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Chapter: Organic Chemistry: Structure and bonding

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Bonds and hybridized centers

Every bond in an organic structure is aσ bond or a π bond. Every atom in a structure is linked to another by a single σ bond. If there is more than one bond between any two atoms, the remaining bonds are π bonds.

BONDS AND HYBRIDIZED CENTERS

Key Notes

σ and π bonds

Every bond in an organic structure is aσ bond or a π bond. Every atom in a structure is linked to another by a single σ bond. If there is more than one bond between any two atoms, the remaining bonds are π bonds.

Hybridized centers

All atoms in an organic structure (except hydrogen) are either sp, sp2 or sp3 hybridized. Atoms linked by single bonds are sp3 hybridized, atoms linked by double bonds are sp2 hybridized* and atoms linked by triple bonds are sp hybridized.*

Shape

sp3Hybridized centers are tetrahedral, sp2hybridized centers are trigonal planar and sp centers are linear. This determines the shape of functional groups.  Functional  groups  containing  sp2  hybridized  centers  are  planar while functional groups containing sp hybridized centers are linear.

Reactivity

Functional groups containing π bonds tend to be reactive since the π bond is weaker than aσ bond and is more easily broken.

 

σ and π bonds

Identifying σ and π bonds in a molecule (Fig. 1) is quite easy as long as you remember the following rules:

 

·           all bonds in organic structures are either sigma (σ) or pi (π) bonds;

·           all single bonds are σ bonds;

·           all double bonds are made up of one σ bond and one π  bond;

·           all triple bonds are made up of one σ bond and two π bonds.


Hybridized centers  

All the atoms in an organic structure (except hydrogen) are eithersp sp2or sp3 hybridized (Fig. 2).


The identification of sp, sp2 and sp3 centers is simple if you remember the following rules:

·           all atoms linked by a single bond are sp3 hybridized (except hydrogen).

·           both carbon atoms involved in the double bond of an alkene (C=C) must be sp2 hybridized.*

·           both the carbon and the oxygen of a carbonyl group (C=O) must be sp2 hybridized.

·           all aromatic carbons must be sp2 hybridized.

·           both atoms involved in a triple bond must be sp hybridized.

·           hydrogen uses a 1s orbital for bonding and is not hybridized.

Hydrogen atoms cannot be hybridized. They can only bond by using an s orbital since there are no p orbitals in the first electron shell. It is therefore impossible for a hydrogen to take part in π bonding. Oxygen, nitrogen and halogens on the other hand can form hybridized orbitals which are either involved in bonding or in holding lone pairs of electrons.

Shape

 The shape of organic molecules and the functional groups within themis determined by the hybridization of the atoms present. For example, functional groups containing trigonal planar sp2  centers are planar while functional groups containing sp centers are linear:

planar  functional  groups  –  aldehyde,  ketone,  alkene,  carboxylic  acid,  acid chloride, acid anhydride, ester, amide, aromatic.

linear functional groups – alkyne, nitrile.

functional groups with tetrahedral carbons – alcohol, ether, alkyl halide.

Reactivity

Functional groups which contain π bonds are reactive since the π bond is weaker than aσ bond and can be broken more easily. Common functional groups which contain πbonds are aromatic rings,alkenes,alkynes, aldehydes,ketones, carboxylic acids, esters, amides, acid chlorides, acid anhydrides, and nitriles.

 

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