Organic Tests

Question 1

test for Alkenes

Answer 1

Bromine water - turns colourless (from orange-brown)

Question 2

test for aldehydes

Answer 2

• Tollens reagent —> silver mirror formed
• Fehlings test / Benedict’s —> red precipitate forms from blue solution (both fehlings and Benedict’s form red precipitates)
  -Heat both solutions gently

Question 3

Test for alcohols (and anything else with an -OH group but traditionally alcohols )

Answer 3

• PCl5 —> white misty fumes of HCl formed

Question 4

Test for carboxylic acids

Answer 4

• react with sodium carbonate, and observe effervescence of CO2
• bubble CO2 into limewater

Question 5

Test for methylketones or ethanal :

Answer 5

• iodoform test
• React methylketone or ethanal with iodine and sodium hydroxide
  -positive test : yellow crystalline precipitate formed

(Also works for secondary alcohols where one of the carbons bonded to the carbon bonded to the OH group is the start of the chain , I.e secondary alcohols with structure R-CH(OH)-CH3

Question 6

Test for carbonyl compounds

Answer 6

-2,4DNP
- Forms an orange precipitate (positive test)

Question 7

How can you find the exact carbonyl in solution using 2,4-DNP

Answer 7

-React 2,4-DNP with a carbonyl (condensation reaction between NH2 group of 2,4-DNP and O from C=O bond in carbonyl)

• Yellow precipitate forms

-Recrystallise products

-determine the melting temperature of the precipitate

-compare it to data booklet of known 2,4DNP- carbonyl precipitates to identify the carbonyl

Question 8

Combustion analysis general method

Answer 8

• do 12/44 x mass CO2 , to find the mass of carbon from combusted compound

-do 2/18 x mass H2O , to find the mass of hydrogen from combusted compound

-if it’s a hydrocarbon proceed to find moles of C and H, to then find molar ratio of C to H then find empirical
formula

If the compound contains oxygen ,minus mass of C and H from compound to find mass of O before doing molar ratios