Organic Chemistry – Some Basic Principles and Techniques

 BASIC PRINCIPLES AND TECHNIQUES IN ORGANIC CHEMISTRY

Organic chemistry is the chemistry of carbon compounds except oxides of carbon and metal carbonates. The term organic literally means derived from living organism. All organic compounds contain carbon as their essential constituent. Carbon atom has unique property to form bonds with other carbon atoms. This property of forming bonds with atoms of the same element is called catenation. 

Carbon form strong bonds with many others elements and especially with other carbon atoms to form chains and rings that gives rise to millions of organic compounds.

CLASSIFICATION OF ORGANIC COMPOUNDS

Organic compounds are broadly classified in two ways.

1] Based on carbon skeleton

Aliphatic compounds

These are the compounds in which carbon atoms are joined to form an open chain. Their structure may consist of straight chain or branched chain.

e.g. CH₃ – CH₂ – CH₂ – CH₃

Cyclic compounds

These are the compounds in which carbon atoms are joined to form one or more rings. They are further classified into two types.

a] Homocyclic or carbocyclic

In these compounds, the ring is made up of carbon atoms only. They are further divided into two types.

i) Alicyclic compounds

These compounds show some of the properties similar to those of aliphatic compounds. Carbon atoms are linked by single bonds only.

ii) Aromatic compounds

These are the compounds which contains at least one aromatic ring which resembles benzene in their chemical behavior.

b] Heterocyclic compounds

These compounds include one or more heteroatoms like O, N, S etc. in the ring of carbon atoms. They are also divided into two types.

i) Hetero – alicyclic compounds

Alicyclic compounds which contain at least one heteroatom in the ring are called hetero-alicyclic compounds.

ii) Hetero – aromatic compounds

Aromatic compounds which contain at least one heteroatom in the ring are called hetero – aromatic compounds.

2] Based on functional group

An atom or a group of atoms in the organic molecule which determines its characteristic chemical properties is called the functional group.

e.g.   – OH, - X, - CHO, - COOH, - NH₂, etc. are functional group. The chemistry of every organic molecule is determined by the functional group it contains.

Homologous series

A series of organic compounds each containing a characteristic functional group, and the successive members differ from each other in molecular formula by a –CH₂– (methylene) group is called homologous series.

 

 

PURIFICATION OF ORGANIC COMPOUNDS

The purification of organic compounds involves the following processes.

1. Crystallization

This is the most common method used to purify organic solids which dissolve in particular solvent. The purification is done on the basis of differences in solubility of a given organic compound and impurities.

 

2. Sublimation

It is a process which is used for solids which directly change into vapor state upon heating without passing the liquid state and vapors on cooling give back the solid Substance. Impure samples of naphthalene, anthracene, camphor, etc. are purified by this method.

 

3. Distillation

The process in which liquid is converted into its vapor phase at its boiling point and the vapors is then condensed back to liquid on cooling is known as distillation. This method is used if the organic liquid is stable at its boiling point and it contains non – volatile impurities.

 

4. Fractional distillation

The process of separating and purifying the components of a mixture of two or more miscible liquids having different boiling points is known as fractional distillation. The liquid which is more volatile distills out first leaving behind the less volatile liquid in the distillation flask.

 

5. Steam distillation

Liquids which are immiscible with water but are steam volatile are separated by this technique. Aniline, nitrobenzene, bromobenzene etc. can be steam distilled.

 

6. Fractional crystallization

The process of separating the components of a mixture of two or more solids, having different solubilities in the same solvent at the same temperature, by step – wise crystallization is known as fractional crystallization.

 

7. Differential extraction

The organic solvents like benzene, chloroform, petroleum ether etc. are immiscible with water such solvents are used to extract an organic compound present in aqueous solution by the method of differential extraction.

 

8. Chromatography

Chromatography is the technique used for the separation, isolation, purification and identification of constituents of a mixture. This technique depends on the distribution of the mixture between two phase, one stationary phase and other moving or mobile phase. Depending upon the principle involved, it is divided into two types.

 

a) Adsorption chromatography

It is based on the principle of differential adsorption. Different compound are adsorbed on adsorbent to different degrees. It also has two types i) column chromatography and ii) thin – layer chromatography (TLC).

 

b) Partition chromatography

This technique is based on continuous differential partitioning of components of a mixture between stationary and mobile phases.

 

 

Determination of empirical formula

The formula of a compound which gives the simple whole number ratio of the atoms of various elements present in one molecule of the compound is called its empirical formula. It involves following steps.

Step – I

The percentage composition of elements present in a given compound is determined by quantitative analysis.

Step – II

The percentage of each element is divided by the respective atomic mass. This is gives the atomic ratio of constituents atoms present in a given compound.

Step – III

The simplest ration is found by dividing all the atomic ratios by the smallest atomic ratio.

 

Molecular formula

The formula which gives the actual number of atoms of various elements present in one molecule of the compound is called its molecular formula. Molecular formula is determined from empirical formula as –

Molecular formula = n x empirical formula

Where n is whole integer given by-

N    =          Molecular mass/ Empirical formula mass

 

 

 

ISOMERISM

The compounds having same molecular formula and different structural formulae are called isomers of each other. This phenomenon is known as isomerism. It has two main types.

 

1. Stereoisomerism

The stereoisomers have the same structural formula but differ in the arrangement of atoms or groups in space.

 

 

2. Structural isomerism

The structural isomers differ from each other in the arrangement of the atoms or groups within the molecules.

It can be further divided as :

 

a) Chain or nuclear isomerism

It is the type of isomerism in which the isomers differ in the chain of carbon atoms. For example, C₄H₁₀ has two isomers.

b) Position isomerism

It is the type of isomerism in which the different isomers differ in the position of the functional group. For example, C₃H₇OH has two isomers.

c) Functional isomerism

It is the type of the isomerism in which isomers have different functional group. For example, C₂H₆O has two isomers :

d) Metamerism

It is the type of isomerism in which the isomers differ in the nature of alkyl group attached to the same functional group. For example, C₅H₁₀O has two isomers.

C₂H₅ – CO – C₂H₅    (Diethyl Ketone) and       CH₃ – CO – C₃H₇ (Methyl propyl Ketone)

 

FUNDAMENTAL CONCEPTS IN ORGANIC REACTION MECHNAISM

Electron displacements in covalent bonds

 

Inductive effect

When an organic molecule has a polar covalent bond in its structure polarity is induced in adjacent carbon – carbon single bond too. This is called inductive effect. The arrow head put in the center of the bond is used to represent the inductive effect. The direction of arrow head indicates the direction of permanent electron displacement. For example,

This effect decreases rapidly as the length of carbon chain increases. There are two types of inductive effect.

i) – I effect

Atoms or group of atoms which are highly electronegative or carry positive charge are electron – withdrawing groups and such groups are said to have – I effect.

e.g. – F, - Cl, -Br, - I , - NO₂, - CN, - COOH, - COOR, - SO₃H, etc.

 

ii) + I effect

Atoms or group of atoms which are electropositive or carry negative charge are donating groups and are said to have + I effect.

e.g. Metals like Na, K, Mg, Zn etc. and alkyl groups such as – CH₃, - CH₂CH₃, - CH(CH₃)₂, - C(CH₃)₃, - C(C₂H₅)_3, etc.

 

Electromeric effect

The complete transference of shared pair of electrons to one of the atom joined by a multiple bond in presence of an attacking reagent this is known as electromeric effect.

This is a temporary effect and takes place only in the presence of an attacking reagent. For example:

 

Resonance effect

The polarity developed within a molecule is due to the interaction of two ∏ - bonds or a ∏ - bonds with the lone pair of electrons present on the adjacent atom is called the resonance effect or mesomeric effect. It is a permanent effect and depending, upon the direction of transfer of electrons it is of two types.

 

i) +R effect

When the transfer of electron is away from an atom or substituent group attached to the conjugated system, it is termed as +R effect. For example +R effect in aniline.

 

 ii) – R effect

When the transfer of electron is towards the atom or substituent group attached to the conjugated system, it is termed as – R effect. For example – R  effect in nitrobenzene.

 

 

Hyper conjugation

The delocalization of electrons due to overlap between a P – orbital and sigma (σ) bond of C – H is called hyper conjugation or no bond resonance. Greater the number of alkyl groups attached to a positively charged carbon atom, greater is the hyper conjugation interaction and greater is the stability of cation. Thus the relative stability of cation decreases in the order.

(CH₃)₃C⁺ > (CH₃)₂CH⁺ > CH₃CH₂⁺ > CH₃⁺

 

 

Bond fission

In any chemical reaction, when a reactant is converted into products one or more bonds in the reactant are broken and new bonds are formed. The process of breaking or cleavage of covalent bond is known as bond fission. The bond fission takes place in two ways.

 

i) Hemolytic fission

The symmetrical breaking of a covalent bond between two atoms such that each atom acquires one electron of the shared pair is called hemolytic fission or homolysis.

Such fission takes place in the presence of ultraviolet light or high temperature. The cleavage of a bond results in the formation of free radicals. A free radical is a neutral species (atom or group) which contain an unpaired electron. A hemolytic fission is represented as:

ii) Heterolytic fission

The unsymmetrical breaking of a covalent bond between two atoms such that the more electronegative atom acquires both the electrons of the shared pairs is called heterolytic fission or heterolysis .

Such a fission takes place in the presence of polar solvent. The cleavage of a bond results in the formation of ions. This ions formed are unstable and reactive. One of the ion has a positive charge called as cation and the other ion has at least one lone pair and a negative charge called as an anion. Organic reactions which proceed by heterolysis are called ionic or  heteropolar simply polar reaction. A heterolytic fission is represented as:

The species in which carbon atom passes a positive charge is called carbanion and the species in which carbon atom possesses negative charge is called carbanion.

Example of heterolysis is,

Reaction intermediates

 

i) Free radicals

An uncharged species which is electrically neutral and contains a single electron is called free radical.

Free radical is highly reactive and therefore has a transitory existence i.e. it is short – lived. The stability of free radicals decreases in the order triphenyl methyl > benzyl > allyl > 3ᵒ > 2ᵒ > 1ᵒ > methyl > vinyl.

 

ii) Carbocation

A species in which carbon atom bears a positive charge is called a carbocation. Positively charged carbon is sp² hybridized. It is electron – deficient. It is Lewis acid and act as an electrophile. It has planer geometry.

e.g. tetra – butyl carbocation (CH₃)₃C⁺.

 

iii) Carbanion

A species in which carbon atom bears a negative charge is called a carbanion. Negatively charged carbon is Sp³ / Sp² hybridized. It is electron – rich. It is Lewis base and act as a nucleophile. It has pyramidal geometry.

e.g. methyl carbanion      CH₃.

 

iv) Carbenes

The highly reactive intermediate containing neutral and divalent carbon atoms is called carbene.

For example, CH₂

 

 

Types of attacking reagent.

1) Electrophiles

Electrophiles are electron – deficient species. They are either positively charge species like H⁺, NO⁺₂ etc. or molecules having incomplete octet of electrons like BF₃, AlCl₃, ZnCl₂ etc.

Since electrophiles are electron deficient they accept a pair of electrons from donor atoms and thus they are electron loving reagents. All electrophiles are basically lewis acids.

 

2) Nucleophiles

Nucleophiles are electron rich species. They are either negatively charged species like OH^-, CN^-, Cl^-, Br^-etc. or molecules containing at least one lone pair of electrons on central atom like H₂O, NH₃, H₂S, R-OH, R-NH_2, etc. since nucleophiles are electron rich, they donates pair of electrons to acceptor atoms and thus they are nucleus loving reagents. All nucleophiles are Lewis base.

 

 

 

Types of organic reactions

1) Substitution reaction

A reaction in which an attacking species replaces another atom or group in substrate is called substitution reaction.

e.g.

2) Addition reaction

When two molecules combine to form one product molecule, it is called an addition reaction.

e.g.

CH₂ = CH₂ (Ethylene)   +     HCl    →    CH₃ – CH₂ – Cl (Ethyl chloride)

  

3) Elimination reaction

When one molecule is split into two fragment molecules, it is called an elimination reaction.

e.g.

4) Rearrangement reaction

A reaction in which either the carbon skeleton or the functional group or both are modified is known as rearrangement reaction.

e.g.

 Stereochemistry PDF Notes

1. Stereo-Chemistry is the branch of the chemistry. Stereochemistry concerned with the 3D arrangement of atoms & molecules and also concerned with the effect of this 

   3D arrangement on chemical reactions. You can download complete stereochemistry PDF notes from our telegram channel @ChemistryNotesInfo

1. What is Stereochemistry ?

2. Stereochemistry is the branch of chemistry. Stereochemistry involves study of the relative spatial arrangement or we can say that 3D (Three-Dimensional) arrangement of atoms that makeup the structure of the molecules. 

What are Stereoisomers ?

Stereoisomers compounds with the same connectivity but different arrangement in the space are known as Stereoisomers.

What are Enantiomers ?

Enantiomers are stereoisomers that have the non-superimposable mirror images. Only property of the enantiomers that differ them are the direction (+ or -) of the optical rotation are known as enantiomers.

What is Diastereomers ?

Diastereomers are the stereoisomers that have not the mirror images but they are different compounds with the different physical properties are known as Diastereomers.

Download Stereochemistry PDF Notes

You can Download Stereochemistry PDF Notes from below link-

Link- https://t.me/chemistrynotesinfo/453 

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Amines Class 12 Notes

In this revision lecture notes, we will learn about Amines Class 12 Chemistry Notes. We will cover all basic scientific knowledge about Amines in this article. So, enjoy learning chemistry with Chemistry Notes Info @ https://chemistrynotesinfo.com

Amines Class 12 Chemistry Notes

AMINES INTRODUCTION

            Amines are the organic derivatives of ammonia (NH3) in which one, two, or all three hydrogen atoms attached to nitrogen are replaced by equivalent number of same or different alkyl and/ or aryl groups.

Like ammonia, amines have pyramidal geometry and the nitrogen atom in amines, the nitrogen is attached to sp3 hybridized carbon of alkyl group and in aromatic amines to sp2 hybridized carbon of aryl group. H – N – H , C – N – H or C – N – C bond angle is less than 109ᵒ28’.

CLASSIFICATION OF AMINES

            Depending upon the number of hydrogen atoms replaced by alkyl or aryl groups attached to nitrogen atom in ammonia molecule, amines are classified as –

1) Primary (1ᵒ) Amines

The functional group present is – NH2 (Amino group)

e.g.     CH3 – NH2 Methylamine

2) Secondary (2ᵒ) Amines

The functional group present is – NH-  (Amino group)

e.g.     CH3 – NH– CH3 Dimethylamine

3) Tertiary (3ᵒ) Amines                

The functional group present is -N= (Tertiary nitrogen atom)

Secondary and tertiary amines are further classified as

Secondary and Tertiary amines of Amines Class 12 Notes are further classified into two groups i.e.

a) Simple / Symmetrical amines

In simple amines same alkyl or aryl groups are attached to the nitrogen.

b) Mixed / Unsymmetrical amines

In mixed amines different alkyl or aryl groups are attached to the nitrogen.

METHOD OF PREPARATION OF AMINES

1) By ammonolysis of alkyl halides

            When ethyl bromide is heated with alcoholic ammonia at 373K in sealed copper tube, it gives mixture of ethylamine, diethyl amine and triethyl amine along with tetraethyl ammonium bromide.

C2H5 – Br + NH3     Δ → C2H5 – NH2 + HBr

C2H5 – NH2 + C2H5 – Br    Δ  → (C2H5)2NH + HBr

(C2H5)2NH + C2H5 – Br    Δ  → (C2H5)3N + HBr

(C2H5)3 + C2H5 -Br    Δ  → (C2H5)4N+Br–

Thus,

NH3     RX→ R – NH2    RX→ R2NH  RX→ R3N    RX→ R4N+X–

2) By reduction of Nitro Compounds

            Both aliphatic and aromatic primary amines can be prepared by the reduction of nitro compounds by either catalytically with H2 in the presence of Raney Ni, Pt or Pd or chemically with active metals.

e.g.       CH3NO2    +     3H2   Raney Ni/Pt→     CH3NH2     +     2H2O         Nitromethane                   Ethanol          Methylamine

3) By Gabriel phthalimide synthesis

            Phthalmide reacts with ethanolic potassium hydroxide to give potassium salt of phthalmide. In this step N – H proton is removed to give imide ion. It is then heated to give N – alkyl phthalimide, which on alkaline hydrolysis give a primary amine. Share these Amines Class 12 Notes of chemistry with your friends.

4) By Reduction of Alkyl nitrites / cyanides

            Nitriles on reduction with lithium aluminium hydride (LiAlH4) or catalytic hydrogenation produce primary amines.

   R – CN (Alkyl cyanide)     — H2 / Ni & Na(Hg) / C2H5OH  →   R – CH2 – NH2  (1ᵒ Amine)                  

5) By reduction of amides

            Acid amides on reduction with lithium aluminium hydride give corresponding amines.

6) By Hoffmann bromamide degradation

            The conversion of amides into amines in the presence of bromine and alkali is known as Hoffmann degradation of amides.

R – CO – NH2 (Amides) +   Br2  +   4NaOH       →   R – NH2  (Amine) +  Na2CO3  +  2NaBr  +  2H2O

PHYSICAL PROPERTIES OF AMINES

1) Aliphatic amines with low molecular weight are colourless, gaseous compounds with fishy odour. High molecular weight aliphatic amines are solid.

2) Pure aromatic amines such as aniline are colourless liquid. Arylamines are toxic in nature.

3) Aliphatic amines are soluble in water. As the molar mass increase solubility decreases.

4) Amines are less polar than the corresponding alcohol but more polar than corresponding alkanes.

5) Amines are higher boiling points than corresponding alkanes but lower than corresponding alcohols or carboxylic acids. The order or boiling points of isomeric amines is 1ᵒ > 2ᵒ > 3ᵒ.

REACTIONS OF AMINES

            The lone pair of electron on nitrogen makes amines both basic and nucleophilic. They react with acids to form salts and react with nucleophiles in many reactions.

1) Basic nature of Amines

            Nitrogen atom of amines contains a lone pair of electron which can be donated. Thus, amines act as Lewis bases and are Lowery – Bronsted bases as they accept a proton.

The parent amine is regenerated when alkyl ammonium halide is treated with NaOH.

R – NH3+X– (Alkyl Ammonium Halide) + NaOH → R – NH2 (Amine)  + NaX  + H2O

Amines are weak bases. They dissolve in water and produce OH– ions. Aqueous Solutions of amines turn the color of litmus paper from red to blue.

  R – NH2     +     H – O – H     ↔     R – NH3+    +     OH–

Weak Base                                                           Strong base

            The equilibrium lies far to the left as OH– is a stronger base than amines. The expressions for the equilibrium constant K, basicity constant Kb and pKb values are as follows.

PKb = -logKb

Strong bases have high values of Kb and low values of PKb.

2) Action of nitrous acid

• On primary (1ᵒ) amines

Except methylamine, primary amines react with nitrous acid, in cold condition to give alcohol and nitrogen gas.

  R – NH2  (1ᵒ amines) +   NaNO2  +  2HCl   273 – 278K→   R – OH  +  HCl  +  N2  ↑

• On secondary (2ᵒ) amines

Secondary amines react with nitrous acid to give the N – nitrosoamines which are generally pale yellow oils.

 R2 – NH (2ᵒ amine) +  HNO2  —NaNO2 + dil. HCl (at 273 – 278K)

→ R2N – N = O  +  H2O

• On tertiary (3ᵒ) amines

Tertiary amines react with nitrous acid to form water soluble nitrite salt. As visible change observed, it is said that there is no reaction.

     R3N (3ᵒ amine) + HNO2  NaNO2 / dil. HCl ( at 273 – 278K )

→    [ R3NH ]+NO–2   (No change)

3) Acylation of amines

            Acylation is the replacement of a hydrogen atom of amino group by acyl group (R – CO). It is nucleophilic substitution reaction. Acetylation of primary / secondary amines with acetyl chloride or acetic anhydride gives corresponding amine.

4) Carbylamine reaction  

            Aliphatic and aromatic primary amines on heating with chloroform and ethanolic potassium hydroxide form isocyanides or carbylamines. This reaction is known as carbylamines reaction or isocyanides test.

R – NH2  +  CHCl3  +  3KOH  Heat→   R – NC   +   3KCl  +   3H2O

5) Reaction with arylsulphonyl Chloride (Hinsberg’s test)

• Primary amines when reacts with benzene Sulphonyl chloride ( Hinsberg’s reagent),

it yields N – ethylbenzene sulphonamide.

• Secondary amines reacts with benzene sulphonyl chloride, it yields N, N – dimethylbenzene sulphonamides.

• Tertiary amines do not react with benzene sulphonyl chloride.

6) Electrophilic aromatic substitution

• Bromination

Reaction of aniline with bromine water at room temperature results in the formation of white precipitate of 2, 4, 6 – tribromoaniline.

• Nitration

In direct nitration of aniline with concentrated nitric acid at 288K in the presence of sulphuric acid, a mixture of ortho, meta and para isomers of nitroaniline are obtained as product. Also, dark coloured tars are obtained due to oxidation.

• Sulphonation

Reaction of aniline with cold concentrated Sulphuric acid gives anilinium hydrogen sulphate. Heating anilinium hydrogen sulphate with Sulphuric acid at 453K to 473K gives p – aminobenzene sulfonic acid (sulphonic acid) as major product.

DIAZONIUM SALTS

            The diazonium salts have the general formula RN2+X– where R stands for aryl group and X– ion may be Cl–, Br–, HSO4, BF4–, etc.

The N2+ group is called diazonium group.

e.g.  C6H5N2+Cl– IS named as benzenediazonium chloride.

The conversion of primary aliphatic or aromatic amines into diazonium salts is known as diazotization.

PREPARATION OF DIAZONIUM SALTS

            Diazonium salts are prepared by the reaction of nitrous acid in cold condition with alkyl/aryl primary amines (other than methylamine).

e.g.  

REACTION OF DIAZONIUM SALTS

Reaction involving displacement of nitrogen (Diazonium group)

• Replacement by -Cl , -Br and –CN

The reaction in which copper (1) salts are used to replace nitrogen in diazonium salt is called sandmeyer reaction

Yield in sandmeyer reaction is better than yield in Gattermann reaction.

•  Replacement by – I

Ar – N+2X–   +   KI       Δ  →    Ar – I   +   KX   +   N2    ↑

• Replacement by – F

Ar – N+2X–       HBr  →  Ar – N+2BF–4     Δ  →    Ar – F   +   BF3   +   N2    ↑

• Replacement by – H

Ar – N+2X  + H3PO2  +  H2O      CuCl  →  ArH   +   H3PO3   +   HX   +   N2    ↑

• Replacement by – OH

Ar – N+2X  + H2O   Dil. H2SO4  →  Ar – OH    +   HX   +   N2    ↑

• Replacement by – NO2

Ar – N+2X      HBF4  →  Ar – N+2BF–4     NaNO2 / Cu &  Δ  →    Ar – NO2   +  NaBF4   +   N2    ↑

IMPORTANCE OF DIAZONIUM SALTS

1) Arene diazonium salts can be prepared from nearly all primary amines and are helpful in the synthesis of variety of organic compounds.

2) Arenes diazonium salts are used as useful intermediates to introduce various groups into aromatic ring. Such as – F, – Cl, -Br, – I, -CN, -NO2, -OH, -H, etc.

3) The halide or Cyano group can be easily introduced in aromatic ring through diazonium salts.

4) The compounds which cannot be prepared by direct electrophilic aromatic substitution can be prepared by replacement of diazo group.

5) Azo compounds obtained from diazonium salts are strongly coloured and are used as dyes. 

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