Question

16. Predict the major products (including stereochemistry) when R-2-Butanol reacts with the following reagents. (a) PBr₃

(b) SOCl₂/

(c) Lucas reagent (d) TsCl / pyridine, then NaI

Answer 1

(a) (S)-2‑Bromobutane
(b) (S)-2‑Chlorobutane
(c) Racemic 2‑Chlorobutane
(d) (S)-2‑Iodobutane

Answer 2

Solution:

Let the starting alcohol be (R)-2-butanol. Recall that:

1. SN2 reactions (e.g. with PBr₃ and when using tosylate → iodide) involve inversion of configuration.
2. SN1 reactions (e.g. with Lucas reagent) proceed via a carbocation so that the chiral center becomes racemic.

Now, step‐by-step:

(a) With PBr₃:

• PBr₃ converts the –OH into a bromide via an SN2 mechanism.
• Thus, (R)-2-butanol gives inversion yielding (S)-2‑bromobutane.

(b) With SOCl₂ in dioxane:

• Thionyl chloride generally reacts by forming a chlorosulfite intermediate and then substitution occurs via an SN2-like pathway.
• Hence, (R)-2-butanol is converted to (S)-2‑chlorobutane (inversion).

(c) With Lucas reagent (ZnCl₂/HCl):

• Lucas reagent reacts via an SN1 mechanism on secondary alcohols, forming a planar carbocation.
• This leads to a racemization of the center so the product is 2‑chlorobutane (racemic mixture).

(d) With TsCl/pyridine then NaI:

• First, TsCl converts the –OH into a tosylate (which retains the configuration; the stereochemistry is that of the starting alcohol).
• Then, NaI displaces the tosylate via an SN2 process causing inversion.
• Overall, (R)-2-butanol is converted to (S)-2‑iodobutane.

Minimal Explanation:
For (a) and (d): SN2 gives inversion (R becomes S). For (b): SOCl₂ in dioxane also proceeds via an SN2-like mechanism (inversion, S product). For (c): Lucas reagent involves an SN1 mechanism yielding a racemic mixture.