Construct reaction pathways linking the functional groups studied in Module 7 and apply retrosynthesis logic to plan multi-step syntheses, including reagents and conditions for each step

What this dot point is asking

NESA wants you to map all the functional-group conversions in Module 7 onto a single tree (alkane to alkene to alcohol to aldehyde to acid to ester, plus side branches to haloalkanes and amines), then plan a synthesis from a given starting material to a given target. The skill being tested is retrosynthesis: working backwards from the target, asking "what could have made this in one step?", and repeating until you reach the starting material.

The answer

The master synthesis tree (Module 7)

Going forward (oxidation direction or chain growth):

alkane <-> alkene -> haloalkane -> alcohol
                  -> alcohol (hydration)
alcohol (primary) -> aldehyde -> carboxylic acid
alcohol (secondary) -> ketone
carboxylic acid + alcohol -> ester (+ H2O)
carboxylic acid + amine -> amide (+ H2O)
haloalkane + NH3 -> amine

Each arrow has a specific reagent and condition.

Forward conversions (reagent reference)

Retrosynthesis: thinking backwards

To plan a synthesis, work backwards from the target. At each step, ask:

1. What functional group is on the target?
2. What is the most common one-step reaction that produces this functional group?
3. What is the precursor (the synthon) for that step?
4. Is that precursor accessible from the given starting material? If not, recurse.

Worked example: butan-2-ol from but-1-ene.

• Target: butan-2-ol ($CH_3CH_2CH(OH)CH_3$). Secondary alcohol.
• Backwards: a secondary alcohol comes from Markovnikov hydration of an alkene (water on the more substituted carbon). The alkene precursor with the C-OH at C2 is but-1-ene.
• Forward synthesis (one step): $CH_2=CHCH_2CH_3 + H_2O \xrightarrow{H_2SO_4} CH_3CH(OH)CH_2CH_3$.

Worked example: ethyl ethanoate from ethene.

• Target: ethyl ethanoate (ester). An ester comes from acid + alcohol with $H_2SO_4$.
• Precursors: ethanoic acid and ethanol.
• Ethanol comes from ethene by hydration.
• Ethanoic acid comes from ethanol by oxidation (reflux with $K_2Cr_2O_7$).
• Full forward synthesis: ethene to ethanol (split into two portions); oxidise half to ethanoic acid; esterify with the other half. Three forward steps.

Worked example: methyl propanoate from propan-1-ol.

• Target: methyl propanoate. From propanoic acid + methanol.
• Propanoic acid: oxidise propan-1-ol with excess acidified dichromate under reflux. We have propan-1-ol.
• Methanol: not given, must come from elsewhere (methane + Cl₂ to chloromethane, then NaOH; or accept the question wording that allows methanol as a reagent).
• Forward: oxidise propan-1-ol to propanoic acid, esterify with methanol and conc $H_2SO_4$.

Strategies for planning

Count the carbons. The target and starting material must have compatible carbon skeletons. HSC does not include carbon-skeleton-changing reactions (no $C-C$ bond formation), so the carbon count is preserved through every step.

Check the oxidation level. Alkane and alkene are at the same level for the carbon involved. Alcohol is one step up. Aldehyde and ketone are another step. Carboxylic acid is one more. Esters and amides are at the same level as carboxylic acids.

Identify the limiting step. Markovnikov hydration always gives the more substituted alcohol. To get the less substituted alcohol, use a haloalkane route (HBr addition followed by hydrolysis) or accept the Markovnikov product and work from there.

Use reflux when stated. Esterification, oxidation to acid, hydrolysis, and base hydrolysis all require reflux. Distillation only is for collecting an aldehyde or separating an ester after the reaction.

Common synthesis sequences

Common traps

Forgetting conditions. "Oxidise to a carboxylic acid" without saying "reflux with excess acidified dichromate" loses marks.

Markovnikov going the wrong way. Hydration of propene gives propan-2-ol (Markovnikov), not propan-1-ol. State the regiochemistry explicitly.

Trying to build $C-C$ bonds. HSC does not include Grignards or coupling reactions. Carbon count must match.

Stopping a primary alcohol at the aldehyde under reflux. Under reflux with excess oxidant, you get the acid. To isolate the aldehyde, distil it off as it forms.

Writing esterification with dilute $H_2SO_4$. It must be concentrated, both for catalysis and for water removal.

In one sentence

Plan an organic synthesis by working backwards from the target functional group through the master tree (alkene to alcohol to aldehyde/ketone to acid to ester or amide), choosing the standard reagent for each one-step conversion (e.g. acidified dichromate to oxidise, conc $H_2SO_4$ to esterify or dehydrate, dilute $H_2SO_4$ to hydrate), and checking that the carbon skeleton matches because HSC scope does not allow $C-C$ bond changes.