CD - Organic reactions *01 *02

Question 1

electrophilic substitution reactions of aromatic compounds: halogenation of the ring

Answer 1

eg. chlorination

reactants and conditions:

• X2 (any halogen)
• AlCl3 catalyst
• warm

to make electrophile
eg. Cl2 + AlCl3 —> Cl+ + AlCl4-

overall reaction
C6H6 + Cl2 —[AlCl3]—> C6H5Cl + HCl (makes chlorobenzene)

Question 2

electrophilic substitution reactions of aromatic compounds: nitration

Answer 2

Reactants and conditions:

• conc. H2SO4 catalyst
• HNO3
• 55 degrees (so only one group)

to make electrophile: NO2+
- HNO3 + 2H2SO4 —> NO2+ + 2HSO4- + H3O+

products
nitrobenzene and water

mechanism

1. NO2+ attacks benzene ring
2. unstable intermediate forms
3. the H+ ion is lost
   - reacts with HSO4- to regenerate H2SO4 catalyst

Question 3

electrophilic substitution reactions of aromatic compounds: sulfonation

Answer 3

reactants and conditions
- fuming H2SO4
- 40 degrees
- 30 mins
OR 
- boil benzene under reflux for several hours with conc. H2SO4

Making electrophile: (SO3)
- H2SO4 —> H2O + SO3

products:
benzene-sulfonic acid and water

same mechanism as Nitration

Question 4

electrophilic substitution reactions of aromatic compounds: friedel-crafts ALKYLATION

Answer 4

conditions and reactants:

• RCL
• AlCl3 cataylst
• reflux

making electrophile: R+ (eg CH3+) in this case its a carbocation
CH3Cl + AlCl3 —–> CH3+ + AlCl4-

products:
methylbenzene and HCl
(AlCl4- reacts with the H+ that is lost with makes HCl and regenerates catalyst)

overall reaction:
C6H6 + RCl —[AlCl3]—> C6H5 + HCl

Question 5

electrophilic substitution reactions of aromatic compounds: freidel-crafts ACYLATION

Answer 5

conditions and reactants

• RCOCl
• AlCl3 catalyst
• reflux

making electrophile: (RCO+ eg. CH3CO+)
- CH3COCl + AlCl3 —-> CH3CO+ + AlCl4-

products:
phenylethanonoe and HCl

Question 6

Making a Diazonium Salt

Answer 6

phenylamine and nitrous acid

nitrous acid is unstable so has to be made in situ
- NaNO2 + HCl —> HNO2 + NaCl
(sodium nitrate + Hydrochloric acid)

Nitrous acid + phenylamine + HCl —–> benezenediazonium chloride

TEMP BELOW 5 DEGREES to prevent phenol forming

Question 7

Making an Azo Dye

Answer 7

coupling reaction
diazonium salt + phenol

1. phenol dissolved in NaOH solution —-> sodium phenoxide solution
2. stood on ice and chilled benzene-diazonium chloride added
3. azo dye precipitates out of solution immediately

Question 8

reactions of carbonyl compounds: making aldehydes

Answer 8

distil primary alcohol with acidified potassium dichromate (VI)

Question 9

reactions of carbonyl compounds: making carboxylic acids

Answer 9

heat a primary alcohol (or aldehyde) under reflux with acidified potassium dichromate (VI)

Question 10

reactions of carbonyl compounds: making ketones

Answer 10

reflux a secondary alcohol with acidified potassium dichromate (VI)

Question 11

testing for aldehydes and ketones: what can be used

Answer 11

• Fehling’s solution

- tollen’s reagent

Question 12

testing for aldehydes and ketones: Fehling’s solution

Answer 12

Fehling’s solution is a blue solution of copper (II) ions dissolved in sodium hydroxide
- If heated with an aldehyde the copper (II) ions are reduced to a brick red precipitate of Copper (I) oxide, Cu2O

method

1. add 2cm3 of Fehling’s solution to a test tube
2. add 5 drops of aldehyde or ketone
3. put the test tube in a hot water bath for a few minutes

aldehyde - brick red precipitate
( Cu2+ + e- —> Cu+) the aldehyde is oxidised to the carboxylate ion ( RCHO —> RCOO-)
ketone - nothing happens

Question 13

testing for aldehydes and ketones: Tollen’s reagent

Answer 13

1. put 2cm3 of 0.1moldm-3 silver nitrate solution into a test tube
2. add a few drops of dilute sodium hydroxide solution, a light brown precipitate should form
3. add drops of dilute ammonia solution until the brown precipitate dissolves completely
   - this solution is tollen’s regent
4. place the test tube in a hot water bath and add 10 drop of aldehyde or ketone, wait for a few mins.

results:
aldehyde - a silver mirror (a thin coating a silver) form on the walls of the test tube
Ag(NH3)2+ (aq) + e- —-> Ag (s) + 2NH3 (aq), aldehyde –> carboxylate ion
ketone - nothing happens

Question 14

why cant aldehydes and ketones be put over a flame

Answer 14

they are flammable

so must be heated in a water bath

Question 15

PAG7

Question 16

reactions of carbonyl compounds: reaction with cyanide ions

Answer 16

Nucleophilic addition
hydrogen cyanide reacts with carbonyl compounds to produce CYANOHYDRINS (molecules with a -CN and -OH group)

hydrogen cyanide is a weak acid
HCN < —> H+ + CN-

1. the CN- attacks the partially +ve carbon atom and donates a pair of electrons
   - two electrons from the double bond transfer to the oxygen
2. H+ (for HCN or H2O) bonds to the oxygen tp form the hydroxyl group

Question 17

dangers of hydrogen cyanide

Answer 17

highly poisonous gas

so reaction needs to be done in fume cupboard