testing for different functional groups

Question 1

describe the test and positive result for an alkene

Answer 1

bromine water
shake unknown compound with bromine water

orange solution will decolourise in presence of alkene

Question 2

describe the test and positive result for an alcohol

Answer 2

use acidified potassium dichromate and hydrochloric acid with a few drops of the alcohol

if nothing happens warm mixture gently in a water bath

orange solution will turn green
( for primary and secondary alcohols)

with tertiary alcohols the solution will stay orange

Question 3

describe the test and positive result for an aldehyde

Answer 3

tollens reagent
( add a drop of sodium hydroxide to silver nitrate solution until a brown precipitate is formed and add ammonia to dissolve the precipitate)

add a few drops of unknown chemical to tollens reagent and gently warm in a water bath for a few minutes

colourless to produce a grey ppt of silver forming a silver mirror

Question 4

explain the procedure for making tollens reagent

Answer 4

add a drop of sodium hydroxide to silver nitrate solution until a brown precipitate is formed and add ammonia to dissolve the precipitate

Question 5

why do aldehydes form a silver mirror in the presence of tollens reagent

Answer 5

they reduce the diamine silver 1 ion of metallic silver

Question 6

describe the test and positive result for fehling’s solution

Answer 6

add a few drops of unknown compound to fehlings solution and warm in a water bath for a few minutes

aldehydes = blue solution produces a dark red ppt of copper 1 oxide

aldehydes reduce the complexed copper 11 ion to the copper 1 oxide

Question 7

describe the test and positive result for a carboxylic acid

Answer 7

add unknown compound to 1-2 cm cubed of sodium carbonate solution

if bubbles of gas are seen this is a good indicator you have a carboxylic acid

bubble the gas evolved through limewater

if limewater turns cloudy this proves the gas produced was CO2