Nitro & Cyno Compounds

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.

Primary nitro alkane        Secondary Nitro alkane            Tertiary nitro alkane

—NO2 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.

What are ambident nucleophiles?

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

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.

Resonating forms                             Hyrbid structure

Out of three 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 π-bond above and below the plane of molecule.

Preparation of Nitro Compounds

(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  1o nitro compounds primarily while 2oand 3o halides give major proportion of alkenes due to β – 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 NaNO2 or KNO2 give good yield (about 60%) of nitro compound.

Reactions:

R—I + AgNO2 ——> RNO2 + Agl

C2H5l + AgNO2  ——> C2H5NO2 + Agl

Nitroethane

(ii) Nitration:

Nitro derivatives of aromatic compounds like nitrobenzene are produced when benzene is allowed to react with nitrating mixture.(conc. HNO3/conc.H2SO4).

Mechanism:

Generation of nitronium ion

Attack of NO2 on benzene molecule

Loss of proton:

Nitrobenzene

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

R — H + HONO2 ———> R — NO2 + H2O

675 K          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. CH3CH2CH3 + HNO3 ——> CH3CH2CH2NO2 + CH3CH2NO2 + CH3NH2 + other products

(iii) From amines:

3o nitroalkanes can be produced as follows:

CH3                                                       CH3

|                                                            |

CH3 ———— C ———— NH2 ————> CH3 ———— C ———— NO2 + 2H2O

|                                                            |

3o butylamine                                        (83% yield)

Distinguish test between nitroalkanes & alkyl nitrites

1. Nitroalkane on reduction with  H2/Ni produce 1o amines while alkyl nitrites produce alcohols and NH3

CH3CH2NO2  CH3CH2NH2 + H2O

1o amine

CH3CH2 — O — N = O  CH3CH2OH2 + NH3 + H2O

Ethyl nitrite

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

CH3 O – N = O + NaOH ——> CH3OH + NaNO2

Cyanides & Isocyanides

Both alkyl cyanides (RCN) and alkyl isocyanides (RNC) are organic derivatives of hydrocyanic acid HCN. Alkali cyanides are ionic  and cyanide ion is ambident in nature (can form covalent bond either from carbon or nitrogen).AgC = N is covalent, hence lone pair on nitrogen is mainly available for covalent bond formation, resulting in predominant formation of isocyanides.

Illustration . 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.

CH3(CH3)6 OCNH2  CH3(CH2)6 CN

2. From RX:

RX + KCN ——> RCN + KX

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

CH3CH2Cl + KCN →  CH3CH2CN + KCl

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

RMgCl + CICIN →  RCN + MgCl2

This is best method for preparing 3o alkyl cyanides.

(CH3)3CMgCl + CICN →  (CH3)3 CCN + MgCl2

4. From Diazonium salt

Methods of Preparation of Isocyanides:

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

Rl + AgCN →  RNC + Agl

C2H5l + AgCN →  C2H5NC + Agl

Ethylisocyanide

2. By carbylamine reaction

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

RNH2 + CHCl2 + 4KOH ——> RNC + 3KCl + 3H2O

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

CHCl3 + KOH ———>KCl + H2O + : CCl2

Properties of Isocyanides

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 + 2H2O  RNH2 + HCO2H

CH3NC + 2H2O  CH3NH2 + HCO2H

RNC are not hydrolysed by alkalis.

2. Reduction:

RNC R NHCH3

2o amine

CH3NC CH3NHCH3

Methyl isocyanide        Dimethyl amine

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

RNC →  R CN

CH3CH2NC →  CH3CH2CN

4. With non metals:

(i)   RNC + X2 ———> RNCX2

CH3NC + Cl2 ——> CH3NCCl2

(ii)   RNC + S ———> RNCS

Alkyl isothiocyanates

CH3NC + S ———> CH3NCS

5. Oxidation with HgO:

RNC + HgO →  RNCO + Hg

Akyl isocyanates

CH3NC + HgO →  CH3NCO + Hg

Look here for Organic Chemistry Revision Notes, Chemistry Syllabus and Best books of Organic Chemistry.

Related Resources
Chemical Properties of Amines

Chemical Properties of Amines Basic Nature of...

Solved Examples

Solved Examples on Amines Question 1 . The...

Basicity of Amines

Basicity of Amine Aliphatic Bases As increasing...

Preparation of Amines

Preparation of Amine Ammonolysis of Alkyl Halides...