Multi-Step Synthesis: Functional Group Interconversions

Learn to design synthetic pathways by converting one functional group to another through a logical sequence of reagent-driven reactions.

Organic Chemistry 1

What Is a Functional Group?

A functional group is a specific arrangement of atoms within a molecule that determines its chemical reactivity. The carbon skeleton stays largely the same: the functional group is what changes during a reaction.

Alkene

\( \text{C}=\text{C} \) double bond: reactive, electron-rich

Alcohol

\( -\text{OH} \) group: polar, hydrogen-bonding

Alkyl Halide

\( -\text{X} \) (X = Cl, Br, I): good leaving group

Functional Group Interconversion (FGI)

FGI means converting one functional group into a different one using a known reagent or set of conditions. This is the fundamental "move" in organic synthesis: each step changes the molecule's reactivity.

Why Multi-Step Synthesis Matters

Rarely can you go from starting material to product in a single reaction. Real synthesis requires planning a route through multiple intermediate functional groups.

Drug Design

Pharmaceuticals are built by assembling functional groups step by step

Materials

Polymers and plastics rely on precise functional group control

Problem Solving

Synthesis questions test your ability to think backward from the product

The Core Reagent Toolkit

Memorize these key transformations. Each converts one functional group to another with a specific reagent:

1

Alkene → Alcohol: Acid-catalyzed hydration (\( \text{H}_2\text{O} / \text{H}_2\text{SO}_4 \)) or hydroboration-oxidation (\( \text{BH}_3 \) then \( \text{H}_2\text{O}_2 / \text{NaOH} \))

2

Alkene → Alkyl Halide: Hydrohalogenation (\( \text{HBr} \), \( \text{HCl} \))

3

Alcohol → Alkyl Halide: \( \text{PBr}_3 \), \( \text{SOCl}_2 \), or \( \text{HBr} \)

4

Alkyl Halide → Alcohol: \( \text{S}_\text{N}2 \) with \( \text{NaOH} \) or \( \text{H}_2\text{O} \)

5

Alcohol → Alkene: Dehydration with \( \text{H}_2\text{SO}_4 \) / heat

The Reagent Map

Markovnikov vs. Anti-Markovnikov

When adding \( \text{HX} \) or \( \text{H}_2\text{O} \) across a double bond, regiochemistry determines where each atom ends up. This distinction is critical for planning your synthesis.

	Markovnikov	Anti-Markovnikov
Rule	\( \text{H} \) adds to the carbon with more H's	\( \text{H} \) adds to the carbon with fewer H's
Reagent (HBr)	\( \text{HBr} \) alone	\( \text{HBr} + \text{ROOR} \) (peroxides)
Reagent (Hydration)	\( \text{H}_2\text{O} / \text{H}_2\text{SO}_4 \)	\( \text{BH}_3 \) then \( \text{H}_2\text{O}_2 / \text{NaOH} \)
Product Type	More substituted carbon bears \( \text{X} \) or \( \text{OH} \)	Less substituted carbon bears \( \text{X} \) or \( \text{OH} \)

How to Apply: Retrosynthetic Analysis

Work backward from the target molecule. Ask: "What functional group do I need in the product? What reaction makes that group? What starting material does that reaction need?"

1

Identify the target: What functional group does the product have?

2

Disconnect backward: What reaction could form this group? What is the immediate precursor?

3

Repeat: Can you reach the given starting material? If not, add another step backward.

4

Write it forward: Reverse the sequence and list reagents for each step.

5

Check regiochemistry and stereochemistry: Does each step give the correct product?

Example 1: Alkene to Alcohol

Goal: Convert 1-butene to 1-butanol (anti-Markovnikov product).

Example 2: Two-Step Synthesis

Goal: Convert propene to 2-propanol via an alkyl halide intermediate.

Example 3: Elimination & Addition

Goal: Convert cyclohexanol to cyclohexene and then to bromocyclohexane.

Substitution vs. Elimination

When converting an alkyl halide, two competing pathways exist. Choosing the right conditions is critical:

	Substitution (\( \text{S}_\text{N} \))	Elimination (E)
Product	Alcohol, ether, or other nucleophile-bearing group	Alkene
Favored by	Strong nucleophile, low temperature, primary substrate	Strong base, high temperature, bulky base, tertiary substrate
Example Reagent	\( \text{NaOH (dilute)} \)	\( \text{KOtBu, heat} \)
Mechanism	\( \text{S}_\text{N}2 \) or \( \text{S}_\text{N}1 \)	E2 or E1

Oxidation and Reduction in Synthesis

Beyond interconversions, oxidation state changes are essential moves in synthesis:

Oxidation

Alcohol → Aldehyde → Carboxylic Acid. Use \( \text{PCC} \) (mild) or \( \text{KMnO}_4 \) (strong).

Reduction

Alkene → Alkane: \( \text{H}_2 / \text{Pd} \). Carbonyl → Alcohol: \( \text{NaBH}_4 \) or \( \text{LiAlH}_4 \).

Common Mistakes in Synthesis

✕ Wrong Regiochemistry

Using \( \text{H}_2\text{O} / \text{H}_2\text{SO}_4 \) when you need the anti-Markovnikov alcohol. Use hydroboration-oxidation instead.

✕ Substitution vs. Elimination

Forgetting that a strong, bulky base at high temperature favors elimination over substitution.

✕ Over-Oxidation

Using \( \text{KMnO}_4 \) to make an aldehyde. It goes all the way to carboxylic acid. Use \( \text{PCC} \) to stop at the aldehyde.

Quick Reference Table

Transformation	Reagent(s)	Notes
Alkene → Alcohol (Markovnikov)	\( \text{H}_2\text{O} / \text{H}_2\text{SO}_4 \)	OH on more substituted C
Alkene → Alcohol (Anti-Mark.)	\( \text{BH}_3 \) then \( \text{H}_2\text{O}_2 / \text{NaOH} \)	OH on less substituted C
Alkene → Alkyl Halide	\( \text{HBr} \) or \( \text{HCl} \)	Markovnikov addition
Alcohol → Alkyl Halide	\( \text{PBr}_3 \), \( \text{SOCl}_2 \)	Inversion at chiral center
Alkyl Halide → Alcohol	\( \text{NaOH} \) (\( \text{S}_\text{N}2 \))	Primary substrates best
Alcohol → Alkene	\( \text{H}_2\text{SO}_4 \), heat	E1 dehydration
Alcohol → Aldehyde	\( \text{PCC} \)	Mild oxidant, stops here
Alkene → Alkane	\( \text{H}_2 / \text{Pd} \)	Catalytic hydrogenation

Question 1 of 5

Which reagent converts an alkene to an anti-Markovnikov alcohol?

Hydroboration-oxidation (\( \text{BH}_3 \) / \( \text{H}_2\text{O}_2 \)) gives the anti-Markovnikov alcohol. Acid-catalyzed hydration gives Markovnikov, HBr gives a halide, and \( \text{NaBH}_4 \) reduces carbonyls, not alkenes.

Question 2 of 5

To convert 1-propanol to 1-bromopropane, which reagent is most appropriate?

\( \text{PBr}_3 \) converts a primary alcohol directly to a primary alkyl bromide in one step with inversion of configuration. NaOH is a nucleophile (not useful here), \( \text{H}_2\text{SO}_4 \) with heat causes dehydration, and \( \text{BH}_3 \) is for hydroboration.

Question 3 of 5

What is the product when cyclohexene reacts with \( \text{HBr} \)?

\( \text{HBr} \) adds across the double bond of cyclohexene via electrophilic addition, producing bromocyclohexane. The alkene is consumed (not preserved), water is not involved (so no alcohol), and reduction to cyclohexane would require \( \text{H}_2 / \text{Pd} \).

Question 4 of 5

You want to oxidize a primary alcohol to an aldehyde without going further to the carboxylic acid. Which reagent should you choose?

PCC is a mild oxidant that stops at the aldehyde stage. \( \text{KMnO}_4 \) is too strong and will oxidize all the way to the carboxylic acid. \( \text{LiAlH}_4 \) and \( \text{H}_2 / \text{Pd} \) are reducing agents, not oxidants.

Question 5 of 5

What is the most direct method to convert 2-butene to 2-butanol?

Acid-catalyzed hydration adds water across the double bond of 2-butene to form 2-butanol in a single step. Since 2-butene is symmetrical, both carbons are equally substituted, so both Markovnikov and anti-Markovnikov additions would yield the same product (2-butanol). Acid-catalyzed hydration is the most direct single-reagent method. \( \text{PBr}_3 \) requires an alcohol starting material, and \( \text{NaBH}_4 \) does not react with alkenes. While hydroboration-oxidation would also work, it involves two steps compared to the one-step acid-catalyzed hydration.