2.5 instrumental analysis

Question 1

what is the empirical formula?

Answer 1

the simplest whole number ratio of the elements found in a compound

Question 2

what are the steps to working out the empirical formula?

Answer 2

1) mass of each element
2) divide each by its GFM
3) divide each by the smallest number

Question 3

what does mass spectrometry tell us about?

Answer 3

the structures within a molecule

Question 4

how are fragments formed in mass spectrometry?

Answer 4

1. sample bombarded with high-energy electrons causing electrons to be knocked off
2. forms positive ions which break down into smaller fragments

Question 5

how are the fragments recorded in mass spectrometry?

Answer 5

deflected onto a detector that records the mass/charge ratio of the fragment.

Question 6

in mass spec, what s the parent ion?

Answer 6

the peak with the highest m/z

Question 7

what does the parent ion tell us about?

Answer 7

the gfm of the compound

Question 8

what is infrared spec used to identify?

Answer 8

certain functional groups in a compound

Question 9

IR radiation is passed though a compound in an IR spectrometer. Depending on the … , …

Answer 9

-type of bonds present, different wavelengths of radiation are absorbed

Question 10

what does an IR detector observe?

Answer 10

the intensity of radiation absorbed by a sample at different wavelengths

Question 11

what is the absorbance of IR measured in?

Answer 11

wavenumbers (cm^-1)

Question 12

IR radiation can cause parts of a molecule to …

Answer 12

vibrate

Question 13

name the types of vibration that occur as a result of IR radiation

Answer 13

stretching, bending, scissoring

Question 14

different vibrations …

Answer 14

absorb different wavelengths of IR

Question 15

the wavelength of IR absorbed depends on:

Answer 15

the strength of the bond
the type of atoms attached to the bond

Question 16

from which region of IR spec should we try to identify peaks from?

Answer 16

above 1500cm^-1

Question 17

what information does proton NMR give us?

Answer 17

info about different chemical environments of hydrogen atoms and how many hydrogen atoms are in each of these environments

Question 18

when exposed to a powerful magnet, hydrogen atoms either:

Answer 18

align themselves with the field (lower energy),
or align themselves against the field (higher energy).

Question 19

describe the process of producing an NMR spectrum

Answer 19

Absorption of radiation in the radio frequency causes the H nuclei to ‘flip’ from
the lower to the higher energy alignment.
As they fall back from the higher to the lower
the emitted radiation is detected and plotted on a spectrum.

Question 20

the amount of energy released by hydrogens falling to low energy state is affected by…

Answer 20

the environment of the H atoms

Question 21

what is the standard reference substance used for proton NMR?

Answer 21

tetramethylsilane (TMS)

Question 22

how many proton environments does TMS have?

Answer 22

1

Question 23

what is the chemical shift value of TMS?

Answer 23

0

Question 24

which page of the data book gives chemical shift values for different proton environments?

Answer 24

page 17

Question 25

what does low-resolution proton NMR show?

Answer 25

the environments of the peaks and the number of hydrogens at each environment

Question 26

what does high-resolution proton NMR use?

Answer 26

higher radio frequencies than low resolution and produces more detailed spectra

Question 27

what does high-resolution NMR show us?/

Answer 27

how many H neighbours each environment has

Question 28

list everything that low res proton NMR tells us

Answer 28

1. the number of different environments that H is found in (no. of peaks - ref)
2. the chemical environment of the hydrogens (see chemical shift values on pg 17)
3. the number of H atoms in each environment (relative heights)

Question 29

what does a doublet mean?

Answer 29

there is 1 H atom on the adjacent C

Question 30

what does a triplet mean?

Answer 30

there are 2 H atoms of the neighbouring c