KA 4: experimental determination of structure

Question 1

what is elemental microanalysis used to determine?

Answer 1

masses of C,H,O,S and N in a sample of an organic compound to determine it’s empirical formula

Question 2

what does empirical formula show?

Answer 2

simplest ratio of elements in a molecule

Question 3

two ways elemental microanalysis can be determined

Answer 3

-combustion product masses
-percentage product by mass

Question 4

what can mass spectrometry be used to determine?

Answer 4

the accurate gfm and structural features of an organic compound

Question 5

describe the process of mass spectrometry

Answer 5

-small sample of organic comp. bombarded by high-energy elecs

-electrons therefore removed from organic molecule generating + charged molecular ions (parent ions)

-these break into smaller + charged ion fragments

Question 6

how is a mass spectrum obtained?

Answer 6

showing a plot of the relative abundance of the ions detected against the mass-to-charge (m/z) ratio

Question 7

what can the mass-to-charge ratio of the parent ion be used to determine?

Answer 7

gfm of molecular ion

.:. molecular formula can be determined using emp. formula

Question 8

the fragmentation data can be interpreted to gain…

Answer 8

structural information

Question 9

what is infrared spectroscopy used to identify?

Answer 9

certain functional groups in an organic compound

Question 10

describe what happens when infrared radiation is absorbed by organic compounds

Answer 10

-bonds within molecule vibrate (stretch and bend)
-wavelengths of i.r that are absorbed depend on type of atoms that make up bond and strength of the bond

Question 11

describe what happens in infrared spectroscopy

Answer 11

-infrared radiation passed through a sample of the organic compound
-then into detector that measures the intensity of the transmitted radiation at different wavelenghts

Question 12

how is the absorbance of infrared radiation measured

Answer 12

wavenumbers (reciprocal of wavelength) cm-1

Question 13

where are charateristic absorptions by particular vibrations given?

Answer 13

data booklet

Question 14

what does proton NMR or 1H NMR stand for?

Answer 14

proton nuclear magnetic resonance spectroscopy

Question 15

what info does proton NMR provide?

Answer 15

-different chemical environments of H atoms (protons or 1H in an organic molecule)
-how many hydrogen atoms there are in each of these environments

Question 16

describe the behaviour of 1H nuclei

Answer 16

-behave like tiny magnets

-in a strong magnetic field, align w/ the field (lower E) while the rest align against it (higher E)

Question 17

describe what happens when 1H nuclei absorb radiation in the radio frequency

Answer 17

-they “flip” from lower energy to higher energy alignment

-as they fall back from higher to lower E alignment the emitted radiation is detected and plotted on the spectrum

Question 18

describe a 1H NMR chemical shift

Answer 18

δ (peak position) is related to the environment of the 1H atom and is measured in parts per million (ppm)

Question 19

what is the area under the peak related to?

Answer 19

the number of 1H atoms in that environment and is often given by an intergration curve in a spectrum

Question 20

what is the height of an intergration curve proportional to?

Answer 20

the NO. of 1H atoms in that environment, and so a ratio of 1H atoms in each environment can be determined

Question 21

what is the standard reference substance used in 1H NMR spectroscopy?

Answer 21

tetramethylsilane (TMS), which is assigned a chemical shift value equal to zero

Question 22

what can 1H NMR spectra can be obtained using?

Answer 22

low-resolution or high-resolution NMR

Question 23

difference between high/low-resolution 1H NMR?

Answer 23

high: uses higher radio frequencies than low res

Question 24

pro of high-res over low-res

Answer 24

provides more detailed spectra

Question 25

in a high res 1H NMR, what results in the splitting if peaks into multiplets?

Answer 25

an interaction w/ 1H atoms on neighbouring carbon atoms

Question 26

what does the NO. of 1H atoms on neighbouring carbon atoms determine?

Answer 26

the NO. of peaks within a mulitplet and can be determined using the n+1 rule, where n is the number of 1H atoms on the neighbouring carbon atom

Question 27

Low- and high-resolution 1H NMR spectra can be _______, and low-resolution 1H NMR
spectra can be ________for any given compound.

Answer 27

-analysed
-sketched