ochem week 7 part 2

Good Leaving Groups (from alcohols)

Alkyl halides formed by $PBr<em>3$ or $SO Cl</em>2$ conversion, and Tosylates.

How does converting an alcohol to a tosylate affect stereochemistry?

It does not change the stereochemistry of the carbon; the -OH is replaced by -OTs with retention of configuration.

How does converting an alcohol to an alkyl halide using $PBr<em>3$ or $SO Cl</em>2$ affect stereochemistry?

It changes the stereochemistry. For example, $PBr_3$ often leads to inversion of configuration at the stereocenter.

What is the E1 Elimination (Acid-Catalyzed Dehydration) reaction?

A common reaction in organic chemistry I that forms alkenes from alcohols, especially predominant with strong acid and heat.

Conditions for E1 Elimination (Acid-Catalyzed Dehydration)

Strong acid (e.g., $H<em>2 SO</em>4$, $H<em>3 PO</em>4$) and heat ($\Delta$).

Outline the E1 Elimination mechanism steps.

1. Protonation of alcohol to form an alkyloxonium ion. 2. Loss of water to form a carbocation intermediate. 3. Deprotonation by a base to form the alkene product (Zaitsev product usually preferred).

Carbocation stability in E1 Elimination

Tertiary carbocations are preferred due to their stability.

Key takeaway of E1 Elimination of Alcohols

Requires heat, forms a carbocation, and can be a desired side reaction when substitution is favored at higher temperatures.

Purpose of E2 Elimination using $POCl_3$/Pyridine

To provide an alternative to E1 dehydration, avoiding harsh acidic conditions and high temperatures, which can cause rearrangements.

Conditions for E2 Elimination using $POCl_3$/Pyridine

Phosphoryl chloride ($POCl<em>3$) and a base (e.g., pyridine or triethylamine, $Et</em>3 N$) at cold temperatures.

Pyridine characteristics in E2 elimination

A nitrogen-containing aromatic base, it acts as a base and a solvent, similar to benzene but with one carbon replaced by nitrogen.

Mechanism of E2 Elimination (Using $POCl_3$/Pyridine)

1. Alcohol oxygen attacks phosphorus of $POCl_3$, and a chlorine leaves. 2. Chloride or base removes H from the alcohol oxygen. 3. Base abstracts a beta-hydrogen, C-H bond forms C=C pi bond, and the oxygen-phosphorus complex leaves.

Characteristics of E2 Elimination (using $POCl_3$/Pyridine)

It is a concerted reaction involving simultaneous removal of a beta-hydrogen, formation of a pi bond, and departure of the leaving group, occurring under milder, basic conditions and cold temperatures.

E1CB Elimination

A much rarer, niche E1 mechanism compared to the typical E1, proceeding via a carbanion intermediate.