Lecture 2Dr. Moona Mehboob Khan

17.1.2013

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Learning ObjectivesStudents will able to understand basic concept of the

followingHydrocarbonsBondingConjugationTypes of conjugationStability characters of conjugated system

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Important TermsAtomBonding

IonicCovalentCo-ordinate covalent

Dipole momentsSigma bondPi-bond

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Organic ChemistryStudy of carbon containing compounds except few as

carbon di oxide ,carbon mono oxide ,carbonates ,bicarbonates,cyanates ,thiocynates etc.

Organic compounds play important role in our life as in food (protein ,carbohydrates ,fats) , ,medicine, pesticides ,harmones , steroids ,enzymes ,vitamins ,antiseptics,dyes etc

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Properties of Organic compoundsThey have low melting and boiling point, low solubility in

water but high in nonpolar solvents ,flammable ,non-conductor of electricity. They exhibit covalent bonding and isomerism ,have slow chemical reaction.

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HydrocarbonsSaturated Alkayne CnH2n+2CnH2n+2Unsaturates Alkene CnH2nCnH2n & Alkyne CnH2n-2CnH2n-2

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Bonding

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Chemical BondingBonding

IonicCovalentCo-ordinate covalent

Dipole momentsSigma bondPi-bondSigma bonding orbitalPi-bonding orbital

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Single bonds are always sigma bonds. Multiple bonds consist of a combination of sigma

and pi bonds:    double: one sigma bond, one pi bond     triple: one sigma bond, two pi bonds

In order to form a pi bond, there must be at least one unhybridized p orbital on each of the two atoms Bonds can also form due to overlap between unhybridized p orbitals oriented perpendicular to the internuclear axis

In these p (pi) bonds, the electron density is concentrated symmetrically above and below the internuclear axis

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Ionization potentialElectron affinityBond Energy (The energy required to break the bond in a

molecule is called bond energy which arises due to attractive forces between the atoms.)

Bond length (The distance between the two bonded atoms is called bond length.This is due to attraction and repulsion forces of atoms.)

Electronegativity of an atom is a measure of it’s power to attract electron in a covalent bond.

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Polar BondsWhen a covalent bond is formed between

two atoms with different electronegativities the more electronegative atom will acquire a partial negative charge and the less electronegative atom will acquire a positive charge .Such a bond having a negative and positive end is said to be a polar bond.

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Non-polar BondsWhen a covalent bond is formed between two atoms with same electronegativities (C-C, H-H, F-F ), the electrons involved in the bond are shared equally. There is no positive and negative end such a bond is said to be a non-

polar bond.

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Cyclic StructureThe molecules which consists of carbon atoms

joined to form close rings. These are called cylic compounds or ring structure e.g

cyclohexane, benzene.

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ConjugationConjugation is the overlap of one p-orbital

with another across an intervening sigma bond

A conjugated system is a system of connected p-orbitals with delocalized electrons in compounds with alternating single and multiple bonds.

This system have a p-orbital on an atom adjacent to double bond-molecules with delocalized pi bonds.

The compound may be cyclic, linear or mixed.

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TypesIsolated / non conjugated; C=C-C-C=CIsolated / non conjugated; C=C-C-C=C

Conjugated ; C=C-C=CConjugated ; C=C-C=C

Cumulated ; C=C=CCumulated ; C=C=C

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ConjugationAllyl radical CH2=CH-CH3 + X2----Low temp----- CH2X-

CHX-CH3 (addition reaction)But when temperature is high &

concentration of halogen is low thenCH2=CH-CH3 + X2------- CH2=CH-CH2X +

HX (Substitution reaction) This is allylic hydrogen atom & substitution

is allylic substitution

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Stability of conjugated system Conjugated Carbonyls: ,-unsaturated

systems containing conjugated double bonds, not just those of dienes, are more stable than those containing unconjugated double bonds.

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3-Cyclohexenone(less stable)

2-Cyclohexenone(more stable)

O O

HyperconjugationThe interaction of the electrons in a sigma

bond (usually C–H or C–C) with an adjacent empty (or partially filled) non-bonding p-orbital to give an extended molecular orbital that increases the stability of the system

It is also known as sigma-pi conjugation no bond resonance. It is a permanent effect.

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ReferencesOrganic chemistry by IL FinarOrganic chemistry by Solomons & FryhleOrganic chemistry by Brown Foote Inversion

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