Chapter 5 2 alcohols and phenols

Alcohols and Phenols

• Can be thought of as organic derivatives of water in which one of the water hydrogens is replaced by an organic group.

• an alcohol is any organic compound in which has hydroxyl group (-OH)

• general formula for a simple acyclic alcohol is CnH2n+1OH

• classified into 10, 20, 30 alcohols

Naming alcohols

• suffix: -ol• select longest chain containing hydroxyl group.

Derive name by replacing –e with –ol). If next letter is not a vowel then –e is not omitted.

• Begin numbering at end nearer hydroxyl group• Number substituents and list alphabetically.• If a higher priority group is present (such as an

aldehyde, ketone or carboxylic acid), then it is necessary to use the prefix hydroxy

Naming Phenols

• Named similar to benzenes• Phenol parent name

Exercise

• (z)-2-ethylbut-2-en-1-ol• Cyclohex-3-en-1-ol

Properties

• same geometry around the oxygen atom as water

• have higher boiling point due to hydrogen bonding: +vely polarized –OH • hydrogen atom from one molecule is attracted to a lone

pair of electrons on the –ve oxygen of another molecule

• Solubility?

• weakly acidic and weakly basic– alkyl substitution effects acidity due to solvation

of alkoxide ion that results from dissociation(more solvated, more stable, more acidic)• steric hinderance reduces solavation • inductive effects (e-withdrawing) more stabilize

• phenol a million times more acidic than alcohol due to resonance stabilization of the phenoxide ion.

• substituted phenols can be either more acidic(e-withdrawing) or less acidic(e-

donating)

Preparation (alcohol)• alkene:– hydration– hydroboration/oxidation– hydroxylation with OsO4 (1,2-diols)– Oxymercuration/reduction

• carbonyl compounds (reduction)– ketone: secondary alcohols– aldehyde: primary alcohols

• add 1 H to carbonyl C, LiAlH4/NaBH4

– carboxylic acid: primary alcohols– ester: primary alcohols

• add 2 H to carbonyl C, only LiAlH4

• alkyl halide– nucleophilic aliphatic substitution to primary alcohols– with grignard reagents + carbonyl groups

• from reaction of carbonyls with Grignard reagent – formaldehyde gives 1o alcohols– aldehydes give 2o alcohols,– ketones give 3o alcohols – esters give 3o alcohols – carboxylic acid give alkanes

• limitations of Grignard reagents: can not be prepared if there is other reactive functional group on the same organohalide – would self react (ex. Carbonyl group on organohalide therefore Grignard adds to these groups)

Preparation (Phenol)

Reaction of alcohols• conversion of alcohols to alkyl halides (3o)• conversion of alcohols to OTos (1o &2o)• dehydration of alcohols to alkenes (H3O+ or POCl3)• conversion of alcohols into esters: carboxylic acid +

alcohol• oxidation of alcohols to yield carbonyl compounds

(KMnO4, CrO3, Na2Cr2O7)– 1o alcohols give aldehydes(PCC) or carboxylic acid (CrO3)– 2o alcohols give ketones– 3o alcohols do not normally react with most oxidizing

agents

Reaction of Phenols

• electrophilic aromatic substitution• oxidation to yield quinones with Fremy’s salts

(potassium nitrosodisulfonate)

Protection of alcohols• complex molecule synthesis: one functional group in a

molecule interferes with the intended reaction on a second functional group. (ex. Grignard reagent can not be prepared from a halo alcohol)

• circumvent problem by protecting interfering group– introduce protecting group to block interfering function– carry out desired reaction– remove protecting group

• alcohol protection is commonly with chlorotrialkylsilane, Cl-SiR3 to yield trialkylether, R’O SiR3

• reaction carried out in presence of base, trietyhlamine, to help form alkoxide ion Followed by SN2 reaction with ClSiR3)

• Removal with aqueous acid or with fluoride ion