Chemistry for Textile

5. Bonding in organic compounds

L8: Primary & secondary bonding, covalent, ionic and coordinate covalent bonding

Organic Chemistry

Organic chemistry includes those compounds whose main structure is based on carbon, while hydrogen is the second most abundant element found in organic compounds.

The compounds containing only carbon and hydrogen are called hydrocarbons.

Frequently elements other than carbon and hydrogen also appear in organic compounds. These are usually oxygen, nitrogen, sulfur and halogens

Most of the bonds in an organic compounds are of covalent nature.

Electronic configurations

1A 2A 3A 4A 5A 6A 7A 8A

1H

1s1

2He1s2

3Li1s2

2s1

4Be1s2

2s2

5B

1s2

2s22p1

6C

1s2

2s22p2

7N

1s2

2s22p3

8O

1s2

2s22p4

9F

1s2

2s22p5

10Ne1s2

2s22p6

11Na[Ne]3s1

12Mg[Ne]3s2

13Al

[Ne]3s23p1

14Si

[Ne]3s23p2

15P

[Ne]3s23p3

16S

[Ne]3s23p4

17Cl

[Ne]3s23p5

18Ar

[Ne]3s23p6

Electronic configuration of carbon in atomic form

Atomic structure

The hydrogen atom has only one electron which is present in the first or K shell. The hydrogen atom requires one more electron to complete it’s duplet.

Atomic number of carbon is 6, so it has six electrons in all out of which two reside in the first shell. The other four electrons occupy the second (the last) shell. To become stable it needs four more electrons to complete it’s octet.

Atomic structure The shape of atom is spherical like

a ball with the electron cloud occupying most of the space taken by the atom. The nucleus of atom is situated at the center of sphere and occupies a very small space as compared to the space occupied by the electrons. The electrons around the nucleus are present in different shells.

The hydrogen atom having a single electron has only one shell around the nucleus.

The carbon with six electron has two electronic shells. The inner shell contain two electrons and the outer shell contains four.

Why compounds are formed

The atoms whose outer shell is incomplete (contains less than 8 electrons) are unstable and tend to react with similar or dissimilar atoms to make compounds.

Why some compounds are ionic while others are covalent? It depends on the

relative electron attracting power of the bonding atoms.

Electro-negativities of elements Since the size and the number of protons and electrons varies in atoms of

different elements, their power to attract electrons towards nucleus also differs. This property of atoms is called electro-negativity and it determines the nature of bond forming between two atoms. Electro-negativity of some of the elements is given in table below.

Covalent bond When there is minor or no

difference of electro-negativity between the bonding atoms, the atoms only share their electrons to form the bond. The resulting bond is called a covalent bond. The bonded atoms cannot move away from each other until the bond between them breaks.

When there is somewhat greater difference of electro-negativity between the bonding atoms the bond is still covalent but becomes polar.

H2.2

Cl3.16

H2.2

C2.55

OHO = 3.44

Cl3.16

Covalent bond

Polar covalent bonds

A covalent bond becomes polar when there is some difference of elecro-negativity between the bonding atoms.

Hydrogen chloride is a covalent compound but since there is greater difference of electro-negativity between these two atoms, the electron cloud of the molecular orbital is slightly shifted towards the more electronegative atom chlorine and hydrogen is slightly deprived of electron cloud.

Due to shifting of electron cloud the chlorine atom acquire a partial negative charge and hydrogen acquires a partial positive charge. Such bonds are called polar bonds.

Effect of varying Electronegativity on polarity The polarity in covalent bonds occurs due

to difference of electro-negativity in bonding atoms.

The strength of polarity varies with varying difference of electro-negativity, i.e. increasing difference will increase polarity and decreasing will decrease polarity.

Non polar and polar bonds in organic compounds The saturated hydrocarbons like

methane and ethane are non polar organic compounds. Similarly unsaturated hydrocarbons like ethene (or ethylene) propene (or propylene) are also non polar compounds.

When carbon is bonded to nitrogen or oxygen the bond becomes polar due to greater difference of electronegativity. Acetone and ethyl amine are polar compounds.

Some characteristics of polar and non-polar covalent compounds

Non-Polar compounds Polar compounds

They have lower melting and boiling points.

They have higher melting and boiling points.

Liquid compounds show greater volatily.

Liquid compounds show lesser volatility.

They tend to dissolve in non-polar solvents.

They tend to dissolve in polar solvents.

They show lower reactivity They show higher reactivity

Polar vs non-polar compounds

Methane is a non-polar compound which occurs in gaseous form while methanol is polar and occurs in liquid form.

Similarly non-polar ethane is a gaseous compound while ethanol is polar and liquid.

Ionic bond

When the difference of electro-negativity becomes too high, the electron from less electronegative atom moves to the more electronegative atom.

Such a bond is called ionic bond because such compounds produce ions in solution.

The ions in solution can move freely, hence the bonded atoms move away from each other.

H2.2

Cl3.16

OHO = 3.44

Na0.90

Ionic bond

Hydrogen chloride (HCl) is a covalent compound where the difference of electo-negativity is less than one, while sodium chloride (NaCl) is an ionic compound where the difference of electronegativity is more than 2.

Co-ordinate covalent or dative bond It is similar to the covalent

bond in that electrons are shared between the bonding atoms but the difference is that both the shared electrons are donated by only one atom.

Since both the shared electrons are donated by one atom, the donor atom becomes electron deficient and hence gains positive charge. The acceptor atom become electron efficient and gains negative charge.

This kind of bond has both ionic and covalent character

Co-ordinate covalent or dative bond

Another interesting example of coordinate covalent or dative bond is the ammonium ion which is formed when ammonia reacts with hydrochloric acid to make ammonium chloride.

Hydrogen atom leaves it’s electron with chlorine and makes coordinate covalent bond with ammonia. The resulting species are ammonium and chloride ions.

Atomic structure A simple structure of atom implies that all

the electrons in a single atomic orbital have same energy.

The simple structure of atom does not however represent a true picture of atom. It is because even in a single atomic orbital all the electrons do not have the same energy level and all atomic orbital except the first one consist of more than one sub levels or orbitals which are designated as s,p,d and f orbitals.

In case of carbon the second (last) shell cotains two sub-shells namely s and p. S is of lower energy and p is of higher energy. Two of the four electrons in last shell occupy s orbital and the other two Px and py orbitals.

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Atomic structure

In atomic state the valance shell of carbon has 2 electrons in 2s orbital, one electron in 2px orbital and one electron in 2py orbital, while the 2pz orbital is empty.

In this situation there are only two unpaired eletrons which suggests the carbon to be bivalent.

Hybridization 22

Atomic structure

In fact in all carbon compounds’ the carbon is found in tetravalent state.

For e.g. in methane carbon is in tetravalent form.

Hybridization 23

Hybridization 24

Electronic configuration and bonding in carbon Carbon can make four single

bonds with four other species or one double and two single bonds with three other species or one triple bond and one single with two other species.

Looking at the electronic configuration it seems difficult for carbon to make such bonds. For making one sigma and one or more pi bonds p orbitals of both atoms must be parallel which is not possible under these conditions.

Hybridization 25

Valence shell atomic orbitals of carbon

Hybridization 26

Mutual orientation of 2s and 2p orbitals

Hybridization 27

Bonding with carbon

Considering the electronic configuration of carbon, we see that 2s orbital is completely filled, 2px and 2py are partially filled while the 2pz orbital is empty. Under this condition carbon can make only two single bonds with two other species.

Hybridization 28

Bonding with carbon

On the other hand if we consider the promotion of one of the 2s electrons to the empty 2pz orbital, four unpaired electrons in the 2s and 2p orbitals will be obtained and hence carbon should now be able to make four single bonds with four other species like hydrogen.

Hybridization 29

Bonding with carbon

The problem which now arises is that the low lying s orbital will not be able to make an effective bond with another species due to hindrance offered by p orbitals.

Furthermore all the four bonds will not be of equal bond energy, however in actual practice all four single bonds which carbon makes with four other similar species, like hydrogen, are of same energy.

Hybridization 30

Hybridization Carbon can make four single bonds with four separate atoms, two

single bonds and one double bond with three separate atoms or one single and a triple bond with two separate atoms.

All sigma bonds posses the same bond energy. This cannot be justified by simply promoting one 2s electron to 2p

orbital. The solution to this problem was provided by suggesting the idea of

hybridization of 2s and 2p atomic orbitals to give the same number of hybridized orbitals having the same bond energy and shape.

Hybridization is the mixing up of atomic orbitals of different energy to give a new set of same number of hybrid orbitals having same energy.

Hybridization 31

Shapes of hybridized and unhybridized orbitals

Hybridization 32

sp3 hybridization

When carbon makes four single bonds, one 2s and three 2p orbitals hybridize to produce four hybrid orbitals called sp3 orbitals.

These four hybrid orbitals have same energy value and have same bond angles (109.5º), directed towards the corners of a regular tetrahedron.

Hybridization 33

sp3 hybridized carbon

Hybridization 34

Methane molecule

Hybridization 35

sp2 hybridization When carbon makes two single bonds and one double

bond, one 2s and two 2p orbitals hybridize to produce three hybrid orbitals called sp2 orbitals. The third 2p orbital do not hybridize.

The three hybridized orbitals have the same energy value and bond angle (120º), while the unhybridized 2p orbital have different energy value and helps in making pi (Л) bond.

Hybridization 36

sp2 hybridization

Hybridization 37

sp hybridization

When carbon makes one single bond and one triple bond, one 2s and one 2p orbital hybridize to produce two hybrid orbitals called sp orbitals. The two remaining 2p orbitals do not hybridize.

The two hybridized orbitals have the same energy value and the bond angle between them is 180º, while the unhybridized 2p orbitals have different energy value and helps in making triple bond.

Hybridization 38

Sp hybridization C – C triple bond

Hybridization 39

Sp3 hybridization in Nitrogen, Oxygen and Halogens Nitrogen has three unpaired electrons in its 2p orbitals and two

paired electrons in s orbital. While bonding it hybridizes to sp3 geometry making three single bonds with other species.

Oxygen has two unpaired electrons in its 2p orbitals and four electrons in two pairs occupying 2s and one of the 2p orbitals. While bonding it hybridizes to sp3 geometry making two single bonds with other species.

Halogens have one unpaired electron in its 2p orbital and six electrons in three pairs occupying 2s and two of the 2p orbitals. While bonding it hybridizes to sp3 geometry making one single bonds with another specie.

Hybridization 40

sp3 Hybridization in Nitrogen, oxygen and halogens

Hybridization 41

sp2 hybridization in nitrogen and oxygen Nitrogen has three unpaired electrons in its 2p orbitals and two paired electrons in 2s

orbital. While making double bond it hybridizes to sp2 geometry making two sigma bond and one pi bond with other species.

Oxygen has two unpaired electrons in its 2p orbitals and four electrons as two pairs occupying 2s and one of the 2p orbitals. While making double bond it hybridizes to sp2 geometry forming one sigma and one pi bond with the other specie.

Hybridization 42

sp hybridization in nitrogen

Nitrogen has three unpaired electrons in its 2p orbitals and two paired electrons in 2s orbital. While making triple bond it hybridizes to sp geometry making one sigma bond and two pi bonds with the other specie.

Primary and secondary bonding A primary bond is a true bond

which is formed by sharing of electrons between two atoms or by transfer of an electron from one atom to the other. Primary bonds are covalent, ionic and coordinate covalent or dative bonds.

A secondary bond on the other hand is one where no true sharing of electrons or transfer does occur. Examples of secondary bonding are hydrogen bonding, dipole-dipole interaction, Van der Waal’s forces etc.