Nitroalkanes

NITROALKANES

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.
 

Physical properties of nitroalkane

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.

Preparation of nitroalkane

(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  1nitro 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 + 2H2

                      |                                                            |

               3o butylamine                                        (83% yield)

 

chemical properties of nitroalkane

Reduction of nitro compounds

 

Alkylation of nitroalkanes

Alkylation of nitroalkanes is a chemical reaction in which an alkyl group (-CH2CH3, -CH(CH3)2, etc.) is added to a nitroalkane molecule (-NO2) in the presence of a strong base. The reaction is typically carried out using an alkyl halide (such as ethyl chloride) and a base (such as sodium ethoxide) in a solvent like ethanol.

The alkylation of nitroalkanes is an important method for the synthesis of a variety of organic compounds, including pharmaceuticals, pesticides, and dyes. The reaction proceeds by a nucleophilic substitution mechanism in which the nitro group acts as a nucleophile, attacking the electrophilic carbon atom of the alkyl halide. The alkyl group then replaces the halide group, forming a new carbon-carbon bond.

The alkylation of nitroalkanes is useful because the nitro group can be easily reduced to an amino group (-NH2) using reducing agents like tin and hydrochloric acid. This reaction provides a convenient way to introduce an alkyl group into an amino compound, which is important for the synthesis of many pharmaceuticals and other organic compounds.

However, the alkylation of nitroalkanes can also lead to the formation of unwanted by-products, such as elimination products, which can decrease the yield of the desired product and create environmental issues. Therefore, it is important to optimize the reaction conditions and control the reaction parameters carefully to maximize the yield and minimize the formation of by-products.