7 - analytical techniques

Question 1

how does the mass spectrum of an element appear?

Answer 1

simple, small number of vertical lines (peaks) each one representing an isotope of the element

Question 2

how does the mass spectrum of an organic compound appear?

Answer 2

complex, large number of peaks

Question 3

what is the molecular ion peak?

Answer 3

the peak furthest to the right with the greatest m/z value - the result of the organic molecule losing an electron in the mass spectrometer
e.g. Butane
C4H10 + e- –> C4H10 + 2e-
electron collides and knocks off another electron. m/z value is 58, butane’s relative molecular mass

Question 4

what is the m + 1 peak?

Answer 4

a very small peak to the right of the molecular ion peak - due to naturally occurring carbon-13 isotope

Question 5

what are other small peaks a result of in mass spectrometry?

Answer 5

fragmentation in the mass spectrometer, rearrangement reactions and the loss of one or more electron

Question 6

what is formed when a carbon-carbon bond in the molecular ion formed from a hydrocarbon is broken?

Answer 6

another positive ion or a neutral species (usually a radical)

Question 7

example of fragmentation…

Answer 7

ethane - form methyl cation and a methyl radical
*radicals are not detected in a mass spectrometer so all the peaks formed by fragmentation are caused by positive ions

Question 8

how do you label the axis of a mass spectrum?

Answer 8

y = relative intensity (%)
x = m/z

Question 9

what is the tallest peak referred to?

Answer 9

base peak - represents the ion with the highest abundance, also most stable fragment

Question 10

what happens when molecules absorb infrared radiation?

Answer 10

stretching - where the bond length increases and decreases
bending - where the bond angle increases and decreases

Question 11

what affects the amount of energy absorbed from infrared radiation?

Answer 11

length and strength of bond and mass of each atom involved in the bond
non polar molecules do not absorb infrared radiation

Question 12

what does infrared spectrum look like?

Answer 12

the bonds in the molecules absorb radiation in some parts of the spectrum but not from others

Question 13

how do you label the axis of infrared spectroscopy?

Answer 13

y - transmittance (%)
x - wavenumber /cm-1 (usually 4000 to 500)

Question 14

what are the dips/troughs in an infrared spectra referred to as?

Answer 14

absorptions - importance is the intensity - strong intensity = low transmittance value

Question 15

what are the three ways of understanding infrared spectra?

Answer 15

• predicting the spectrum of an organic compound
• deducing the functional groups from a list of wave numbers
• deducing the structure from wavenumbers and molecular formula

Question 16

what is the fingerprint region?

Answer 16

ignored region where very slight absorptions may occur - different for every molecule