoic acid (carboxylic acid has higher priority).

Common Names for Alcohols

• Identify the alkyl group and add the word "alcohol."
• Useful for simple alcohols.
• Examples:
  • Isobutyl alcohol
  • Secondary butyl alcohol

Alcohols with Multiple Hydroxyl Groups

• Two numbers are needed to locate the two hydroxyl groups, and the prefix "diol" is used instead of "ol."
• Example: 1,6-hexanediol.
• Vicinal glycols: Two hydroxyl groups are located on two different, adjacent carbons.
  • Example: 1,2-ethanediol (ethylene glycol).
  • 1,2-propanediol (propylene glycol).

Physical Properties of Alcohols

• Alcohols associate with each other via hydrogen bonding due to the presence of the $-OH$ group.
• Alcohols generally have high boiling points due to hydrogen bonding.
• Small alcohols are miscible in water, but as the size of the alkyl group increases, the solubility decreases.
• Boiling points increase as the number of carbon atoms increases.
• Alcohols have higher boiling points than ethers of comparable molecular weight due to hydrogen bonding.
• Only the hydroxyl group associates with water, not the alkyl group.
• Examples:
  • Hexane (6 carbons): not miscible
  • Cyclohexanol (6-carbon cyclic): more miscible
  • Cyclopentanol (5-carbon cyclic): even more miscible
  • 1-butanol: even more miscible.

Increasing Boiling Point and Solubility

• To arrange substances in order of increasing boiling point and increasing solubility, consider the number of carbon atoms and the presence of hydroxyl groups.
• More alkyl groups lead to higher boiling points.
• More hydroxyl groups and smaller alkyl groups lead to higher solubility in water.

Common Alcohols and Their Uses

• Ethyl alcohol (ethanol): Used as an antiseptic and is found in alcoholic beverages.
• Ethanol: Found in Listerine, perfumes, and bone.
• Ethanol is derived from sugar or carbohydrates in corn through hydrolysis and fermentation under anaerobic conditions.

Production of Ethanol from Corn

• The sugar or carbohydrates (e.g., amylose) in corn grains undergo hydrolysis and fermentation.
• Hydrolysis breaks down polysaccharides into monosaccharides.
• Fermentation converts monosaccharides to ethanol under anaerobic conditions.
  • $C<em>6H</em>{12}O<em>6 \rightarrow 2 C</em>2H<em>5OH + 2 CO</em>2$
• Ethanol is often mixed with gasoline to produce gasohol or E85.
• Combustion of gasohol in engines:
  • $C<em>2H</em>5OH + 3 O<em>2 \rightarrow 2 CO</em>2 + 3 H_2O + \text{Energy}$

Biological Processes

• When we eat corn, amylose undergoes hydrolysis in the presence of water, breaking glycosidic bonds.
• In the stomach, fermentation occurs, converting some products to ethanol.
• Glucose units are converted and undergo oxidation, yielding carbon dioxide, water, and energy.

Photosynthesis in Plants

• Plants convert carbon dioxide and water into glucose (carbohydrates) through photosynthesis, using sunlight and chlorophyll.

Other Common Alcohols

• Methanol (wood alcohol): Very toxic.
• Isopropyl alcohol: Found in rubbing alcohol, skin tonics, and astringents.
• Menthol: Found in cough drops, shaving cream, and toothpaste.
• Ethylene glycol: A diol (1,2-ethanediol) used as antifreeze in car radiators.
• Glycerol: A triol (1,2,3-propanetriol) used in mock-up syrup and cosmetics.
• Benzyl alcohol: Found in fragrances of flowers.
• Farnesol: Found in fragrances of flowers like lily of the valley (15-carbon compound made of isoprene units).

Reactions of Alcohols

Acidity of Alcohols

• Alcohols can function as Bronsted-Lowry acids because they can donate a proton ($H^+$).
• The hydrogen atom attached to the oxygen atom is the acidic hydrogen.
• When an alcohol dissociates, it forms a hydrogen ion ($H^+$) and a conjugate base (alkoxide ion).
• Reaction with metals (Na, K, Mg) leads to dissociation and formation of alkoxides.
• $R-O-H + Na \rightarrow R-O^-Na^+ + \frac{1}{2} H_2$
• Reactivity Trend: Methanol > Primary > Secondary > Tertiary alcohol.
• Larger alcohols are generally less acidic because alkyl groups contribute electron density and stabilize the bond between hydrogen and oxygen.
• Electron-donating alkyl groups decrease acidity.
• Electron-withdrawing groups (halides) increase acidity because they withdraw electron density, weakening the $O-H$ bond.
• Alkoxide ions are stronger bases and good nucleophiles.

Oxidation of Alcohols

• Alcohols undergo oxidation.
• Chromic acid test is used to determine if the alcohol has undergone oxidation.
• Primary alcohols are oxidized to aldehydes, and further oxidation converts aldehydes to carboxylic acids.
  • Primary Alcohol $\xrightarrow{\text{Oxidation}}$ Aldehyde $\xrightarrow{\text{Oxidation}}$ Carboxylic Acid
• Secondary alcohols are oxidized to ketones.
  • Secondary Alcohol $\xrightarrow{\text{Oxidation}}$ Ketone
• Tertiary alcohols do not easily undergo oxidation due to steric hindrance.

Oxidation in the Body

• Ethanol in alcoholic beverages is oxidized in the body to acetaldehyde, which is then further oxidized to acetic acid.
• Acetaldehyde is toxic and can cause liver damage.
• Excess acetic acid is converted to acetyl CoA (the active form of acetate), which is a precursor in fat synthesis.
• Heavy drinkers may develop fatty liver and cirrhosis due to the accumulation of fat.

Oxidation of Alcohols (Examples)

Primary Alcohols

• Ethanol is oxidized to acetaldehyde, then to acetic acid.
  • This process occurs in the liver after drinking alcoholic beverages.
• If ethanol will be converted to acetic acid when exposed to air.

Secondary Alcohols

• 2-propanol is oxidized to acetone.

Tertiary Alcohols

• Tertiary alcohols do not oxidize readily because of steric hindrance at the alpha carbon.

Chromic Acid Test

• Changes from a yellow-orange solution to an opaque green solution for primary and secondary alcohols.
• There is no color change observed for tertiary alcohols.