Volume 25, Number 2, November 2015
Answers
Revision quizzes
Anne Hodgson
See how you did on the revision questions in the magazine.
Global warming: reconstructing the past
23
1
There are 6.02 × 10 molecules of water in a mole. If 0.2% of them contain
23
21
there will be 0.2% × 6.02 × 10 = 1.2 × 10 heavy oxygen atoms.
2
a
CaO(s) + CO2(g)
b
CaCO3(s) + H2O(l) (this is the limewater test for CO2)
c
Ca(HCO3)2
18
O atoms, then
3
Lab page
a
2,3-dihydroxypropyl octadecanoate (glyceryl monostearate)
b
Hexadecan-1-ol (cetyl alcohol)
c
2-hydroxypropane-1,2,3-tricarboxylic acid (citric acid)
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Top tips
1
There are four possible products from a reaction to form dipeptides from glycine and alanine:
Water is a by-product of this condensation reaction.
2
The building blocks of a polypeptide are referred to as ‘amino acid residues’ rather than
‘amino acids’ because water was lost in the formation of each peptide bond, so the amino acids no
longer exist as such.
3
a
b
Remember when working out the R/S designation of a chiral centre you first of all
must assign ‘priorities’ to each of the four groups attached to the chiral carbon. The higher the
atomic number, the higher the priority, but if two atoms have the same atomic number, you
need to look at the atoms attached to these atoms and compare their atomic numbers.
Number the groups so that the highest priority is 1 and the lowest priority is 4. For alanine the
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highest priority group is NH2 (1), followed by COOH (2), then CH3 (3) and H is the lowest
priority (4).
Imagine that you are holding the molecule (building a model helps here) so that you are
looking at the chiral carbon with the lowest priority group (4) pointing away from you and then
trace around the other groups from highest (1) downwards (i.e. 1, 2, 3). If you traced out a
clockwise circle, then the molecule is assigned as the R form (R for rectus, Latin for right). If
you went anticlockwise then the molecule is the S form (S for sinister, Latin for left).
So for alanine we can assign the enantiomers thus:
c
Glycine (which has H as its side chain) is the only ‘standard’ amino acid that is not
chiral, as it does not have four different groups around the alpha-carbon.
d
L-cysteine is the only ‘standard’ amino acid that is an R enantiomer. This is because
the sulfur in its side chain has a higher atomic number (16) than carbon (6), nitrogen (7) or
oxygen (8), and so increases its priority when assigning R/S. For this reason it makes sense
to use L to designate amino acids in a biological context, so that all those found in normal
proteins are the same.
Investigating with isotopes
1
Testosterone has six chiral centres:
2
The average mass of an atom of chlorine is
(75.78% × 34.968853) + (24.22% × 36.965903) / 100
= 35.452539
So for a molecule of Cl2 the relative molecular mass will be 2 × 35.452539 = 70.905078
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3
a
The relative molecular masses of nitrogen (N2), carbon monoxide (CO) and ethene
(C2H4) to the nearest whole integer are all 28.
b
Their relative molecular masses to six decimal places (assuming that the molecules
are composed entirely of the most common isotopes) are:
N2
2 × 14.003074 = 28.006148
CO
12.000000 + 15.994915 = 27.994915
C 2H 4
(2 × 12.000000) + (4 × 1.007825) = 28.031300
Focus on industry
1
22
2
+4
3
In order of increasing density (density values are given for the metals at room temperature):
–3
–3
–3
–3
aluminium (2702 kg m ), titanium (4506 kg m ), iron (7860 kg m ), copper (8960 kg m )
4
a
TiH2 is titanium dihydride (titanium(II) hydride or titanium hydride), the titanium is in
oxidation state +2.
b
TiF3 is titanium(III) fluoride (trifluorotitanium or titanium trifluoride), the titanium is in
oxidation state +3.
c
Ti2S3, titanium(III) sulfide (dititanium trisulfide or titanium trisulfide), the titanium is in
oxidation state +3.
d
TiBr4, titanium(IV) bromide (titanium tetrabromide), the titanium is in oxidation state +4.
e
Ti3O5, titanium (III, IV) oxide (trititanium pentoxide), the titanium is in oxidation state +3
and +4.
f
Ti(CO)6, titanium hexacarbonyl (titanium(O) carbonyl or titanium carbonyl). This is an
organometallic carbonyl complex and the titanium is in oxidation state 0.
5
46
Ti (8.0%),
47
Ti (7.3%),
48
Ti (73.8%),
49
Ti (5.5%),
50
Ti (5.4%)
Encounter
1
The carbon dioxide breathed out by rodent could be taken in through the leaves of a sugar
cane plant and, through the process of photosynthesis, be incorporated into a molecule of sugar —
initially glucose and then the disaccharide sucrose. The harvested sugar cane crop is quickly
processed to extract a sugar solution (containing our carbon atom), which is then fermented to
produce ethanol (also known as bioethanol), so the carbon atom could now be part of CH3CH2OH.
This ethanol can be blended into petrol (about 5% ethanol in the fuel mixture) and used to power a car
— perhaps along the highways of Britain.
2
Besides carbon dioxide (CO2), other important greenhouse gases include methane (CH4)
chlorofluorocarbons (CFCs, e.g. CCl2F2), ozone (O3), dinitrogen oxide (nitrous oxide, N2O), water
vapour (H2O), fluorocarbons (sometimes called perfluorocarbons or PFCs, e.g. CF4), sulfur
hexafluoride (SF6) and nitrogen trifluoride (NF3).
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25 years of…
1
2
Prostaglandin F2α:
Propolisbenzofuran B:
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3
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