()
The equation for this with a tertiary alcohol and zinc chloride solution with concentrated hydrochloric acid will be :
Apparatus needed:
- Acidified potassium dichromate
- Test tube
- Unknown substance
- Water bath
Presence of a carbonyl group?
I will do this test in order to see whether a carbonyl group is present. If a carbonyl group is present this means that the unknown organic compound will be a ketone or an aldehyde.
The test I will do will be using 2-4-dinitrophenylhydrazine. First of all I will add 5 drops of 2-4-dinitrophenylhydrazine in a test tube and then add 3 drops of the unknown organic compound and then allow it to stand for 2-3 minutes. A positive test will show an orange crystalline precipitate which is called a hydrazone.
Apparatus needed:
- Test tube
- 2-4-Dinitrophenylhyrazine
- Unknown organic compound
This shows the reaction with 2-4-dinitrophenylhydrazine with 2-propanone. A orange precipitate is positive for a carbonyl group.
Aldehydes
Aldehydes contain carbonyl (-C=O) groups and are therefore named as the carbonyl groups. Aldehydes are unsaturated as they contain less than maximum possible number of hydrogen’s. When naming Aldehydes, they have the suffix of –al. E.G. ethanal and they have the common structure of:
O
//
R---C
\
H
The bond angles in Aldehydes are close to 120 and are trigonar planar in shape (Revise A2 chemistry for OCR A by Helen Eccles and Mike Wooster) Aldehydes can be oxidized by reflux to carboxylic acids or be reduced to primary alcohols. The tests which I can use to identify an aldehyde are:
-
Tollens reagent – to about 4cm3 of silver nitrate solution in a test tube, add one drop of sodium hydroxide solution followed by 2M ammonia solution drop by drop, with shaking, until the initial precipitate just dissolves. To about 0.5cm3 of this solution in a semi-micro test tube, add one drop of the substance and heat in a water bath. If the test is positive the solution a silver mirror will form.
-
Fehling’s solution – Mix 0.5cm3 of Fehling’s solution A with 0.5cm3 of Fehling’s solution B in a small test tube. Add 2 or 3 drops of the unknown organic compound and heat the tube. If the test is positive the solution will go from blue to red.
I have decided to use the Fehlings test and the tollens reagent as these two tests will make sure it has a aldehyde present we can eliminate that to find out whether the unknown compound is a ketone . Fehlings test, in the the blue Cu2+(aq) complex solution forms a brown precipitate of copper(I) oxide Cu2O(s). The aldehyde reduces the copper ion and in the process gets oxidised to a carboxylic acid.
Apparatus needed:
- Fehlings solution A
- Fehlings solution B
- Test tube
- Unknown organic compound
- Hot water bath
- Beaker
For tollens
- Silver nitrate solution
- Sodium hydroxide
- Ammonia solution
- Water bath
- Unknown organic compound
When these two have been used and we have found out that it is not an aldehyde we will be use the 2-4-dinitrophenylhydrazine test to test positive for a ketone as aldehyde has been eliminated
Equation for tollen’s :
RCHO + 2{Ag(NH3) } OH → RCOO NH4 + 2Ag + H2O + 3NH3
Where R is a functional group
Equation for fehlings solution is ??
Ketones
Ketones also contain the carbonyl group C=O and are closely related homologous series as they both are carbonyl compounds. As they are closely related this means Ketones are also unsaturated and trigonar planar in shape. However, they do not have the same structure, as aldehydes have hydrogen attached to the carbonyl carbon but ketones do not. Ketones have two alkyl groups:
O
//
R1---C
\
R2
All names of Ketones end in –one as the carbon chain is named with –one at the end.
There are no tests which can identify Ketones from the AS/A2 syllabus. Therefore is there if no reaction from the Fehlings test and all other functional groups have been ruled out. We would assume a ketone is the functional group of the unknown organic compound.
Apparatus needed would be same as the apparatus needed for the aldehyde test – Fehling’s solution. If both of these tests were negative, we would then use the 2-4dinitrophenylhydrazine to see whether a carbonyl group was present. If this tests positive it is a ketone.
Carboxylic acid
Aldehydes and primary alcohols oxidize by reflux to give carboxylic acids. They normally smell like vinegar. There structural formula is:
O
//
R---C
\
O—H
Water is formed when carboxylic acids are reacted with alkali’s e.g. sodium hydroxide because the hydrogen on the –COOH group is highly electron deficient due to the 2 electronegative oxygen’s pulling the electrons towards themselves and away from the hydrogen in the –OH group. Therefore the hydrogen is lost as an ion and forms water with alkalis.
Carboxylic acids are typically weak acids and they partially dissociate into hydrogen ion cations and RCOO anions in aqueous solutions. R is an alkyl group or can b hydrogen atom. The suffix for carboxylic acids is –oic ( ). Carboxylic acids form salts when reacted with metals, alkalis, carbonates and basic metal oxides.
- Metals produce hydrogen
- Alkali’s and basic metal oxides produce water
- Carbonates produce carbon dioxide and water
The test I will be using to identify a carboxylic acid will be using a bi-carbonate. I will mix a few drops of the substance with about 2-3cm of water. Add a little sodium hydrogen bi-carbonate and then test for the identity of any gas released. As I am using a bi-carbonate, I will test for carbon dioxide present; if carbon dioxide is present when bubbles through lime water the test is positive for the presence of a carboxylic acid.
Apparatus needed:
- Water
- Unknown organic compound
- Sodium hydrogen carbonate
- Test tube
- Limewater
E.g CH3COOH + NaHCO3 → CH3COONA 2 + CO2 +H2O
Esters
Esters can be formed from the reaction of carboxylic acid and alcohol in the presence of an acid catalyst and heat to give an ester and water. This process is called esterification.
Carboxylic acid + alcohol Ester + water
E.g. Ethanoic acid + ethanal Ethyl ethanoate + water
CH3COOH + C2H5OH CH3COOC2H5 + H2O
Esters have the functional group of:
O
//
R---C
\
O--R
R would either be hydrogen or an alkyl group. Esters have a pleasant fruity smell e.g. apple and evaporate very easily due to their low boiling point, therefore they are used in the manufacture of perfumes.
The identification test I will be using will be: Mix the ester (unknown organic compound → U.O.C) with 2-3cm3 of water. Add a drop of phenolphthalein and shake the test tube. Add sodium hydroxide till there’s a change in color. Heat in a water bath and if the test is positive the substance will go clear again.
Apparatus needed
- Water
- Phenolphthalein indicator
- Water bath
- Sodium hydroxide
- Unknown organic compound
- Test tube
Others tests I could have used would be using litmus paper as when litmus is put into an ester substance, it is neutral.
Equation between sodium hydroxide and ethyl ethanoate
NaOH + CH3COOC2H5 → CH3COONa + C2H5OH
Phenol
The structure of phenol is:
Where –OH is joined to a benzene ring. The formula of phenol is C6H6OH. With a base, phenol acts as a acid and forms water and a phenoxide ion as the hydrogen from the –oh is substituted to give 0 base + water
The reason why phenol produces water is because it is partially sparingly soluble in water. Therefore phenol ionizes slightly in water so the –OH bond breaks to form a hydrogen ion and phenoxide ion. The bond is more readily in phenol than water so phenol is more acidic than water (chemistry 2 endorsed by OCR page 17 in the blue box)
The identification test I will use for this will be using ferric (iron iii) chloride. I will add 5 drops of ferric chloride; add 2 drops of substance and 2 drops of water. If the test is positive the color of the solution will turn purple. The Iron(III) ions form strongly coloured complexes with several organic compounds including phenol. The colour of the complexes vary from compound to compound. You get an intense violet-purple solution formed when phenol is added to the solution.
(http://www.chemguide.co.uk/organicprops/phenol/other.html)
Apparatus needed:
- Ferric chloride
- Test tube
- Water
- Unknown organic compound
Other tests I could have used would have been adding the substance to sodium hydrogen carbonate. If it doesn’t dissolve this would have suggested a phenol was present.
Flow chart of tests
