· Nitro Compounds
· Cyanides and Isocyanides
· Diazonium salts
· Some commercially important Compounds
Functional groups containing nitrogen are present in a variety of naturally occurring and man made organic compounds. These functional groups impart physicochemical characteristics to these molecules. These groups are responsible for their unique chemical reactivity patterns and play crucial roles in the preparation of drugs, agrochemicals, dyes and molecules of life. There are many functional groups , which contain one or more nitrogen atoms. Some categories of compounds based on these functional groups include nitro compounds, amines, cyanides , isocyanides and diazo compounds.
are called nitro compounds. Because of their easy availability, conversion to other functional groups and the influence of nitro group on the overall reactivity of the molecule, they play a central role in organic synthesis. They may be aliphatic or aromatic compounds according to the nitro group is attached to alkyl or aryl group. For example,
Nitroalkanes may be further classified as primary, secondary or tertiary depending upon whether the nitro group is attached to primary, secondary or tertiary carbon.
Nitro alkanes are isomeric with alkyl nitrites, R - O - N = O, in which alkyl or aryl group is attached to - O - N = O group. Thus - NO2 group is an ambident group. Therefore - NO2 group forms two isomers.
These are named by prefixing ‘nitro’ to the name of parent alkane. The position of the nitro group of the carbon chain is indicated by a number.
Methods of preparation
The methods of syntheses of aromatic and aliphatic nitro compounds are quite different.
Aliphatic Nitro Compounds
1. Vapour phase nitration of alkanes
Nitro compounds are prepared industrially by the vapour phase nitration of hydrocarbons with nitric acid at about 675 K. However, it gives a mixture of nitroalkanes and various oxidation products.
Lower alkanes can be nitrated with HNO3 at 675 K.
The major limitation of this method is that a mixture of nitroalkanes is produced. This is because the reaction occurs at very high temperature and there is also a cleavage of C - C bonds forming a mixture of nitro compounds.
2. From alkyl halides
Alkyl halide (specially iodides) react with silver nitrite (AgNO2) in alcohol give nitroalkanes.
In the above reaction a small amount of alkyl nitrite is also formed. For example, ethyl iodide reacts with AgNO2 to give about 80% nitroethane and 20% ethyl nitrite.
However, when alkyl halides are treated with NaNO2, nitrites are formed as the major product along with nitroalkanes as minor product.
CH3I + NaNO2 ® CH3ONO + NaI
Aromatic nitro compounds cannot be prepared by this method because of smaller reactivity of aryl halides towards nucleophilic substitution reactions.
The reaction of nitrite ion with alkyl halide is a nucleophilic substitution reaction. Nitrite ion (-O-N=O) acts as an ambident nucleophile because it can attack the alkyl halide through either oxygen or nitrogen.
When the attack occurs through N-atom, nitroalkanes (RNO2) are formed as the major product while when the attack occurs through oxygen, alkyl nitrite (RONO) are formed.
Alkali nitrites are ionic compounds (NaNO2 or KNO2) , therefore, both N and O are available for attack. However, the attack mainly occurs through oxygen because C-O bond is relatively stronger. Therefore, alkyl nitrites are formed as the major products. On the other hand, silver nitrite is a covalent compound and therefore only N is available for attack. As a result, silver nitrite gives mainly nitroalkanes.
3. Oxidation of t-alkyl amines with KMnO4
Here the amine must be primary and -NH2 group should be attached to a tertiary carbon.
AROMATIC NITRO COMPOUNDS
Arenes , like benzene can be easily nitrated with a mixture of HNO3 and H2SO4. For example,
For direct nitration of aromatic compounds the choice of nitrating agent depends upon the reactivity of the aromatic compound. Nitration is performed with a mixture of concentrated nitric acid and sulphuric acid which is a source of nitronium ion.
Electron releasing substituents like –CH3, -OCOCH3, -OH , -NH2 etc activate the ring and stabilise the carbocation while electron withdrawing groups like –NO2, -CN, -SO3H, -X deactivate the carbocation.
The nitration of benzene is an electrophilic substitution reaction in which nitronium ion (NO2+) is an electrophile. The different steps of the reaction are :
(i) Formation of electrophile : It is formed from the nitrating mixture of Con HNO3 and H2SO4.
(ii) Attack of nitronium ion on benzene molecule : The electrophile +NO2 attacks the benzene ring forming carbocation which gets resonance stabilised.
This is a slow step and is the rate determining step.
(iii) Loss of H+ ion intermediate cation : This step leads to the formation of nitrobenzene.
1. Write three canonical forms of carbocation intermediate for m-attack on nitrobenzene.
2. Draw the structures of isomeric nitroalkanes and alkyl nitrites having the molecular formula C3H7NO2.
3. Write the IUPAC names of following compounds:
4. Which of the following have higher boiling point in each of the following pairs ?
(i) CH3NO2 and butane
(ii) Nitrobenzene and benzene
(iii) Nitromethane and methyl nitrite
(iv) Nitromethane and acetic acid
5. Write the major products in the following reactions.
Electronic structure and Properties
General structure of a nitro group as represented below is a resonance hybrid of two equivalent zwitter ionic , polar structures.
The hybrid structure has a positively charged nitrogen and two equivalent negatively charged oxygens. As a result of the above structural character, the nitro group shows the following physical properties.
The two N - O bonds have equal bond lengths ( 121 pm in nitromethane) which is intermediate between N - O single bond (136 pm) and N = O double bond (115 pm) .
Nitro compounds have large dipole moments amongst simple organic compounds. Due to polarity , their boiling points are unusually high in comparison with other compounds of same molecular mass. Lower members are liquids and higher members are solids. They are soluble in most organic solvents but only lower liquid members have some solubility in water.
Dipole moment (D)
The formal positive charge on nitrogen of nitro group makes it strong electron-withdrawing group and exerts a strong pull on neighbouring electrons affecting physicochemical properties of molecule. The presence of nitro group in phenols enhances their acidity and in aromatic compounds deactivates them towards electrophilic substitution.
Reactions of Nitro Compounds
In aromatic and aliphatic nitro compounds , nitro group undergoes similar reactions, however, the different way in which it affects the reactivity of the attached aryl or alkyl group is very important in synthesis.
One of the most important reactions of nitro compounds is their reduction to primary amines. The reduction is believed to occur through the following stages.
The final product , however, depends upon the pH of the reaction medium and nature of the reducing agent.
(a) Reduction in acidic medium
Both aliphatic and aromatic nitro compounds can be reduced to the corresponding primary amines by a combination of some active metals like zinc, iron or tin and concentrated hydrochloric acid. For example,
Under acidic conditions , the intermediate products ( i.e., nitroso compounds and hydroxylamines) are reduced to even more readily than the parent nitro compound and hence only the primary amine is obtained as the final product.
(b) Reduction in neutral medium
In the neutral medium , i.e., Zn dust and NH4Cl solution, both aliphatic and aromatic nitro compounds are reduced to the corresponding hydroxyl amines. For example,
These hydroxyl amines when warmed with Tollen’s reagent are easily oxidised back to the corresponding nitroso compounds and thus reduce Tollen’s reagent to metallic silver. This reaction is used as a test for nitro compounds under the name Barker-Mulliken’s test.
(c) Reduction in the alkaline medium
The reduction of the aromatic nitro compounds in alkaline medium gives different products (bimolecular reduction) depending upon the nature of the reducing agent employed. For example,
(d) Catalytic reduction
Both aliphatic and aromatic nitro compounds can also be reduced to the corresponding primary amines with hydrogen in presence of Raney nickel, platinum or palladium as catalyst. For example,
(e) Reduction with metal hydrides
Nitroalkanes can be easily reduced to corresponding primary amines with lithium aluminium hydride. For example,
In contrast, aromatic nitro compounds on reduction with LiAlH4 give azo compounds and not primary amines. For example,
(f) Electrolytic reduction
Electrolytic reduction of nitrobenzene in weakly acidic medium gives aniline but in strongly acidic medium , it gives para-amino phenol obviously through the acid catalysed rearrangement of initially formed phenyl hydroxyl amine.
(g) Selective Reduction
If two or more nitro groups are present in the benzene ring, it is possible to reduce one of them without affecting the others. Such reductions are called selective reductions. For example, reduction of m-dinitrobenzene with sodium or ammonium sulphide gives m-nitroaniline.
This reduction of nitro compounds with sulphides and polysulphides is called Zinin reduction.
(i) Primary nitroalkanes when treated with boiling HCl or 85 % H2SO4 undergo hydrolysis to form carboxylic acid and corresponding salt of the hydroxyl amine.
This reaction is used for the manufacture of hydroxylamine.
This reaction is used to manufacture of hydroxyl amine.
(ii) In constrast, secondary nitroalkanes upon hydrolysis with boiling HCl gas give a ketone and nitrous oxide.
(iii) Tertiary nitroalkanes , however do not generally undergo hydrolysis with hydrochloric acid.
2. Action with nitrous acid
Primary , secondary and tertiary nitroalkanes behave differently towards nitrous acid.
(i) Primary nitroalkanes react with nitrous acid to form nitrolic acids which dissolve in alkalies to form a red solution.
(Dissolves in NaOH to give blood red colouration)
(ii) Secondary nitroalkanes react with nitrous acid to form blue coloured pseudonitroles which do not dissolve in alkali.
(Blue colouration, does not dissolve in alkali)
(iii) Tertiary nitroalkanes do not react with nitrous acid since they do not have a-hydrogen atoms.
Nitroalkanes containing a-hydrogen atoms , i.e., primary and secondary nitroalkanes , show tatomerism. For example, nitromethane exists in two tautomeric forms , i.e., I and II.
The nitro-form (I) is often called pseudo acid form while the isonitroform (II) is called nitronic acid. The equilibrium, however, almost completely lies towards left due to resonance(within the nitro groups) stabilisation of nitro-form (I). Similarly, nitroethane , 1-nitropropane, 2-nitropropane, show tautomerism while aromatic nitro compounds, i.e., nitrobenzene, m-dinitrobenzene etc. which do not contain a-hydrogen atoms , do not show tautomerism.
5. Acidic character
The a-hydrogen atoms of primary and secondary nitroalkanes are weakly acidic and thus can be abstracted by strong alkalies such as aqu. NaOH . Therefore 1° and 2° nitroalkanes dissolves in aq. NaOH to form salts. For example,
The main reasons for acidic nature of 1° and 2° nitroalkanes are :
(i) The strong electron withdrawing inductive effect of the nitro group and
(ii) The resonance stabilisation of carbanion (I) formed after the removal of a proton.
Although 1° and 2° nitroalkanes are weakly acidic yet as compared to aldehydes and ketones , they are twice as strong. For example, pKa of nitromethane is 10 while that of acetone is 20. Nevertheless, nitroalkanes are neutral to litmus.
Primary and secondary nitroalkanes on treatment with halogen (chlorine or bromine) in presence of alkali form halonitroalkanes. During this reaction, all the a-hydrogen atoms of nitroalkanes are successively replaced by the halogen atoms. For example,
Chloropicrin is an important insecticide.
7. Ring substitution reactions of nitrobenzene
a) Electrophilic substitution reactions
Nitrobenzene like halobenzene also undergoes electrophilic substitution reactions(i.e., halogenation, nitration and sulphonation). Since , the –NO2 group is strongly deactivating and meta-directing, therefore, the incoming group enters the meta-position. This may be explained as follows:
Nitrobenzene can be represented as a resonance hybrid of the following structures :
It is evident that nitro group because of its electron-withdrawing nature reduces electron density more at o- and p-positions than that at m-positions. In other words, electron density is comparatively more at m-positions than at o- and p-positions. Therefore, nitro group is m-directing. Further since the –NO2 group is strongly deactivating , therefore the reactions occur only under drastic conditions. For example,
(b). Nucleophilic substitution reactions
The presence of positive charge at o- and p-positions in resonance structures (I – IV) suggests that nitrobenzene can also undergo nucleophilic substitution reactions at these positions. These reactions , however occur with strong nucleophiles under drastic conditions. For example, nitrobenzene when fused with solid KOH gives a low yield of a mixture of o- and p-nitrophenols. Thus,
7. Reductive removal of Nitro Group
The nitro group can be removed from an aromatic ring via reduction to amine followed by diazotisation with HNO2 and then reductive removal of the diazonium group using sodium borohydride or hypophosphorus acid/Cu+ mixture.
1. Lower nitroalkanes such as nitromethane, nitroethane etc. and nitrobenzene are widely used as solvent in industry.
2. Nitroarenes are important intermediates in the manufacture of detergents, dyes, pharmaceuticals and explosives. 2,4,6-Trinitrotoluene (TNT) , 1,3,5-Trinitrobenzene (TNB), RDX (Research and Development Explosive) and HMX (Her Majesty’s Explosive) are important commercial explosives.
3. Nitroalkanes are also used as propellants. For example, nitromethane is used as a liquid propellant while nitrocellulose gels in nitroglycerine are used as a solid propellant.
4. Chloropicrin is used as an insecticide.
Some commercially important compounds
1. 2,4,6-Trinitrotoluene (TNT)
It is manufactured by three stage nitration of toluene with Con. HNO3 and Con. H2SO4 as shown below.
This three stage nitration is prefered over one stage nitration because of the following reasons :
(i) The waste acid from final stage may be recycled to be used in first and second stages.
(ii) One - stage nitration will require not only large volumes of strong mixed acids at high temperature but would also partially oxidise TNT.
(i) Because of its safe and low cost of production, high explosive power, low melting point and low toxic effects, it is widely used as military explosive.
(ii) A mixture of TNT (20%) and NH4NO3 (80%) called amatol is used in coal mining.
(iii) A mixture of TNT (30%) , NH4NO3 (47%) , aluminium (22%) and charcoal (1%) is called aminonal and is used for blasting purposes.
2. Nitroglycerine and Dynamite
When glycerol is added slowly to a mixture of con. H2SO4 and con. HNO3 at 283-298 K, glyceryl trinitrate (Nobels oil) is formed which is commonly, but incorrectly called nitroglycerine.
Glyceryl trinitrate is a colourless oily liquid. It explodes violently on heating or detonation. Alfred Nobel, the scientist who instituted Nobel prize , found that nitroglycerine can be stabilised by absorbing on kieselguhr . A mixture of glyceryl trinitrate and glyceryl dinitrate absorbed on kieselguhr is called dynamite.
However, these days dynamite is manufactured by using saw dust or wood pulp as the adsorbent and adding solid ammonium nitrate.
3. RDX (Research and Development Explosive): It is also called cyclonite. It is prepared by controlled nitration of hexamethylenetetramine with fuming nitric acid at 298 K.
RDX is about 35% more powerful than TNT. However, in most of the cases, it is mixed with TNT and other ingredients. For example, a mixture of RDX, TNT and Al powder is used in torpedo heads.
4. HMX (Her Majestey’s Explosive)
It is obtained as a coproduct during the preparation of RDX.
Since it has four
N-NO2 groups instead of three in RDX, therefore , it is even powerful than RDX.
06. Write three cannonical forms of intermediate for attack of hydroxide on o-chloronitrobenzene.
07. How will you convert nitrobenzene into :
(v) metanilic acid
08. Suggest steps for conversion of toluene into sym-trinitrobenzene .
09. During nitration of benzene with a mixture of concentrated nitric acid and concentrated sulphuric acid , nitric acid acts as a base . Explain .
10. In constrast to arenes, aliphatic hydrocarbons do not undergo nitration easily.
11. Toluene is more easily nitrated than benzene. Explain.
Amines are regarded as derivatives of ammonia in which one, two or three hydrogen atoms are replaced by alkyl or aryl group.
These amines are classified as primary, secondary or tertiary according as one, two or three hydrogen atoms of ammonia molecule are replaced by alkyl or aryl group.
(where R may be alkyl or aryl group)
The characteristic groups in primary, secondary and tertiary amines are :
Apart from these three types of amines, there is another class of compounds known as quaternary ammonium compounds. These compounds may be regarded as derivatives of ammonium salts in which all the four H-atoms are replaced by alkyl or aryl groups. For example,
Classification of Amines
Amines may be classified into two categories.
1. Aliphatic amines
Amines in which the nitrogen atom is directly bonded to one or more alkyl groups are called aliphatic amines. For example,
2. Aromatic Amines
(a) Aryl amines : Amines in which the nitrogen atom is directly bonded to one or more aromatic rings or aryl groups are called aromatic amines.
(b) Arylalkyl amine or side chain substituted amines : Amines in which the nitrogen atom is bonded to the side chain of the aromatic ring are called arylalkyl amines. For example,
Simple and Mixed amines
Secondary and tertiary amines may be classified as simple or mixed amines according as all the alkyl or aryl groups attached to the nitrogen atom are same or different . For example,
Nomenclature of Amines
Aliphatic amines are named by appending the suffix –amine to the name of the alkyl group and is written as one word. When two or more alkyl groups in secondary or tertiary amines are same, the prefix di or tri is appended and when these are different, amines are named as N-substituted derivatives of the largest group of primary amine. Some examples are given below.
Aromatic amines are named as derivatives of the parent member, aniline, but in some cases other names o / m / p – toluidine for o / m / p –methylaniline and o / m / p –anisidine for o / m / p –methoxy anilines are assigned. Even N-phenyl derivative of aniline is generally called diphenylamine.
The ending e of the alkane is replaced by suffix –amine. In case of diamines, the ending e of the hydrocarbon name is retained and suffix diamine is appended. When additional functional groups such as OH or double bond are present in an amine, the prevailing priority order for nomenclature is observed.
The ground state electronic configuration of N is :
N : 1s22s22Px12Py12Pz1
The 2s–orbital and three 2p-orbitals hybridise to form four sp3-hybridised orbitals ; three of these orbitals contain one electron each, while the fourth contains a lone pair of electrons.
In primary amines, one of the three half-filled sp3-orbitals overlaps with sp3-hybridised orbital of the carbon atom of the alkyl group (or sp2-hybridised orbital of the carbon atom of the aryl groups) and the remaining two overlap with s-orbitals of hydrogen atoms thereby forming one C-N and two N-H s-bonds.
In secondary amines, two of the three sp3-hybridised orbitals of nitrogen overlap with sp3-hybridised orbitals of the carbon atoms of two alkyl groups(or sp2-hybridised orbital of the carbon atom of the aryl groups) and one with s-orbital of the H-atom thereby forming two C-N and one N-H s-bonds.
In tertiary amines , all the three sp3-hybridised orbitals of N overlap with sp3-hybridised orbitals of carbon atoms of three alkyl groups (or sp2-hybridised orbital of the carbon atom of the aryl groups). In all these amines , forth sp3-orbital contains the lone pair of electrons as shown in Fig.
Orbital structure of 1°, 2° and 3° amines
Since lone-pair-bond pair repulsions are much greater than bond-pair-bond-pair repulsions, therefore , bond angle between any two adjacent H-atoms or alkyl groups decreases from tetrahedral angle of 109°28’ to 107° in 1° and 2° amines. However, in the case of 3°amines, due to steric hindrance between three bulky groups, the bond angle increases from 107° in ammonia to 108° in triethylamine. Thus, all the three amines(1°, 2°, 3°) like ammonia have pyramidal shape.
Aliphatic amines have a pyramidal shape that is approximately tetrahedral when we assume electron pair on nitrogen as a group. Thus, an amine which has three different groups attached to nitrogen has chiral nitrogen..
But such amines cannot be resolved into enantiomers because of the rapid conversion of an enantiomer to its mirror image. This is known as flipping.
Methods of preparation
Amines are prepared by various methods. Some methods commonly used for amines in the laboratory using nitro compounds, alkyl halides, aldehydes and ketones, amides, nitriles and azides are outlined below:
1. From Alkyl halides
(a) By ammonolysis (Hofmann’s method) : When an aqueous or alcoholic solution of ammonia heated with an alkyl halide at 373 K in a sealed tube, all the three types of amines are obtained. This type of reaction is called ammonolysis.
Tertiary amines also combine with methyl iodide to form quarternary ammonium salts.
This reaction is an example of nucleophilic substitution reaction in which ammonia or amine molecule acts as a nucleophile due to the presence of lone pair on the nitrogen atom.
The order of reactivity of halides is :
RI > RBr > RCl
(i) The method gives a mixture of amines and it is very difficult to separate the mixture in the laboratory. The composition of the reaction mixture depends upon the amounts of alkyl halide and ammonia taken. If ammonia is taken in large excess and the alkyl halide in small amount, then the primary amine is the main product.
(ii) The method is not suitable for the preparation of arylamines because aryl halides are relatively less reactive than alkyl halides towards nucleophilic substitution reactions.
(c) By Gabriels phthalimide synthesis
This method is used for preparing only primary amines. In this method, phthalimide is treated with alcoholic KOH to give potassium phthalimide, which is treated with alkyl halide to form N-alkylphthalimide. The hydrolysis of N-alkylphthalimide with 20% HCl under pressure or refluxing with NaOH give primary amine. The phthalimide can also be hydrazinolysed to get pure primary amine.
Phthalic acid is again be converted into phthalimide and used again and again. This method is very useful because it gives pure amines.
2. Reduction of nitrocompounds
Aliphatic and aromatic amines can be easily prepared by the reduction of corresponding nitro compounds. The reduction can be carried out in a number of ways as discussed below:
(a) Nitro compounds can be catalytically reduced with hydrogen in presence of Raney nickel Ni, Pt or Pd as catalyst at room temperature.
(b) Nitro compounds can be reduced with active metals as Fe, Sn , Zn etc. and con. Hydrochloric acid.
This is a good method for the preparation of aromatic amines because they cannot be prepared from the corresponding aryl halides on treatment with ammonia.
(c) Nitro compounds can also be reduced to amines with LiAlH4 .
2. Reduction of nitriles (cyanides) and isonitriles(isocyanides)
Nitriles can be reduced to corresponding amines using H2/Raney Ni or Pt, LiAlH4 or Na/C2H5OH.
The method of reduction of cyanide with sodium and alcohol is called Mendius reaction.
The reduction of isocyanides under similar conditions gives secondary amines.
4. From amides
(i) Reduction of amides : Amides are reduced to the
corresponding amines by LiAlH4 or Na / C2H5OH.
It may be noted that the product contains the same number of carbon atoms as the original amide.
Secondary and tertiary amines can be prepared by the reduction of secondary and tertiary amides respectively.
(ii) From amides by Hofmann degradation method : Primary amines can be prepared by treatment with Br2 and KOH. The amine formed contains one carbon atom less than the parent amide. Therefore , this method is used for stepping down the series in organic conversions.
Note : It may be noted that primary amines can be obtained from amides by reduction with LiAlH4 or by treating with Br2 and NaOH. The reduction with LiAlH4 gives amine having same number of C-atoms as the original amide while reduction with NaOH and Br2 gives amine having one carbon atom less than the original amide.
5. Reduction of Oximes
Primary amines can be prepared by the reduction of oximes of aldehydes and ketones with either Na/C2H5OH or LiAlH4. The oximes can be obtained from aldehydes and ketones by reaction with hydroxylamine.
6. Reductive amination of aldehydes and ketones
Primary amines may be prepared by reacting aldehydes with ammonia to form imines which can be reduced with H2, Ni.
This reaction is called reductive amination .
7. Reduction of Azides
On treatment with sodium azide, alkyl halides undergo nucleophilic substitution to form alkyl azides which on catalytic hydrogenation give primary amines. Primary amine has the same number of carbons as the precursor alkyl halide.
Industrial Preparation of Amines
1. From alcohols : On a large scale, aliphatic amines are prepared by passing vapours of an alcohol and ammonia over alumina at 723 K. This reaction is called Sabatier and Mailhe method.
The mixture of three amines is separated by fractional distillation.
On an industrial scale , aniline is prepared by the reduction of nitrobenzene by catalytic hydrogenation (H2/Pt or V or CuO) or by chemical means using Fe/HCl or Sn/HCl.
Aniline is also prepared on a large scale by treating chlorobenzene with ammonia at 473 K and 60 atm pressure using copper oxide as catalyst.
Physical Properties of Amines
The important physical properties of amines are given below :
1. Physical state and smell : Lower members of the family such as methylamine, dimethylamine and ethyl amine are gases at ordinary temperatures have smell like ammonia. The higher members are mostly liquids having fishy smell.
2. Boiling points : Amines are polar compounds with the exception of tertiary amines can form intermolecular hydrogen bonds.
As a result , amines have higher boiling points than non-polar compounds of the same molecular mass. However, amines have lower boiling points than those of alcohols or carboxylic acids. This is due to the reason that O - H bonds present in alcohols and carboxylic acids are more polar than N- H bond in amines and therefore , the hydrogen bonds in alcohols and carboxylic acids are stronger and consequently they have higher boiling points. This is given below :
Among isomeric amines, the primary amines have highest boiling point due to their large tendency to form hydrogen bonds whereas tertiary amines have lowest boiling points because of their inability to form hydrogen bonds.
3. The lower amines are soluble in water because they are capable of forming hydrogen bonds with water. Hower, the higher amines containing six or more carbon atoms are insoluble. The amines are also soluble in less polar solvents like ether, alcohol or benzene, etc.
Aromatic amines are insoluble in water but are soluble in ether, alcohols or benzene.
Basic character of amines
Amines are derivatives of ammonia in which one or more hydrogen atoms have been replaced by alkyl or aryl group. Therefore amines have pyramidal structure like ammonia. In this case , N involves sp3 hybridisation . Three of the four sp3 hybrid orbitals form s-bonds with three orbitals of alkyl (or aryl or H) while the fourth hybrid orbital contains a lone pair. Thus amines have pyramidal shape (Fig)
Pyramidal shape of amine (R may be H)
All classes of amines contain nitrogen atoms that bears unshared pair of electrons. The tendency of nitrogen to share these electrons with acids is responsible for the basic character of amines which is affected by the number and the nature of their alkyl or aryl groups.
Like ammonia, amines are strong bases and react with mineral acids to form ammonium salts from which they can be liberated by treatment with a strong base (such as sodium hydroxide).
For comparison of the basic character of amines, the equilibrium constant of following reaction is taken as a measure of their basic character. The amines react with water to form hydroxides which ionise to furnish hydroxyl ions.
The basic strength of an amine is expressed in terms of dissociation constant , Kb . For reaction ,
Greater the Kb value , the stronger the base.
The basic strength of amines can also be expressed in terms of pKb values which is defined as :
The smaller the value of pKb, more is the basic strength of amine.
All classes of aliphatic amines (Kb of about 10-3 to 10-4 ) are somewhat stronger bases than ammonia ( Kb = 1.8 x 10-5) . Aromatic amines are weaker bases ( Kb ~ 10-9) and the substituents on their aromatic ring have a marked effect on their basicity as p-nitroaline is only 1/4000 as basic as aniline.
Comparison of basic strength of compounds
(a) Comparison of aliphatic amines and ammonia Aliphatic amines are stronger bases than ammonia. This is due to the reason that alkyl groups are electron releasing ( + I effect). As a result of electron releasing effect of alkyl group, it increases the electron density on the nitrogen atom and therefore , they can donate electron pairs more easily than ammonia. Thus, alkyl amines are more basic than ammonia. For example, Kb value of NH3 ( 1.8 x 10-5) and ethyl amine ( Kb = 5.5 x 10-4) shows ethyl amine is stronger base.
(b) Comparison of Basic strength of Primary, secondary and Tertiary amines
Due to the presence of lone pair of electrons in amines, they are basic in character. The methyl group is electron releasing group ( + I effect) and will increase the electron density on nitrogen and therefore, its basic character should increase. As a result, the basic character should decrease in the order :
Tertiary amine > secondary amine > primary amine
Hower, it has been observed that tertiary amines are unexpectedly less basic than others. For example, the correct order of methyl amines is :
(CH3)2NH > CH3NH2 > (CH3)3N > NH3
The correct order ethyl amines are :
(C2H5)2NH > C2H5)3N > C2H5NH2 > NH3
The Kb values of some amines are given in TABLE.
1.8 x 10-5
4.5 x 10-4
5.4 x 10-4
6.0 x 10-5
5.1 x 10-4
10.0 x 10-4
5.6 x 10-4
4.2 x 10-10
5.0 x 10-10
1.15 x 10-9
2.0 x 10-5
This may be explained on the basis of following factors :
1. Steric factor : The size of alkyl group is more than that of hydrogen and therefore , it hinders the attack of acid on the amine and therefore , basic strength decreases. Now crowding of alkyl group increases from primary to tertiary amines. As a result, their basic strength decreases. This is called steric hindrance. According to this effect, tertiary alkylamine should be least basic.
2. Solvation of ions : When amines are dissolved in water, they undergo hydration through hydrogen bonding. The protonated amines form hydrogen bonds with water molecules and release energy called hydration energy. Now greater the extent of hydrogen bonding in protonated amine, more will be its stabilization and consequently greater will be the tendency of amine to change into cation and greater will be the strength of the corresponding amine. As is seen below, the hydration of protonated amine due to hydrogen bonding decreases as
Thus , tertiary ammonium ion is less hydrated than secondary ammonium ion, which is less hydrated than primary ammonium ion. Thus, tertiary amines has less tendency to form ammonium ion and consequently they are less basic.
As a consequence of combined effects of inductive effect and solvation, the secondary amines are the strongest bases among amines and basic strength varies as :
2°-amine > 1° - amine > 3° - amine
(c) Comparison of Basic strength of Aniline and Ethylamine
Both aniline and ethylamine are basic in nature due to the presence of lone pair on N-atom. But aniline is less basic than ethylamine as shown by Kb values.
Ethylamine : Kb = 5.1 x 10-4 Aniline : Kb = 4.2 x 10-10
The less basic character of aniline can be explained on the basis of aromatic ring present in aniline. Aniline can have the following resonating structures.
It is clear from the above resonating structures that three of these (III, IV and V) acquire some positive charge on nitrogen. As a result, the pair of the electrons become less available for protonation. Hence, aniline is less basic than ethylamine in which there is no resonance.
Note : The basic character of aniline can be understod in terms of orbital theory. According to this concept, , orbital containing the lone pair of electrons on the nitrogen atom interacts with p-bonding of the benzene ring system. The delocalised p-cloud gets extended as shown below.
As a result, the lone pair of electrons are not readily available for protonation and therefore, aniline is less basic than ethylamine (alkyl amines)