Hybridisation & Shapes of Organic Molecules | Chemistry Notes for IITJEE/NEET

HYBRIDISATION AND SHAPES OF ORGANIC MOLECULES

HYBRIDISATION

Sigma bonds are the most common bonds in organic chemistry. All single bonds are sigma (σ) bonds and formed by the overlapping between s-s, s-p and p-p (head on) atomic orbitals present on different atoms. A pi () bond results from the overlap of two p-orbitals that are oriented perpendicular to the axis of the nuclei. A p bond is not cylindrically symmetrical. A σ bond is stronger than p bond  due to better overlap. All multiple bonds contain one σ bond and others bond(s).

To have more efficient overlapping and to provide more symmetrical structure to the molecule the atomic orbitals on the same atom interact to provide hybrid atomic orbitals and the interaction is known as hybridisation. The hybrid atomic orbitals have enhanced electron density.

HYBRIDISATION OF CARBON

The ground state electronic configuration of carbon is . The electronic configuration of carbon in excited state is .

sp3 HYBRIDISATION

If we superimpose one s and three p atomic orbitals we get 4sp3 hybrid orbitals.

Each hybrid orbital contains single electron, has 25% s character and 75% p character. They are directed towards the four corners of a regular tetrahedron with the carbon located in the centre. The angle between any two sp3 hybrid orbitals is 109º 28' (109.5º).

These hybrid orbitals can overlap with four s atomic orbitals provided by four hydrogen atoms to form methane molecule.

sp2 HYBRIDISATION

If we superimpose one s and two p atomic orbitals we get 3sp2 hybrid orbitals

Each sp2 hybrid orbital has 33% s character and 67% p character. They lie in the same plane with their axis directed towards the corner of an equilateral triangle and are 120º apart from each other. The unhybridized pz atomic orbital is perpendicular to the plane of sp2 hybrid orbitals.

BONDING IN ETHYLENE

Consider two sp2 hybridised carbon atoms approaching to each other and four hydrogen atoms which provide four s atomic orbitals

sp HYBRIDISATION

If we superimpose one s and one p atomic orbitals we get 2sp hybrid orbitals.

 

Each sp hybrid orbital has 50% s character and 50% p character. They are diagonally present with their axis forming an angle of 180º. The unhybridized 2py and 2pz atomic orbitals are perpendicular to each other and perpendicular to hybrid orbitals also.

BONDING IN ACETYLENE

HYBRIDISATION OF NITROGEN

The ground state electronic configuration of nitrogen is

7N = 1s2, 2s2 2px1 2py1 pz1

One s and three p atomic orbitals superimpose and give 4sp3 hybrid orbitals. These are tetrahedrally present.

sp2 HYBRIDISATION

When nitrogen attaches itself to two other atoms it is present in the sp2 hybridised form. Consider the formation of methylimine CH2 = NH in which carbon and nitrogen both are in sp2 hybrid state

sp HYBRIDISATION

When nitrogen is attached to only one atom its hybridisation is sp. In  both carbon and nitrogen are in sp hybridised form

HYBRIDISATION OF OXYGEN

The electronic configuration of oxygen is .

sp3 HYBRIDISATION

When oxygen is attached to two atoms the hybridisation is sp3.

  

sp2 HYBRIDISATION

When oxygen is attached to one atom as in case of aldehydes and ketones e.g. in Formaldehyde carbon and oxygen, both are in sp2 hybrid form.

BOND LENGTHS

Some importants bond lengths are as follows

C–C sp3 – sp3 1.54 Å C–O sp3 – O 1.41 Å

sp3 – sp2 1.50 Å sp2 – O 1.34 Å

sp3 – sp 1.46 Å C=O sp2 – O 1.20 Å

sp2 – sp2 1.48 Å sp – O 1.16 Å

sp2 – sp 1.43 Å C–N sp3 – N 1.47 Å

sp – sp 1.38 Å sp2 – N 1.36 Å

C=C sp2 – sp2 1.34 Å C=N sp2 – N 1.28 Å

sp2 – sp    1.31 Å CºN sp – N 1.16 Å

sp – sp     1.28 Å

CºC sp – sp 1.21 Å

C–H sp3– H 1.11 Å

sp2 – H 1.10 Å

sp – H 1.08 Å

BOND ANGLES IN SELECTED MOLECULES

AROMATICITY AND AROMATIC COMPOUNDS

Aromatic indicates a stable system which undergoes substitution rather than addition, retaining the closed p-electron system. Many such systems contain only six p electrons, but generally they contain (4n+2) p electrons, where n is an integer.

Non-benzenoid heterocyclic compounds with 6electrons are aromatics

The hetero atom contributes to non bonded electrons, to complete the sextet.

In general, higher polycyclic aromatic compounds are somewhat less stable than benzene.

n = 0 cyclopropenyl cation contains 2- electrons and is aromatic

ANTIAROMATICITY

The less stability of monocyclic compounds containing (4n) electrons than their acyclic analogues is called anti aromaticity. For example

Cyclobutadiene    is less stable than 1,3-Butadiene

Here Resonance is the cause of destabilisation (hence the concept of antiaromaticity)

         

More examples of antiaromatic compounds

       

The electronic configuration of carbon in excited state is .

Chemistry Study Notes

• Some Basic Concepts of Chemistry
• Structure of Atom
• Classification of Elements and Periodicity in Properties
• Chemical Bonding and Molecular Structure
• States of Matter
• Thermodynamics
• Chemical Equilibrium
• Ionic Equilibrium
• Redox Reactions
• Hydrogen
• s-block Elements – Alkali Metals
• s-block Elements – Alkaline Earth Metals
• p-Block Elements – Boron Family
• p-Block Elements – Carbon Family
• Purification, Qualitative and Quantitative Analysis of Organic Compounds
• Classification and Nomenclature of Organic Compounds
• Hybridisation & Shapes of Organic Molecules
• General Organic (Basic Concepts)
• Isomerism
• Hydrocarbons
• Environmental Chemistry
• Solid State
• Solutions
• Electrochemistry
• Chemical Kinetics
• Surface Chemistry
• General Principles & Processes of Isolation of Elements
• p-Block Elements - Nitrogen Family
• p-Block Elements - Oxygen Family
• p-Block Elements - Halogens
• p-Block Elements – Noble Gases
• d & f-Block Elements
• Coordination Compounds
• Haloalkanes and Haloarenes
• Alcohols, Phenols and Ethers
• Aldehydes & Ketones
• Carboxylic Acids and their Derivatives
• Amines
• Biomolecules
• Polymers
• Chemistry in Everyday Life
• Volumetric Analysis
• Principles related to Practical Chemistry (Part-1)
• Principles related to Practical Chemistry (Part-2)
• Principles related to Practical Chemistry (Part-3)

Study Notes for JEE Main/JEE Advanced

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