---
title: "Retrosynthesis: Disconnections and Synthons"
description: "Planning a synthesis runs backwards from the target. Each disconnection is a bond you break on paper, chosen because a real reaction can make it, and the fragments it leaves tell you what starting mat"
canonical: https://lightmysky.com/learn/science/retrosynthesis-disconnections-and-synthons-mt_0EJbkDPo2P
source: https://lightmysky.com/learn/science/retrosynthesis-disconnections-and-synthons-mt_0EJbkDPo2P.md
retrieved: 2026-09-12
---

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# Retrosynthesis: Disconnections and Synthons

Planning a synthesis runs backwards from the target. Each disconnection is a bond you break on paper, chosen because a real reaction can make it, and the fragments it leaves tell you what starting materials to look for.

Subject: Science · Area: Chemistry · Ages 21 to 22
Page: https://lightmysky.com/learn/science/retrosynthesis-disconnections-and-synthons-mt_0EJbkDPo2P

## Ready when they can

- Marks a disconnection on a target and names the reaction that would make that bond forwards
- Converts a pair of synthons into real reagents with the right polarity at each carbon
- Recognises a carbonyl compound as a two-carbon or three-carbon disconnection point and uses it
- Compares two routes to the same target and argues for one on step count and available starting materials

## Lesson: Planning backwards from the target

When a lab needs to make a medicine, the team plans backwards from the target to cheap starting materials. You draw the target, break one bond on paper into two fragments, and name the forward reaction that would rejoin them, such as Grignard, aldol, or esterification. Each disconnection must match a reaction you can actually run.

Each fragment leaves a synthon, an idealized piece with the polarity your bond formation needs, often one you cannot buy. You convert it into a real reagent you can buy or make. Carbonyl carbons are natural acceptors, while Grignard and enolate carbons are donors, so you match donor to acceptor.

**Example.** Carbonyls are your signposts for where to cut. An alcohol next to a former carbonyl suggests a Grignard disconnection, while a beta-hydroxy carbonyl suggests an aldol one and a 1,3-dicarbonyl suggests a Claisen one. Esters come from acids plus alcohols, amides from acids plus amines, and an alpha beta unsaturated carbonyl points to retro aldol.

When two routes reach the same target, you choose by judgment. You count steps, check starting material cost and availability, and prefer reliable high yield steps with little protecting group juggling. A short route from the catalogue beats a clever long one.

**Recap.** You cut one bond at a time into honest reactions, fix polarity with real reagents, cut next to carbonyls, and pick the shorter cheaper route.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Pericyclic Reactions: The Diels-Alder and Orbital Symmetry](https://lightmysky.com/learn/science/pericyclic-reactions-the-diels-alder-and-orbital-symmetry-mt_I4i9d6aDgD)

## Opens up

- [Planning a Multi-Step Synthesis: Order, Protection and Yield](https://lightmysky.com/learn/science/planning-a-multi-step-synthesis-order-protection-and-yield-mt_3PYRELbNfu)
- [Searching the Chemical Literature by Structure and by Reaction](https://lightmysky.com/learn/science/searching-the-chemical-literature-by-structure-and-by-reaction-mt_7vkNiUQ2uw)
- [Strategy in Retrosynthesis: Strategic Bonds and Convergent Routes](https://lightmysky.com/learn/science/strategy-in-retrosynthesis-strategic-bonds-and-convergent-routes-mt_9ZRKVbdP72)