Chapter 5: Alcohols

Hydroxy-

Phenols. Ortho, o: adjacent carbons Meta, m: separated by two groups Para, p: groups on opposite side of ring

Alcohols are capable of intermolecular hydrogen bonding, resulting in higher melting and boiling point than those of analogous hydrocarbons. Molecules with more than one hydroxy Group show greater degree of hydrogen bonding, which is evident from the boiling points in the image.

Hydrogen bonding causes increased melting points, boiling points, and solubility and water.

The hydroxyl hydrogen is weekly acidic, and alcohols can dissociate into protons and alkoxide ions in the same way that water dissociates into protons and hydroxide ions. Recall that a high Ka means strong acid and a low pKa (-2 and under) is a strong acid. Low Ka and high pKa are weak acids.

Phenol, at the bottom, has the lowest pKa and is thus the strongest acid on that table.

Aromatic rings, such as phenol, allow for resonance stabilization of the negative charge of oxygen, stabilizing the anion, and thus producing a stronger acid. As with other compounds, electron withdrawing substituent increase acidity, and electron donating groups decrease acidity (acidity decreases as more alkyl groups are attached because they are electron donating, which destabilizes the alkoxide ion. The presence of more alkyl groups in non-aromatic alcohols produce less acidic molecules. Alkyl groups donate electron density, destabilizing the negative charge. Additionally, alkyl groups helped stabilize positive charges, explaining why more substituted carbocations have higher stability than less substituted carbocations. (For example: methanol is more acidic than hexanol due to the increased stability of the conjugate base)

That it. That’s the card. Rate it high if you know what those are.

Primary alcohols can oxidize to aldehydes (with pyridinium chlorochroamte) and carboxylic acids (with stronger oxidizing agents like dichromate). Secondary alcohols can be oxidized to ketones. Tertiary alcohols do not typically oxidize.

The hydroxyl group of alcohols are fairly poor leaving groups for nucleophilic substitution reactions. However, they can be protonated, or reacted to form much better leaving groups called mesylates and tosylates. Mesyl and tosyl groups can also serve as protecting groups when we do not want alcohols to react.

Aldehydes and ketones can be reacted with two equivalent of an alcohol or diol forming acetals (primary carbons with two -OR groups) or ketals (secondary carbons with two -OR groups). Carbonyls are very reactive with strong reducing agents (like LiAlH4), whereas acetals and ketals do not react with LiAlH4. The acetal and ketal protects the aldehyde or ketone from reaction. After reducing other functionalities in the molecule, the acetal or ketal can be reverted back to a carbonyl with aqueous acid, a step called deprotection.

The hydrogen in the hydroxyl group of phenols is particularly acidic because the oxygen containing anion is resonance stabilized by the ring.

Treatment of phenols with oxidizing agents produces quinones (2,5-cyclohexadiene-1,4-diones)

Yep. Into hydroxyquinones. Hydroxyquinones have biological activity. Some are used to synthesize medications.

Vitamins K1 (phylloquinone) and K2 (menaquinone). Fat soluble vitamins that play a role in carboxylation of clotting factors.

Photosynthesis and carboxylation of clotting factors in blood.