topic 6: organic analysis

Question 1

RP6 - test for an alkene and positive result

Answer 1

add bromine water
shake test tube
orange solution decolourises
result: orange to colourless

Question 2

test for primary, secondary and tertiary alcohol

Answer 2

reagent: acidified potassium dichromate (K2Cr2O7 (VI)) and sulfuric acid (H2SO4)

result
primary alcohol: orange to green
secondary alcohol: orange to green
tertiary alcohol: remains orange (no reaction)

the green to orange is the orange dichromate (VI) being reduced to green chromium (III) Cr3+, same result for primary and secondary alcohol so need further test

Question 3

RP6 - test for an aldehyde (T) and positive result

Answer 3

-add tollen’s reagent and heat gently
forms a silver mirror

result
tollens: silver mirror forms (Ag 0)

-to test between aldehydes and ketones - ketones do not get oxidised so nothing happens

Question 4

RP6 - active complex ion in tollen’s reagent

Answer 4

[Ag(NH3 )2]+

Question 5

RP6 - how does the silver mirror form using tollen’s reagent?

Answer 5

aldehydes are oxidised by Tollen’s reagent into a carboxylic acid and the silver(I) ions are reduced to silver atoms

Question 6

RP6 - test for an aldehyde (F) and positive result?

Answer 6

-add fehling’s solution and heat gently
result: blue solution to brick red precipitate

Question 7

RP6 - active ion in fehling’s solution

Answer 7

Cu2+

Question 8

RP6 - how does a brick red ppt form using fehling’s solution?

Answer 8

aldehydes only are oxidised by Fehling’s Solution into a carboxylic acid and the copper (II) ions are reduced to copper(I) oxide

Question 9

RP6 - what must be done to the solutions to test for an aldehyde using tollens or fehling’s solution?

Answer 9

heat gently in water bath

Question 10

RP6 - what is a primary alcohol oxidised to using K2Cr2O7 and H2SO4?

Answer 10

aldehyde

Question 11

RP6 - what is a secondary alcohol oxidised to using K2Cr2O7 and H2SO4?

Answer 11

ketone

Question 12

RP6 - what is a tertairy alcohol oxidised to using K2Cr2O7 and H2SO4?

Answer 12

cannot happen as no H

Question 13

RP6 - how does the solution change from orange to green in the test for alcohols/aldehydes?

Answer 13

orange Cr6+ in Cr2O72- ion is reduced to green Cr3+

Question 14

RP6 - the positive result for the potassium dichromate and sulfuric acid test for primary and secondary alcohols is the same, how do you confirm which one you started with?

Answer 14

collect the product from the distillation set up then test with fehling’s or tollen’s

Question 15

RP6 - test for alcohol using a metal and positive result

Answer 15

add a small piece of metallic sodium to alcohol it will fizz and produce hydrogen gas which is shown by ‘squeaky pop’ test

Question 16

RP6 - test for a haloalkane and positive results

Answer 16

add NaOH, HNO3 and warm solution
then add AgNO3
-AgCl is white
-AgBr is cream
-AgI is yellow

Question 17

RP6 - which silver halide ppt and why?

Answer 17

silver iodide because iodine is more reactive due to a weaker C-X bond so it is easier to react and the substitution happens fastest

Question 18

RP6 - test for a carboxylic acid and positive result

Answer 18

reagent: NaHCO3 (carbonates)
add sodium hydrogen carbonate NaHCO3 fizzing and effervescence of CO2 (limewater cloudy)

Question 19

what is formed in the mass spectrometry?

Answer 19

-a molecular ion (M+) is formed when a molecule loses an ELECTRON
-the molecular ion produces a MOLECULAR ION PEAK (highest peak - molecular mass) on the mass spectrum of the compound

Question 20

what is the mass/charge (mz) value of the molecular ion peak?

Answer 20

-the mass/charge (mz) value of the molecular ion peak, will be the same as the MOLECULAR MASS of the compound, since the charge on the ions is normally +1

Question 21

describe the bonds during IR spectroscopy

Answer 21

-the bonds in a molecule absorb infrared radiation at characteristic wave numbers

-in infrared (IR) spectroscopy, a beam of IR RADIATION is passed through a sample of a chemical
-the IR radiation is absorbed by the COVALENT BONDS in the molecules, increasing their VIBRATIONAL ENERGY
-BONDS BETWEEN DIFFERENT ATOMS absorb DIFFERENT FREQUENCIES of IR radiation
-bonds in different PLACES in a molecule absorb different frequencies

example: O-H group in an alcohol and the O-H in a carboxylic acid absorb different frequencies

Question 22

fingerprinting infrared spectroscopy

Answer 22

-fingerprinting allows identification of a molecule by comparison of spectra
-It usually has many peaks caused by complex vibrations of the whole molecule
-This shape is unique for any particular substance
-It can be used to identify the specific chemical after you’ve found the functional group
-A computer database checks the fingerprint region of the unknown compound’s IR spectrum against known compounds

Question 23

State why the precise relative atomic mass quoted in the table for the 12C isotope is exactly 12.00000

Answer 23

the standard value of the isotope