Organic Analysis

• alkenes test

  c=c, add Br2 (aq), orange-colourless
• test for 1 and 2 alcohols

  acidified potassium dichromate, warm in a water bath

  orange->green
• test for 3 alcohols

  H2SO4/K2Cr2O7

  warm

  stays orange
• tests for aldehydes

  Tollens' reagent- warm in water bath

  silver mirror forms

  Fehling's solution, warm

  blue -> brick red ppt
• test for carboxylic acids

  NaCO3/ NaHCO3

  fizzing
• test for halogenoalkanes

  NaOH (aq)

  add NaNO3

  White/ cream/ yellow ppt
• infrared spectroscopy

  analytical technique used to identify types of organic compounds. Used to detect presence of specific covalent bonds within a compound

  Each bond vibrates at a specific frequency and so it absorbs a specific frequency of IR radiation

  An infrared spectrum is a graph of % transmittance against wave number (cm-1). The position of each inverted peak tells us which bonds are present and therefore which functional groups are present in organic compound
• A table of IR absorption data

  the c=o is an intense peak. the c=c is often small. the c-h bond confirms the presence of an organic compound

  
• what is the fingerprint region?

  the region between 1500 cm-1 and 400 cm-1. It's unique for each compound. To identify a compound, compare the spectrum to a database
• why are peaks below 1500cm-1 ignored?

  the fingerprint region- unique to each compound
• O-H (alcohols) typical peak


  here:

  
• O-H (acids) Peak


  here:

  
• what are the many lines due to in electron impact mass spectrometry?

  fragment ions
• molecular ion peak

  the line with the largest m/z ratio- this gives the Mr of the compound. The line furthest to the right (except an isotope peak)
• why does the mass spectrum of chloromethane have peaks at m/z=50 and 52

  Mr of chloromethane= 50.5

  peak at m/z 50= due to CH3 35Cl

  peak at m/z 52= due to CH3 37Cl

  the m/z 50 peak is 3x bigger because 35Cl is 3x more abundant

  
• high resolution mass spectrometry

  can measure Ar and Mr up to 5 decimal places. Very useful when compounds have the same Mr when rounded to the nearest whole number
• Some accurate Ar values

  1H= 1.00794

  12C= 12.00000

  16H= 15.99491
• using high resolution mass spectrometry, calculate Mr of propan-1-ol and propanoic acid

  both have an Mr of 60.0 using periodic table Mr values, but with accurate Ar values: propan-1-ol= 60.05843 and propanoic acid= 60.02158
• the precise relative atomic mass of carbon-12 is exactly 12.00000 by defintion
• limitations of HR mass spec

  compounds with the same molecular formula will have the exact same precise mass values
• combustion analysis

  organic substances can be burnt in a large excess of oxygen and the products collected. The results can be used to determine the empirical formula of the compound
• how to calculate masses from combustion analysis?

  CxHyOz + excess O2 -> XCO2 + y/2 H2O

  • mass of C in CxHyOz= mass of C in CO2 produced. mass of C in CO2 produced= 12/44 x mass of CO2 produced
  • mass of H in CxHyOz= mass of H in H2O produced. mass of H in H2O produced= 2/18 x mass of H2O produced
  • mass of O= original amount- mass of C - mass of H