Question

Show how each alcohol or diol can be prepared from an alkene. (a) 2 -Pentanol (b) 1-Pentanol (c) 2-Methyl-2-pentanol (d) 2-Methyl-2-butanol (e) 3-Pentanol (f) 3 -Ethyl-3-pentanol (g) 1,2 -Hexanediol

Short answer

Question: Show how each alcohol or diol can be prepared from an alkene: a) 2-Pentanol b) 1-Pentanol c) 2-Methyl-2-pentanol d) 2-Methyl-2-butanol e) 3-Pentanol f) 3-Ethyl-3-pentanol g) 1,2-Hexanediol Answer: a) pent-2-ene → BH_3, THF → H_2O_2, OH^- → 2-Pentanol b) pent-1-ene → H_2SO_4, H_2O → 1-Pentanol c) 3-methylpent-2-ene → BH_3, THF → H_2O_2, OH^- → 2-Methyl-2-pentanol d) 2-methylbut-2-ene → BH_3, THF → H_2O_2, OH^- → 2-Methyl-2-butanol e) pent-2-ene → Hg(OAc)_2, H_2O → NaBH_4 → 3-Pentanol f) 3-ethylpent-2-ene → BH_3, THF → H_2O_2, OH^- → 3-Ethyl-3-pentanol g) hex-1-ene → OsO_4 → Na_2O_2 → 1,2-Hexanediol

Step-by-step solution

(a) 2-Pentanol Preparation

To prepare 2-pentanol, start with the alkene pent-2-ene, and proceed as follows: Perform hydroboration-oxidation using borane (BH_3) followed by hydrogen peroxide (H_2O_2) and hydroxide ion (OH^{-}) to produce 2-pentanol. This reaction follows Markovnikov's rule, adding the hydroxyl group to the less substituted end of the double bond. pent-2-ene → BH_3, THF → H_2O_2, OH^- → 2-Pentanol

(b) 1-Pentanol Preparation

To prepare 1-pentanol, start with the alkene pent-1-ene, and proceed as follows: Perform an acid-catalyzed hydration using a dilute sulphuric acid (H_2SO_4) and water (H_2O) to give the desired 1-pentanol. This reaction follows Markovnikov's rule, adding the hydroxyl group to the more substituted end of the double bond. pent-1-ene → H_2SO_4, H_2O → 1-Pentanol

(c) 2-Methyl-2-pentanol Preparation

To prepare 2-methyl-2-pentanol, start with the alkene 3-methylpent-2-ene and proceed as follows: Perform hydroboration-oxidation using borane (BH_3) followed by hydrogen peroxide (H_2O_2) and hydroxide ion (OH^{-}) to produce 2-Methyl-2-pentanol. This reaction follows Markovnikov's rule, adding the hydroxyl group to the less substituted end of the double bond. 3-methylpent-2-ene → BH_3, THF → H_2O_2, OH^- → 2-Methyl-2-pentanol

(d) 2-Methyl-2-butanol Preparation

To prepare 2-methyl-2-butanol, start with the alkene 2-methylbut-2-ene and proceed as follows: Perform hydroboration-oxidation using borane (BH_3) followed by hydrogen peroxide (H_2O_2) and hydroxide ion (OH^{-}) to produce 2-Methyl-2-butanol. This reaction follows Markovnikov's rule, adding the hydroxyl group to the less substituted end of the double bond. 2-methylbut-2-ene → BH_3, THF → H_2O_2, OH^- → 2-Methyl-2-butanol

(e) 3-Pentanol Preparation

To prepare 3-pentanol, start with the alkene pent-2-ene and proceed as follows: Perform an oxymercuration-demercuration reaction using mercuric acetate (Hg(OAc)_2) and water (H_2O), followed by sodium borohydride (NaBH_4) to give the desired 3-pentanol. This reaction follows Markovnikov's rule, adding the hydroxyl group to the more substituted end of the double bond. pent-2-ene → Hg(OAc)_2, H_2O → NaBH_4 → 3-Pentanol

(f) 3-Ethyl-3-pentanol Preparation

To prepare 3-ethyl-3-pentanol, start with the alkene 3-ethylpent-2-ene and proceed as follows: Perform hydroboration-oxidation using borane (BH_3) followed by hydrogen peroxide (H_2O_2) and hydroxide ion (OH^{-}) to produce 3-Ethyl-3-pentanol. This reaction follows Markovnikov's rule, adding the hydroxyl group to the less substituted end of the double bond. 3-ethylpent-2-ene → BH_3, THF → H_2O_2, OH^- → 3-Ethyl-3-pentanol

(g) 1,2-Hexanediol Preparation

To prepare 1,2-hexanediol, start with the alkene hex-1-ene and proceed as follows: Perform a two-step oxydation reaction, first using osmium tetroxide (OsO_4) followed by sodium peroxide (Na_2O_2) to anti dihydroxylate the alkene, producing the desired 1,2-hexanediol. hex-1-ene → OsO_4 → Na_2O_2 → 1,2-Hexanediol