Special Reactions and Diagnostic Reactions

19.   Oxidation of an Alcohol
          This is a neat reaction that allows you to transform an alcohol into a ketone.


The placement of the ketone in the starting molecule is important because you get aldehydes or carboxylic acids.  Primary alcohols can be oxidized into aldehydes first and then further oxidized into carboxylic acids as seen below. This same reaction occurs in apples. The alcohol groups on the sugars oxidize into aldehydes. This turns the white pulp into brown. The smell also becomes "musty" which is a characteristic smell of rotting apples. Further oxidation converts the aldehydes into carboxylic acids which makes the pulp go mushy.
eg.  propan-1-ol is first oxidized into propanal and then further oxidized into propanoic acid



20.  Substitution of OH with a Halogen
This reaction is useful in changing alcohols, which are readily available, commercially into more valuable and harder to find halocarbons. The R group below can be any hydrocarbon chain. X can be any halogen.

R-OH + PBr3 --------> R-Br

R-OH + SOCl2 --------> R-Cl

R-OH + CH3X --------> R-X
eg.  pentan-3-ol is converted into 3-chloro-pentane




21.  Change a Ketone into an Alcohol
This is also used as a positive test for a ketone, but since LiAlH2 is hard to prepare, extremely expensive and explosive it is not allowed in high school labs.

eg.  penta-3-one is converted into penta-3-ol



22.  Tollen's Silver Mirror Test
This test was used commerically to plate silver onto the back of plate glass to make mirrors. It was also used to convert aldehydes into carboxylic acids. It is not used any more because silver is too expensive. The aldehyde most often used was formaldehyde which is considered to be a poison now.  The silver mirror reagent must be made fresh because it tends to be mildly explosive if left sitting too long and there are better ways to coat glass. i.e. vapourized metallic spray.

eg. benzaldehyde is converted into benzoic acid



23.  Fehling's or Benedict's Test for Aldehydes
These reactions both test for the presence of aldehydes by oxidizing Cu+2 ions which are blue into copper metal which is in stages green then brown. See the organic labs for more details.


24. Reduction of a Carboxylic Acid to an Aldehyde
    Reduction is the opposite of oxidation.  Oxygen is removed from a molecule instead of being added in this reaction.

eg.  heptanoic acid is reduced to heptanal


25.  Iodoform Test for a Ketone
Iodoform is a specialized test that reacts only with ketones. It will not react with the ketone group found in an aldehyde or carboxylic acid therefore it is a definitive test. R can be any carbon chain grouping.

The iodoform precipitate is usually brightly coloured


26. Removal of Halogens
This reaction is used to remove halogens from an alkane chain. The Mg will only attack terminal halogens, i.e. ones that are on the end carbon. The ether is there to provide a H source for the replacement of the halogen and too prevent other side reactants from adding to the alkane chain. R can be any alkane chain.

             R-CH2-X     R-CH3

eg.      CH3CH2Cl     CH3CH3 + MgCl2



27.  Ester Formation
The formation of an ester involves a carboxylic acid and an alcohol. The reaction requires concentrated sulphuric acid as a dehydrating agent.

eg.  pentanoic acid reacts with phenol in an esterification reaction to create phenyl pentanoate

The best alcohol's are primary followed by secondary and tertiary alcohols.


28.   Rehydration of an Ester
Since an ester is made by the removal of water in a concentrated acid, it is resonable to expect that it can be split by adding water in a basic environment.

eg.   phenyl propanoate reacts with water to create propanoic acid and phenol



29.  Test for a Carboxylic Acid
The action of sodium carbonate and sodium bicarbonate both of which neutralize acids can react to produce CO2 bubbles which is an indication that there was carboxylic acid present.



30.  Anhydride Creation
This reaction takes two carboxylic acids and using a strong inorganic acid dehydrates a water molecule from between the two acids.  This creates an anhydride linkage.

eg.  Propanoic acid and hexanoic acid are dehydrated into hexanoic propanoic anhydride



31.  Anhydride Cleavage
This reaction is the opposite of the carboxylic acid dehydration that originally made the anhydride. By adding water and adding just enough heat to cause bond disruption the anhydride will cleave vigourously yielding the two original carboxylic acids.
eg. dipropanoic anhydride is hydrated into two molecules of propanoic acid



32.  Test for a Ketone
This is a conclusive test for a ketone. The regent displaces the O from a ketone and repleaces it with a rather complex group which always gives a bright yellow precipitate. It will not work on an aldehyde. R1 and R2 can be anything.  The reagent is 2,4-dinitrophenylhydrazine. (2,4-DNH)



33.   Cyano Converion to a Halogen
This reaction starts with a cyano compound and removes the cyano group by substitution with a halogen.  This may also result in the decrease of the C parent chain length by one carbon.

eg 1-cyano-3-methyl-butane is converted to 3-methyl-butanol



34.   Creation of an amine
Starting with a cyanide compound the reagent LiAlH2  with NH3 is used to create an amine
eg. 2-methyl-pentyl cyanide is converted into 2-methyl-pentamine



35.  Deamination to an Alkene
This reaction will deaminate or remove an amino group from a amine compound and will create an alkene bond.

eg.



36.  Grignard Reaction
This is a reaction that is used to create long molecules from shorter ones.  It uses a "Grignard Reagent" which is a chain of carbons that has been linked with MgBr to make it more reactive.  This MgBr sequence then attacks a ketone group leaving behind an alcohol and one chain linked to another.

eg.  1-benzyl-propan-2-one and butyl-MgBr combine to form 1-benzyl-1-hydroxy-1-methyl-pentane

eg.  butanoic acid and 4-benzyl-butyl-MgBr combine to form 1-benzyl-heptan-4,4-diol

Go to the Organic Reactions Worksheet