IMAT Chemistry · Organic Chemistry

Organic Reactions: Types, Oxidation, Esters and Combustion

Most organic reactions are one of a few types (substitution, addition, elimination, condensation, hydrolysis, oxidation), and the type can be read from what goes in and what comes out.

All Organic Chemistry notes

Why this matters

The Cambridge papers asked to classify the steps of a reaction scheme, to name the products of oxidising an alcohol or reacting an acid, and in 2022 about making biodiesel from a fat. The ministry papers since 2023 have not asked a reaction question yet, so expect the simpler named facts if one appears.

Concept 1 of 4: The main types of organic reaction

Compare the reactants with the products. If one atom has swapped for another, it is substitution. If two molecules became one and a double bond disappeared, it is addition. If a double bond appeared and a small molecule left, it is elimination. If two molecules joined and water came out, it is condensation.

Definition

Alkanes undergo substitution with halogens in UV light. Haloalkanes undergo substitution with nucleophiles such as $\mathrm{OH^-}$ or $\mathrm{CN^-}$ .
Alkenes undergo addition: $\mathrm{H_2}$ (nickel catalyst) gives an alkane; $\mathrm{Br_2}$ gives a dibromoalkane and decolourises orange bromine water (the test for C=C); HBr gives a bromoalkane; steam (acid catalyst) gives an alcohol. With HBr on an unsymmetrical alkene, the H goes mainly to the carbon that already has more H (Markovnikov's rule).
Elimination makes a C=C: an alcohol with hot concentrated $\mathrm{H_2SO_4}$ loses water (dehydration); a haloalkane with hot KOH in ethanol loses HBr. The same haloalkane with aqueous NaOH gives substitution instead.
Oxidation of carbon: gaining O, losing H, or gaining a halogen (the oxidation number of carbon rises). Reduction is the reverse: adding $\mathrm{H_2}$ to a C=C, or turning a nitrile into an amine.
Many alkene molecules can add to each other: addition polymerisation, e.g. ethene to poly(ethene).

Type	What happens	Example	How to spot it
Substitution	One atom or group replaces another	$\mathrm{CH_3CH_2Br + OH^- \rightarrow CH_3CH_2OH + Br^-}$	Two reactants, two products; no change in multiple bonds
Addition	Two molecules join across a C=C or C≡C	$\mathrm{CH_2{=}CH_2 + Br_2 \rightarrow CH_2BrCH_2Br}$	One product; a multiple bond is used up
Elimination	A small molecule leaves neighbouring carbons, making a C=C	$\mathrm{CH_3CH_2OH \rightarrow CH_2{=}CH_2 + H_2O}$	A new C=C appears
Condensation	Two molecules join and release a small molecule, usually water	Acid + alcohol $\rightarrow$ ester + water	Water among the products
Hydrolysis	Water splits a molecule; the reverse of condensation	Ester + water $\rightarrow$ acid + alcohol	Water among the reactants
Oxidation	Carbon gains O or loses H	Ethanol $\rightarrow$ ethanal $\rightarrow$ ethanoic acid	More O or fewer H in the product

Practice this conceptself-check · 4 quick reps

Adding bromine to an alkene is also an oxidation of carbon

In ethene each carbon has oxidation number −2; in 1,2-dibromoethane each has −1. The carbons have been oxidised, even though no oxygen is involved. Adding

\mathrm{H_2}

to make ethane is the opposite: a reduction.

Condensation joins molecules; elimination splits one

Both release water. In condensation two molecules join into a bigger one (acid + alcohol to ester). In elimination a single molecule loses water and gains a C=C (ethanol to ethene).

Concept 2 of 4: Oxidation of primary, secondary and tertiary alcohols

To oxidise an alcohol, the oxidising agent removes the H from the OH and an H from the carbon carrying it, making a C=O. So the carbon carrying the OH must have at least one H. A primary alcohol has two such H and can be oxidised twice; a tertiary alcohol has none.

Definition

Classify an alcohol by how many carbons are bonded to the carbon carrying the OH:

Primary (one carbon, or none for methanol): oxidised to an aldehyde, then to a carboxylic acid. Distil the aldehyde off as it forms to stop at the aldehyde; heat under reflux to go all the way to the acid.
Secondary (two carbons): oxidised to a ketone, and no further.
Tertiary (three carbons, no H on that carbon): not oxidised under normal conditions.
The usual oxidising agent is acidified potassium dichromate, $\mathrm{K_2Cr_2O_7/H^+}$ ; it turns from orange to green when it reacts.
Reduction reverses these steps: aldehydes give primary alcohols, ketones give secondary alcohols.

Alcohol class	Example	Product of oxidation	Dichromate colour
Primary	Propan-1-ol $\mathrm{CH_3CH_2CH_2OH}$	Propanal, then propanoic acid	Orange to green
Secondary	Propan-2-ol $\mathrm{CH_3CH(OH)CH_3}$	Propanone (a ketone) only	Orange to green
Tertiary	2-methylpropan-2-ol $\mathrm{(CH_3)_3COH}$	No reaction	Stays orange

Practice this conceptself-check · 3 quick reps

A primary alcohol never gives a ketone

The carbon carrying the OH in a primary alcohol is at the end of the chain, so its C=O can only be an aldehyde, then an acid. Ketones come only from secondary alcohols, and ketones are not oxidised further.

Concept 3 of 4: Esterification, hydrolysis and amide formation

An acid and an alcohol can join by losing a water molecule between them: the OH comes from the acid and the H from the alcohol. Adding water back splits the ester again. The same idea joins an acid to an amine to give an amide, and amino acids to make proteins.

Definition

Esterification: carboxylic acid + alcohol $\rightleftharpoons$ ester + water. A condensation reaction, catalysed by concentrated $\mathrm{H_2SO_4}$ , and reversible.
Acid hydrolysis of an ester (water, acid catalyst) is the reverse and is also reversible.
Base hydrolysis (NaOH) goes to completion and gives the carboxylate salt and the alcohol. Base hydrolysis of fats gives glycerol and soaps (saponification).
Transesterification: an ester reacts with another alcohol and swaps its alcohol part. Biodiesel is made this way: a triglyceride + 3 methanol $\rightarrow$ 3 methyl esters + glycerol, usually with a base catalyst (an enzyme can be used but is not required).
Amides: an acid (or a more reactive acid derivative) with ammonia or an amine gives an amide; heating the ammonium salt of an acid gives the amide and water. Amides are hydrolysed back to an acid and an amine (or their salts) by hot acid or base.
Amines are bases: they react with acids to form salts.

Esterification

\mathrm{RCOOH + R'OH \rightleftharpoons RCOOR' + H_2O}

$\mathrm{RCOOH}$ the carboxylic acid; gives the -oate half of the name
$\mathrm{R'OH}$ the alcohol; gives the alkyl half of the name

Worked example

Propanoic acid reacts with methanol in the presence of a little concentrated sulfuric acid. Name the ester, give its formula, and check that mass is conserved.

Practice this conceptself-check · 4 quick reps

Base hydrolysis gives a salt, not the free acid

Boiling an ester with NaOH gives the sodium carboxylate and the alcohol, and the reaction goes to completion. The free carboxylic acid appears only after acidifying, or in acid hydrolysis, which stays reversible.

Concept 4 of 4: Combustion of organic compounds

Burning in plenty of oxygen turns every carbon into

\mathrm{CO_2}

and every pair of hydrogens into

\mathrm{H_2O}

. Balance carbon first, then hydrogen, and finally count the oxygen atoms needed on the right. Any oxygen already in the fuel reduces what must come from the air.

Definition

Complete combustion (plenty of $\mathrm{O_2}$ ): products $\mathrm{CO_2}$ and $\mathrm{H_2O}$ only.
Incomplete combustion (limited $\mathrm{O_2}$ ): also carbon monoxide CO (toxic) and carbon (soot).
Balance in the order C, H, O. A half in front of $\mathrm{O_2}$ is allowed, or double every coefficient.
Combustion is exothermic and is an oxidation.

Complete combustion of a fuel containing C, H and O

\mathrm{C_xH_yO_z} + \left(x + \frac{y}{4} - \frac{z}{2}\right)\mathrm{O_2} \rightarrow x\,\mathrm{CO_2} + \frac{y}{2}\,\mathrm{H_2O}

$x, y, z$ numbers of C, H and O atoms in the fuel ( $z = 0$ for a hydrocarbon)

Worked example

Write the equation for the complete combustion of propane, and find the mass of

\mathrm{CO_2}

made by burning 11 g of propane (C = 12, H = 1, O = 16).

Practice this conceptself-check · 4 quick reps

Subtract the oxygen the fuel already contains

For alcohols, aldehydes and acids, part of the oxygen in the products comes from the fuel itself. Use

x + y/4 - z/2

; leaving out the

z/2

gives the answer for the hydrocarbon with the same C and H.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Esterification, hydrolysis and amide formation
Esterification
$\mathrm{RCOOH + R'OH \rightleftharpoons RCOOR' + H_2O}$
Combustion of organic compounds
Complete combustion of a fuel containing C, H and O
$\mathrm{C_xH_yO_z} + \left(x + \frac{y}{4} - \frac{z}{2}\right)\mathrm{O_2} \rightarrow x\,\mathrm{CO_2} + \frac{y}{2}\,\mathrm{H_2O}$

Reference tables (2)

The main types of organic reaction6 rows

Type	What happens	Example	How to spot it
Substitution	One atom or group replaces another	$\mathrm{CH_3CH_2Br + OH^- \rightarrow CH_3CH_2OH + Br^-}$	Two reactants, two products; no change in multiple bonds
Addition	Two molecules join across a C=C or C≡C	$\mathrm{CH_2{=}CH_2 + Br_2 \rightarrow CH_2BrCH_2Br}$	One product; a multiple bond is used up
Elimination	A small molecule leaves neighbouring carbons, making a C=C	$\mathrm{CH_3CH_2OH \rightarrow CH_2{=}CH_2 + H_2O}$	A new C=C appears
Condensation	Two molecules join and release a small molecule, usually water	Acid + alcohol $\rightarrow$ ester + water	Water among the products
Hydrolysis	Water splits a molecule; the reverse of condensation	Ester + water $\rightarrow$ acid + alcohol	Water among the reactants
Oxidation	Carbon gains O or loses H	Ethanol $\rightarrow$ ethanal $\rightarrow$ ethanoic acid	More O or fewer H in the product

Oxidation of primary, secondary and tertiary alcohols3 rows

Alcohol class	Example	Product of oxidation	Dichromate colour
Primary	Propan-1-ol $\mathrm{CH_3CH_2CH_2OH}$	Propanal, then propanoic acid	Orange to green
Secondary	Propan-2-ol $\mathrm{CH_3CH(OH)CH_3}$	Propanone (a ketone) only	Orange to green
Tertiary	2-methylpropan-2-ol $\mathrm{(CH_3)_3COH}$	No reaction	Stays orange

Watch out for (5)

Adding bromine to an alkene is also an oxidation of carbon
→ The main types of organic reaction
Condensation joins molecules; elimination splits one
→ The main types of organic reaction
A primary alcohol never gives a ketone
→ Oxidation of primary, secondary and tertiary alcohols
Base hydrolysis gives a salt, not the free acid
→ Esterification, hydrolysis and amide formation
Subtract the oxygen the fuel already contains
→ Combustion of organic compounds

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