Organic analysis

Test for alkenes

• Add bromine water

  • Alkenes decolourise bromine water

Test for primary and secondary alcohols

• Warm with K₂Cr₂O₇ and H₂SO₄ (acidified potassium dichromate)

  • Orange solution turns green (remains orange for tertiary alcohols)

Test for aldehydes

• Warm with Tollens’ reagent

  • Silver mirror

• Warm with Fehling’s solution

  • Brick red precipitate

• (They also turn acidified potassium dichromate green)

Test for carboxylic acids

• Add Na₂CO₃ or NaHCO₃

  • Effervescence (due to CO_2⁾

Test for halogenoalkanes

• Warm with AgNO₃ in ethanol

  • Precipitate is seen: (white for RCl, cream for RBr, yellow for RI)

How to write equations for functional group tests?

• Bromine water to alkene is addition (star fish does the splits)

• Alcohols go to carboxylic acids (1º) or ketone (2º) using [O] as oxidising agent

• Tollens’ and Fehling’s oxidise aldehyde to COOH, using [O] as oxidising agent

• COOH oxidised form RCOONa and water and CO₂

What is infrared spectrometry?

• Used to identify types of organic compounds

• The position of each peak/absorption tells us which bonds are present and which functional groups in organic compounds

• Remember there are two different absorptions for O-H bonds

What is the effect of infrared radiation on covalent bonds?

Covalent bonds vibrate and absorb infrared radiation

What is the fingerprint region?

Each compound has a unique fingerprint region (<1500cm^-1). A computer database can be used for identification of compounds

Where would you find absorptions of C-H on the infrared spectrum?

Sharp jagged edges

Between 2800-3300 cm^-1

Where would you find absorptions of C=O on the infrared spectrum?

Usually quite intense (sharp and long)

Looks similar to C≡N but closer to the 1500 line (before 2000 is reached)

Where would you find absorptions of O-H (alcohol) on the infrared spectrum?

• Broad and smooth like a finger

• C-H absorption is next door (join together slightly)

Where would you find absorptions of O-H (acid) on the infrared spectrum?

• Upside down mountain

• Very broad with jagged edge on right hand side

• C-H absorption has been overlapped (swallowed by O-H absorption)

Where would you find absorptions of C=C on the infrared spectrum?

• Often small and usually very sharp (like a little needle)

• Similar space to C=O but much smaller

Where would you find absorptions of N-H on the infrared spectrum?

• Looks like a little pincer

• Similar place to O-H but much more jagged and smaller in size than O-H

Where would you find absorptions of C≡N on the infrared spectrum?

• After 2000 mark

• Very sharp and long (not common)

How to describe infrared spectrum?

• Identify bonds you think are present

• Use data sheet (back of periodic table)

• There is absorption between__ - __ cm^-1 due to _ = _ bond

• Or this is a ___ because ‘’ ‘’

Link between infrared and global warming

• Greenhouse gases (CO₂, CH₄ and water vapour) absorb IR because their bonds absorb IR and vibrate

• Infrared spectra of these compounds show big absorptions as the bonds are very efficient at absorbing IR

What is electron impact mass spectrometry?

• When organic compounds pass through mass spectrometer, the spectrum contains many lines. Lines before are due to fragment peaks (smaller molecules)

• Molecular ion peak has biggest m/z ratio NOT the most abundant - (gives Mr)

• Isotopic peaks after could be due to ¹³C or ²H

• Could have multiple peaks due to ³⁷Cl and ³⁵Cl

What is high resolution mass spectrometry used for?

• Different compounds that have different molecular formulas but the same Mr, so uses precise A_r values to distinguish between Mr

• High resolution spec would give 2 separate peaks for these compounds

• The precise relative atomic mass of ¹²C is exactly 12.000000 by definition

• Compounds which are functional group isomers of each other would have the same high resolution mass spec as they have the same molecular formula so the same precise Mr

How can combustion analysis be used to determine the empirical formula of compounds?

• Find the mass of C by 12/44 x mass of CO₂

• Find mass of H by 2/18 x mass of H₂O

• Mass of O = mass of compound - mass C - mass H

• Calculate the normal empirical formula using the chart