topic 6: organic analysis

Question 1

test for alkene

Answer 1

add bromine then shake
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

test for aldehyde

Answer 3

-warm with fehlings reagent and tollens solution

result
fehlings reagent: blue to red precipitate
tollens: silver mirror forms (Ag 0)

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

Question 4

test for carboxylic acid

Answer 4

reagent: NaHCO3 (carbonates)
-fizzes and produces CO2 which is bubbles through limewater - cloudy

Question 5

what is formed in the mass spectrometry?

Answer 5

-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 6

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

Answer 6

-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 7

describe the bonds during IR spectroscopy

Answer 7

-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 8

fingerprinting infrared spectroscopy

Answer 8

-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 9

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

Answer 9

the standard value of the isotope