RI 2025 Hydroxy Cpds Tutorial - Anwers to Self-check Questions

-7- Answers to Hydroxy Compounds self-check questions 1 (a) primary (1o) primary (1o) primary (1o) primary (1o) secondary (2o) CC C C* H H H O H C H H H H H H H H 3-methylbutan-2-ol secondary (2o) tertiary (3o) secondary (2o) (b) pentan-1-ol, 3-methylbutan-1-ol, 2-methylbutan-1-ol (Note: Pentan-3-ol is not accepted as it forms an alkene which can exhibit cis-trans isomerism.) (c) 2,2-dimethylpropan-1-ol (Note: For dehydration to occur, there must be a H atom and a –OH group on adjacent carbon atoms) (d) (e) (i) 2-methylbutan-2-ol (Note: 3o alcohols do not undergo oxidation) (ii) pentan–1–ol, 3–methylbutan–1–ol, 2–methylbutan–1–ol, 2,2–dimethylpropan–1–ol (Note: Only carboxylic acids will react with Na2CO3(aq) to form CO2 gas)

-8- 2 (a) (b) 3 Relative acidity: alcohol < water < phenol The relative acid strength depends on the relative stability of the respective conjugate bases, namely the alkoxide, hydroxide and phenoxide ions. The more stable the conjugate base, the more acidic the compound. Phenylmethanol is an alcohol while 4-methylphenol and 4-nitrophenol are substituted phenols. Phenols are more acidic than alcohols.  The p-orbital containing the lone pair of electrons on th e oxygen atom of the phenoxide ions overlaps with the  electron cloud of the benzene ring and the lone pair of electrons is delocalised into the ring.  This results in the delocalisation of negative charge on oxygen into the ring, i.e. dispersal of the negative charge over the ring.  The phenoxide ion is resonance–stabilised and more stable than alkoxide ion and the equilibrium position for the acid dissociation of phenol lies more to the right.  Hence, phenols have larger Ka value than alcohols. 4-methylphenol is less acidic than 4-nitrophenol.  Presence of electron-withdrawing group, NO2, on the benzene ring disperses the negative charge on the oxygen atom.  Thus is more stable than .  The equilibrium position for the acid dissociation of 4-nitrophenol lies more to the right.  4-nitrophenol has a larger Ka value. OR  Presence of electron-donating group, CH3, on the benzene ring intensifies the negative charge on the oxygen atom.  Thus is less stable than .  The equilibrium position for the acid dissociation of 4-methylphenol lies more to the left.  4-methylphenol has a smaller Ka value.

-9- 4 (Ans: C) A B no reaction (Note: Phenols do not undergo dehydration when heated with conc. H2SO4.) C Acid-base reaction occurs. D No reaction. (Note: Phenols do not undergo oxidation reaction.) 5 (Ans: A) ROH + Na(s)  RONa+ + ½ H2(g) For C6H12O6, there is one degree of unsaturation. Since there are no C=C bonds, the compound must have one C=O bond. The remaining O atoms must be in -OH groups  There are a total of 5 -OH groups. mole ratio of -OH : Na : H2 = 1 : 1 : ½ He