Organic Chemistry – Some Basic Principles and Techniques

 SOME 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 other 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

Aliphatic compounds 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

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

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 compounds 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 presents 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 giving the atomic ratio of constituent's 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.

Organic chemistry class 12 conversion questions with answers

Dear Science Students! Here are some organic chemistry conversion questions for Class 12 with answers:

1. Convert Ethanol to Ethanoic Acid:

   

Answer: Ethanol can be converted to ethanoic acid by oxidizing it using an oxidizing agent like potassium dichromate (K2Cr2O7) and sulfuric acid (H2SO4).

   Equation:

   CH3CH2OH + [O] → CH3COOH + H2O

2. Convert Benzene to Nitrobenzene:

Answer: Benzene can be converted to nitrobenzene through nitration using a mixture of concentrated nitric acid (HNO3) and concentrated sulfuric acid (H2SO4).

   Equation:

   C6H6 + HNO3 → C6H5NO2 + H2O

3. Convert Propanone to Propene:

Answer: Propanone (acetone) can be converted to propene through the dehydrogenation process using a strong base like potassium hydroxide (KOH).

   Equation:

   (CH3)2CO → CH2=CH2 + H2O

4. Convert Toluene to Benzyl Alcohol:

Answer: Toluene can be converted to benzyl alcohol through reduction using a reducing agent like sodium borohydride (NaBH4).

   Equation:

   C6H5CH3 + NaBH4 → C6H5CH2OH + NaBO2 + H2

5. Convert Ethylamine to Acetamide:

Answer: Ethylamine can be converted to acetamide through the reaction with acetic anhydride.

   Equation:

   CH3COOC2H5 + NH2CH2CH3 → CH3CONHC2H5 + CH3COOH

6. Convert Butanal to Butanoic Acid:

Answer: Butanal can be converted to butanoic acid through oxidation using an oxidizing agent like potassium dichromate (K2Cr2O7) and sulfuric acid (H2SO4).

   Equation:

   CH3CH2CH2CHO + [O] → CH3CH2CH2COOH

7. Convert Ethene to Ethanol:

Answer: Ethene can be converted to ethanol through hydration, typically in the presence of a catalyst like sulfuric acid (H2SO4).

   Equation:

   CH2=CH2 + H2O → CH3CH2OH

8. Convert Ethylbenzene to Benzoic Acid:

Answer: Ethylbenzene can be converted to benzoic acid through oxidation using potassium permanganate (KMnO4) and sulfuric acid (H2SO4).

   Equation:

   C6H5CH2CH3 + [O] → C6H5COOH + H2O

These conversion reactions are common examples in organic chemistry and are frequently encountered in Class 12 syllabus. Remember to balance the chemical equations and pay attention to reaction conditions and reagents used in each conversion.

Points to be remember from organic Chemistry

• Atoms Stick Together: In organic chemistry, things called "atoms," like carbon and hydrogen, like to share their outer parts with each other, forming bonds. This sharing is called "covalent bonding."
• Carbon's Special Trait: Carbon is like a friendly glue because it can make four bonds with other atoms. This makes it the building block for lots of different organic compounds.
• Different Arrangements: Organic compounds can look different even if they have the same ingredients. This is because atoms can be arranged in various ways.
• Special Groups Matter: Some groups of atoms, like OH or CO, give organic compounds their special qualities. These groups are called "functional groups."
• Naming Rules: There are rules for naming organic compounds, like calling them by specific names using the IUPAC system.
• Naming Different Shapes: Compounds that look similar but are arranged differently need different names too.
• Shifting Electrons: Sometimes, electrons in compounds move around, creating different versions of the same compound. This is called "resonance."
• Acids and Bases: Some compounds can be sour (acidic), and some can be bitter (basic). Knowing which ones which are is important.
• How Things Change: Learn how organic compounds transform during reactions, like switching one piece for another.
• Left and Right-Handed: Some molecules can come in two versions that are like mirror images. These are called "left-handed" and "right-handed."
• Polarity Matters: Think of molecules like magnets – some are attracted to water, and some aren't. This affects how they dissolve.
• Cleaning Up: Scientists have ways to clean up messy mixtures of compounds, like using special tools to separate them.
• Using Light and Machines: Special tools like flashlights (infrared spectroscopy), magnets (NMR spectroscopy), and scales (mass spectrometry) help scientists understand compounds better.
• How Reactions Happen: Find out how different reactions work, like what needs to be added and what conditions they like.
• Inside the Reaction: Learn what happens at the tiny level during a reaction, including things that form in the middle.
• Who's Who in Reactions: Some compounds like to take electrons (called electrophiles), while others like to share (called nucleophiles).
• Switching Groups: Discover how different parts of molecules can be changed into other parts through different reactions.
• Making New Stuff: Organic chemistry is like building with blocks – you can make new things by putting pieces together in special ways.
• Staying Safe: When working with chemicals, always be safe. Wear the right gear and follow safety rules in the lab.
• Practice Makes Perfect: Keep practicing problems and learning about how things react to get better at organic chemistry.