Nitro Compounds | 12th Chemistry

13.1 NITRO COMPOUNDS#

Nitro compounds are considered as the derivatives of hydrocarbons. If one of the hydrogen atom of hydrocarbon is replaced by the - $ \mathrm{NO_2} $ group, the resultant organic compound is called a nitrocompound.

13.1.1 Classification of nitrocompounds#

Nitroalkanes are represented by the formula, R-NO2 where R is an alkyl group $(\mathrm{C_nH_{2n+1}})-$.Nitroalkanes are further classified into primary, secondary, tertiary nitroalkanes on the basis of type of carbon atom to which the nitro (-NO )2 group is attached.

13.1.2 Nomenclature of nitroalkanes#

In the IUPAC nomenclature, the nitroalkanes are named by adding prefix nitro before the name of alkane, the position of the nitro group is indicated by number.

13.1.3 ISOMERISM#

Nitroalkanes exhibit chain and position isomerism among their own class and functional isomerism with alkyl nitrites and special type tautomerism can also exist in nitro alkanes having an $ \alpha $ - H atom. For example, nitro compounds having the molecular formula $ \mathrm{C_4H_9NO_2} $ exhibit the following isomerisms.

Tautomerism: Primary and secondary nitroalkanes, having $ \alpha $ - H, also show an equilibrium mixture of two tautomers namely nitro - and aci - form.

Tertiary nitro alkanes do not exhibit tautomerism due to absence of $ \alpha $ - H atom.

S.No.	Nitro form	Aci – form
1.	Less acidic	More acidic
2.	Dissolves in NaOH slowly	Dissolves in NaOH instantly
3.	Decolourises $ \mathrm{FeCl_3} $ solution	With $ \mathrm{FeCl_3} $ gives reddish brown colour
4.	Electrical conductivity is low	Electrical conductivity is high

13.1.4 Acidic nature of nitro alkanes#

The $ \alpha $ - H atom of $ 1^{\circ} $ & $ 2^{\circ} $ nitroalkanes show acidic character because of the electron withdrawing effect of $ \mathrm{NO_2} $ group. These are more acidic than aldehydes, ketones, ester and cyanides. Nitroalkanes dissolve in NaOH solution to form a salt. Aci - nitro derivatives are more acidic than nitro form. When the number of alkyl group attached to $ \alpha $ carbon increases, acidity decreases due to $ +I $ effect of alkyl groups.

Evaluate yourself

Write all possible isomers for the following compounds.

\[ \mathrm{i)\ C_2H_5-NO_2 \qquad ii)\ C_3H_7-NO_2} \]

13.1.5 Preparation of nitroalkanes#

1) From alkyl halides: (Laboratory method)

a) Alkyl bromides (or) iodides on heating with ethanolic solution of potassium nitrite gives nitroethane.

$\mathrm{CH_3CH_2{-}Br + KNO_2 \xrightarrow[\mathrm{SN^2}]{\mathrm{ethanol}/\Delta} CH_3CH_2{-}NO_2 + KBr}$

$\text{Ethyl bromide} \qquad\qquad\qquad\qquad\qquad\qquad \text{Nitroethane}$

The reaction follows $ \mathrm{SN2} $ mechanism.

This method is not suitable for preparing nitrobenzene because the bromine directly attached to the benzene ring cannot be cleaved easily.

2) Vapour phase nitration of alkanes: (Industrial method)

Gaseous mixture of methane and nitric acid passed through a red hot metal tube to give nitromethane.

$\mathrm{CH_4(g) + HNO_3(g) \xrightarrow[\mathrm{Red\ hot\ Si\ tube}]{675\ K} CH_3{-}NO_2 + H_2O}$

Except methane, other alkanes (upto $ n $-hexane) give a mixture of nitroalkanes due to C-C cleavage. The individual nitro alkanes can be separated by fractional distillation.

$$\text{CH}_3\text{-CH}_3 + \text{HNO}_3 \xrightarrow{675\text{ K}} \underset{\text{Nitroethane}(73\%)}{\text{CH}_3\text{CH}_2\text{-NO}_2} + \underset{\text{Nitromethane (27\%)}}{\text{CH}_3\text{NO}_2}$$

3) From $ \alpha $-halocarboxylic acid

$ \alpha $-chloroacetic acid when boiled with aqueous solution of sodium nitrite gives nitromethane.

$$\underset{\alpha\text{ - chloro acetic acid}}{\text{Cl - CH}_2\text{-COOH}} + \text{NaNO}_2 \xrightarrow[\text{SN2}]{\text{H}_2\text{O/Heat}} \underset{\text{Nitromethane}}{\text{CH}_3\text{-NO}_2} + \text{CO}_2 + \text{NaCl}$$

4) Oxidation of tert-alkyl amines

tert-butyl amine is oxidised with aqueous $ \mathrm{KMnO_4} $ to give tert-nitro alkanes.

5) Oxidation of Oximes

Oxidation of acetaldoxime and acetone oxime with trifluoroperoxy acetic acid gives nitroethane $ (1^{\circ}) $ and 2-nitropropane $ (2^{\circ}) $ respectively.

$$\underset{\text{Acetaldoxime}}{\text{CH}_3\text{-CH=N-OH}} \xrightarrow[{[\text{O}]}]{\text{CF}_3\text{COOOH}} \underset{\text{Nitroethane}}{\text{CH}_3\text{CH}_2\text{-NO}_2}$$

13.1.6 Preparation of Nitroarenes#

1) By Direct nitration

When benzene is heated at 330K with a nitrating mixture $ \mathrm{(Conc.\ HNO_3 + Conc.\ H_2SO_4)} $, electrophilic substitution takes place to form nitrobenzene. (Oil of mirbane)

On direct nitration of nitrobenzene m-dinitrobenzene is obtained.

2) Indirect method

Nitration of nitrobenzene gives m-dinitrobenzene. The following method is adopted for the preparation of p-dinitrobenzene.

For example

Amino group can be directly converted into nitro group, using caro’s acid $(\text{H}_2\text{SO}_5)$ (or) persulphuric acid $(\text{H}_2\text{S}_2\text{O}_8)$(or)peroxytrifluro acetic acid $(\text{F}_3\text{C.CO}_3\text{H})$ as oxidising agent.

13.1.7 Physical properties of nitroalkanes#

The lower nitroalkanes are colourless pleasant smelling liquids, sparingly soluble in water, but readily soluble in organic solvents like benzene, acetone etc. They have high boiling points because of their highly polar nature. Alkyl nitrites have lower boiling points than nitro alkanes.

13.1.8 Chemical properties of nitroalkanes#

Nitroalkanes undergo the following common reactions.

i. Reduction ii. Hydrolysis iii. Halogenation

i. Reduction of nitroalkanes

Reduction of nitroalkanes has important synthetic applications. The various reduction stages of nitro group are given below.

The final product depends upon the nature of reducing agent as well as the pH of the medium.

Reduction of alkyl nitrites

Ethyl nitrite on reduction with $ \mathrm{Sn/HCl} $ gives ethanol

\[ \mathrm{CH_3CH_2-O-N=O + 6[H] \xrightarrow{Sn/HCl} CH_3CH_2-OH + NH_3 + H_2O} \]

ii. Hydrolysis of nitroalkanes

Hydrolysis can be effected using conc. HCl or conc. $ \mathrm{H_2SO_4} $. Primary nitroalkanes on hydrolysis gives carboxylic acid, and the secondary nitroalkanes give ketones. The tertiary nitroalkanes have no reaction.

On the other hand, the acid or base hydrolysis of ethyl nitrite gives ethanol.

$$\begin{aligned} &\text{CH}_3\text{CH}_2{\color{black}\boldsymbol{-}}\text{O}{\color{black}\boldsymbol{-}}\text{N=O} + \text{HOH} \xrightarrow[(\text{or) H}^+]{\text{OH}^-} \text{CH}_3\text{CH}_2{\color{black}\boldsymbol{-}}\text{OH} + \text{HNO}_2 \\ &\underset{\text{\vphantom{Ethanol}}}{\text{Ethylnitrite}} \qquad\qquad\qquad\qquad\qquad\qquad\qquad\ \ \ \ \ \underset{\text{Ethanol}}{\text{}} \end{aligned}$$

iii. Halogenation of nitroalkanes

Primary and secondary nitroalkanes on treatment with $ \mathrm{Cl_2} $ or $ \mathrm{Br_2} $ in the presence of NaOH give halonitroalkanes. The $ \alpha $-H atom of nitroalkanes are successively replaced by halogen atoms.

$$\text{CH}_3{\color{black}\boldsymbol{-}}\text{NO}_2 + 3\text{Cl}_2 \xrightarrow{\text{NaOH}} \underset{\text{Chloropicrin (trichloronitromethane)}}{\text{CCl}_3{\color{black}\boldsymbol{-}}\text{NO}_2} + 3\text{HCl}$$

Toxicity: Nitroethane is suspected to cause genetic damage and be harmful to the nervous system.

iv. Nef carbonyl synthesis

Chemical Properties of nitrobenzene#

Electrolytic reduction:

Reduction with catalytic and metal hydrides

Nitrobenzene reduction with Ni (or) Pt, (or) $ \mathrm{LiAlH_4} $ gives aniline

$$\text{C}_6\text{H}_5{\color{black}\boldsymbol{-}}\text{NO}_2 + 6\text{ [H]} \xrightarrow[(\text{or) LiAlH}_4]{\text{Ni (or) Pt / H}_2} \text{C}_6\text{H}_5{\color{black}\boldsymbol{-}}\text{NH}_2 + 2\text{ H}_2\text{O}$$

Selective reduction of polynitro compounds

Electrophilic substitution reaction

The electrophilic substitution reactions of nitrobenzene are usually very slow and vigorous reaction condition have to be employed (-$ \mathrm{NO_2} $ group is strongly deactivating and m-directing).

Nitrobenzene does not undergo Friedel-Crafts reactions due to the strong deactivating nature of -$ \mathrm{NO_2} $ group.