Chap 21C - Hydroxy

Describe condensation of alcohols with carboxylic acids

Equation	alcohol + carboxylic acid ⇌ ester + water Eg. CH3CH2OH + CH3COOH ⇌ CH3COOCH2CH3 + H2O ethyl ethanoate
Reagents and conditions	Heat the alcohol with carboxylic acid and conc H2SO4 catalyst NOTE:  The catalyst provides H+ ions which catalyse the reaction The catalyst serves as a dehydrating agent, removing water from the equilibrium reaction, shifting POE to the right

Describe condensation of alcohols with acyl chlorides

alcohol + acyl chloride → ester + hydrogen chloride gas

Just add acyl chloride  NOTE: Using acyl chlorides to form esters from alcohols is the preferred as acyl chlorides are more reactive and the reaction goes to completion

Describe tri-iodomethane formation

• This oxidation OR oxidative cleavage reaction applies ONLY to alcohols with a methyl group and a hydrogen atom attached to the carbon atom at which the hydroxy group is also attached

• Reagents and conditions: I2(aq) in NaOH(aq), warm 

• Observation: Yellow or pale yellow precipitate of tri-iodomethane (iodoform)

Draw reaction for tri-iodomethane formation (use CRHCH3OH) (overall)

NOTE:

• Ethanol is the ONLY primary alcohol to show positive iodoform test 

• The product RCOO– has one C atom less than the reactant RCH(OH)CH3 (step-down reaction) due to the formation of CHI3

Test for alcohols which contain a          (check image)           group

• Warm the alcohol with I2(aq) in NaOH(aq)

• Observation:

  • If the alcohol contains a –CH(OH)CH3 group, a (pale) yellow precipitate (of CHI3) is formed

  • If –CH(OH)CH3 group is absent, no visible reaction is observed

• NOTE: Carbonyl compounds containing a methyl carbonyl group also give a positive iodoform test

Describe phenols

• Compounds containing a hydroxy group attached directly to an aromatic ring

• Since the hydroxy group attached directly to the benzene ring -> lone pair of electrons on O is delocalised into the benzene ring -> C–O bond has partial double bond character

Describe appearance, bp and solubility of phenols

1. Appearance 

• Phenol is a colourless, hygroscopic and crystalline solid (m.p. 42 °C)

2. Boiling point 

• Higher bp than hydrocarbons with similar Mr due to intermolecular HB between molecules 

3. Solubility 

• Partially soluble in water

• The –OH group form HB with water but the large non-polar aryl group cannot

• Solubility in water increases with temperature

Describe aciditiy of phenol and why it is more acidic than water

• Relative acidity: phenol > water > alcohol

• Phenol can behave as an acid by donating protons

• Greater acidity of phenol is due to the stabilisation of phenoxide anion (C6H5O–) by charge dispersal

  • The p-orbital of oxygen overlaps with the pi electron cloud of the benzene ring -> delocalisation of negative charge on the oxygen into the benzene ring -> disperses the negative charge on the O atom -> phenoxide anion is stabilised by charge dispersal

  • Hence, phenol is more acidic than water as such delocalisation effect and dispersion of charge is not found in water

Explain effect of electron-donating and withdrawing subsituents on acidity benzene ring

Electron-donating substituents	Electron-withdrawing substituents
Increase the electron density in the benzene ring -> decrease the acidity of phenol	Decrease the electron density in the benzene ring AND -ve charge -> increase the acidity of phenol

Explain which compound is more acidic

• Comparing the conjugate bases of 2-chlorophenol and the conjugate base of phenol, the electron density on O atom is lower in the conjugate base of 2-chlorophenol due to the electron-withdrawing –Cl group

• Hence, the conjugate base of 2-chlorophenol is more stable than the conjugate base of phenol -> 2-chlorophenol more acidic than phenol

State reactions of phenols

Reactions with hydroxy grp:

1. Redox with Na

2. Acid-base with NaOH

3. Acid-base with NaCO3

Elec sub at benzene:

1. Nitation

2. Halogenation