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Flashcards in Organic Analysis Deck (29)
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1

What is a mass spectrometer used for?

To measure the relative mass of isotopes

2

Describe electron impact ionisation

High energy e- fired from an electron gun at a sample to knock off an e-

X(g)—-> X+(g) + e-

3

When does fragmentation occur?

When covalently bonded molecules have their bonds broken during electron impact ionisation and these molecules break into fragments

4

Does fragmentation occur to all molecules?

Nope

5

What happens to the molecules that don’t get fragmented?

What do these give?

They remain intact and gain a positive charge

The peak with the highest m/z value
This will tell us the Mr

6

Describe electrospray ionisation

Substance dissolved in volatile substance and injected through hypodermic needle

Tip of needle has a high voltage

Each particle ionised by gaining a proton

7

Generic equation for electrospray ionisation

Does fragmentation occur?

X(g) + H+ —-> XH+(g)

No

8

When you find the m/z value of a molecule after electrospray ionisation, what must you do and why?

Minus 1

To account for added H+

9

With electrospray ionisation what does the Mr=?

The peak with the greatest Mr value minus one

10

With both techniques there will be a very small peak at a m/z value of 1 greater than the molecular ion peak. What causes this and what do we do with it?

Some molecules contain either C13 or H2

We ignore it

11

Do isotopes have exactly whole number atomic masses?

What is the exception?

No

Carbon-12 by definition

12

Define molecular ion

A molecule with 1 electron removed

13

Why is high resolution mass spectrometry more useful?

Some molecules have the same Mr to one decimal place so more precision is necessary

14

How does IR spectrometry work?

Causes bonds to vibrate

The stronger they are the faster they vibrate

15

What is the shape of the O-H wavelength peak?

Always broad

16

What bond must you always clarify for IR?

O-H
Whether it’s acid or alcohol

17

Where is the fingerprint region?

Below 1500

18

Do you acknowledge peaks below 1500?

Why?

No

This is the fingerprint region so peaks are hard to distinguish

19

Why is a C-O Bond hard to identify in IR spectrometry

It is in the fingerprint region

20

How can IR spectrometry be used to identify impurities?

Reveals absorptions that should not be there

21

How is IR used to identify greenhouse gases

The more effective the greenhouse gas the more IR radiation it absorbs

22

What peaks should you ignore around 2850-3300?

C-H peaks as they are in almost every organic molecule

23

Does O-H and C=O peaks always indicate carboxylic acids?

No

24

How to differentiate between O-H (alcohol) peaks and N-H peaks?

Why would you need to?

O-H is broad
N-H appears as 2 peaks

They appear in same region

25

For IR what should you use if unsure?

The Mr

26

Which bond peak tends to be small?

C=C

DONT MISS IT

27

Explain how infrared spectroscopy can be used to show that an aldehyde is definitely pentanal

Use the compound’s fingerprint region and use an internet data base search to find an EXACT match

28

How to test if an organic product is pure

Measure melting point using melting point equipment apparatus

A sharp melting point/ a melting point that matches the data book indicates a pure substance

29

You are provided with a small sample of pure aspirin in a melting point tube. Describe briefly how you would determine an accurate value for the melting point of aspirin (2)

Heat using melting point apparatus

Heat slowly near melting point