Organic Anaylisis Flashcards
(22 cards)
What is mass spectrometry used for in organic chemistry
To determine the molecular mass and molecular formula of an organic compound
What is the molecular ion (M+)?
The molecule formed when a molecule looses one electron
(M –> M+ + e-)
It has the same mass as the Mr of the compound
What peak corresponds to the molecular ions in a mass spectrum
The peak with the highest m/z value (excluding very small peaks from isotopes)
What is m/z value
Mass to charge ratio. Since most ions have a +1 charge, m/z = mass
What causes fragmentation in mass spectrometry
Molecular ions breaking into smaller ions and neutral fragments due to instability
What do they peaks other than the M+ peak represent
Fragment ions from the breakdown the molecular ion
How can fragment peaks help identify a compound
Look in the finger print regions. Each compound fragments in a unique way. The pattern of fragments is like a fingerprint region
What is the base peak
The tallest peak in the spectrum (most abundant ion). It’s given an intensity of 100%.
What is the M+1 peak?
A small peaks one unit higher than the molecular ion peak due to the C¹³ isotope
How does the M+1 peak determine the number of carbon atoms?
The bigger the M+1 peak, the more carbon atoms the molecule obtains
How does high resolution mass spectrometry differ from low resolution?
High resolution mass spectrometry can give the molecular formula by measuring masses to 4+ decimal places
Example: What does a molecular ion peak at m/z = 88 suggest?
The Mr of the compound is 88. Look for compounds that fit this Mr
What information is essential from a spectrum
. Molecular ion peak (gives Mr)
. Fragment peaks (suggests structure or groups)
. M+1 or isotope peaks (confirms elements)
How do you tesr for alkenes (C=C)
Add bromine water
Positive result: orange to colourless
How do you test for halogenoalkanes
Add NaOH and warm it. Acidify with HNO³, then add AgNO³
Positive result:
. Chloride: white precipitate
. Bromide: cream precipitate
. Iodide: yellow precipitate
How do you test for alcohols
Add acidified potassium dichromate (K²Cr²O⁷ + H²SO⁴)
. Primary or secondary alcohols: orange to green
. Tertiary alcohols: no change
How do you distinguish between 1°, 2° and 3° alcohols
Oxidation with acidified K²Cr²O⁷:
. 1° aclohol: aldehyde –> carboxylic acid
. 2° alcohol: kentone
. 3° alcohol: no oxidation
How do you test for aldehydes
. Tollens reagent (AgNO³ +NH³):
Silver mirror forms
. Fehlings solution: blue to brick red precipitate
How do you test for kentones
Do not react with tollens or fehlings - no visible change
How do you test for carboxylic acids
Add sodium carbonate or hydrogen carbonate.
Positive result: effervescence
How do you test for phenol (Ar-OH)?
. Doesn’t react with sodium carbonate (not acidic enough)
. Add FeCl³ (aq): a purple complex forms
How do you test for carbonyl groups in general
Add 2,4 - DNPH
(2,4-diniteophenylhydrazine)
. Positive result: orange precipitate (hydrozone derivative)
