Organic Chemistry Exam II - Official Flashcards

why do we use mass spectrometry?

to determine chemical formula of a compound

what does mass spectrometry measure

it measures masses that are shown on a spectrum

what scale is used in mass spectrometry

u or amu

describe the 7 steps of mass spectrometry as explained in the lecture

1. start with the molecule of interest

2. atomize the sample (spread out molecules in space)

3. use an electron beam to ionize the molecule (knock off an electron from each particle)

4. a radical cation is produced (M+*)

5. using a negative plate, accelerate the cations into the mass spectrometer

6. the inoic beam interacts with a magnetic plate and deflects. deflection depends on mass

7. particles reach the detector and we can come up with experimental data

when a beam of electrons is added to a molecule, what happens in mass spectometry?

an electron is kiked off, forming a radical cation (a compound with a radical that is positively charged)

why do radical cations fragment? which fragments are detect in mass spec?

radical cations fragment because they are high energy and want to break down into smaller, more stable fragments. only positively charged fragments are analyzed in mass spec

what is the highest mass value on a mass spec graph associated with?

it is associated with the original sample. we can use the mass given by the graph to determine the molecular formula by using the masses of elements from the periodic table

if there are clusters around certain peaks, what could these represent?

these could represent isotopes of that particular element, for example carbon has the isotope C13

what do the following values represent: 15, 29, 43, 57, 71

15 - 1 carbon fragment removed

29 - 2 carbon fragment removed

43 - 3 carbon fragment removed

57 - 4 carbon fragment removed

71 - 5 carbon fragment removed

what is the base peak associated with on mass spec graph

it is a reference peak that has a relative abundance of 100%. all other peaks on the graph have an abundance relative to the base peak. largest peak is the base peak.

why is the base peak the most abundant particle?

molecules can be cut in anyway to produce a radical and a cation. this explains why the base peak is the most abundant. was the preferred pathway used to create stable fragments? if there are more stable particles, there are more abundant peaks and stable fragments

how can we achieve molecular formula without a diagram of the molecule given?

1. zoom in on the molecular ion peak

2. use the rule of 13’s (13 mass units for one C-H bond)

3. any remainder is equivalent to additional hydrogens if the molecule given is a hydrocarbon

if the molecular ion peak is 86amu, and there is a peak labelled (M-15), what does this indicate?

M-15 indicates that a one carbon fragment has been chopped off from the original molecule

the same concept applies to the following numbers 29, 43, 57, 71, where each corresponds to a carbon fragment that has been cut off from the original fragment

how can we find molecular formula in mass spectrometry if heteroatoms are involved?

if a heteroatom is involved, it will be indicated by an IR spectrum, for example a carbonyl peak at 1710cm-1 would indicate the presence of a heteroatom

take the mass of the heteroatom and equal it to the number of C-H bonds it will equal to. for example one oxygen atom weighs 16 mass units, which is equal to a CH4 unit. then take that CH4 unit off of your original molecular formula for a hydrocarbon fragment

what hydrocarbon fragment are oxygen and nitrogen equal to?

oxygen is equal to CH4

nitrogen is equal to CH2

outside of looking at the fragments and molecular ion peak to find molecular formula, what is a second method that can be used to determine the number of carbon atoms?

use a plug and chug formula where

((M+1) / (M)) / (1.1%) x 100 = # of carbons

the molecular ion peak will be 100, while you must find the percentage that of the isotopic abundance in relation to the molecular ion peak

what are the isotopic abundances for chlorine atoms?

Cl35:Cl37

Ratio of 1:3

(M+2) Peak

what are the isotopic abundances for bromine atoms?

Br79:Br81

Ratio of 1:1

(M+2) peak and molecular ion peak are equal in size

how is presence of iodine identified

large gap between molecular ion and iodine

the gap is -127 large

(M-127) would indicate iodine presence

be able to draw heterolysis (important for the exam) with bromine

bromine is attach to a Ch3-Ch2-Ch2 fragment

an electron is knocked off to form the molecular ion. an electron leaves Bromine (bromines can be of either isotope)

knock off bromine from the molecular ion fragment to form a bromine radical and a positively charged Ch3-Ch2-Ch2 fragment.

only the hydrocarbon fragment would be observed on the mass spec graph

how can we tell if there are nitrogen present in a molecule in mass spec

molecular ions will have an odd mass

fragments will have an even mass

nitrogens like to form three bonds

heterolysis with a chlorine atom

CH3CHCLCH3 fragment

an electron is kicked off, and leaves Chlorine with a radical and a positive charge

kick off chlorine and the positive charge moves to the middle CH

the hydrocarbon fragment would be observed since it has a positive charge

explain the process of alpha cleavage and draw it out

alpha carbon carries a heteroatom

CH3-ChCl - CH3

heteroatom carries the positive charge

Rearrange the molecule to form a CH3 radical and a positively charged CH3-Ch-Cl fragment

refer to notebook, but you might have to upload an actual image later