Topic 19: Modern Analytical Techniques II

Question 1

What analytical technique provides information about the positions of ¹H or ¹³C atoms in a molecule?

Answer 1

Nuclear magnetic resonance (NMR) spectroscopy

NMR identifies chemical environments of nuclei in organic compounds.

Question 2

Why does ¹³C NMR give simpler spectra than ¹H NMR?

Answer 2

Fewer signals and no coupling

¹³C atoms are less abundant and coupling with ¹H is often decoupled.

Question 3

Fill in the blank:

In NMR spectroscopy, chemical shifts are recorded on the ______ scale.

Answer 3

δ

The δ scale measures resonance relative to TMS in ppm.

Question 4

True or False:

The chemical shift of a nucleus depends on its molecular environment.

Answer 4

True

Electron density around the nucleus affects the magnetic shielding.

Question 5

What does the integration of ¹H NMR signals indicate?

Answer 5

Relative number of protons

The area under a peak is proportional to the number of equivalent hydrogens.

Question 6

True or False:

NMR samples are typically dissolved in deuterated solvents to avoid interference from ¹H.

Answer 6

True

Solvents such as CDCl₃ or CCl₄ are commonly used.

Question 7

Fill in the blank:

Tetramethylsilane (TMS) is used as a standard because it produces a single signal at δ = ______.

Answer 7

0

It is inert, volatile and gives a sharp reference peak.

Question 8

How can ¹H NMR spectra be used to deduce the number of protons in different environments?

Answer 8

Integration and chemical shift

Each unique hydrogen gives a peak; integration shows relative numbers.

Question 9

True or False:

The n+1 rule predicts the number of peaks due to spin–spin splitting in ¹H NMR.

Answer 9

True

A proton with n adjacent non-equivalent protons splits into n+1 peaks.

Question 10

Fill in the blank:

In aliphatic compounds, spin–spin splitting can produce doublets, triplets and ______.

Answer 10

quartets

These arise from coupling with neighbouring protons.

Question 11

Why is TMS suitable as an NMR standard?

Answer 11

• Chemically inert and produces a single signal
• It is non-toxic
• It has a low boiling point (26°C) and so can be easily removed from the sample afterwards

Its δ = 0 peak provides a reference point for chemical shifts.

Question 12

How can ¹H and ¹³C NMR data be used to suggest molecular structures?

Answer 12

By comparing chemical shifts and splitting

Peaks, integration and multiplicity reveal the types and connectivity of hydrogens and carbons.

Question 13

When would you use CCl4 as a solvent?

Answer 13

When analysing non polar compounds.

Question 14

When would you use deuterated solvents in NMR?

Answer 14

When analysing polar compounds.

Question 15

What is the purpose of chromatography in chemistry?

Answer 15

To separate and identify components of a mixture.

Different types of chromatography are chosen depending on the sample and stationary/mobile phases.

Question 16

Name three common types of chromatography.

Answer 16

• Thin-layer chromatography
• Column chromatography
• Gas chromatography

TLC uses a plate, CC uses a packed column, GC uses a gas mobile phase with a column.

Question 17

Fill in the blank:

In TLC, a plate is coated with a ______ and a solvent moves up the plate.

Answer 17

solid

The solid acts as the stationary phase while the solvent is the mobile phase.

Question 18

True or False:

In column chromatography, the solvent moves down the column.

Answer 18

True

The stationary phase (solid) is packed in the column and components are separated by interaction with it.

Question 19

What factor determines the separation of substances in chromatography?

Answer 19

Relative solubility and retention

Compounds with higher solubility in the mobile phase move faster; retention is determined by affinity for the stationary phase.

Question 20

Fill in the blank:

The ratio used to quantify the movement of a spot in TLC is called the ______ value.

Answer 20

Rf

Rf = distance travelled by substance / distance travelled by solvent front.

Question 21

True or False:

Retention time in gas chromatography can be compared with standards to identify substances.

Answer 21

True

Each compound has a characteristic retention time under specific conditions.

Question 22

How can mass spectrometry be combined with gas chromatography?

Answer 22

To analyse separated components.

GC separates compounds and MS provides molecular weight and structural information.

Question 23

Fill in the blank:

In TLC, the Rf value is always between ______ and 1.

Answer 23

0

Rf = 0 if the substance does not move and Rf = 1 if it moves with the solvent front.

Question 24

What is the mobile phase in gas chromatography?

Answer 24

Gas

A carrier gas, such as helium or nitrogen, transports components through the column.

Question 25

# True or False: Rf values and **retention times** are compared with standards to identify unknown substances.

Answer 25

True ## Footnote This allows identification of compounds in mixtures.

Question 26

What laboratory application uses **TLC** to identify analgesics or metal ions?

Answer 26

Test-tube separation and identification ## Footnote TLC separates components for visualisation and comparison with known standards.