Identification tests Flashcards
What are the various tests to identify a carboxylic acid
Carboxylic acids change the colour of many indicators and react with active metals to produce hydrogen gas (confirmed by the pop test). These acids also react with alcohol to produce strong-smelling esters. The addition of sodium carbonate will produce carbon dioxide gas, indicated by effervescence or when bubbled gas turns limewater cloudy.
How to identify whether alcohol is primary, secondary or tertiary alcohol
React with acidified potassium permanganate, the permanganate ion which is initially purple will see the following changes, depending on whether alcohols oxidise:
primary -> aldehyde -> carboxylic acid, soln. decolourises
secondary -> ketone, soln. decolourises
tertiary -> no reaction, soln. remains purple
How to identify aldehyde
Add benedict’s solution to a sample, if the compound is an aldehyde, then a red precipitate will form
What are the various tests to identify an alkene?
Alkenes are ‘unsaturated’ compounds and unlike alkanes, contain highly reactive carbon-carbon double bonds. Hence can undergo addition reactions with halogens.
Bromine water is used to test for the presence of alkenes, as it decolourises when added to alkenes, however, must be conducted in the absence of UV light.
How to merely test/confirm for the presence of OH
Sodium test, reacting alcohol with a small amount of dry sodium will produce a sodium alkoxide and hydrogen gas, the presence of hydrogen can be confirmed by the pop test. Note, that alcohol must not contain water, as sodium will react violently with water.
How can we distinguish between primary and secondary alcohol?
Lucas Reagent, react with a zinc chloride/hcl mixture, tertiary alcohols will react immediately, secondary react slowly and primary does not react at all. This is a substitution reaction which forms an alkyl chloride. This forms an organic layer that is immiscible with water and less dense hence sits atop, a separate layer. The insoluble ppt settles at the bottom, creating a turbid bottom layer and clear top layer in a secondary alcohol solution, while the primary alcohol solution remains clear.
How is NMR used to obtain information about the structure of compounds?
The nuclei of atoms containing an odd number of nuclear particles spin. Spinning nuclei develop a magnetic field which causes them to behave like tiny bar magnets. In the presence of an external magnetic field, nuclei with spin can line up either in the same direction as the field (lower energy) or in the opposite direction (higher energy). When nuclei are inside an NMR spectrometer, radio waves are applied to provide the energy to ‘flip’ the nuclei into a higher energy state. Over time, the nuclei flip back into a lower energy state and in doing so release a pulse of energy. This pulse is measured and displayed in graphical form as an NMR spectrum (give an example of a group and its shift). The difference in energy between higher and lower energy spin states depends on the type of nucleus and the chemical environment surrounding it. There are two types of NMR spectroscopy, carbon-13 and hydrogen-1. Which provides info about the number, position and type of carbon and hydrogen nuclei in an organic compound.
How is Infrared spectra used to obtain information about the structure of compounds?
In Infrared spectroscopy, an IR lamp passes infrared waves through a sample of a compound. Some frequencies make some of the bonds vibrate. When these bonds vibrate, they absorb energy from the IR light source. A detector recreates these as a spectrum with each absorbance peak representing a particular bond vibrating. Hence, IR spec provides info about the functional groups and type of bonds between atoms, e.g. C=C group generally occurs between 1620 and 1680 cm^-1.
