rp6 + other tests for functional groups

other tests from oxidation of alcohols topicc

describe the test for an alcohol and a +ve result:

• add solution you are testing to clean, dry test tube

• place small piece of metallic sodium into test tube

• conduct ‘squeaky pop’ test - place lit splint into test tube - if squeaky pop is heard, hydrogen has been produced, indicating the presence of an alcohol

• OR w/o squeaky pop test: Na fizzes

describe the test for an aldehyde using Fehling’s solution and a +ve result:

• add equal volumes Fehling’s solution A and B to a clean, dry test tube - this should form a dark blue solution

• add 5 drops of this solution and some anti bumping granules to a different clean, dry test tube

• add solution you are testing into test tube

• prepare a water bath and place test tube inside, bringing water to boiling

• leave contents of test tube to stand for a few minutes

• if aldehyde present → brick red precipitate

• (if ketone present → no change)

describe the test for an alkene and a +ve result:

• add 2 cm³ of solution you are testing to a clean, dry test tube

• shake w/ 2 cm³ of bromine water

• if solution changes from orange → colourless, C=C bonds are present, highlighting the presence of an alkene

describe the test for a carboxylic acid and a +ve result:

• add 2 cm³ solution you are testing to clean, dry test tube, then add 2 cm³ sodium carbonate solution (or a small spatula of solid sodium hydrogencarbonate)

• if successful, fizzing of CO₂ being produced should occur, highlighting the presence of a carboxylic acid

• OR CO₂ produced turns limewater cloudy

• (note that this test shows a +ve result for all acids!)

give the general eqn for the test for (carboxylic) acids:

(carboxylic) acid + carbonate → salt + carbon dioxide + water

describe the test for a haloalkane and a +ve result:

• add solution you are testing to NaOH solution in a test tube

• prepare a water bath at approx 60^oC and place the test tube and its contents into water bath

• add dilute nitric acid and silver nitrate solution (acidifies solution)

• if a haloalkane is present, a cream precipitate should form

how can we test for 1^o alcohols? give a +ve result:

• add 10 drops alcohol to 2 cm³ acidified potassium dichromate (VI)

• warm gently in hot water bath

• if 1^o: solution goes from orange → green as an aldehyde forms

• we must repeat this and distill the product to separate it and carry out a further test to find out whether it is actually a aldehyde and so whether or not the alcohol was 1^o

how can we test for 2^o alcohols? give a +ve result:

• add 10 drops alcohol to 2 cm³ acidified potassium dichromate (VI)

• warm gently in hot water bath

• if 2^o: solution goes from orange → green as a ketone forms

• we must repeat this and distill the product to separate it and carry out a further test to find out whether it is actually a ketone and so whether or not the alcohol was 2^o

give the eqn for and explain the colour change of 1^o/2^o alcohols when added to acidified potassium dichromate (VI):

• Cr₂O₇^2- → Cr³⁺ + e^-

• orange

how can we test for 3^o alcohols? give a +ve result:

• add 10 drops alcohol to 2 cm³ acidified potassium (VI) dichromate

• warm gently in hot water bath

• if 3^o: nothing happens

summarise the process of oxidising a 1^o/2^o alcohol to distill the product (aldehyde/ketone - respectively)

• add XS alcohol to 2 cm³ acidified potassium (VI). dichromate solution in a round bottom flask

• set up flask as part of distillation apparatus

• gently heat flask - alcohol will be oxidised and product will be distilled to be collected

what are the 2 tests we can use to distinguish between an aldehyde and ketone after distillation? what happens if a ketone is present?

• Fehling’s solution test (same as aldehyde test in RP6)

• Tollen’s reagent test

• if a ketone is present for both: no change

describe the Tollens’ reagent test and a +ve result:

make Tollens’ reagent:

• add 2 cm³ 0.1 mol dm^-3 silver nitrate solution to a test tube

• add a few drops of dilute NaOH solution - light brown precipitate should form

• add drops of dilute ammonia solution until brown precipitate completely dissolves

test:

• place test tube containing Tollens’ reagent into hot water bath and add 10 drops of aldehye/ketone

• wait for a few minutes

• if aldehyde present - silver mirror forms on test tube walls

• if ketone present - nothing happens :(

suggest why a water bath may be used to heat the mixture in a test tube test:

most organic compounds (specific) flammable

give and explain the equation for the formation of the silver mirror in the presence of an aldehyde in the Tollens’ reagent test:

• Ag⁺ + e^- → Ag _(s)

• aldehyde reduces Ag⁺ to silver ions

give the eqn for and explain the colour change when 1^o/2^o alcohols are added to acidified potassium dichromate (VI):

• orange dichromate (VI) ion is reduced to the green chromium (III) ion

• Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O