Elucidating Organic Molecules
- You are expected to be able to identify organic functional groups, their properties, how to test for their presence and how they are made
Functional groups
- The table below summarises the tests to identify the presence of certain functional groups and the reactions to make them
Functional groups, their reactions & identifying tests table
| Functional Group | Test | Production |
| Alkane |
Hydrogenation of alkenes using H2, Pt/Ni catalyst and heat Cracking of crude oil using heat and Al2O3 catalyst |
|
| Alkene | Decolourises bromine water |
Elimination of halogenoalkanes by heating it with NaOH(ethanol) Dehydration of alcohols using hot Al2O3 catalyst Cracking of crude oil using heat and Al2O3 catalyst |
|
Halogenoalkane (primary, secondary and tertiary) |
Form silver halide precipitate with dilute nitric acid, silver nitrate and dilute ammonia solution (AgCI = white, AgBr = cream and AgI = yellow) |
Fee-radical substitution of alkanes using UV and a halogen Electrophilic addition of hydrogen halides to alkenes Nucleophilic substitution of an alcohol, e.g. by reaction with HX (g) or with KCl and concentrated H2SO4 or concentrated H3PO4 or with PCl3 and heat or with PCl5 or with SOCl2 |
|
Alcohol (primary, secondary and tertiary) |
React with carboxylic acid and sulfuric acid to make esters which have fruity smells Primary alcohols get oxidised to aldehydes (give positive test with Fehling's and Tollens' solution and carboxylic acids Secondary alcohols are oxidised to ketones (positive test with 2,4-DNPH but not Fehling's and Tollens' solution) Tertiary alcohols can not be oxidised |
Electrophilic addition of alkenes using hot steam, concentrated phosphoric(VI) acid as a catalyst Oxidation of alkenes using cold, dilute KMnO4 to form a diol Nucleophilic substitution of halogenoalkanes using heat and NaOH (aq) Reduction of aldehydes and ketones using NaBH4 and LiAlH4 (aldehydes are reduced to primary alcohols and ketones are reduced to secondary alcohols) Reduction of carboxylic acid using LiAlH4 Hydrolysis of esters using dilute acid or dilute alkali |
| Aldehyde |
Silver mirror in Tollens' reagent and red precipitate in Fehling's solution Orange precipitate with 2,4-DNPH |
Oxidation of primary alcohols by reagent and red slowly adding warm alcohol to acidified K2Cr2O7 or KMnO4 and distillation |
| Ketone | Orange precipitate with 2,4-DNPH | Oxidation of secondary alcohols by reagent and red slowly adding warm alcohol to acidified K2Cr2O7 or KMnO4 and distillation |
| Carboxylic Acid | React with carbonates to form CO2 gas which will turn limewater cloudy |
Oxidation of aldehydes and ketone acidified K2Cr2O7 or KMnO4 and refluxing Hydrolysis of nitriles with dilute acid or dilute alkali followed by acidification Hydrolysis of ester with dilute acid or dilute alkali followed by acidification |
| Ester | Have sweet, fruity smells | Condensation reaction of alcohols and carboxylic acids with hot, concentrated H2SO4 as catalyst |
| Amine | Turns universal indicator blue / purple | Nucleophilic substitution of halogenoalkanes when heated under pressure with NH3 (ethanol) |
| Nitrile | Nucleophilic substitution of halogenoalkanes when heated under pressure with KCN (ethanol) and heat |
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Types of reactions
- You should also be aware of the different type of reactions that functional groups can undergo
Reactions of functional groups table
| Type of reaction | Definition | Reagents | Products |
| Hydrogenation | The addition of alkenes with hydrogen | H2, Pt / Ni catalyst | Alkanes |
| Cracking | The process in which large, less useful hydrocarbon molecules are broken down into smaller, more useful molecules in an oil refinery | Al2O3 catalyst and heat | Alkanes and alkenes |
| Free-radical substitution | The reaction in which halogen atoms substitute for hydrogen atoms in alkanes, The mechanism involves steps in which reactive free radicals are produced (initiation), regenerated (propagation) and consumed (termination) | Halogen and UV light | Halogenoalkane |
| Electrophilic Addition | The mechanism of the reaction in which an electrophile attacks the C=C bond and addition across the double bond occurs | Electrophile (eg. Br2 , NaOH, KCN, NH3 and HBr) | Halogenoalkane |
| Nucleophilic Addition | The mechanism of the reaction in which a nucleophile attacks the carbon atom in a carbonyl group and addition across the C=O bond occurs | Nucleophile (e.g. HCN) | Halogenoalkane, alcohol, nitrile or amine |
| Electrophilic Substitution | The replacement of an atom by another atom or group of atoms after initial attack by an electron—deficient species | Electrophile (eg. Br2, NaOH, KCN, NH3 and HBr | |
| Nucleophilic Substitution | The mechanism of the organic reaction in which a nucleophile attacks a carbon atom carrying a potential positive charge. This results in the replacement of an atom carrying a partial negative charge by the nucleophile | Nucleophile (eg. HCN) | |
| Oxidation | The loss of electrons or gain of oxygen of an atom, ion or molecule | Oxidising agent such as acidified K2Cr2O7 or KMnO4 | Alcohol, aldehyde, ketone or carboxylic acid |
| Reduction | The gain of electrons or loss of oxygen of an atom, ion or molecules | Reducing agent such as NaBH4 or LiAlH4 | Alkene, aldehyde, primary and secondary alcohol, carboxylic acid |
| Hydrolysis | The breakdown of a compound by water or by dilute acids or alkali | Water or dilute acid or alkali | |
| Condensation | A reaction in which two organic molecules join together an din the process eliminate a small molecule such as water or hydrogen chloride | Two molecules that can react with each other | Small molecule and a larger molecule |
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Oxidising & reducing agents
- Certain functional groups only react with specific oxidising and reducing agents which you should be aware of
Oxidising & reducing agents table
| Oxidising Agent | |||
| Oxidises | Oxidation Product | Colour Change | |
| Acidified potassium dichromate (K2Cr2O7 / H2SO4) | Primary alcohols | Aldehydes then carboxylic acids | Orange to green |
| Secondary alcohols | Ketones | Orange to green | |
| Tertiary alcohols | X | ||
| Aldehydes | Carboxylic acids | Orange to green | |
| Ketones | X | ||
| Acidified potassium permanganate (K2Mn2O4 / H2SO4) | Primary alcohols | Aldehydes then carboxylic acids | Purple to colourless |
| Secondary alcohols | Ketones | Purple to colourless | |
| Tertiary alcohols | X | ||
| Aldehydes | Carboxylic acids | Purple to colourless | |
| Ketones | X | ||
| Alkenes | Diol | Purple to colourless | |
| Reducing Agent | ||
| Reduces | Reduction Product | |
| Sodium borohydride (NaBH4) | Carboxylic acids | X |
| Aldehyde | Primary alcohol | |
| Ketone | Secondary alcohol | |
| Alkene | X | |
| Lithium aluminium hydride (LiAlH4) | Carboxylic acids | Aldehyde then primary alcohol |
| Aldehyde | Primary alcohol | |
| Ketone | Secondary alcohol | |
| Alkene | X | |
| Hydrogen and Pt / Ni Catalyst (H2, Pt / Ni catalyst) | Carboxylic acids | X |
| Aldehyde | X | |
| Ketone | X | |
| Alkene | Alkane | |
Tests
- The test also requires you to distinguish between the different tests that identify functional groups in a compound
Tests identifying functional groups in a compound table
| Functional Group Tested | Colour Change | |
| Fehling's Solution | Aldehydes | Clear blue solution turns opaque red/orange as a precipitate is formed |
| Tollens' Reagent | Aldehydes | Silver mirror |
| Bromine water | Alkenes | Bromine water changes from orange/yellow |
| 2,4—dinitrophenylhydrazine (2,4—DNPH) | Carbonyl compounds (aldehydes and ketones) | Orange precipitate |
| lodoform | Methyl ketone (CH3CO—) group | Yellow precipitate of triiodomethane |
| Silver nitrate and ammonia | Halogens | AgCl = white AgBr = cream & Agl = yellow |
Worked example
Identify which tests give positive results with the following compounds and which oxidising and reducing agents the compounds react with.
Answers:
