Chapter 11/21 Spectroscopy

Deduce the number of 1H NMR signals produced by the zwitterion form of alanine.

3

[1 mark]

Outline why TMS (tetramethylsilane) may be added to the sample to carry out 1H NMR spectroscopy and why it is particularly suited to this role.

acts as internal standard

OR

acts as reference point

one strong signal

OR

12 H atoms in same environment

OR

signal is well away from other absorptions

Accept "inert" or "readily removed" or "non-toxic" for M1.

[2 marks]

State two techniques, other than IR spectroscopy, which could be used to confirm the identity of aspirin.

Any two of:

melting point

mass spectrometry/MS

high-performance liquid chromatography/HPLC

NMR/nuclear magnetic resonance

X-ray crystallography

elemental analysis

Accept "spectroscopy" instead of "spectrometry" where mentioned but not "spectrum".

Accept "ultraviolet «-visible» spectroscopy/UV/UV-Vis".

Do not accept "gas chromatography/GC".

Accept "thin-layer chromatography/TLC" as an alternative to "HPLC".

[2 marks]

a.iv.

A sample of 2 methyl propene was prepared in which the 12C in the CH2 group was replaced by 13C.

(i) State the main difference between the mass spectrum of this sample and that of normal 2 methyl propene.

«molecular ion» peak at «m/z =» 57, «not 56»OR«molecular ion» peak at one «m/z» higherORwill not have a «large» peak at 56

Accept a peak at m/z one greater than the 12C one for any likely fragment.

Consider the compound chloroethene, CH2=CHCl.

Deduce two features you would expect to observe in its mass spectrum.

[2]

a.

Predict two features you would expect to observe in its infrared (IR) spectrum.

peaks at (m/z =) 62 and 64 / two molecular ion peaks;

height ratio of molecular ion peaks is 3 :1;

peak at (m/z =) 27 (due to loss of Cl);

Accept peaks at 63/65 (13C)/peak(s) resulting from 13C / peaks at 61/63 (loss of H)/peak(s) resulting from loss of H / peaks at 48/50 (loss of CH2)/peak(s) resulting from loss of CH2/ 35/37 Cl+ as an alternative to final marking point.

a.

absorption at 600 - 800 (cm−1cm−1 C-Cl);

absorption at 1610 - 1680 (cm−1cm−1 >C=C<);

absorption at 2850 - 3100 (cm−1cm−1 C-H);

Deduce the number of different chemical environments of the hydrogen atoms in the 1HNMR1HNMR spectrum of 3-methylbutanoic acid.

4;

Describe the essential difference between the emission spectrum of sodium and the absorption spectrum of sodium.

Emission spectrum: coloured lines and Absorption spectrum: black/dark lines;

OR

Emission spectrum: lines and Absorption spectrum: continuous;

The integration trace for A:B:C:D was found to be 1:1:1:3.

Deduce what information can be obtained about the hydrogen atoms responsible for peak D at 1.2 ppm from the integration trace in the 1HNMR1HNMR spectrum of X.

[1]

b.

three hydrogens in same (chemical) environment / CH3CH3/methyl (group);

(i) State the information about this particular compound that can be derived from a mass spectrum and outline how it is found.

molar/molecular mass/M=74 (g mol−1)M=74 (g mol−1) / relative molecular mass/Mr=74Mr=74;

peak with highest m/z (ignoring any peak attributable to 13C) / found from parent/molecular ion peak;

Allow mass for m/z.

OR

compound has methyl/CH3CH3;

m/z = 15 due to CH+3CH3+;

OR

compound has propyl/C3H7C3H7/isopropyl/CH(CH3)2CH(CH3)2/acetyl/CH3COCH3CO;

m/z = 43 due to C3H+7/CH(CH3)+2/CH3CO+C3H7+/CH(CH3)2+/CH3CO+;

OR

compound has acetoxy/CH3COOCH3COO;

m/z = 59 due to CH3COO+CH3COO+;

Fragment must contain + sign in relevant marks above.

Penalize missing charges where relevant once only in (a)(i) and (ii).

State the region of the electromagnetic spectrum that is used in HNMR.

(i) radio (wave);

Explain why the infrared spectra of the structural isomers for C3H6O are very similar.

Explain how the mass spectra of the structures in (a) can be used to distinguish between them.

same/similar (types of) bonds / both contain the carbonyl group/C=O;

Do not accept same functional group.

(mass spectrum of) CH3CH2CHOCH3CH2CHO contains peak at m/z=29m/z=29 / CH3COCH3CH3COCH3 does not contain peak at m/z=29m/z=29;

(corresponding to) loss of C2H5C2H5 / Mr−C2H5Mr−C2H5 / CHO+CHO+ / loss of CHO / Mr−CHOMr−CHO / C2H+5C2H5+;

OR

(mass spectrum of) CH3CH2CHOCH3CH2CHO contains a (strong) peak at m/z=57m/z=57 / CH3COCH3CH3COCH3 does not contain a (strong) peak at m/z=57m/z=57;

(corresponding to) loss of H / Mr−HMr−H / CH3CH2CO+CH3CH2CO+;

Penalize missing + once only in A1.

Suggest a reason for the peak at m/z = 43 having an exceptionally high relative abundance for IR spectrum for pentan-2-one.

CH3CH2CH+2CH3CH2CH2+ and CH3CO+CH3CO+/two species have this mass/m/z;

Do not penalize missing + in this part.

Identify what information from the spectrum allows the determination of the relative numbers of hydrogen atoms producing each peak.

(ratio of) area under each peak / integration trace;

Accept size of peak but not height of peak.

Explain why tetramethylsilane (TMS) is suitable as a reference standard.

all (12) protons/hydrogens in same chemical environment (and hence gives 1 peak);

absorbs upfield/away from most other protons/H's;

low boiling point/bp / volatile / easily removed from sample;

not toxic;

highly unreactive (and hence does not interfere with sample) / inert;

Explain what happens at a molecular level during the absorption of IR radiation by carbon dioxide, CO2.

change in bond length / bond stretching / asymmetric stretch;

change in bond angle / bending (of molecule);

Allow [1 max] for only stating vibrations.

induces molecular polarity/dipole moment / OWTTE;

Comment on the absence of a peak at m/z = 59 for a mass spectrum that has an empirical formula C2H4O.

C2H3O2 produced has no charge / fragment produced after loss of C2H5 from molecular ion has no charge;

Accept fragment(s) too unstable, fragment breaks up etc.

Do not accept answers with reference to 13C/14C isotopes and peak at m/z = 61.

Do not accept C2H3O2+ / C3H7O+ does not exist.

Explain what occurs at a molecular level during the absorption of infrared (IR) radiation by the sulfur dioxide molecule, SO2SO2.

(O-S-O) bond angle changes;

(S-O) bond (length) stretches;

Allow [1] for S-O bond vibrations if neither of the above points are scored.

Distinguish between the 1HNMR1HNMR spectra of 1-bromopropane and 2-bromopropane (splitting patterns are not required).

2-bromopropane will show two separate absorptions/peaks;

in the ratio 6:1;

1-bromopropane will show three separate absorptions/peaks;

in the ratio 3:2:2;

Deduce two similarities and one difference in the 1HNMR1HNMR spectra of the two isomers CH3COOHCH3COOH, a carboxylic acid, and HCOOCH3HCOOCH3, an ester. 1HNMR1HNMR data are given in Table 18 of the Data Booklet.

Similarities: [2 max]

both have two peaks;

in the same/1:3 ratio;

both have only singlet peaks;

Difference:

CH3COOHCH3COOH will have an absorption/chemical shift/δδ in the range 2.0-2.5, (HCOOCH3HCOOCH3 will not) / HCOOCH3HCOOCH3 will have an absorption in the range 3.8-4.1, (CH3COOHCH3COOH will not) / CH3COOHCH3COOH will have an absorption in the range 9.0-13.0, (HCOOCH3HCOOCH3 will not);

Outline why a real gas differs from ideal behaviour at low temperature and high pressure.

Any one of:

finite volume of particles «requires adjustment to volume of gas» ✔

short-range attractive forces «overcomes low kinetic energy» ✔

Which technique involves breaking covalent bonds when carried out on an organic compound?

mass spectrometry

List the type of atomic or molecular process and the region of electromagnetic spectrum for:

-HNMR

-IR

HNMR:

-(change in) nuclear spin

- radiowave

IR:

-bond vibrations/stretching and bending of bonds

-infra-red

Define index of hydrogen deficiency.

-the degree of unsaturation for an organic compound

Deduce the splitting patterns in the 1H NMR spectrum of C2H5Cl.

triplet AND quartet

Explain the splitting pattern, indicating the hydrogen responsible for a quartet.

Formula: CH3CH(OH)COOH

quartet means next C has 3 H atoms / is CH3CH3;

due to the CH group;

due to relative orientation of spinning nuclei/protons;

with relative probabilities of 1,3,3,1;

OH group results in no splitting (due to rapid proton exchange);

State the type of bond fission that takes place in a SN1 reaction.

heterolytic

State the type of solvent most suitable for SN1.

polar protic

Outline, in terms of the bonding present, why the reaction conditions of halogenation are different for alkanes and benzene.

benzene has «delocalized» ππ bonds «that are susceptible to electrophile attack» AND alkanes do not

Outline why the signal at 1.0 ppm is a triplet.

next to a carbon atom bonded to two H atoms/protons / OWTTE;

Suggest why it is necessary to include an internal reference standard in a sample when its 1H1H NMR spectrum is being determined and why tetramethylsilane (TMS) is used for this purpose.

changes in absorption frequency (very) small / magnetic field can vary (slightly);

Accept "providing a point from which to measure the shift" / OWTTE.

only single signal / well removed from other signals / very strong absorption / chemically inert / non-toxic / easily removed from sample;

Explain why tetramethylsilane (TMS) is used as a reference standard in 1HNMR spectroscopy.

strong single peak as there are 12 protons in identical chemical environment;

absorbs upfield/away from most other protons/H's;

low boiling point/bp / volatile (so easily removed from sample);

not toxic / unreactive / does not interfere with sample;