Functional Group Analysis

Question 1

how to test for the presence of an alkene

Answer 1

add bromine water to the sample and shake

Question 2

what is a positive test for an alkene with bromine water

Answer 2

solution goes from orange/ brown to colourless if alkene present

Question 3

why do alkenes react with bromine water

Answer 3

bromine undergoes an addition reaction with the double bond to form a colourless bromoalkane

Question 4

how to test for haloalkanes

Answer 4

add aqueous silver nitrate and ethanol to the sample and gently warm the mixture

Question 5

how does the halogenalkane test work

Answer 5

the halide ion is oxidised to form an insoluble silver halide precipitate

Question 6

what is the result of the halogenalkane test for chloroalkane

Answer 6

AgCl precipitate - white in colour

Question 7

what is the result of the halogenalkane test for bromoalkane

Answer 7

AgBr precipitate - cream in colour

Question 8

what is the result of the halogenalkane test for iodoalkane

Answer 8

AgI precipitate - yellow in colour

Question 9

what is the test for primary or secondary alcohols

Answer 9

add acidified orange potassium dichromate solution to the sample. then heat gently for a few minuted

Question 10

how does the primary/ secondary alcohol test work

Answer 10

the alcohol undergoes an oxidation reaction to form carbonyl compounds

Question 11

what is a positive result of the primary and secondary alcohols test

Answer 11

the orange solution will turn green if a primary or secondary alcohol is present

Question 12

why does the primary/secondary alcohol test not work on tertiary alcohols

Answer 12

tertiary alcohols cannot be oxidised by potassium dichromate

Question 13

what is the test for carbonyl compounds

Answer 13

dissolve 2,4-dinitrophenylhydrazine (2,4-DNPH) in sulfuric acid, water and metanol to prepare Brady’s reagent

add the Brady’s reagent to the sample and shake the mixture

Question 14

what is the positive result for thr carbonyl compound test

Answer 14

2,4-DNPH reacts with carbonyls to form an orange precipitate.

Question 15

what does the formation of an orange precipitate in the carbonyls test indicate

Answer 15

shows the sample is an aldehyde or ketone

Question 16

how to tell which specific aldehyde or ketone a sample that is positive for the carbonyls test is

Answer 16

filter and purify the precipitate by recrystallisation. determine its melting point and compare it to the melting points of known values

Question 17

how to test for aldehydes

Answer 17

prepare Tollens’ reagent [Ag(NH3)2]+(aq),by mixing solutions of silver nitrate, sodium hydroxide and dilute ammonia

add the Tollens’ reagent to the sample and gently warm the mixture

Question 18

how does the aldehyde test work

Answer 18

the Tollen’s reagent oxidises aldehydes to carboxylic acids as Ag+ ions are reduced to silver metals

Question 19

what is a positive result for the aldehydes test

Answer 19

formation of a silver mirror inside the test tube

Question 20

what happens if a ketone undergos the aldehyde test

Answer 20

ketones cannot be oxidies further by Tolens’ reagent so solution remains colourless

Question 21

what is the test for carboxylic acids

Answer 21

add a small spatula of solid sodium carbonate to the sample. carboxylic acids react with carbonates to from carbon dioxide gas

bubble the gas produced through limewater in a second test tube

Question 22

what is the positive test for carboxylic acids

Answer 22

limewater turns cloudy when the gas is bubbled through

Question 23

what is the test for phenols and how is a positive test shown

Answer 23

add one small spatula of solid sodium hydroxide to a sample. phenols are weakly acidic and are neutralised by bases to form colourless phenolate salts

if the solid sodium hydroxide dissolves to form a colourless solution, the sample may be a phenol

to confirm, add a small spatula of sodium carbonate to a sepearte sample

a lack of effervescence confirms the sample is a phenol since carbonates are weaker bases than sodium hydroxide so do not react with weak acids like phenols