In a retrosynthetic analysis, you disconnect a C–C bond in a target alcohol to give a carbanion synthon and an electrophilic carbonyl synthon. What is the next step?
AWrite the reaction conditions for combining the carbanion and carbonyl directly in the flask
BIdentify synthetic equivalents — real reagents that deliver carbanion and electrophilic carbonyl reactivity (e.g., a Grignard reagent and an aldehyde)
CRepeat the disconnection on the carbanion synthon until it reduces to a single carbon
DVerify that the retrosynthetic arrow is written in the correct forward direction
Synthons are idealized species — a carbanion may not exist as a stable molecule. The next step is to match each synthon to a synthetic equivalent: a real reagent that delivers that reactivity. A Grignard reagent (RMgBr) is the synthetic equivalent of a carbanion; an aldehyde or ketone is the synthetic equivalent of an electrophilic carbonyl. Only after identifying real reagents can you verify the forward synthesis.
Question 2 Multiple Choice
A student identifies a beta-hydroxy carbonyl pattern in a target molecule. What disconnection does this structural motif suggest, and why?
AA Michael addition disconnection, because 1,4-additions always produce beta-hydroxy carbonyls
BAn aldol disconnection, because the beta-hydroxy carbonyl is the direct product of an aldol reaction between an enolizable carbonyl and an aldehyde
CA Grignard disconnection, because all C–C bonds adjacent to oxygen are best made with Grignard reagents
DNo strategic disconnection is implied; any C–C bond is equally productive
Structural patterns are 'signposts' pointing to known reactions. A beta-hydroxy carbonyl — an OH on the carbon beta to a carbonyl — is the hallmark product of an aldol reaction. Disconnecting the C–C bond between the alpha-carbon and the beta-carbon reveals an enolizable carbonyl nucleophile and an aldehyde electrophile. Recognizing these patterns is the core skill that makes retrosynthesis systematic rather than random.
Question 3 True / False
A student draws A → B → C (target) using forward reaction arrows throughout. This is an example of retrosynthetic analysis.
TTrue
FFalse
Answer: False
Retrosynthetic analysis works backward: from target C to precursor B to starting material A, using the retrosynthetic arrow (⇒), not forward reaction arrows. Working forward (A → B → C) is the forward synthesis. The retrosynthetic approach reverses the direction of thinking — each step asks 'what simpler molecule could give me this target?' rather than 'what can I make from this starting material?'
Question 4 True / False
A retrosynthetic disconnection and its corresponding forward reaction should use identical reagents and conditions.
TTrue
FFalse
Answer: False
Retrosynthetic arrows show logical disconnections, not actual reactions. A disconnection reveals the structural relationship between target and precursor, but the forward reaction requires specific reagents, solvents, temperatures, and workup not captured in the disconnection notation. For example, disconnecting an alcohol's C–C bond implies Grignard chemistry, but the forward step also requires anhydrous conditions, careful addition order, and acidic workup.
Question 5 Short Answer
Why does retrosynthetic analysis begin at the target molecule and work backward, rather than starting from available reagents and working forward?
Think about your answer, then reveal below.
Model answer: Working forward from reagents is combinatorially explosive — there are too many possible reactions to consider, and no guarantee of reaching the target. Starting at the target and asking 'what bond can I disconnect?' reduces complexity in a directed way at each step, generating simpler precursors guided by functional-group recognition. This transforms an open-ended search into a manageable decision tree.
Corey developed retrosynthetic analysis because forward planning fails to scale to complex targets. Each disconnection is guided by recognizing patterns (beta-hydroxy carbonyls signal aldol, 1,5-dicarbonyls signal Michael, etc.), dramatically limiting the search space. Working forward from reagents offers no such systematic guidance.