AS Chemistry: Organic Synthesis Routes — Revision Notes

Condensed for the final weeks. For the full explanation, use the Organic Synthesis: Planning Multi-Step Routes study guide.

The AS reaction map

                    ALKANE
                      | free-radical substitution (UV, X2)
                      v
              HALOGENOALKANE  <--- HX ---  ALKENE
                 |     |    \                ^  |
     aq NaOH     |     |     \ KCN(ethanol)  |  | H2/Ni
     reflux      |     |      \              |  | (reverse: conc H2SO4, 170C)
                 v     |       v             |  v
             ALCOHOL   |    NITRILE          ALKANE
              |   |    |
   [O] distil |   | [O] reflux
              v   v
        ALDEHYDE -> CARBOXYLIC ACID
              |            |
     NaBH4    |            | + alcohol, conc H2SO4
              v            v
           ALCOHOL       ESTER

Learn the reagent and conditions for each arrow — that is what the marks are for.

The conditions that decide the product

Starting material	Reagent	Conditions	Product
Halogenoalkane	NaOH	Aqueous, reflux	Alcohol
Halogenoalkane	NaOH	Ethanolic, reflux	Alkene
Halogenoalkane	KCN	Ethanolic, reflux	Nitrile (+1 C)
Primary alcohol	K₂Cr₂O₇/H₂SO₄	Distil	Aldehyde
Primary alcohol	K₂Cr₂O₇/H₂SO₄	Reflux	Carboxylic acid
Alcohol	conc H₂SO₄	170 °C	Alkene
Alkene	Steam, H₃PO₄	300 °C, 60 atm	Alcohol

Three pairs turn on a single word — aqueous vs ethanolic, distil vs reflux, and the direction of alcohol ⇄ alkene. Getting those right is most of the topic.

Changing the carbon chain length

Change	Route
+1 carbon	Halogenoalkane + KCN → nitrile (then hydrolyse to acid)
+1 carbon	Carbonyl + HCN → hydroxynitrile
No change	All other AS reactions

If a question needs one more carbon, a nitrile step is compulsory — spotting that immediately usually reveals the whole route.

Using reactions as identification tests

Every characteristic reaction already learned doubles as an identification test — no separate list needs memorising:

Observation	Functional group indicated
Decolourises aqueous bromine	C=C (alkene)
Effervesces with a carbonate	–COOH (carboxylic acid)
Orange precipitate with 2,4-DNPH	C=O (aldehyde or ketone)
Silver mirror with Tollens’ reagent	–CHO (aldehyde, specifically)
Yellow precipitate with alkaline I₂(aq)	CH₃CO– or CH₃CH(OH)–
Orange → green with acidified K₂Cr₂O₇	–OH on a primary/secondary carbon
AgNO₃(aq)/ethanol precipitate: white = Cl, cream = Br, yellow = I	halogenoalkane (C–X); colour identifies which halogen

Given an unknown molecule’s reactions with several reagents, work through this table in reverse: each positive or negative result rules functional groups in or out, narrowing down what the molecule must contain.

Analysing a given route — spotting by-products

The reverse skill to planning a route is being given one and asked to identify what’s happening, and what by-products might form, at each step:

Ethane --Br2/UV--> bromoethane   FREE-RADICAL SUBSTITUTION
        by-product: further substitution -> some dibromoethane

Bromoethane --NaOH(aq), heat--> ethanol   NUCLEOPHILIC SUBSTITUTION
        (if ETHANOLIC NaOH used instead: ELIMINATION -> ethene)

Checking whether a given step used aqueous or ethanolic conditions is always worth doing explicitly, since it changes both the product and the type of by-product a route might generate.

Planning a multi-step route

1. Identify the functional group in the starting material and the target.
2. Compare carbon counts — different means a nitrile step.
3. Work backwards from the target: what makes this group?
4. Chain the steps, giving reagent and conditions for each.
5. Check no step destroys a group you need later.

Exam traps

• Omitting conditions — “NaOH” alone does not say whether you get an alcohol or an alkene.
• Forgetting that KCN adds a carbon.
• Writing “oxidise” instead of naming the reagent and stating distil or reflux.
• Proposing a route that changes carbon count without a nitrile step.
• Using reflux when the aldehyde is the target.
• Using 2,4-DNPH alone to claim a molecule is specifically an aldehyde — it only confirms a carbonyl (aldehyde or ketone) is present; Tollens’ is needed to identify an aldehyde specifically.
• Forgetting that free-radical substitution is poorly selective, so proposing a route via this step without acknowledging further-substitution by-products.
• Proposing a plausible-sounding but non-syllabus step (“it gets converted directly”) instead of a real, named reagent and condition.

Self-test

1. Convert 1-bromopropane to propan-1-ol. Reagent and conditions?
2. Convert propan-1-ol to propanal, and say how you avoid the acid.
3. How would you turn 1-bromoethane into propanoic acid?
4. Which two AS reactions lengthen the carbon chain?
5. What single word distinguishes substitution from elimination with hydroxide?
6. A molecule decolourises bromine water and effervesces with sodium carbonate. What can you deduce about its functional groups?
7. What by-product might form alongside 1-bromoethane in the free-radical substitution of ethane with Br₂/UV?

Answers: 1. Aqueous sodium hydroxide, heat under reflux. 2. Acidified potassium dichromate(VI), and distil the aldehyde off as it forms so it cannot be oxidised further. 3. Ethanolic KCN under reflux to give propanenitrile (adding one carbon), then acid hydrolysis to propanoic acid. 4. Halogenoalkane + KCN, and carbonyl + HCN. 5. Aqueous (substitution → alcohol) versus ethanolic (elimination → alkene). 6. It contains both a C=C double bond (from decolourising bromine water) and a –COOH group (from effervescing with a carbonate) — the two pieces of evidence are independent and can be combined. 7. Further substitution — some 1,1-dibromoethane (or another dibromoethane isomer) forming as the reaction continues on the monobromo product.

For the full worked examples of planning and analysing multi-step routes, see the Organic Synthesis: Planning Multi-Step Routes study guide; for further exam-style practice, see the Organic Synthesis Routes practice questions.