How is 1-propoxypropane synthesised from propan-1-ol? Write mechanism of this reaction.

Exercises · 7.26

Q. How is 1-propoxypropane synthesised from propan-1-ol? Write mechanism of this reaction.

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Concept understanding — Williamson Ether Synthesis

Williamson Ether Synthesis: From Intuition to Mechanism

Imagine you want to build a simple bridge between two carbon chains — an oxygen atom linking them together. That bridge is an ether ( $R-O-R'$ ). The Williamson ether synthesis is the most reliable way to build that bridge in a lab.

The Core Idea

You have two pieces: an alkoxide ion ( $RO^-$ ) and an alkyl halide ( $R'X$ ). The alkoxide is a strong nucleophile — it loves positive charge. The alkyl halide has a carbon attached to a halogen (like Cl, Br, I) that is slightly positive because the halogen pulls electrons away.

When you mix them, the alkoxide attacks that slightly positive carbon, kicks out the halide ion, and forms a new $C-O$ bond. The result? An ether.

R-O^- + R'-X \longrightarrow R-O-R' + X^-

That's the entire reaction in one line. But the devil is in the details — especially which alkyl halide you choose.

The Mechanism (SN2)

This is a classic SN2 reaction — one step, no intermediates. The alkoxide approaches the carbon from the opposite side of the halogen. As the $C-O$ bond forms, the $C-X$ bond breaks. The halide leaves as a stable anion.

Because it's SN2, the reaction is sensitive to steric hindrance. The carbon being attacked must be accessible.

Watch out

If the alkyl halide is tertiary (3°), the reaction will not work via SN2. The bulky carbon blocks the backside attack. Instead, the alkoxide will act as a base and cause elimination (forming an alkene). You'll get no ether.

The Practical Rule

Alkyl halide	Works?	Why
Methyl ( $CH_3X$ )	Yes	Least hindered, fastest SN2
Primary (1°)	Yes	Clean SN2
Secondary (2°)	Sometimes	Works if not too bulky; elimination competes
Tertiary (3°)	No	Elimination dominates
Aryl (e.g., bromobenzene)	No	SN2 impossible on $sp^2$ carbon

Tip

To make an ether like $R-O-R'$ , always use the less hindered alkyl halide and the more hindered alkoxide. For example, to make $CH_3CH_2-O-CH(CH_3)_2$ , use $CH_3CH_2O^-$ (primary alkoxide) + $(CH_3)_2CHBr$ (secondary halide) — not the other way around.

How to Choose the Alkoxide

You can't just buy alkoxide ions in a bottle. You make them by reacting an alcohol with a strong base like sodium hydride ( $NaH$ ) or sodium metal.

ROH + NaH \longrightarrow RO^-Na^+ + H_2

The alkoxide is then used immediately with the alkyl halide.

A Common Exam Trap …

Why this formula?

Williamson Ether Synthesis: Why the Key Principles Hold

The Williamson Ether Synthesis is a classic method to prepare ethers. The core reaction is:

\text{R-O}^- + \text{R'-X} \rightarrow \text{R-O-R'} + \text{X}^-

Where:

$\text{R-O}^-$ is an alkoxide ion (strong nucleophile)
$\text{R'-X}$ is an alkyl halide (electrophile)
$\text{X}^-$ is a halide ion (leaving group)

Let's break down why this works — the reasoning behind the key principles.

1. Why an Alkoxide (Not an Alcohol) is Needed

The Problem with Alcohols

Alcohols ( $\text{R-OH}$ ) are weak nucleophiles. The oxygen has a partial negative charge, but the O–H bond is strong. If you mix an alcohol with an alkyl halide, the reaction is extremely slow or doesn't happen at all.

The Solution: Deprotonation

By treating the alcohol with a strong base (like $\text{NaH}$ , $\text{Na}$ , or $\text{KOH}$ ), you remove the proton:

\text{R-OH} + \text{NaH} \rightarrow \text{R-O}^- \text{Na}^+ + \text{H}_2

The alkoxide ion ( $\text{R-O}^-$ ) has a full negative charge on oxygen. This makes it:

A much stronger nucleophile (higher electron density)
More reactive toward the electrophilic carbon in the alkyl halide

Key takeaway: The alkoxide's full negative charge is what drives the reaction — it's not just about having oxygen, but about having a charged, electron-rich oxygen.

2. Why the Alkyl Halide Must Be Primary (or Methyl)

The Mechanism: $\text{S}_\text{N}2$ is the Only Path

The Williamson synthesis proceeds exclusively via an $\text{S}_\text{N}2$ mechanism (bimolecular nucleophilic substitution). This means:

The nucleophile attacks the carbon from the backside
The leaving group departs from the opposite side
The reaction is concerted (one step, no intermediates)

Why Primary Halides Work Best

In $\text{S}_\text{N}2$ reactions, the rate depends on steric hindrance:

Alkyl Halide Type	Steric Hindrance	$\text{S}_\text{N}2$ Reactivity
Methyl ( $\text{CH}_3\text{X}$ )	Minimal	Very fast
Primary ( $\text{RCH}_2\text{X}$ )	Low	Fast
Secondary ( $\text{R}_2\text{CHX}$ )	Moderate	Slow
Tertiary ( $\text{R}_3\text{CX}$ )	High	Does not occur

Why Tertiary Halides Fail

With a tertiary halide, the bulky alkyl groups block the backside attack. Instead, the alkoxide (a strong base) will eliminate a proton from the halide, forming an alkene:

\text{R-O}^- + \text{R'}_3\text{C-X} \rightarrow \text{R-OH} + \text{alkene} + \text{X}^-

This is an E2 elimination — not the desired ether formation.

Key takeaway: The Williamson synthesis works only when the alkyl halide is primary or methyl because $\text{S}_\text{N}2$ requires an unhindered backside.

3. Why the Leaving Group Must Be Good

The Role of the Halide

The halide ( $\text{X}^-$ ) must be a good leaving group — meaning it can stabilize the negative charge after departure.

Halide	Leaving Group Ability	Reason
$\text{I}^-$	Excellent	Large, polarizable, weak base
$\text{Br}^-$	Good	Moderate size, weak base
$\text{Cl}^-$	Fair	Smaller, stronger base
$\text{F}^-$	Poor	Small, strong base, holds tightly

Why Fluoride Fails

Fluoride is a strong base and a poor leaving group. The C–F bond is very strong, and $\text{F}^-$ does not depart easily. So alkyl fluorides are unreactive in Williamson synthesis.

Key takeaway: The leaving group must be weakly basic and polarizable — iodide and bromide are ideal.

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1-Propoxypropane (dipropyl ether) is synthesised from propan-1-ol via Williamson Ether Synthesis — reacting sodium propoxide (from propan-1-ol and Na) with 1-bromopropane. The final product is $\text{CH}_3\text{CH}_2\text{CH}_2\text{OCH}_2\text{CH}_2\text{CH}_3$ .

The Williamson Ether Synthesis is the most reliable laboratory method for making unsymmetrical ethers. The key idea: you need an alkoxide ion (a strong nucleophile) and a primary alkyl halide (a good electrophile). The alkoxide attacks the halide in an $\text{S}_\text{N}2$ reaction, forming the ether.

Why does this work so well for 1-propoxypropane? Because both the alkoxide and the alkyl halide are derived from the same alcohol — propan-1-ol. You just need to convert half the alcohol into the nucleophile and the other half into the electrophile.

Here’s the step-by-step:

Form the alkoxide: Treat propan-1-ol with a strong base like sodium metal ( $\text{Na}$ ) or sodium hydride ( $\text{NaH}$ ). The base deprotonates the alcohol, giving sodium propoxide ( $\text{CH}_3\text{CH}_2\text{CH}_2\text{O}^-\text{Na}^+$ ) and hydrogen gas (if using Na).

\text{CH}_3\text{CH}_2\text{CH}_2\text{OH} + \text{Na} \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{O}^-\text{Na}^+ + \frac{1}{2}\text{H}_2

Prepare the alkyl halide: Convert another portion of propan-1-ol into 1-bromopropane. This is typically done using $\text{PBr}_3$ or $\text{HBr}$ (with $\text{H}_2\text{SO}_4$ ). The reaction follows an $\text{S}_\text{N}2$ mechanism because propan-1-ol is primary.

\text{CH}_3\text{CH}_2\text{CH}_2\text{OH} + \text{HBr} \xrightarrow{\text{H}_2\text{SO}_4} \text{CH}_3\text{CH}_2\text{CH}_2\text{Br} + \text{H}_2\text{O}

Perform the Williamson synthesis: Mix the sodium propoxide (from step 1) with 1-bromopropane (from step 2). The propoxide ion acts as a strong nucleophile and attacks the electrophilic carbon of the alkyl halide in an $\text{S}_\text{N}2$ displacement. The bromide ion leaves, and the ether forms.

\text{CH}_3\text{CH}_2\text{CH}_2\text{O}^- + \text{CH}_3\text{CH}_2\text{CH}_2\text{Br} \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{OCH}_2\text{CH}_2\text{CH}_3 + \text{Br}^-

Watch out

A common mistake is to try using propan-2-ol (isopropyl alcohol) or a secondary/tertiory halide. Secondary halides give significant elimination (alkene) instead of substitution, and tertiary halides almost exclusively eliminate. Always use a primary alkyl halide for the $\text{S}_\text{N}2$ step.

Mechanism of the Williamson step (the key reaction): …

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Williamson Ether Synthesis — Method & Mechanism

Method: Williamson Ether Synthesis

This is an SN2 reaction between an alkoxide ion and a primary alkyl halide (or tosylate).

Steps to synthesise 1-propoxypropane from propan-1-ol

Deprotonate propan-1-ol Treat propan-1-ol ( $\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}$ ) with a strong base like NaH or Na metal to form the sodium propoxide ion:

\text{CH}_3\text{CH}_2\text{CH}_2\text{OH} + \text{NaH} \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{O}^- \text{Na}^+ + \text{H}_2

Prepare the electrophile Convert another molecule of propan-1-ol into 1-bromopropane (or 1-chloropropane) using $\text{PBr}_3$ or $\text{HBr}$ :

\text{CH}_3\text{CH}_2\text{CH}_2\text{OH} + \text{PBr}_3 \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{Br}

Perform SN2 reaction Mix the sodium propoxide with 1-bromopropane. The alkoxide attacks the primary carbon, displacing bromide:

\text{CH}_3\text{CH}_2\text{CH}_2\text{O}^- + \text{BrCH}_2\text{CH}_2\text{CH}_3 \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{OCH}_2\text{CH}_2\text{CH}_3 + \text{Br}^-

Final product: 1-propoxypropane (dipropyl ether)

Mechanism (step-by-step)

Step 1 — Formation of alkoxide

Propan-1-ol reacts with NaH (strong base). The base abstracts the acidic proton of the alcohol:

\text{CH}_3\text{CH}_2\text{CH}_2\text{OH} + \text{H}^- \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{O}^- + \text{H}_2

Step 2 — SN2 attack

The alkoxide ion acts as a nucleophile. It attacks the electrophilic carbon of 1-bromopropane from the back side (SN2 stereochemistry):

$\text{CH}_3\text{CH}_2\text{CH}_2\text{O}^- \quad \xrightarrow{\text{attacks}} \quad \text{Br} - \text{CH}_2\text{CH}_2\text{CH}_3$ …

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Here are the common mistakes students make in Williamson Ether Synthesis (specifically for making 1-propoxypropane from propan-1-ol) and how to avoid each.

1. Choosing the Wrong Halide/Alcohol Pair (The "Symmetry Trap")

The Mistake: Students often try to react two molecules of propan-1-ol directly with each other (e.g., $CH_3CH_2CH_2OH + CH_3CH_2CH_2OH \rightarrow$ ether). This does not work under Williamson conditions.

Why it's wrong: The Williamson synthesis requires an alkoxide ion (strong nucleophile) and an alkyl halide (electrophile). Two neutral alcohols cannot react to form an ether without a strong base to deprotonate one of them first.

How to Avoid: Always identify the two fragments:

Alkoxide: Comes from the less hindered alcohol (usually the one with the smaller alkyl group or the one you deprotonate).
Alkyl halide: Comes from the other alcohol (converted to a halide, e.g., using $PBr_3$ or $SOCl_2$ ).

For 1-propoxypropane ( $CH_3CH_2CH_2-O-CH_2CH_2CH_3$ ), both sides are identical. The correct approach:

Convert one molecule of propan-1-ol to propyl bromide ( $CH_3CH_2CH_2Br$ ).
Deprotonate the other molecule of propan-1-ol with a strong base (e.g., $NaH$ ) to get sodium propoxide ( $CH_3CH_2CH_2O^-Na^+$ ).
React them: $CH_3CH_2CH_2O^-Na^+ + CH_3CH_2CH_2Br \rightarrow CH_3CH_2CH_2-O-CH_2CH_2CH_3 + NaBr$ .

2. Using the Wrong Base (or No Base at All)

The Mistake: Using a weak base like $NaOH$ or $K_2CO_3$ in water, or forgetting to add a base entirely.

Why it's wrong: Propan-1-ol is a weak acid ( $pK_a \approx 16$ ). To form the alkoxide ion ( $RO^-$ ), you need a base strong enough to deprotonate it. $NaOH$ in water is not strong enough because water is a stronger acid ( $pK_a \approx 15.7$ ) than the alcohol — the equilibrium lies with the alcohol, not the alkoxide.

How to Avoid: Use a strong, non-nucleophilic base in an anhydrous (dry) solvent:

Best choices: $NaH$ (sodium hydride), $Na$ metal, or $KOH$ in dry ethanol (if the alcohol is the solvent).
Avoid: Aqueous bases (water will protonate the alkoxide back to alcohol).

Correct step: $CH_3CH_2CH_2OH + NaH \rightarrow CH_3CH_2CH_2O^-Na^+ + H_2 \uparrow$

3. Forgetting the $S_N2$ Mechanism Details (Especially Stereochemistry)

The Mistake: Drawing the mechanism as a simple "swap" without showing the backside attack of the alkoxide on the alkyl halide, or ignoring that it's an $S_N2$ reaction.

Why it's wrong: The Williamson synthesis is a classic $S_N2$ reaction. If you draw it as a one-step bond-breaking/bond-forming without showing the nucleophile approaching from the opposite side of the leaving group, you lose marks. Also, if the alkyl halide is secondary or tertiary, elimination ( $E2$ ) will dominate, and you won't get the ether.

How to Avoid:

Draw the curved arrow from the alkoxide oxygen lone pair to the carbon attached to the halogen (the $\alpha$ -carbon).
Show the leaving group (Br $^-$ or I $^-$ ) departing simultaneously.
Use a primary alkyl halide (like propyl bromide) to ensure $S_N2$ is fast and elimination is minimal.

Mechanism (correct):

CH_3CH_2CH_2O^- \quad \xrightarrow{\text{backside attack}} \quad CH_3CH_2CH_2\text{-}O\text{-}CH_2CH_2CH_3 + Br^-

(Show the Br leaving as the C-O bond forms.)

4. Ignoring the "Elimination vs. Substitution" Competition

The Mistake: Using a secondary or tertiary alkyl halide (e.g., 2-bromopropane or 2-bromo-2-methylpropane) with a strong alkoxide base.

Why it's wrong: Strong bases (like alkoxides) are also strong bases. With secondary or tertiary halides, E2 elimination is much faster than $S_N2$ substitution. You'll get an alkene instead of the ether.

How to Avoid: Always use a primary alkyl halide (like $CH_3CH_2CH_2Br$ ) for the electrophile. If you must use a secondary halide, use a weaker base (like silver oxide, $Ag_2O$ ) or a different method (e.g., alkoxymercuration).

For 1-propoxypropane: Both fragments are primary — safe.

5. Writing the Mechanism Without Showing the Deprotonation Step …

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Q.How is 1-propoxypropane synthesised from propan-1-ol? Write mechanism of this reaction.

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