Alcohols, Ethers, Phenols, Aldehydes, and Ketones: Structure, Properties, and Reactions

Alcohols, Ethers, and Phenols

Introduction to Oxygen-Containing Organic Compounds

Organic compounds containing oxygen, such as alcohols, ethers, and phenols, play a crucial role in biochemistry and general chemistry. Their properties and reactivity are influenced by the presence and position of oxygen atoms within the molecule.

• Alcohols: Compounds with a hydroxyl (-OH) group attached to a saturated carbon atom.

• Ethers: Compounds with an oxygen atom connected to two alkyl or aryl groups (R-O-R').

• Phenols: Compounds with a hydroxyl group attached directly to an aromatic ring.

Range of Carbon Oxidation States

The oxidation state of carbon in organic molecules varies depending on the number of bonds to oxygen. This affects the molecule's reactivity and classification.

Structure	Bonds to O	Oxidation State
CH3-CH3	0	Most reduced
CH3-OH	1	1
CH3-CHO	2	2
CH3-COOH	3	3
CO2	4	Most oxidized

Groups Containing Oxygen Single Bonds

Alcohols, ethers, and phenols all contain single bonds between carbon and oxygen. The position and environment of the oxygen atom determine the compound's properties.

• Alcohol: R-OH (hydroxyl group attached to alkane carbon)

• Ether: R-O-R' (oxygen between two carbons)

• Phenol: Ar-OH (hydroxyl group attached to aromatic ring)

Note: Gaining a bond to oxygen makes a compound more oxidized than an alkane.

Alcohol Properties

Alcohols exhibit unique physical and chemical properties due to their ability to form hydrogen bonds.

• Intermolecular forces: Alcohols form strong hydrogen bonds due to the presence of the -OH group.

• Hydrogen bond donor and acceptor: Alcohols can both donate and accept hydrogen bonds.

• Solubility: Alcohols are usually water-soluble unless the alcohol group is a small percentage of the total mass.

• Hydrophobic vs. Hydrophilic regions: Long hydrocarbon chains are hydrophobic, while the -OH group is hydrophilic.

Example: Cholesterol contains both hydrophobic and hydrophilic regions, affecting its solubility and biological function.

Alcohols and Ethers as Structural Isomers

Alcohols and ethers with the same molecular formula (e.g., C2H6O) are structural isomers, differing in the connectivity of atoms.

Compound	Structure
Alcohol	CH3CH2OH (O at end of C chain)
Ether	CH3OCH3 (O between C atoms)

Summary

• Alcohols, ethers, and phenols all contain single bonds between carbon and oxygen, making these compounds polar.

• Alcohols hydrogen bond with each other and water; ethers only hydrogen bond with water. Intermolecular forces in ethers (dipole-dipole) are weaker than in alcohols (hydrogen bonding).

Phenols and Alcohols: Acidity and Classification

Acidity of Phenols and Alcohols

Both phenols and alcohols can act as acids, donating a proton to water. Phenols are generally more acidic than alcohols due to resonance stabilization of the conjugate base.

• Alcohol pKa: ~16

• Phenol pKa: ~10

Example: The aromatic ring in phenol stabilizes the negative charge on the oxygen after deprotonation.

Examples: Amino Acids with Alcohol and Phenol Groups

Amino Acid	Functional Group
Serine, Threonine	Alcohol (hydroxyl group)
Tyrosine	Phenol

Alcohol Classification

Type	Structure	Description
Primary (1°)	R-CH2OH	OH on carbon attached to one other carbon
Secondary (2°)	R2CHOH	OH on carbon attached to two other carbons
Tertiary (3°)	R3COH	OH on carbon attached to three other carbons

Reactions of Alcohols

• Dehydration: Loss of water to produce a double bond (alkene)

  • Elimination reaction

  • Reverse of hydration (addition); non-redox

Summary

• The carbon-oxygen bond in phenols is more polarized than in alcohols due to the aromatic ring.

• Phenols are more acidic than alcohols because the conjugate base is stabilized by the electron density of the aromatic ring.

Oxidation and Reduction of Alcohols

Alcohol Oxidation

Alcohols can be oxidized to carbonyl groups (compounds with a carbon-oxygen double bond). The product depends on whether the alcohol is primary, secondary, or tertiary.

• Primary (1°) alcohol: Can be oxidized to an aldehyde, then further to a carboxylic acid.

• Secondary (2°) alcohol: Can be oxidized to a ketone.

• Tertiary (3°) alcohol: Cannot be oxidized (no hydrogen on the carbon bearing the OH group).

Oxidation Equations

• 1° alcohol: $\ce{R-CH2OH} \xrightarrow{[O]} \ce{R-CHO} \xrightarrow{[O]} \ce{R-COOH}$

• 2° alcohol: $\ce{R2CHOH} \xrightarrow{[O]} \ce{R2CO}$

• 3° alcohol: No reaction

Example: Breathalyzer Test

• Oxidizer: $\ce{K2Cr2O7}$ (potassium dichromate)

• Ethanol (1° alcohol) is oxidized to carboxylic acid; $\ce{Cr2O7^{2-}}$ is reduced to $\ce{Cr^{3+}}$ (color change)

• Amount of green color correlates to amount of 1° alcohol present

Carbonyl Compounds: Aldehydes and Ketones

Introduction to Carbonyl Groups

Carbonyl groups (C=O) are highly polarized and define the reactivity of aldehydes and ketones. The identity of the groups attached to the carbonyl carbon determines the functional group.

• Aldehyde: R-CHO (carbonyl at end of chain)

• Ketone: R-CO-R' (carbonyl in middle of chain)

• Carboxylic acid, ester, amide: Other carbonyl-containing groups

Properties of Aldehydes and Ketones

• Only hydrogen bond acceptors (no -OH group)

• Polar, higher boiling points than alkanes but lower than alcohols

• Weaker intermolecular forces than alcohols, but stronger than ethers

Reactions of Aldehydes and Ketones

• Oxidation: Increases number of bonds to oxygen, decreases number of bonds to hydrogen

• Reduction: Decreases number of bonds to oxygen, increases number of bonds to hydrogen

• Addition of alcohol: Forms hemiacetals, hemiketals, acetals, and ketals

Summary

• Carbonyl groups are polar and define aldehydes, ketones, carboxylic acids, esters, and amides.

• Aldehydes and ketones are polar but do not contain hydrogen-bond donors.

• Aldehydes are at the end of the chain, ketones are in the middle.

Oxidation and Reduction of Aldehydes and Ketones

Aldehyde Oxidation

• Aldehydes can be oxidized to carboxylic acids.

• Equation: $\ce{R-CHO} \xrightarrow{[O]} \ce{R-COOH}$

Ketone Oxidation

• Ketones cannot be oxidized further under normal conditions.

Distinguishing Aldehydes and Ketones

• Tollens' test: Tollens' reagent reacts with aldehydes (producing silver mirror), but not with ketones.

• Benedict's test: Benedict's reagent reacts with "alpha-hydroxy" aldehydes and ketones, producing a red-brown precipitate.

Summary

• Aldehydes can be oxidized to carboxylic acids and reduced to 1° alcohols.

• Ketones cannot be oxidized but can be reduced to 2° alcohols.

• Tollens' test selectively oxidizes aldehydes; Benedict's test is selective for alpha-hydroxy aldehydes and ketones.

Condensation and Hydrolysis Reactions

Condensation and Hydrolysis

• Condensation reaction: Two molecules combine with the loss of water (substitution).

• Hydrolysis reaction: Addition of water breaks a molecule into two parts.

Example: Acetal formation from aldehyde and alcohol.

Addition of Alcohols to Aldehydes and Ketones

• Alcohols add to aldehydes to form hemiacetals and acetals.

• Alcohols add to ketones to form hemiketals and ketals.

• Intramolecular additions form cyclic hemiacetals and hemiketals.

Summary

• Condensation reactions combine two molecules with elimination of water; hydrolysis reactions add water to break a molecule into two.

• Alcohols add to aldehydes and ketones to form hemiacetals/acetals and hemiketals/ketals, respectively.

• Intramolecular additions form cyclic hemiacetals and ketals, as seen in glucose.