Introduction to Organic chemistry

Importance of Organic Chemistry

Field of organic chemistry is very vast; some industrial applications of organic chemistry are given below:

• Food Industry: Carbohydrates, fats, proteins, sweetening & flavoring agents, food products like milk, etc. are organic compounds.

1. Textile Industry: Synthetic fibers like nylon, terylene, rayon, etc. and natural fibers like cotton, silk, etc. are all organic in nature.
2. Petroleum industry: LPG, gasoline, kerosene, etc. are all organic compounds.
3. Pharmaceutical industry: Synthetic drugs like aspirin, sulpha drugs, antibiotics, vitamins, hormones, etc. are basically organic compounds.
4. Plastic industry: PVC, polythene, Bakelite, etc. are polymers prepared from simple organic compounds.
5. Dye industry: Natural and synthetic dyes like indigo, eosin, fluorescein, azodyes, etc. have organic background.
6. Fertilizer industry: Urea is an important organic compound which is used as fertilizer.
7. Cosmetic and perfume industry: Cosmetics, perfumes, creams, talcum powders, etc. are organic compounds.

General characteristics of organic compounds

1. Composition: All organic compounds contain carbon. Other elements present may be hydrogen, oxygen, halogen, sulphur, phosphorus, nitrogen, etc.
2. Type of linkage: Most of the organic compounds contain covalent linkages in their molecules.
3. Catenation: Ability of carbon atoms to combine with one another to form long chains, branched chains and rings is known as catenation
4. Isomerism: Compounds having the same molecular formula but different structural formula are called isomers and this property of organic compounds is known as isomerism.
5. Polymerization: A large number of small organic molecules (monomers) add up to form a high molecular weight compound called polymer and this process is known as Polymerization.
6. Solubility: Organic compounds are generally insoluble in water but soluble in organic solvents like ether, alcohol, benzene, etc.
7. Melting and boiling points: Organic compounds usually have low melting and boiling points due to their covalent nature.
8. Odour: Some organic compounds possess characteristic odour. For example, esters have a sweet and pleasant odour.
9. Functional group: organic compounds are characterized by functional groups that determine its chemical properties.
10. Combustibility: Most of the organic compounds are combustible in nature.
11. Homologous series: A series of organic compounds each containing a characteristic functional group and successive members differing from each other in molecular formula by –CH2 group is called homologous series.

Types of hybridisation in Carbon Atom

Carbon atoms exhibit three types of hybridization.

sp3 Hybridisation

• Carbon atom shows sp3 hybridisation when it links to four atoms or groups.
• Carbon atoms have one 2s and three 2p (2px, 2py, and 2pz) orbitals.
• These four atomic orbitals mix up to give four new sp3 hybrid orbitals.
• Each sp3 hybrid orbital has 25% s-character, and 75% p-character.
• These hybrid orbitals are oriented at an angle of 1090 28’ to each other and are directed towards four corners of a regular tetrahedron with carbon atom atcenter of the tetrahedron.

sp2 Hybridisation

• When carbon links itself to three atoms or groups, each containing only one unpaired electron, it shows sp2 hybridisation.
• In sp2 hybridisation, one s and two p orbitals undergo mixing to produce 3 equivalent sp2 hybridised orbitals.
• Three sp2 hybrid orbitals are oriented in a plane along the three corners of an equilateral triangle.
• They are inclined to each other at an angle of 1200.
• Third p-orbital remains unchanged.

sp Hybridisation

• In sp hybridization, one 2s and one 2porbitals hybridizeto give two sp hybrid orbitals.
• sp hybrid orbitals are oriented at an angle of 1800 to each other.
• Each hybrid orbital has 50% s- and 50% p-character.
• Other two p-orbitals remain un-hybridised.

Structural Representations of Organic Compounds

Structures of organic compounds are represented in several ways;

1. The Lewis structure or dot structure 2) dash structure

1. condensed structure and 4) bond line structural formulas,

Fission of Covalent Bond

A bond can be cleaved or broken in two different ways;

• Heterolytic cleavage
• Homolytic cleavage

Heterolytic Cleavage

• Bond breaks in such a way that the shared pair of electrons remains with one of the fragments.
• Thus after Heterolytic cleavage one of the atoms has a sextet and another has an octet with at least one lone pair of electrons and one –ve charge.
• A species having carbon atom and possessing sextet and +ve charge is called carbocation for example
• A species having carbon atom and possessing octet and –ve charge is called carbanion for example
• Organic reactions which proceed through heterolytic bond cleavage are called ionic / hetero-polar / polar reactions.

Homolytic Cleavage

• In homolytic cleavage, movement of single electron takes place instead of a pair, such movement is shown by half head arrow (fish hook)
• Homolytic cleavage results in formation of neutral species having unpaired electrons called free radicals.
• Organic reactions, which proceed by homolytic fission are called free radical or homo-polar or nonpolar reactions

Resonance Energy

• Most important consequence of resonance is that, resonance hybrid (i.e. the actual molecule) has much lower energy than the energy of any of the contributing structures.
• Lower energy of resonance hybrid and increased stabilization is expressed in terms of resonance energy.
• Resonance energy may be defined as the difference between energy of an actual molecule (resonance hybrid) and energy of the most stable contributing structure.
• Resonance energy can be computed from heats of hydrogenation and heats of formation.

Resonance or Mesomeric Effect (R or M Effect)

• Resonance effect produced under the influence of certain groups attached to the conjugated system is known as mesomeric effect or mesomerism.
• Like inductive effect, Mesomeric effect (denoted by M) may be + M or – M.
• Atoms or groups which donate electrons to the double bond or to a conjugated system are said to have +M effect or +R effect. Groups or atoms which contain either negative charge or lone pair of electrons are +M groups
• Atoms or groups which withdraw electrons from double bonds or from a conjugated system are said to have −M effect or −R effect. Groups or atoms which contain either positive charge or those groups in which central atom is bonded with electronegative atom by multiple bonds are −M groups 
• Some common atoms or groups which cause + M and –M effect are given below.

+M Effect Order:

–O− > –NH2 > –NHR > –OR > –NHCOR > –OCOR > –Ph > –F > –Cl > –Br > –I

-M Effect Order:

–NO2 > –CN > –S(=O)2−OH > –CHO > –C=O > –COOCOR > –COOR > –COOH > –CONH2 > –COO−

Similarities between Inductive and Mesomeric Effects

• Mesomeric effect is a permanent effect and always operates in a non-reacting molecule.
• It affects the physical properties of a molecule.
• Like an inductive effect, it may either aid or hinder a particular reaction.

Electromeric Effect

• It is temporary displacement of electrons in compounds containing multiple covalent bonds (e.g. C=C, C=O, C≡N, etc.) or in atoms with lone pair of electrons adjacent to covalent bond.
• It involves complete transference of a pair of electrons from a multiple bond to an atom, or from a multiple bond to another bond, or from an atom with a free pair of electrons to a bond.
• In this effect π-electrons of a multiple bond, or p-electrons of an atom are transferred.
• Since the effect involves complete transference of electrons, it leads to the development of full + and – charges within the molecule.
• It is important to note that electromeric effect is purely a temporary effect and is brought into play only at the requirement of attacking reagent; it vanishes as soon as attacking reagent is removed from reaction mixture.

Hyper conjugation

• It is also known as no-bond resonance or Baker-Nathan effect.
• Delocalization of electrons due to overlap between a p orbital and a sigma (σ) bond (α C-H) is called hyper-conjugation.
• Hyper-conjugation occurs only if σ bond and empty p orbital have proper orientation.
• Electrons of sigma bond enter into partial conjugation with an unshared p orbital or with an attached unsaturated system.
• It is a permanent effect and a stabilizing interaction.
• Consider ethyl cation CH3H2. Positively charged carbon atom has six electrons, it is sp2 hybridized and has an empty p orbital.
• One of the C-H bonds of the adjacent methyl group is in alignment with the plane of empty p orbital.
• Electrons of σ bond (C-H bond) are delocalized into empty p orbital and stabilze the cation.

• Positive charge is dispersed by the electron density of adjacent σ bonds due to the overlap which stabilizes the cation. 
• 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, relative stability of following actions decreases in the order.

(CH3)3C+> (CH3)2H > CH3H2>H3

• Evidence for hyper-conjugation
  • Heat of hydrogenation
  • Stability of alkenes (Propene more stable than ethene)
  • Stability of carbocation
  • Stability of alkyl free radicals
  • Bond lengths (shortening of C−C single bonds adjacent to multiple bonds)

Types of Organic Reactions

• Substitution reactions
• Addition reactions
• Elimination reactions
• Rearrangement reactions
• Condensation reactions

Conclusion

Organic chemistry studies the structure, properties and reactions of organic compounds,  that contain carbon-carbon covalent bonds, the study of structure determines their structural formula and the study of properties includes physical and chemical properties, and evaluation of chemical reactivity to understand the behavior. The study of organic reactions includes the chemical synthesis