All The Mechanisms

Question 1

Reaction between halogen and alkane condition

Answer 1

Presence of UV light
Produce halogenoalkanes
(homolytic fission)
produce free radicals

Question 2

Changes from alkenes → alkane
Reagent , condition , type of reaction

Answer 2

Reagent : hydrogen
Conditions : nickel catalyst
Type of reaction : Addition / Reduction

Question 3

Alkene → (di)halogenoalkane
condition , mechanism , type of reagent , type of bond fission

Answer 3

Reagent : halogen eg bromine
( dissolve in organic solvent )
Conditions : room temperature
Mechanism : electrophilic addition
Type of reagent : electrophile
Type of bond fission : heterolytic

Question 4

Alkene → halogenoalkane
Condition , mechanism , type of reagent , type of bond fission

Answer 4

Reagent : hydrogen halide
Condition : room temperature
Mechanism : electrophilic addition
Type of reagent : electrophilic H delta+
Type of bond fission : heterolytic

Question 5

Alkenes → alcohol
Reaction , condition

Answer 5

Alkenes are reacted with steam in the presence of an acid catalyst
Hydration reaction
Condition : high temperature 300 - 600C
Condition : high pressure 70 atm
Condition : catalyst of concentrated H3PO4

No waste product hence has a high atom economy
Separation of product is easy hence cheaper

Question 6

Alkene → diol
Reagent , condition , type of reaction , observation

Answer 6

Reagent : KMnO4 (Potassium permanganate) in an acidified solution
Condition : room temperature
Type of reaction : oxidation
Observation : purple colour of MnO4- ion will decolourise to colourless

Question 7

Qualitative test for a C=C double bond
Reagent , condition , type of reaction , observation

Answer 7

Reagent : bromine dissolved in water
Condition : room temperature
Type of reaction : oxidation
Observation : orange colour of bromine water will decolourise to colourless

Question 8

Alcohol → halogenoalkane / chloroalkane
Reagent and mechanism

Answer 8

Reagent : halogenating agents / PCl5
Mechanism : nucleophilic substitution

The -OH group is replaced by a halogen
produces chloroalkane
ROH(l) + PCl5(s) → RCl(l) + POCl3(l) + HCl(g)

It can be used to test for alcohol
As the product produces steamy fumes
Turns damp blue litmus paper red

Question 9

Alcohol → bromoalkane
What is the reaction

Answer 9

Reaction : 50% concentrated sulfuric acid + potassium bromide → HBr
HBr + alcohol → bromoalkane
CH3CH2OH + HBr → CH3CH2Br + H2O

Question 10

Alcohol → iodoalkane
Reaction

Answer 10

Reaction : phosphorus + iodine → phosphorus (III) iodide
phosphorus (III) iodide + alcohol → iodoalkane
2P + 3l2 → 2Pl3
3CH3CH2OH + Pl3 → 3CH3CH2I + H3PO3

Question 11

Primary alcohol → aldehyde
Reagent , condition

Answer 11

Reagent : Potassium dichromate (VI) solution and dilute sulfuric acid
Conditions : ( use a limited amount of dichromate ) warm gently and distil out the aldehyde as it forms

Question 12

Primary alcohol → carboxylic acid
Reagent , condition

Answer 12

Reagent : Potassium dichromate (VI) solution and dilute sulfuric acid
Conditions : use an excess of dichromate and heat under reflux ( distil off product after the reaction has finished )

Question 13

Change from secondary alcohol → ketone
Reagent , condition

Answer 13

Reagent : Potassium dichromate (VI) solution and dilute sulfuric acid
Condition : heat under reflux

Question 14

Fehling’s (Benedict’s) solution
Reagent , reaction , observation

Answer 14

Reagent : fehling solution containing blue Cu 2+ ions
Condition : heat gently
Reaction : aldehydes only are oxidised by Fehling’s solution into a carboxylic acid and the copper (II) ions are reduced to copper(I) oxide
Observation : Aldehydes : Blue Cu 2+ ions in solution change to a red precipitate of Cu2O. Ketones do not react
CH3CHO + 2Cu2+ + 2H2O CH3COOH + Cu2O + 4H+

Question 15

Alcohol → alkene
Reagent , condition , role of reagent , type of reaction

Answer 15

Reagents: Concentrated phosphoric acid
Conditions: warm (under reflux)
Role of reagent: dehydrating agent/catalyst
Type of reaction: acid catalysed elimination

Question 16

Halogenoalkane → alcohol
Reagent , condition , mechanism , role of reagent

Answer 16

Reagent : potassium / sodium hydroxide
Conditions : in aqueous solution ; heat under reflux
Mechanism : nucleophilic substitution
Role of reagent : nucleophile , OH- ( strong nucleophile than water as it has a full negative charge )

Question 17

Aqueous silver nitrate in ethanol
Rate of reaction

Answer 17

Aqueous silver nitrate is added to a halogenoalkane
The halide leaving group combines with a silver ion to form a silver halide precipitate

The quicker the precipitate is formed the faster the substitution reaction and the more reactive than haloalkanes
The rate of substitution depends on the strength of the C-X bond.
Weaker the faster

Question 18

Halogenoalkane → nitrile
Reagent , condition , mechanism , type of reagent

Answer 18

Reagent: Alcoholic potassium cyanide (KCN)
Conditions: Heating under reflux
Mechanism: Nucleophilic substitution
Type of reagent: Nucleophile (cyanide ion CN-)
This reaction adds on a carbon atom

Question 19

Halogenoalkane → amine
Reagent , condition , mechanism , type of reagent

Answer 19

Reagent : NH3 dissolved in ethanol
Conditions : heating under pressure
in a sealed tube
Mechanism : nucleophilic substitution
Type of reagent : Nucleophile

Question 20

Halogenoalkane → alkene
Reagent , condition , mechanics , role of reagent

Answer 20

Reagent : potassium / sodium hydroxide
Condition : in ethanol / heat
Mechanism : elimination
Role of reagent : base