Hydrolysing Amide

HYDROLYSING AMIDES

This page describes the hydrolysis of amides under both acidic and alkaline conditions. It also describes the use of alkaline hydrolysis in testing for amides.

The hydrolysis of amides

What is hydrolysis?

Technically, hydrolysis is a reaction with water. That is exactly what happens when amides are hydrolysed in the presence of dilute acids such as dilute hydrochloric acid. The acid acts as a catalyst for the reaction between the amide and water.

The alkaline hydrolysis of amides actually involves reaction with hydroxide ions, but the result is similar enough that it is still classed as hydrolysis.

Hydrolysis under acidic conditions

Taking ethanamide as a typical amide:

If ethanamide is heated with a dilute acid (such as dilute hydrochloric acid), ethanoic acid is formed together with ammonium ions. So, if you were using hydrochloric acid, the final solution would contain ammonium chloride and ethanoic acid.

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Note:  You might argue that because the hydrochloric acid is changed during the reaction, it isn't acting as a catalyst. In fact, it is doing two things. It is acting as a catalyst in a reaction between the amide and water which would produce ammonium ethanoate (containing ammonium ions and ethanoate ions). It is secondly reacting with those ethanoate ions to make ethanoic acid.

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Hydrolysis under alkaline conditions

Again, taking ethanamide as a typical amide:

If ethanamide is heated with sodium hydroxide solution, ammonia gas is given off and you are left with a solution containing sodium ethanoate.

Using alkaline hydrolysis to test for an amide

If you add sodium hydroxide solution to an unknown organic compound, and it gives off ammonia on heating (but not immediately in the cold), then it is an amide.

You can recognise the ammonia by smell and because it turns red litmus paper blue.

The possible confusion using this test is with ammonium salts. Ammonium salts also produce ammonia with sodium hydroxide solution, but in this case there is always enough ammonia produced in the cold for the smell to be immediately obvious.

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Note:  This test is OK for UK A level purposes, but there are other things which also give off ammonia on heating with sodium hydroxide solution - for example, nitriles (but you won't come across them in a practical situation at this level) and imides (but they are beyond the scope of courses at this level).

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OTHER REACTIONS OF AMIDES

This page explains the reason for the lack of basic character in amides, and describes their dehydration to give nitriles, and their reaction with bromine and sodium hydroxide solution to form primary amines with one less carbon atom (the Hofmann degradation).

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Note:  The hydrolysis of amides is described on a separate page.

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The lack of base character in amides

Unusually for compounds containing the -NH₂ group, amides are neutral. This section explains why -NH₂ groups are usually basic and why amides are different.

The usual basic character of the -NH₂ group

Simple compounds containing an -NH₂ group such as ammonia, NH₃, or a primary amine like methylamine, CH₃NH₂, are weak bases. A primary amine is a compound where the -NH₂ group is attached to a hydrocarbon group.

The active lone pair of electrons on the nitrogen atom in ammonia can combine with a hydrogen ion (a proton) from some other source - in other words it acts as a base.

With a compound like methylamine, all that has happened is that one of the hydrogen atoms attached to the nitrogen has been replaced by a methyl group. It doesn't make a huge amount of difference to the lone pair and so ammonia and methylamine behave similarly.

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Note:  The reasons that these are bases and the differences between them (because there are slight differences) are explored in some detail on a page about organic bases. It would be useful to read this page before you go on because it is relevant to what is coming next.

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For example, if you dissolve these compounds in water, the nitrogen lone pair takes a hydrogen ion from a water molecule - and equilibria like these are set up:

Notice that the reactions are reversible. In both cases the positions of equilibrium lie well to the left. These compounds are weak bases because they don't hang on to the incoming hydrogen ion very well.

Both ammonia and the amines are alkaline in solution because of the presence of the hydroxide ions, and both of them turn red litmus blue.

Why doesn't something similar happen with amides?

Amides are neutral to litmus and have virtually no basic character at all - despite having the -NH₂ group. Their tendency to attract hydrogen ions is so slight that it can be ignored for most purposes.

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Note:  If you haven't already done so, follow the link mentioned above to the page about organic bases, and read the bit about phenylamine. It is directly relevant to what's next.

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We need to look at the bonding in the -CONH₂ group.

Like any other double bond, a carbon-oxygen double bond is made up of two different parts. One electron pair is found on the line between the two nuclei - this is known as a sigma bond. The other electron pair is found above and below the plane of the molecule in a pi bond.

A pi bond is made by sideways overlap between p orbitals on the carbon and the oxygen.

In an amide, the lone pair on the nitrogen atom ends up almost parallel to these p orbitals, and overlaps with them as they form the pi bond.

The result of this is that the nitrogen lone pair becomes delocalised - in other words it is no longer found located on the nitrogen atom, but the electrons from it are spread out over the whole of that part of the molecule.

This has two effects which prevent the lone pair accepting hydrogen ions and acting as a base:

• Because the lone pair is no longer located on a single atom as an intensely negative region of space, it isn't anything like as attractive for a nearby hydrogen ion.
• Delocalisation makes molecules more stable. For the nitrogen to reclaim its lone pair and join to a hydrogen ion, the delocalisation would have to be broken, and that will cost energy.

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Note:  If you want to look in more detail at the bonding in the carbon-oxygen double bond, you could follow this link.

If you do choose to follow this link, it will probably take you to several other pages before you are ready to come back here again. Use the BACK button (or HISTORY file or GO menu) on your browser to return to this page later.

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The dehydration of amides

Amides are dehydrated by heating a solid mixture of the amide and phosphorus(V) oxide, P₄O₁₀.

Water is removed from the amide group to leave a nitrile group, -CN. The liquid nitrile is collected by simple distillation.

For example, with ethanamide, you will get ethanenitrile.

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Note:  This is a just a flow scheme rather than a proper equation. I haven't been able to find a single example of the use of the full equation for this reaction. In fact the phosphorus(V) oxide reacts with the water to produce mixtures of phosphorus-containing acids.

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The Hofmann Degradation

The Hofmann degradation is a reaction between an amide and a mixture of bromine and sodium hydroxide solution. Heat is needed.

The net effect of the reaction is a loss of the -CO- part of the amide group. You get a primary amine with one less carbon atom than the original amide had.

The general case would be (as a flow scheme):

If you started with ethanamide, you would get methylamine. The full equation for the reaction is:

The Hofmann degradation is used as a way of cutting a single carbon atom out of a chain.

Posted by : VEBRIA ARDINA (RSA1C110020)

Hydronium

Is water an acid or a base according to the arrhenius theory?

water can be both an acid and a base.
H₂O + H⁺ = H₃O⁺
H₂O + HCO₃^-= H₂CO₃ +OH^-

Why water is called an amphoteric substance?

According to Bronsted concept

Water can act as an acid by losing a proton as
H₂O-------->OH^- + H⁺
Water can act as a base by gaining a proton as
H₂O+ H⁺---------------->H₃O⁺

Why/How is Hydronium (H3O) positively charged?

H₂O + H⁺ --> H₃O⁺

I see that due to the conversation of charge, both sides should have a net charge of +1, but I don't see how is the hydronium positively charged.

The H⁺ has a dative covalent bond with the H₂O so hydrogen's s-orbital should be complete, hence the charge should be zero.

Is this due to the electronegativity of the H₂O molecule?

I will appreciate it if you could explain to me what I am missing here

well H20 has a charge of 0

Two H's give you a +2 Charge and then you have one O (oxide has a charge of -2) therefore H₂0 has a zero charge

your adding something that has a charge of 0 to something that has a charge of 1(H)

thats why the result is H₃O⁺

Hydronium

In chemistry, a hydronium ion is the cation H₃O⁺, a type of oxonium ion produced by protonation of water and isoelectronic with ammonia. This cation is often used to represent the nature of the proton in aqueous solution, where the proton is highly solvated (bound to a solvent). The reality is far more complicated, and a proton is bound to several molecules of water, such that other descriptions such as H₅O₂⁺, H₇O₃⁺ and H₉O₄⁺ are increasingly accurate descriptions of the environment of a proton in water.^[3] The ion H₃O⁺ has been detected in the gas phase.

Determination of pH

It is the presence of hydronium ion relative to hydroxide that determines a solution's pH. Water molecules auto-dissociate into hydronium and hydroxide ions in the following equilibrium:

2 H₂O OH⁻ + H₃O⁺

In pure water, there is an equal number of hydroxide and hydronium ions. At 25 °C and atmospheric pressure their concentrations are approximately equal to 1.0 × 10⁻⁷ mol∙dm⁻³. For these conditions, [H₃O⁺] = 10^−pH so pH = 7 is defined as neutral. A pH value less than 7 indicates an acidic solution, and a pH value more than 7 indicates a basic solution. Note that [H₃O⁺]×[OH⁻], the ionic product of water, strongly increases with temperature so [H₃O⁺] is not equal to 10^−pH for temperatures other than 25 °C.

Structure

Since O⁺ and N have the same number of electrons, H₃O⁺ is isoelectronic with ammonia. As shown in the images above, H₃O⁺ has a trigonal pyramid geometry with the oxygen atom at its apex. The H-O-H bond angle is approximately 113°,^[6] and the center of mass is very close to the oxygen atom. Because the base of the pyramid is made up of three identical hydrogen atoms, the H₃O⁺ molecule's symmetric top configuration is such that it belongs to the C_3v point group. Because of this symmetry and the fact that it has a dipole moment, the rotational selection rules are ΔJ = ±1 and ΔK = 0. The transition dipole lies along the c axis and, because the negative charge is localized near the oxygen atom, the dipole moment points to the apex, perpendicular to the base plane.

Acids and acidity

Hydronium is the cation that forms from water in the presence of hydrogen ions. These hydrons do not exist in a free state: they are extremely reactive and are solvated by water. An acidic solute is generally the source of these hydrons; however, hydroniums exist even in pure water. This special case of water reacting with water to produce hydronium (and hydroxide) ions is commonly known as the self-ionization of water. The resulting hydronium ions are few and short-lived. pH is a measure of the relative activity of hydronium and hydroxide ions in aqueous solutions. In acidic solutions, hydronium is the more active, its excess proton being readily available for reaction with basic species.

Hydronium is very acidic: at 25 °C, its pKa is -1.74. It is also the most acidic species that can exist in water (assuming sufficient water for dissolution)(see leveling effect): any stronger acid will ionize and protonate a water molecule to form hydronium. The acidity of hydronium is the implicit standard used to judge the strength of an acid in water: strong acids must be better proton donors than hydronium, otherwise a significant portion of acid will exist in a non-ionized state. Unlike hydronium in neutral solutions that result from water's autodissociation, hydronium ions in acidic solutions are long-lasting and concentrated, in proportion to the strength of the dissolved acid.

pH was originally conceived to be a measure of the hydrogen ion concentration of aqueous solution.^[7] We now know that virtually all such free protons quickly react with water to form hydronium; acidity of an aqueous solution is therefore more accurately characterized by its hydronium concentration. In organic syntheses, such as acid catalyzed reactions, the hydronium ion (H₃O⁺) can be used interchangeably with the H⁺ ion; choosing one over the other has no significant effect on the mechanism of reaction.

Solvation

Researchers have yet to fully characterize the solvation of hydronium ion in water, in part because many different meanings of solvation exist. A freezing-point depression study determined that the mean hydration ion in cold water is approximately H₃O⁺(H₂O)₆:^[8] on average, each hydronium ion is solvated by 6 water molecules which are unable to solvate other solute molecules.

Some hydration structures are quite large: the H₃O⁺(H₂O)₂₀ magic ion number structure (called magic because of its increased stability with respect to hydration structures involving a comparable number of water molecules) might place the hydronium inside a dodecahedral cage.^[9] However, more recent ab initio method molecular dynamics simulations have shown that, on average, the hydrated proton resides on the surface of the H₃O⁺(H₂O)₂₀ cluster.^[10] Further, several disparate features of these simulations agree with their experimental counterparts suggesting an alternative interpretation of the experimental results.

picture

Zundel cation

Two other well-known structures are the Zundel cations and Eigen cations. The Eigen solvation structure has the hydronium ion at the center of an H₉O+4 complex in which the hydronium is strongly hydrogen-bonded to three neighbouring water molecules. In the Zundel H₅O+ complex the proton is shared equally by two water molecules in a symmetric hydrogen bond. Recent work indicates that both of these complexes represent ideal structures in a more general hydrogen bond network defect.

Isolation of the hydronium ion monomer in liquid phase was achieved in a nonaqueous, low nucleophilicity superacid solution (HF-SbF₅SO₂). The ion was characterized by high resolution O-17 nuclear magnetic resonance.

A 2007 calculation of the enthalpies and free energies of the various hydrogen bonds around the hydronium cation in liquid protonated water at room temperature and a study of the proton hopping mechanism using molecular dynamics showed that the hydrogen-bonds around the hydronium ion (formed with the three water ligands in the first solvation shell of the hydronium) are quite strong compared to those of bulk water.

A new model was proposed by Stoyanov based on infrared spectroscopy in which the proton exists as an H₁₃O+6 ion. The positive charge is thus delocalized over six water molecules.

Solid hydronium salts

For many strong acids, it is possible to form crystals of their hydronium salt that are relatively stable. Sometimes these salts are called acid monohydrates. As a rule, any acid with an ionization constant of 10⁹ or higher may do this. Acids whose ionization constant is below 10⁹ generally cannot form stable H₃O⁺ salts. For example, hydrochloric acid has an ionization constant of 10⁷, and mixtures with water at all proportions are liquid at room temperature. However, perchloric acid has an ionization constant of 10¹⁰, and if liquid anhydrous perchloric acid and water are combined in a 1:1 molar ratio, solid hydronium perchlorate forms.

The hydronium ion also forms stable compounds with the carborane superacid H(CB₁₁H(CH₃)₅B₆). X-ray crystallography shows a C_3v symmetry for the hydronium ion with each proton interacting with a bromine atom each from three carborane anions 320 pm apart on average. The [H₃O][H(CB₁₁HCl)]11 salt is also soluble in benzene. In crystals grown from a benzene solution the solvent co-crystallizes and a H₃O·(benzene)₃ cation is completely separated from the anion. In the cation three benzene molecules surround hydronium forming pi-cation interactions with the hydrogen atoms. The closest (non-bonding) approach of the anion at chlorine to the cation at oxygen is 348 pm.

There are also many examples of hydrated hydronium ions known, such as the H₅O+2 ion in HCl·2H₂O, the H₇O+3 and H₉O+4 ions both found in HBr·4H₂O.