Some Important Organic Compounds Containing Nitrogen

INTRODUCTION

Important functional groups containing nitrogen are:

NITRO COMPOUNDS

INTRODUCTION

Nitro alkanes are derivatives of alkanes. They are isomeric to nitrites (esters) classified as primary, secondary and tertiary depending on the nature of carbon atom to which nitro group is linked.

$-N{O}_2$group is an ambident group. If it attacks through nitrogen

It is called nitro and if it attacks through oxygen atom, it is called nitrite. Hence nitrites and nitro compounds are isomers.

Illustration 1. What is ambident nucleophile?

Solution: Nucleophiles which can attack from two sites such as CN^-, NO₂^- are called ambident nucleophiles.

STRUCTURES

Evidences show that nitrogen is attached to one of the oxygen atoms by a double bond and to the other by a dative bond. The resonance hybrid is shown as under which confirms the spectroscopic evidence that both nitrogen – oxygen bonds have same bond length.

Out of three $s{p}^2$hybrid orbitals of nitrogen one overlaps with alkyl group and two with oxygens while the unhybridized p orbital of N – atom containing a pair of electrons and lying perpendicular to the plane of hybrid orbitals overlaps sideway with half filled 2 p – orbitals of two oxygen atoms. This forms $\pi -bond$above and below the plane of molecule.

Illustration 2. p-chloronitrobenzene has less dipole moment (2.4D) than p-nitrotoluene. Explain.

Solution:

In p – chloronitrobenzene, the two vector dipole moment act opposite to each other which partially cancel each other. On the other hand, in p – nitrotoluene, the two vectors acts in the same direction to give additive dipole resultant.

METHODS OF PREPARATION:

(i) From alkyl halides:

Alkyl halides react with silver nitrite in ethanolic solution to give nitro compounds. Alkyl nitrite is formed in minor quantity. This reaction is used to prepare$1^o$ nitro compounds primarily while $2^{o}and{3}^o$halides give major proportion of alkenes due to – elimination. Contrary to this alkali nitrites give alkyl nitrites as major product. This is due to ionic nature of alkali nitrite.

But if the reaction is carried out in solvents like DMF or DMSO, then even $NaN{O}_{2}orKN{O}_2$ give good yield (about 60%) of nitro compound.

Reactions:

$R-I+AgN{O}_2\to RN{O}_2+AgI$

$C_2{H}_{5}I+AgN{O}_2\to C_2{H}_{5}N{O}_2+AgI$

Nitroethane

(ii) Nitration:

Nitro derivatives of aromatic compounds like nitrobenzene are produced when benzene is allowed to react with nitrating mixture.$(conc.HN{O}_3/conc.H_{2}S{O}_4)$.

Mechanism:

Generation of nitronium ion

Attack of $\stackrel{\oplus }{N}{O}_2$ on benzene molecule

Loss of proton:

$N{O}_2^{+}Cl{O}_4^{-},N{O}_2^{+}B{F}_4^{-},C{H}_{3}COON{O}_2,$ etc are other nitrating agents used.

Direct nitration of alkane involves vapour phase nitration at high temperature.

$R-H+HON{O}_2\underset{675K}{\to }R-N{O}_2+H_{2}O$

low yield

Problem faced in the method is that at such high temperature, a mixture of nitro alkanes is formed due to C – C cleavage.

e.g. $C{H}_{3}C{H}_{2}C{H}_3+HN{O}_3\to C{H}_{3}C{H}_{2}C{H}_{2}N{O}_2+C{H}_{3}C{H}_{2}N{O}_2+C{H}_{3}N{O}_2+otherproducts$

(iii) From amines:

$3^o$nitroalkanes can be produced as follows:

PROPERTIES OF NITRO COMPOUNDS

Nitroalkanes are colourless liquids with pleasant smell while aromatic nitro compounds have characteristic odour. Nitro alkanes are sparingly soluble in water, are highly polar with strong dipole – dipole interactions due to which they have high boiling points. Most of nitroalkanes are quite stable and can be distilled without decomposition but alkyl nitrites are unstable and explode on heating. Nitro compounds like nitrobenzene, o – nitrophenol are steam volatile and can be purified by steam distillation.

Reactions of nitro alkanes are given below:

Reduction:

Note:

Hydroxylamine on warming with ammonical silver nitrate solution (Tollen's reagent) gets oxidized to nitroso compound and reduces Tollen's reagent to metallic silver. This reaction is a test for nitro compounds and is known as Baker – Milliken's test.

$C{H}_{3}C{H}_2-N{O}_2\stackrel{LiAl{H}_4/ether}{\to }C{H}_{3}C{H}_{2}N{H}_2$

$1^{o}a\min e$

Aromatic nitrocompounds give azo compounds on above reduction

$2C_6{H}_{5}N{O}_2\stackrel{LiAl{H}_4}{\to }{C}_6{H}_{5}N=N-C_6{H}_5+2H_{2}O$

Aromatic nitro compounds undergo bimolecular reduction in alkaline medium.

Very mild reducing agents like ${(N{H}_4)}_{2}S$ are used to reduce one nitro group without affecting the other.

This reduction is called selective reduction named as Zinin reduction.

Illustration 3. Nitrobenzene when reduced with Zn and $N{H}_{4}OH$yields a product (A). Identify (A). Will (A) reduce Tollen's reagent?

Solution: The product (A) is phenyl hydroxylamine. It reduces Tollen's reagent.

This reaction is Mulliken Test for distinguishing $-N{O}_2$ group.

(ii) Action with nitrous acid:

$1^{o}nitroalkane\stackrel{HONO}{\to }nitrolicacid$

$2^{o}nitroalkane\stackrel{HONO}{\to }pseudonitroles$

$3^{o}nitroalkane\stackrel{HONO}{\to }Noreaction$

(iii) Hydrolysis in acidic medium:

$1^{o}nitroalkanes\to Carboxylicacid+hydroxyla\min e$

$2^{o}nitroalkanes\to nitrousoxide+ketone$

$3^{o}nitroalkanes\to Noreaction$

e.g. $C{H}_{3}C{H}_2-N{O}_2+H_{2}O\stackrel{HCl}{\to }C{H}_{3}COOH+N{H}_{2}OH.HCl$

(iv) Tautomerism and acidic nature:

Nitroalkanes are called pseudoacids as although being neutral to litmus, in very strong alkaline medium, they behave as acids and form salts.

$3^o$nitroalkanes do not have $\alpha -H$ atom and therefore do not exhibit tautomerism.

(v) Condensation with aldehydes:

In alkaline medium, 1° and 2° nitroalkanes condense with aldehydes which on further condensation give unsaturated nitroalkenes

(vi) Ring substitution in nitrobenzene:

Nitro group being electron withdrawing group is a m – directing group. Some of the reactions of nitrobenzene which makes the m – directing nature of $-N{O}_2$ group clear are:

DISTINGUISH TESTS BETWEEN NITROALKANES AND ALKYL NITRITES

1. Nitroalkane on reduction with $H_2/Ni$ produce $1^o$ amines while alkyl nitrites produce alcohols and $N{H}_3.$

$C{H}_{3}C{H}_{2}N{O}_2\stackrel{6(H)}{\to }\underset{1^{o}a\min e}{C{H}_{3}C{H}_{2}N{H}_2}+H_{2}O$

$\underset{Ethylnitrite}{C{H}_{3}C{H}_2-O-N=O}\stackrel{6(H)}{\to }C{H}_{3}C{H}_{2}OH+N{H}_3+H_{2}O$

2. Nitroalkanes do not get hydrolysed in basic conditions while nitrites produce alcohols

Illustration 4. Nitrobenzene, but not benzene, is used as a solvent for Friedel – Craft alkylation of bromobenzene. Why?

Solution: If during alkylation of bromobenzene benzene is used as a solvent, alkylation of benzene will take place because benzene is more reactive in electrophilic substitution reactions than bromobenzene. On the other hand, nitrobenzene does not under goes Friedel – Craft reaction and can be used as solvent for alkylation of bromobenzene.

CYANIDES AND ISOCYANIDES

Both alkyl cyanides (RCN) and alkyl isocyanides (RNC) are organic derivatives of hydrocyanic acid HCN. Alkali cyanides are ionic $\left(:C\equiv \stackrel{-}{N}:\right)$ and cyanide ion is ambident in nature (can form covalent bond either from carbon or nitrogen). $AgC\equiv N$is covalent, hence lone pair on nitrogen is mainly available for covalent bond formation, resulting in predominant formation of isocyanides.

Illustration 5. How would you account for the fact that alkyl cyanides are soluble in water but alkyl isocyanides are insoluble in water?

Solution: Alkyl cyanides possess the tendency to form H – bonding with water which is absent with isocyanides

Methods of preparation of Cynaides :

1. Dehydration of Amides:

High molecular weight acid amides are dehydrated to the corresponding cyanide by heat alone.

$C{H}_3{(C{H}_2)}_{6}CON{H}_2\stackrel{Heat}{\to }C{H}_3{(C{H}_2)}_{6}CN$

2. From RX:

$RX+KCN\to RCN+KX$

This method is satisfactory only if R is $1^{o}or{2}^o$ group. If it is $3^o$ group, then it is converted into alkene.

$C{H}_{3}C{H}_{2}Cl+KCN\to C{H}_{3}C{H}_{2}CN+KCl$

3. By Grignard's reagent and Cyanogen chloride reaction:

$RMgCl+ClCN\to RCN+MgC{l}_2$

This is best method for preparing $3^o$ alkyl cyanides.

${(C{H}_3)}_{3}CMgCl+ClCN\to {(C{H}_3)}_{3}CCN+MgC{l}_2$

4. From Diazonium salt

Properties:

1. Lower members are liquids which are soluble in water but solubility decreases as molecular weight increases.

2. Since alkyl cyanides cannot form intermolecular hydrogen bonds, hence they are much more volatile than their corresponding acids.

Reactions:

1. Hydrolysis:

$RCN\underset{HCl}{\overset{H_{2}O}{\to }}RCON{H}_2\stackrel{H_{2}O}{\to }RC{O}_{2}H+N{H}_3$

Partial hydrolysis to acid amides is achieved with alkaline H₂O₂.

Alkaline hydrolysis of $RC\equiv N$ gives sodium salt.

$RC\equiv N\underset{NaOH}{\overset{dilute}{\to }}RCOONa+N{H}_3$

2. Reduction:

3. With Grignard's reagent (ketone formation):

4. With alcohol:

When a solution of RCN in an alcohol is heated with conc. H₂SO₄ or HCl, ester is obtained.

$RC\equiv N+R^{\prime }OH+H_{2}O\stackrel{H_{2}S{O}_4}{\to }RC{O}_2{R}^{\prime }+N{H}_3$

$C{H}_{3}CN+C_2{H}_{5}OH+H_{2}O\stackrel{H_{2}S{O}_4}{\to }C{H}_{3}C{O}_2{C}_2{H}_5+N{H}_3$

Illustration 6. The products (A) and (B) formed in the given reaction is:

$RCOCl\stackrel{Na{N}_3}{\to }(A)\stackrel{C_2{H}_{5}OH/\Delta }{\to }(B)$

(A) RCON and RCN (B) RCN and RNC

(C) RCON₃ and RCN (D) RCON₃ and RNCO

Solution:

Hence (C) is correct.

Methods of Preparation of Isocyanides:

1. By heating an alkyl iodide with AgCN in aqueous ethanolic solution

$RI+AgCN\to RNC+AgI$

\begin{align}  {{C}_{2}}{{H}_{5}}I+AgCN\xrightarrow{{}}{{C}_{2}}{{H}_{5}}NC+AgI \\  \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,ethylisocyanide \\ \end{align}

2. By carbylamine reaction

Heating a mixture of $1^o$amine and chloroform with ethanolic potassium hydroxide

$RN{H}_2+CHC{l}_3+3KOH\to RNC+3KCl+3H_{2}O$

Mechanism proceeds via intermediate formation of dichloromethylene or, dichloro carbene produced from chloroform in alkaline solution. (Via a-elimination)

$CHC{l}_3+KOH\to KCl+H_{2}O+:CC{l}_2$

Properties:

1. Alkyl isocyanides are poisonous, unpleasant smelling, with lower boiling points than isomeric cyanides.

2. RNC are not very soluble in water, nitrogen atom not having a lone pair of electrons available for hydrogen bonding.

Reactions:

1. Hydrolysis:

$RNC+2H_{2}O\underset{dilute}{\overset{acid}{\to }}RN{H}_2+HC{O}_{2}H$

$C{H}_{3}NC+2H_{2}O\stackrel{H^{+}}{\to }C{H}_{3}N{H}_2+HC{O}_{2}H$

RNC are not hydrolysed by alkalis.

2. Reduction:

\begin{align} RNC\xrightarrow[{}]{{{H}_{2}}/Pt}RNHC{{H}_{3}} \\  \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{{2}^{o}}\,\,a\min e \\ \end{align}

\begin{align}  C{{H}_{3}}NC\,\,\,\,\,\,\,\,\,\,\xrightarrow{{{H}_{2}}/Pt}\,\,\,\,\,\,\,C{{H}_{3}}NHC{{H}_{3}} \\  Methyl\,\,isocyanide\,\,\,\,\,\,\,\,\,Dimethyl\,\,a\min e \\ \end{align}

3. When alkyl isocyanides are heated for a long time, they arrange to form cyanide

$RNC\to RCN$

$C{H}_{3}C{H}_{2}NC\to C{H}_{3}C{H}_{2}CN$

4. With non metals:

(i) $RNC+X_2\to RNC{X}_2$

$C{H}_{3}NC+C{l}_2\to C{H}_{3}NCC{l}_2$

\begin{align} (ii)\,\,\,\,\,\,\,\,RNC+S\xrightarrow{{}}RNCS \\  \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,Alkyl\,\,isothiocyanates \\ \end{align}

$C{H}_{3}NC+S\to C{H}_{3}NCS$

5. Oxidation with HgO:

\begin{align} RNC+HgO\xrightarrow{{}}RNCO\,\,\,\,+\,\,\,\,\,Hg \\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,Alkyl\,\,isocyanates \\\end{align}

$C{H}_{3}NC+HgO\to C{H}_{3}NCO+Hg$

Illustration 7.

Solution:

Hence (C) is correct.

Amino acids and Proteins:

Amino acids are derivatives of carboxylic acids in which a hydrogen atom at the $\alpha -carbon$ is replaced by an amino group. Amino acids are obtained by hydrolysis of proteins. The amino acids can be represented by

About twenty five amino acids are obtained from proteins, of which about ten are essential, i.e. a deficiency in any one prevents growth in young animals and may even cause death. Simplest amino acids are

Alanine and all amino acids except glycine contain an asymmetric carbon atom and exhibit optical isomerism (naturally occurring amino acids are laevorotatory). Amino acids contain amino and carboxyl groups and show the properties of both groups. Amino acids have high melting points and are usually water soluble. They usually exist as inner salts or zwitter ions or dipolar ion (a species containing both positive and negative charges).

Amino acids are amphoteric.

Amino acids are linked together by peptide linkages.

Peptides are condensation products of two or more amino acids through their amino and carboxylic groups involving elimination of water molecules. Amino acid polymers with relative molecular masses 10,000 or less are called polypeptides. Those with larger molecular masses are called proteins e.g. egg albumen 40,000.

Illustration 8. Explain why does glycine exist as a Zwitter ion but anthranilic acid does not.

Solution: The $-$COOH group of glycine releases H⁺ ion which is accepted by $-$NH₂ group. Thus glycine exists as a zwitter ion.

However, the electron withdrawing nature of $-$COOH group reduces electron density on N in anthranilic acid. Thus nitrogen atom of amino group fails to accepts the proton released by the $-$COOH group. That is why anthranilic acid does not exist as zwitter ion.

Test for Proteins:

Presence of proteins can be shown by warming the suspected protein with dilute NaOH solution and CuSO₄ solution. A violet colouration confirms presence of proteins.

Methods of preparation of amino acids:

(i) Amination of $\alpha -haloacids.$

\begin{align} Cl-C{{H}_{2}}COOH+2N{{H}_{3}}\xrightarrow[{}]{}{{H}_{2}}NC{{H}_{2}}COOH+N{{H}_{4}}Cl \\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,Glycine \\ \end{align}

(ii) Gabriel phthalimide synthesis:

$\alpha -ha\log enated$acid or ester combines with potassium phthalimide. Product on hydrolysis gives $\alpha -a\min oacid.$

(iii) Knoop synthesis:

$\alpha -$keto acid on treatment with NH₃ forms an imine which on catalytic reduction gives amino acid.

(iv) Strecker synthesis:

Aldehydes react with HCN and ammonia or NH₄CN and the product on hydrolysis yields $\alpha -a\min o$acids.

Some of the reactions given by amino acids are as follows:

Action of heat:

$\alpha -$amino acids lose two molecules of water and form cyclic amides.

$\beta -$amino acids lose a molecule of ammonia per molecule of amino acid to yield $\alpha ,\beta -$unsaturated acids.