Organic Analysis questions Flashcards
Describe fully how infrared spectra can be used to distinguish between
isomers R, S and T (R and S are alcohols T has a C=O)
- R & S have an O-H alcohols peak at 3230-3550 cm−1
- T has C=O peak at 1680-1750 cm−1
- R & S (unique) fingerprint region or below 1500 cm−1
- Compare to a database / known spectra
Give a reagent and observations for a test-tube reaction to distinguish
between 2-methylbutan-1-ol and 2-methylbutan-2-ol.
Reagent: acidified potassium dichromate
1-ol (orange to) green solution OR goes green
-2-ol no (visible/observed) reaction/change or NVR or stays
orange
two compounds, all colourless liquids, are
* butanone
* 2-methylpropan-2-ol
Suggest how the results of a spectroscopic technique could be used to distinguish between the the two compounds
IR (infra-red) spectroscopy If different functional groups: need
to identify wavenumber and bond of key functional group signal (e.g.
(alcohol) O-H 3230-3550 or C=O 1680-1750 (cm–1
If the same functional group, need idea
of using the fingerprint region to look for match to known compounds /comparing region to samples in a database
Mass spectrometry
If different, can use different Mr
values with values of Mr given
butanone 72(.0),
2-methylpropan-2-ol = 74(.0),
butan-2-ol = 74(.0), butanal = 72(.0)
If compounds have same Mr, then
would have to use idea that
fragmentation patterns would be
different (not on specification but
may be mentioned)
Test-tube reactions can be used to identify the functional groups in organic molecules.
You are provided with samples of each of the four compounds K,L,N,M. K=carboxylic acid + alcohol L= haloalkeane +alcohol n= haloalkeane plus aldehyde m=carboxylic
COOH
a) NaHCO3 / Na2CO3 (or correct alternative)
b) effervescence /gas turns limewater milky
c) K and /or M but not L and/or N
-OH and -CHO
d) acidified K2Cr2O7
e) solution turns green
f) K and/or L and/or N but not M
-CHO
g) Fehlings OR Tollens
h) red ppt OR silver mirror
i) N only but not K and/or L and/or M
-Br
j) Silver nitrate
k) cream ppt
l) L and/or N but not K and/or M
Identify the functional group(s) present in each isomer T, U and V of
C6H12O2 using Table A in the Data Booklet.
(Isomer T)
signals due to OH (alcohol) at 3230−3350 and C=O at 1680−1750
1
OH and C=O (functional groups) separated in molecule.
Allow not a carboxylic acid.
1
(Isomer U)
(only) signal for OH (alcohol) at 3230−3350
1
2 × OH groups present / diol / OH & cyclo(ether) structure
(Isomer V)
signals due to OH (acid) at 2500−3000 (and C=O at 1680−1750)
carboxylic acid group / −COOH present.
