Summary of Key Organic Chemistry Reactions

Question 1

Aldehydes with HCN/KCN

Answer 1

Reagents: HCN/KCN
Conditions: Carefully buffered at pH 9, room temperature, in a fume cupboard
Hydroxynitrile produced

Question 2

Reduction of aldehydes

Answer 2

Reagents: LiAlH4 and dry ether (at 0°C) OR H2 and a platinum catalyst
Primary alcohol produced

Question 3

Aldehydes with 2,4-DNPH

Answer 3

Orange precipitate of 2,4-dinitrophenylhydrazone produced

Question 4

Aldehydes with K2Cr2O7^2- (potassium dichromate) and H^+ ions

Answer 4

Carboxylic acid produced, colour change from orange to green seen

Question 5

Aldehydes with Fehling’s solution

Answer 5

Aldehyde is oxidised

Red precipitate of copper (I) oxide produced when aldehyde is warmed with Fehling’s solution

Question 6

Aldehydes with Tollens’ reagent (ammoniacal silver nitrate)

Answer 6

Aldehyde is oxidised
Add a few drops of sodium hydroxide to silver nitrate solution then dissolving the precipitate in dilute ammonia
The silver is reduced to silver metal on warming with the aldehyde
Silver mirror seen on inside of test tube

Question 7

Aldehydes with iodine and NaOH

Answer 7

Iodoform reaction
CHI3 and salt of a carboxylic acid produced
ONLY works with ethanal

Question 8

Ketones with HCN/KCN at pH=9

Answer 8

Reagents: HCN/KCN
Conditions: Carefully buffered at pH 9, room temperature, in a fume cupboard
Hydroxynitrile produced

Question 9

Reduction of ketones

Answer 9

Reagents: LiAlH4 and dry ether (at 0°C) OR H2 and a platinum catalyst
Secondary alcohol produced

Question 10

Ketones with 2,4-DNPH

Answer 10

Orange precipitate of 2,4-dinitrophenylhydrazone produced

Question 11

Ketones with K2Cr2O7^2- and H^+ ions

Answer 11

No reaction, solution stays orange, ketones cannot be oxidised

Question 12

Ketones with Fehling’s solution

Answer 12

No change, ketones cannot be oxidised

Question 13

Ketones with Tollens’ reagent

Answer 13

No change, ketones cannot be oxidised

Question 14

Ketones with iodine and NaOH

Answer 14

Iodoform reaction
CHI3 and salt of carboxylic acid produced
ONLY works with methyl ketones

Question 15

Halogenoalkanes with aqueous KOH

Answer 15

Nucleophilic substitution to form an alcohol, heat.

Question 16

Halogenoalkanes with KCN

Answer 16

Heat under reflux with ethanol. Forms a nitrile by replacing halide ion

Question 17

Halogenoalkanes with NH3 in ethanol

Answer 17

Heat in a high pressure, sealed apparatus. Forms a primary amine

Question 18

Halogenoalkanes with ethanolic KOH

Answer 18

Heat under reflux. Elimination reaction to form an alkene, halide ion and water

Question 19

Chlorination of alcohols

Answer 19

Vigorous reaction with PCl5 to produce a chloroalkane, POCl3 and HCl, which gives off steamy fumes when HCl is exposed to ammonia. Acts as a test for the hydroxyl group,

Question 20

Bromination of alcohols

Answer 20

Heat under reflux with concentrated/50% sulfuric acid and KBr. 2 stage reaction to eventually form a bromoalkane and water (HBr and K2SO4 are formed from first reaction, HBr goes on to react with alcohol)

Question 21

Iodination of alcohols

Answer 21

Heat under reflux with phosphorus and red iodine. 2 stages to eventually form iodoalkane and phosphoric acid (phosphorus (III) iodide made in situ in first reaction)

Question 22

Oxidation of primary alcohols

Answer 22

Heat with potassium dichromate (VI) and dilute H2SO4
In a distillation apparatus, an aldehyde is formed.
In a reflux apparatus, a carboxylic acid is formed.
Both reactions have a colour change from orange (Cr^6+) to green (Cr^3+)

Question 23

Oxidation of secondary alcohols

Answer 23

Heat under reflux with potassium dichromate (VI) and dilute H2SO4. A ketone is formed and an orange to green colour change is seen

Question 24

Practical techniques: Heating under reflux

Answer 24

• The reflux apparatus is used to heat a reaction mixture safely for long periods of time
• It ensures that the reaction is complete
• Vapours condense in the condenser and fall back into the reaction vessel
• Ensure the apparatus is open (!!) or it may explode - important when drawing apparatus

Question 25

Practical techniques: Solvent extraction using a separating funnel

Answer 25

• Ideal for use when product is more soluble in organic solvents but contains impurities that readily dissolve in water (like ionic compounds)
• Transfer your product to a separating funnel, add water and shake
• 2 layers will form, one organic and one aqueous, and the impurities will move into the organic layer where they are more soluble
• Open the tap of the separating funnel to remove the lower layer which will separate the purified product from the impurities.
• This method is not completely exact.
• As a general rule, water is more dense than organic solvents that do not form hydrogen bonds, so it will be the lower layer

Question 26

Practical techniques: Distillation

Answer 26

• Can be used to separate mixtures of substances with different boiling temperatures
• When drawing apparatus, make sure thermometer is in the right place, water goes in at the bottom of the condenser (which is diagonal) and the apparatus cannot be sealed
• The temperature reading on the thermometer will match the boiling temperature of the substance being distilled at that time

Question 27

Practical techniques: Drying with an anhydrous salt

Answer 27

• ‘Drying’ means removing water
• There will always be a small amount of water mixed with the product, especially if if you have carried out solvent extraction using water
• Add an anhydrous salt (e.g. anhydrous magnesium sulfate) shake, then leave to settle and filter off the solid
• The salt will absorb any water

Question 28

Practical techniques: Boiling point determination

Answer 28

• Used to find out how pure a product is
• Boiling points of pure substances at atmospheric pressure have been accurately measure and are listed in data booklets
• Measuring the final product’s boiling temperature and comparing it with the data book’s value will let you know how pure the sample is
• Pure substances will boil at precise temperatures rather than over a range

Question 29

Production of an alkane from an alkene

Answer 29

Heat at 150°C with hydrogen and a nickel catalyst

Question 30

Thermal cracking

Answer 30

High temperature (up to 1000°C) and high pressure (up to 70 atm). Produces lots of alkenes

Question 31

Catalytic cracking

Answer 31

Zeolite catalyst (hydrated aluminosilicate), slight pressure and high temperature (450°). Mainly produces aromatic hydrocarbons and motor fuels

Question 32

Reforming straight chain alkanes

Answer 32

Converting straight chain alkanes into branched chain alkanes and cyclic hydrocarbons, using a catalyst like platinum stuck on aluminium oxide

Question 33

Halogens with alkenes

Answer 33

Form dihalogenoalkanes, halogens add across the double bond. Decolourises bromine water