Grade by Grade
B. Organic Chemistry · Organic Chemistry

Organic Reaction Types and Multi-Step Pathways

B3.3 B2.4 B3.1 A1.10 A1.11About 70 minutes
Learning goals
  • I can classify an organic reaction as addition, substitution, elimination, oxidation, reduction, condensation, hydrolysis or combustion.
  • I can predict the products of each type of reaction for the families in this unit.
  • I can plan a two- or three-step pathway to make a target compound from a given starting material.
  • I can calculate atom economy and use it to compare reactions.

Words to know

reaction pathway (synthesis)
A sequence of reactions that converts a starting material into a desired product.
retrosynthesis
Planning a synthesis by working backwards from the target molecule to simpler starting materials.
reduction (organic)
Gain of H or loss of O by a carbon compound, such as hydrogenating an alkene or a ketone.
atom economy
The percentage of the total mass of reactants that ends up in the desired product: (mass of desired product ÷ total mass of reactants) × 100%.
by-product
A product formed along with the desired product, such as HCl in a substitution or water in a condensation.

A toolbox of reactions

Over the last eight lessons you have collected a set of reactions. A synthetic chemist, the kind of scientist who makes new medicines, dyes and materials, thinks of these reactions as tools: each one changes one functional group into another. Chaining them together lets you build almost any molecule from simple starting materials, such as ethene and propene from petroleum or ethanol from fermentation.

This lesson organizes the toolbox, gives you a strategy for recognizing each reaction type, and shows how to plan multi-step pathways.

additionA + B → Cone product; a C=C or C≡C isused upsubstitutionA–X + Y → A–Y + Xan atom or group is swappedeliminationC → D + small moleculea C=C forms; H₂O or HX leavescondensationA + B → C + H₂Otwo molecules join, waterleaveshydrolysisC + H₂O → A + Bwater splits a molecule(reverse of condensation)oxidationgains O or loses Halcohol → aldehyde/ketone →acid
Six reaction types and how to recognize them from the reactants and products. Combustion (fuel + O₂ → CO₂ + H₂O) and reduction (gain of H or loss of O) complete the set.

Recognizing reaction types

Ask three questions about any organic reaction:

  1. How many reactants and products? Two → one is addition. One → two (with H₂O or HX) is elimination. Two → two, with a swap, is substitution.
  2. Was water involved? Two molecules joining and releasing water is condensation (ether, ester and amide formation). A molecule reacting with water and splitting in two is hydrolysis.
  3. Did the carbon gain O or lose H? That is oxidation (alcohol → aldehyde or ketone → acid). Gaining H or losing O is reduction (alkene + H₂ → alkane; ketone + H₂ → alcohol).

Combustion is a complete oxidation: everything ends as CO₂ and H₂O.

Notice the reverse pairs: addition ⇄ elimination (hydration and dehydration), condensation ⇄ hydrolysis (esterification and ester hydrolysis), oxidation ⇄ reduction.

alkanecarboxylic acidesteralkenealdehydeamidehaloalkanealcoholketoneether+H₂X₂, UV+HX−HX+H₂O−H₂O1° [O]2° [O][O]+alcohol+amine+alcohol[O] = oxidizing agentX = Cl or Br1°, 2° = primary,secondary alcohol
A map of the main reactions in this unit. Each arrow is a tool that converts one family into another. Follow arrows to plan a pathway, for example alkene → alcohol → aldehyde → carboxylic acid.
Worked example

Example 1: Classifying reactions

Classify each reaction: (a) CH₃CH₂Br + KOH → CH₂=CH₂ + KBr + H₂O; (b) CH₃CH₂CHO + [O] → CH₃CH₂COOH; (c) HCOOCH₃ + H₂O → HCOOH + CH₃OH; (d) CH₃CH=CH₂ + Cl₂ → CH₃CHClCH₂Cl; (e) CH₃COOH + NH₃ → CH₃CONH₂ + H₂O.

  1. (a) A C=C forms and HBr is removed (as KBr + H₂O): elimination.
  2. (b) The aldehyde gains an O: oxidation.
  3. (c) Water splits the ester into an acid and an alcohol: hydrolysis.
  4. (d) Two reactants, one product, C=C used up: addition.
  5. (e) Two molecules join and release water, forming an amide: condensation.

(a) elimination, (b) oxidation, (c) hydrolysis, (d) addition, (e) condensation.

Planning a pathway: think backwards

To make a target compound, start from the target and ask: "What family is this, and which reaction makes that family?" Keep stepping back until you reach the starting material you have. Then write the steps forward, with reagents and conditions. This backwards method is called retrosynthesis.

Useful questions:

  • Is the carbon skeleton the same? None of the reactions in this unit makes new C–C bonds, except polymerization. If the target has the same number of carbons as the starting material, you only need to change functional groups.
  • Where must the functional group go? Markovnikov's rule (additions) and the alcohol classes (oxidation) control the position.
  • Do you need two pieces? Esters, amides and ethers are made from two molecules, so you may need to make each piece first.
Worked example

Example 2: A two-step pathway

Starting from propene, plan a synthesis of propanone.

  1. Target: propanone, CH₃COCH₃, a ketone. Ketones come from oxidizing a secondary alcohol: propan-2-ol.
  2. Propan-2-ol comes from hydration of propene: Markovnikov's rule puts the –OH on C2. ✓
  3. Forward: Step 1: CH₂=CHCH₃ + H₂O —(H₂SO₄)→ CH₃CH(OH)CH₃ (addition).
  4. Step 2: CH₃CH(OH)CH₃ + [O] —(acidified K₂Cr₂O₇)→ CH₃COCH₃ + H₂O (oxidation).

propene → (hydration) → propan-2-ol → (oxidation) → propanone.

Worked example

Example 3: A pathway that needs two pieces

Using ethene as the only organic starting material, plan a synthesis of ethyl ethanoate.

  1. Target: an ester, from ethanoic acid + ethanol. Both have two carbons, so both can come from ethene.
  2. Ethanol: hydrate ethene: CH₂=CH₂ + H₂O —(H₂SO₄)→ CH₃CH₂OH.
  3. Ethanoic acid: oxidize some of that ethanol fully: CH₃CH₂OH + 2 [O] —(excess K₂Cr₂O₇, H⁺, reflux)→ CH₃COOH + H₂O.
  4. Esterify: CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O (conc. H₂SO₄, heat).

ethene → ethanol; part of the ethanol → ethanoic acid; then acid + ethanol → ethyl ethanoate (three reactions: hydration, oxidation and esterification).

Worked example

Example 4: Moving a functional group

Plan a synthesis of 2-bromopropane from 1-bromopropane.

  1. The skeleton is the same; the Br must move from C1 to C2.
  2. Remove HBr to make an alkene: CH₃CH₂CH₂Br + KOH (in ethanol, heat) → CH₃CH=CH₂ + KBr + H₂O (elimination).
  3. Add HBr back: by Markovnikov's rule, Br goes to C2: CH₃CH=CH₂ + HBr → CH₃CHBrCH₃ (addition).

1-bromopropane → (elimination) → propene → (addition of HBr) → 2-bromopropane.

Greener reactions: atom economy

Industrial chemists also ask, "How much of what I put in ends up in the product I want?" The atom economy of a reaction is:

atom economy = (molar mass of desired product ÷ total molar mass of all reactants) × 100%

(using the coefficients of the balanced equation).

  • Addition reactions have 100% atom economy: every atom ends up in the one product. Hydration of ethene to make ethanol wastes no atoms.
  • Substitution, elimination and condensation always make a by-product (HX or H₂O), so their atom economy is below 100%.

Atom economy is different from percentage yield (Grade 11): yield measures how much product you actually got compared with the theoretical amount; atom economy measures how much of the reactants could possibly end up in the product even with a perfect yield. Both matter for reducing waste, one of the goals of green chemistry.

Worked example

Example 5: Calculating atom economy

Compare the atom economy of two ways to make ethanol: (a) CH₂=CH₂ + H₂O → C₂H₅OH; (b) C₂H₅Br + NaOH → C₂H₅OH + NaBr. Molar masses (g/mol): C₂H₄ 28.06, H₂O 18.02, C₂H₅OH 46.08, C₂H₅Br 108.97, NaOH 40.00.

  1. (a) Reactants: 28.06 + 18.02 = 46.08 g. Product: 46.08 g. Atom economy = 46.08 ÷ 46.08 × 100% = 100%.
  2. (b) Reactants: 108.97 + 40.00 = 148.97 g. Product: 46.08 g. Atom economy = 46.08 ÷ 148.97 × 100% = 30.9%.
  3. In (b), most of the reactant mass ends up in the by-product NaBr.

(a) 100% (addition) vs (b) 30.9% (substitution). Hydration is the greener route.

Hands-on activity

Reaction-map card game

You need: 20 index cards (or paper cut into cards), two coloured pens, a coin, and a partner.

  1. Make the cards. On 10 cards, write one family each: alkane, alkene, haloalkane, alcohol (1°), alcohol (2°), aldehyde, ketone, carboxylic acid, ester, amide. On the other 10, write a reagent or condition: H₂/Pt; H₂O/H₂SO₄; HBr; Br₂; conc. H₂SO₄ + heat; KOH in ethanol; [O] (K₂Cr₂O₇/H⁺); alcohol + H₂SO₄; amine + heat; NaOH(aq) + heat.
  2. Play. Shuffle each pile. Draw a family card and a reagent card. If the reagent reacts with that family, name the reaction type and the product family to score 2 points. If it does not react, say "no reaction" and explain why for 1 point. Your partner checks with the reaction map.
  3. Challenge round: draw two family cards, a start and a target. Plan a pathway of up to three steps. A correct pathway scores 5 points.
  4. After the game, write the three pathways you found hardest, with full equations (A1.11).

What to look for: examples of correct calls: alkene + HBr → haloalkane (addition); 2° alcohol + [O] → ketone (oxidation); ketone + [O] → no reaction (no H on the carbonyl carbon); ester + NaOH → alcohol + carboxylate salt (hydrolysis); alkane + H₂/Pt → no reaction (already saturated).

Just for fun

Planning a synthesis backwards is called retrosynthesis. It is a lot like planning a trip to the cottage: you start from the dock and work back to your own driveway. E. J. Corey won the 1990 Nobel Prize in Chemistry partly for turning this way of thinking into a system.

Your turn

1

Classify: CH₃CH₂OH → CH₂=CH₂ + H₂O.

2

Classify: CH₃COOH + CH₃OH → CH₃COOCH₃ + H₂O.

3

Classify: CH₃CH=CH₂ + H₂ → CH₃CH₂CH₃. Is it also a reduction?

4

Predict the product: butan-2-ol + [O].

5

Predict the products: ethyl propanoate + NaOH(aq), heated.

6

Give a two-step pathway from ethene to ethanal.

7

What is the atom economy of CH₂=CH₂ + Br₂ → CH₂BrCH₂Br? Why?

More practice: warm-up

1

Which reaction type has two reactants and one product?

2

Which reaction type is the reverse of condensation?

3

What is the product family when a primary alcohol is fully oxidized?

4

What two compounds make an amide?

5

What does "reduction" mean for an organic compound?

More practice: on your own

1

Plan a pathway from propan-1-ol to propan-2-ol.

2

Plan a pathway from but-1-ene to butanone.

3

Which starting materials would you use to make N-methylpropanamide?

4

Calculate the atom economy for making ethene by dehydrating ethanol: C₂H₅OH → C₂H₄ + H₂O (46.08 → 28.06 + 18.02 g/mol).

5

A student wants to make 2-methylpropan-1-ol by hydrating 2-methylpropene. Explain why this will not work well.

Challenge

1

Using ethene as the only organic starting material, plan a two-step synthesis of ethane-1,2-diol (antifreeze). Hint: a haloalkane reacts with NaOH(aq) by substitution, swapping the halogen for –OH, as in Example 5(b).

2

Compare two routes to chloroethane: (A) CH₃CH₃ + Cl₂ —(UV)→ CH₃CH₂Cl + HCl; (B) CH₂=CH₂ + HCl → CH₃CH₂Cl. Use atom economy and product purity to decide which is better. (Molar masses: C₂H₆ 30.08, Cl₂ 70.90, C₂H₅Cl 64.52, C₂H₄ 28.06, HCl 36.46.)

3

An unknown ester, C₅H₁₀O₂, is hydrolysed. One product is oxidized by acidified dichromate to propanone. Identify the ester and the other hydrolysis product.

Check your understanding

Quiz

1.

What type of reaction is CH₃CH₂CH₂OH + [O] → CH₃CH₂CHO + H₂O?

2.

Which pair of reaction types are the reverse of each other?

3.

Which reagent converts an alkene into an alcohol?

4.

Which reaction type always has 100% atom economy?

5.

Which pathway makes butanone from but-2-ene?

6.

What are the products of the acid hydrolysis of methyl ethanoate?

For parents and teachers

Big ideas (B3.3, B2.4): organic reactions fall into a few types (addition, substitution, elimination, oxidation, reduction, condensation, hydrolysis, combustion). Chaining them gives pathways from simple starting materials to useful products. Atom economy measures how much of the reactants can end up in the product.

How to help: Play the card game with your teen. Ask them to explain why each "no reaction" card really does not react.

Common difficulties:

  • Confusing condensation (water released) with hydrolysis (water used).
  • Forgetting Markovnikov's rule when planning a hydration step.
  • Forgetting that esters and amides need two starting pieces.
  • Confusing atom economy with percentage yield.

When your teen plans a pathway, ask them to justify each step ("Why does the OH end up on that carbon?"). Drawing a conclusion and defending it with evidence is a key science skill (A1.10).