What is Hydrolysing?
Hydrolysis usually means the rupture of chemical bonds by the addition of water. Generally, hydrolysis is a step in the degradation of a substance. In terms of the word's derivation, hydrolysis comes from Greek roots hydro "water" + lysis "separation"
Usually hydrolysis is a chemical process in which a molecule of water molecule adds to a substance. Sometimes this addition causes the substance to split into two parts. In such reactions, one fragment of the target molecule (or parent molecule) gains a hydrogen ion (H+) from the split water molecule. The other portion of the target molecule collects the hydroxyl group (OH−) of the split water molecule. In effect an acid and a base are formed (http://en.wikipedia.org/wiki/Hydrolysis)
What is Hydrolysing Nitriles?
The hydrolysis of nitrilesIntroduction
When nitriles are hydrolysed you can think of them reacting with water in two stages - first to produce an amide, and then the ammonium salt of a carboxylic acid.
For example, ethanenitrile would end up as ammonium ethanoate going via ethanamide.
In practice, the reaction between nitriles and water would be so slow as to be completely negligible. The nitrile is instead heated with either a dilute acid such as dilute hydrochloric acid, or with an alkali such as sodium hydroxide solution.
The end result is similar in all the cases, but the exact nature of the final product varies depending on the conditions you use for the reaction.
Acidic hydrolysis of nitriles
The nitrile is heated under reflux with dilute hydrochloric acid. Instead of getting an ammonium salt as you would do if the reaction only involved water, you produce the free carboxylic acid.
For example, with ethanenitrile and hydrochloric acid you would get ethanoic acid and ammonium chloride.
Alkaline hydrolysis of nitrilesThe nitrile is heated under reflux with sodium hydroxide solution. This time, instead of getting an ammonium salt as you would do if the reaction only involved water, you get the sodium salt. Ammonia gas is given off as well.
For example, with ethanenitrile and sodium hydroxide solution you would get sodium ethanoate and ammonia.
If you wanted the free carboxylic acid in this case, you would have to acidify the final solution with a strong acid such as dilute hydrochloric acid or dilute sulphuric acid. The ethanoate ion in the sodium ethanoate will react with hydrogen ions as mentioned above.
Hydrolysis of nitriles with aqueous acid to give carboxylic acids
Description: Addition of water and acid to a nitrile leads to formation of a carboxylic acid.
Notes:
- This reaction is referred to as “acidic hydrolysis”.
- The reaction is generally used with water as solvent, so an excess of water is present. The acid used is often written as “H3O(+)”
Examples:
Mechanism:
Protonation of the nitrile nitrogen by acid (Step 1, arrows A and B) makes the nitrile carbon a better electrophile. Attack at the carbon by water (Step 2, arrows C and D) followed by proton transfer (Step 3, arrows E and F) gives a species that is in resonance with a protonated amide (arrows G and H). Addition of water to the protonated amide (Step 4, arrows I and J) followed by proton transfer (Step 5, arrows K and L) result in formation of NH3(+) which is an excellent leaving group. Expulsion of NH3 through 1,2-addition (Step 6, arrows M and N) followed by deprotonation (Step 7, arrows O and P) give the carboxylic acid.
Hydrolysing Nitriles In Basic Condition
The cyano group has an electropositive carbon atom due to the higher electronegativity of the nitrogen atom. The resonance dipole of the cyano group also contributes to the electropositive character of the carbon atom.
The imine anion is a strong base and will easily pull a hydrogen atom from a water molecule in the solution.
The imine product will undergo tautomerization to form an amide. This is favored by the bond energies of the reactant versus the product.
The amide carbonyl is electropositive and will be attacked by hydroxide anion. Use a double barbed arrow to show the movement of the electron pair from the hydroxide anion to the carbonyl carbon atom. Show another double barbed arrow to indicate the movement of the p electrons to the oxygen atom.
The product of this reaction can reform the carbonyl and eliminate either a hydroxide anion or an amide anion. If the hydroxide ion is the leaving group, the amide is reformed. The amide anion is a much stronger base and is a very poor leaving group making this equilibrium unfavorable. A small amount can leave and this leads to the next step of the mechanism.
The imine anion is a strong base and will easily pull a hydrogen atom from a water molecule in the solution.
The imine product will undergo tautomerization to form an amide. This is favored by the bond energies of the reactant versus the product.
The amide carbonyl is electropositive and will be attacked by hydroxide anion. Use a double barbed arrow to show the movement of the electron pair from the hydroxide anion to the carbonyl carbon atom. Show another double barbed arrow to indicate the movement of the p electrons to the oxygen atom.
The product of this reaction can reform the carbonyl and eliminate either a hydroxide anion or an amide anion. If the hydroxide ion is the leaving group, the amide is reformed. The amide anion is a much stronger base and is a very poor leaving group making this equilibrium unfavorable. A small amount can leave and this leads to the next step of the mechanism.
My problem:
I still confused about Hydrolysing Nitriles in basic Condition why easily pull a hydrogen atom from a water molecule in the solution.?
8 komentar:
12 Juni 2012 pukul 06.11
Ok via, judging from his article in the imine anion formed under alkaline conditions, where the imine anion is a strong base and will easily attract the hydrogen atoms of water molecules in solution. Imine is a form of amines and aldehydes. But for the reaction is less well understood
http://id.wikipedia.org/wiki/Reaksi_Mannich
12 Juni 2012 pukul 08.53
it is good Vebri, from your writing I got newest information. But I want to ask "In what Condition is more easy to hydrolysing Nitriles? In acid condition or base condition?" Thanks Before Vebri
12 Juni 2012 pukul 09.52
haii vebria cantik, can u explain how the imine formation in acidic conditions?
12 Juni 2012 pukul 10.59
12 Juni 2012 pukul 11.08
Yani Permata Dewi: good Question, I ill try to answer your Question according me Hydrolysis of nitriles proceeds via the corresponding amide.
We already know the mechanism of amide hydrolysis.
Therefore, all we need to do is to see how amides are formed from nitriles under the conditions of hydrolysis.
The mechanism of amide formation is analogous to that of conversion of alkynes to ketones.
It begins with the addition of water across the carbon-nitrogen triple bond.
The product of this addition is the nitrogen analog of an enol. It is transformed to an amide under the reaction conditions.
12 Juni 2012 pukul 11.13
Putri Mutiara Ishak : hehe Thanks before :* according me Imines are formed when any primary amine reacts with an aldehyde or ketone under appropriate conditions. Imine formation requires an acid catalyst, otherwise the reaction is very slow. The acid is needed for the elimination of water. for complete answer please check http://masterorganicchemistry.com/reaction-guide/formation-of-imines-from-primary-amines-and-ketones/
12 Juni 2012 pukul 20.03
Pauline
Can you explain why carboxylic acids are highly acidic, what caused the carboxylic acid is highly acidic??
14 Juni 2012 pukul 02.46
Ika silvi: thanks before, for the reaction actually i also less understand but i will give link mecanism hidrolysing Nitriles in basic condition. Please check this http://chemistry.boisestate.edu/people/richardbanks/organic/nitrilehydrolysisbasicReview3.htm
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