Substitution week 2 prelab

Question

1. Treatment of 1—butanol with phosphoric acid (H₃PO₄) mixed with sodium chloride should result in the formation of 1-chlorobutane. Write a mechanism for this reaction.
2. With an eye on the process used here for preparing a 1-bromoalkane, comment on whether each of the following reactions might lead to 1—bromobutane:

a. 1-butanol + NaBr

b. 1-butanol + NaBr/H₃PO₄

c. 1-butanol + KBr/H₂SO₄

d. 1-butanol + NaBr/HCl

Answer 1

Mechanism for the Formation of 1-Chlorobutane

1. Protonation of 1-Butanol: Phosphoric acid (H₃PO₄) acts as a proton source, protonating the hydroxyl group (–OH) of 1-butanol, converting it into a better leaving group (water).
2. Formation of a Carbocation: Once the hydroxyl group is protonated, water (H₂O) leaves, resulting in the formation of a carbocation at the primary carbon. However, since primary carbocations are typically unstable, this step may involve a concerted mechanism in the presence of a nucleophile.
3. Nucleophilic Attack: Chloride ions (Cl⁻) from sodium chloride (NaCl) act as the nucleophile, attacking the carbocation (or the protonated alcohol in a concerted mechanism) to form 1-chlorobutane.

Evaluating Reactions for 1-Bromobutane Formation

Now let’s evaluate each given reaction for the formation of 1-bromobutane by considering if there’s a suitable proton source to activate the hydroxyl group and a bromide source to serve as a nucleophile.

a. 1-butanol + NaBr

1. Without an acid catalyst, the hydroxyl group in 1-butanol is not easily converted into a good leaving group. Therefore, 1-bromobutane is unlikely to form under these conditions.

b. 1-butanol + NaBr/H₃PO₄

1. Here, H₃PO₄ can protonate the hydroxyl group, making it a better leaving group (water). Once the hydroxyl is protonated, bromide ions (Br⁻) from NaBr can act as nucleophiles and attack the carbon, potentially leading to the formation of 1-bromobutane.
2. Expected Result: Likely to lead to 1-bromobutane formation.

c. 1-butanol + KBr/H₂SO₄

1. Similar to the reaction above, H₂SO₄ is a strong acid that can protonate the hydroxyl group, creating a good leaving group. The Br⁻ from KBr can then attack the carbocation or the protonated alcohol, leading to the formation of 1-bromobutane.
2. Expected Result: Likely to lead to 1-bromobutane formation.

d. 1-butanol + NaBr/HCl

1. HCl can protonate the hydroxyl group in 1-butanol. However, in the presence of HCl, the formation of 1-chlorobutane (from Cl⁻ ions) may be more favorable than the formation of 1-bromobutane due to the higher concentration of chloride ions.
2. Expected Result: More likely to produce 1-chlorobutane rather than 1-bromobutane due to competing nucleophiles (Cl⁻ and Br⁻), with Cl⁻ being more prevalent.

Summary

1. (a) Unlikely to produce 1-bromobutane.
2. (b) Likely to produce 1-bromobutane.
3. (c) Likely to produce 1-bromobutane.
4. (d) More likely to produce 1-chlorobutane due to the presence of HCl.