Alcohols, Thiols, Phenols, and Ethers

Reactions of Alcohols and Thiols

Alcohols and thiols are organic compounds containing hydroxyl (-OH) and sulfhydryl (-SH) groups, respectively. Their chemical reactivity is important in both biological and synthetic chemistry.

• Oxidation of Alcohols: Oxidation involves the loss of hydrogen or the gain of oxygen.

  • Primary alcohols oxidize to aldehydes, which can further oxidize to carboxylic acids.

  • Secondary alcohols oxidize to ketones.

  • Tertiary alcohols do not oxidize under normal conditions.

• Oxidation of Thiols: Thiols oxidize to form disulfide bonds (R-S-S-R), important in protein structure.

Acidity of Phenols

• Phenols are weakly acidic due to the resonance stabilization of the phenoxide ion formed after deprotonation.

• They react with bases to form a salt and water.

Aldehydes and Ketones

The Carbonyl Group

The carbonyl group (C=O) is a functional group where a carbon atom is double-bonded to an oxygen atom. It is highly polar due to the difference in electronegativity between carbon and oxygen.

• In aldehydes, the carbonyl group is always at the end of the carbon chain.

• In ketones, the carbonyl group is on an internal carbon.

Structural Isomers

• Aldehydes and ketones with the same molecular formula but different structures are called structural isomers.

Nomenclature

• Aldehydes: Replace the "e" of the parent alkane with "al" (e.g., propane → propanal).

• Common names for 1–4 carbon aldehydes use prefixes: form-, acet-, propion-, butyr- (e.g., formaldehyde, acetaldehyde).

• Ketones: Replace the "e" of the parent alkane with "one" (e.g., pentane → pentanone). Number the chain from the end nearest the carbonyl group.

• Cyclic ketones: The carbonyl carbon is always carbon 1.

Physical Properties

• Boiling Points: Aldehydes and ketones have higher boiling points than alkanes (due to dipole-dipole interactions), but lower than alcohols (cannot hydrogen bond with each other).

• Solubility: Compounds with 1–4 carbons are soluble in water; those with 5 or more carbons are insoluble due to increased nonpolar character.

Oxidation and Reduction

• Oxidation: Aldehydes can be oxidized to carboxylic acids using reagents such as , , or Tollen’s reagent. Ketones generally cannot be oxidized further.

• Reduction: Both aldehydes and ketones can be reduced to alcohols in the presence of hydrogen and a metal catalyst (Ni, Pd, or Pt):

  • Aldehydes → primary alcohols

  • Ketones → secondary alcohols

Carbohydrates

Definition and Types

Carbohydrates are polyhydroxy aldehydes or ketones, or compounds that yield them upon hydrolysis. They are a major source of energy in living organisms.

• Monosaccharides: Simplest carbohydrates (e.g., glucose, fructose).

• Disaccharides: Two monosaccharides linked by a glycosidic bond (e.g., sucrose, lactose).

• Polysaccharides: Many monosaccharides joined together (e.g., starch, cellulose).

Classification of Monosaccharides

• By functional group: Aldose (aldehyde group) or ketose (ketone group).

• By carbon number:

  • 3 carbons: triose

  • 4 carbons: tetrose

  • 5 carbons: pentose

  • 6 carbons: hexose

Chirality and Enantiomers

• A chiral carbon is bonded to four different groups.

• Chiral compounds have non-superimposable mirror images called enantiomers.

Fischer Projections

• Used to represent 3D structures of sugars.

• D-isomer: OH on the last chiral carbon is on the right.

• L-isomer: OH on the last chiral carbon is on the left.

• Glucose and galactose differ at carbon 4; fructose is a ketose, not an aldose.

Haworth Structures

• Show the cyclic (ring) forms of monosaccharides.

• Anomeric carbon: The carbon that was the carbonyl carbon in the open-chain form.

• Glucose and galactose form six-membered rings (pyranose); fructose forms a five-membered ring (furanose).

• If the OH on the anomeric carbon is up, it is a β-isomer; if down, it is an α-isomer.

• Mutarotation: Interconversion between α and β isomers in solution.

Oxidation and Reduction of Monosaccharides

• Oxidation: Aldoses oxidize to sugar acids (replace "ose" with "onic acid"). These are called reducing sugars because they reduce other substances while being oxidized.

• Glucose and galactose are reducing sugars; fructose can rearrange to glucose and is also a reducing sugar.

• Reduction: Forms sugar alcohols (replace "ose" with "itol").

Disaccharides

• Two monosaccharides linked by a glycosidic bond.

• Maltose and lactose are reducing sugars (free anomeric carbon).

• Sucrose is not a reducing sugar (both anomeric carbons involved in the bond).

Polysaccharides

• Polymers of glucose units; differ in linkage and branching.

• Humans cannot hydrolyze cellulose due to lack of enzyme for β(1→4) linkages.

Additional info: The study guide covers key reactions, structures, and properties of alcohols, thiols, phenols, ethers, aldehydes, ketones, and carbohydrates, including their nomenclature, physical properties, and biological significance. Tables have been added for clarity and completeness.