Organic analysis

Question 1

What is the reagent and positive test result for primary and secondary alcohols

Answer 1

Reagent: acidified pottasium dichromate (K₂Cr₂O₇)
Primary and secondary alcohols are oxidised by K₂Cr₂O₇.
Will turn from orange to green (Cr₂O₇²⁻ to Cr³⁺)

Question 2

What is the reagent and positive test result for a tertiary alcohol

Answer 2

Will remain orange on addition of acidified potassium dichromate

Question 3

What is the reagent and positive test result for alkenes

Answer 3

Reagent: Bromine water - goes from orange to colourless in the presence of an alkene

Question 4

What is the reagent and positive test result for an aldehyde

Answer 4

Reagent: Fehling’s solution (in hot water bath) - goes from a blue solution to a red precipitate in the presence of an aldehyde
or
Reagent: Tollens reagent (in hot water bath)- formation of a silver mirror in the presence of an aldehyde

Question 5

What is the reagent and positive test result for a carboxylic acid
Write an equation for the positive test with CH3CO2H

Answer 5

Reagent: Add sodium carbonate
Effervescence of CO2 formed - can then bubble through limewater to test this
2CH3CO2H + Na2CO3 —. 2CH3CO2Na + H2O + CO2

Question 6

What is the reagent and positive test result for chloroalkanes

Answer 6

Reagent: Warm with silver nitrate
Slow formation of white precipitate AgCl

Question 7

Write an equation for the reaction of CH3CHO with tollens. Write the practical steps to test it and what occurs

Answer 7

Tollens’ Reagent formed by mixing aqueous
ammonia and silver nitrate, which we heat gently in a water bath then add what were testing

Aldehydes are oxidised by tollens into a carboxylic acid, and silver ions are reduced to silver atoms (silver mirror)

CH3CHO + 2Ag+ + H2O —> CH3COOH + 2Ag + 2H+

Question 8

Write an equation for the reaction of CH3CHO with Fehlings. Write the practical steps to test it and what occurs

Answer 8

We heat gently in a water bath then add what we are testing

Aldehydes are oxidised by Fehling’s Solution
into a carboxylic acid. The copper (II) ions are reduced
to copper(I) oxide (blue to brick red)

CH3CHO + 2Cu2+ + 2H2O —-> CH3COOH + Cu2O + 4H+

Question 9

What is the m+1 or molecular ion peak

Answer 9

The peak with the highest mass/charge ratio which will be normally due to the original molecule that hasn’t fragmented, and therefore be equal to Mr

Question 10

When is high resolution mass spectrometry useful

Answer 10

When identifying different molecules with he same Mr rounded to the nearest whole number

Question 11

When does M+2 peak apply

Answer 11

If a compound contains a chlorine or a bromine atom then two molecular ion peaks will occur : a M and a M+2 peak will occur due to the two naturally occurring isotopes of chlorine or bromine.

Question 12

What is the ratio of the 2 isotopes of Cl

Answer 12

Chlorine exists as Cl35 (75%) and Cl37 (25%)
Bromine exists as Br79 (50%) and Br81 (50%)

Question 13

What is the ratio of the heights of the M and M+2 peak in CH3Cl

Answer 13

CH3Cl will have a m/z value of M of 50 (CH3Cl 35) and M+2 of 52 (CH3Cl 37)
The ratio of heights M:M+2 will be 3:1

Question 14

What is the ratio of the heights of the M and M+2 peak in CH3Br

Answer 14

CH3Br will have m/z value of M of 94 (CH3Br 79) and M+2 of 96 (CH3Br 81) The ratio of heights M:M+2 will be 1:1

Question 15

When will an M+4 peak occur

Answer 15

If a compound contains two chlorine or bromine atoms

Question 16

What is the ratio of the heights of the M, M+2 and M+4 peak in C2H4Cl2

Answer 16

C2H4Cl2 will have a m/z value of M of 98 (C2H4Cl(35)Cl(35)) , a
M+2 of 100 (C2H4Cl(35)Cl(37)) and a M+4 of 102 (C2H4Cl(35)Cl(37))
The ratio of heights M:M+2: M+4 will be 9:6:1

Question 17

What is the ratio of the heights of the M, M+2 and M+4 peak in C2H4Br2

Answer 17

C2H4Br2 will have a m/z value of M of 186 (C2H4Br(79)Br(79)) , a M+2 of 188 (C2H4Br(79)Br(81)) and a M+4 of 190 (C2H4Br(81)Br(81))
The ratio of heights M:M+2: M+4 will be 1:2:1

Question 18

What is the ratio of the heights of the M, M+2 and M+4 peak in C2H3Cl3

Answer 18

C2H3Cl3 will have a m/z value of M of 132 (C2H4Cl(35)Cl(35)Cl (35)) , a M+2 of 134 (C2H4Cl(35)Cl(35)Cl(37) , a M+4 of 136 (C2H4Cl(35)Cl(37)Cl(37)) and a M+6 of 138 (C2H4Cl(37)Cl(37)Cl(37))
The ratio of heights M:M+2:M+4:M+6 will be 27:27:9:1

Question 19

What does the frequency of infrared radiation absorbed by a covalent bond depend on

Answer 19

The atoms on either side of the bond
The position of the bond in the molecule

Question 20

Where do we look to identify a functional group in infrared spectroscopy

Answer 20

Above 1500 cm-1

Question 21

What does the fingerprint region in infrared spectrometry tell us

Answer 21

The size and position of the peaks are unique to a particular molecule, we can compare this region against a known library of spectra to identify a molecule

Question 22

Where is the fingerprint region in infrared spectrometry

Answer 22

Between 500cm-1 and 1500cm-1

Question 23

What do extra peaks in the fingerprint region of an infrared spectra indicate

Answer 23

Impurities in the sample

Question 24

Explain how the greenhouse effect works

Answer 24

Electromagnetic radiation from the sun reaches the earth and is absorbed by land and sea. Some of this radiation is re-emitted as infrared
Covalent bonds in greenhouse gasses absorb this radiation and re-emit this back towards earth (which warms the earth up as it cant escape)

Question 25

Give 3 examples of greenhouse gasses

Answer 25

CO2
H2O (water vapour)
CH4 (methane)

Question 26

Give an example of 2 things which have increased the levels of greenhouse gasses in the atmosphere

Answer 26

Burning fossil fuels
Landfill

Question 27

What do we call the gradual warming of the earth

Answer 27

The greenhouse effect / Global warming