structure & formulae

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K E A N E A . C A M P B E L L

M S c ; B S c ; A S c

S E P T E M B E R 8 , 2 0 1 3

Structure & Formulae

Catenation “Explain the occurrence of carbon compounds with straight chains and

branched chains and rings.”

Catenation

This is the ability of a chemical element to form long chain-like structure via covalent bonds with itself. This results in ring and chain structures. Catenation occurs most readily in carbon, which forms covalent bonds with other carbon atoms.

Catenation is the reason for the presence of large number of organic compounds in nature. Carbon is most well known for its properties of catenation, with organic chemistry essentially being the study of catenated carbon structures.

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Tetravalency “Explain the occurrence of carbon compounds with straight chains and

branched chains and rings.”

Tetravalency

A carbon atom has a total of six electrons occupying the first two shells. Carbon displays tetravalency in the combine state. Therefore, a carbon atom has four valence electrons. It could gain four electrons to form C4- anion or lose four electrons to form C4+ cation. As a result of these two aforementioned conditions, it would take carbon far away from achieving stability by the octet rule.

To overcome this problem, students, carbon undergoes bonding by sharing (covalent) its four valence electrons. This allows it to be covalently bonded to one, two, three or four carbon atoms or atoms of other elements or groups of atoms.

Therefore, students, with these aforementioned information, it explains the concept of tetravalency in the occurrence of carbon compounds.

Hybridization

This is the process of mixing a set of atomic orbitals to form a new set of atomic orbitals called hybrid orbitals.

These hybrid orbitals have the same total electron capacity.

At this level we will only look at the following hybrid orbitals:

1. sp3 hybrid orbitals,

2. sp2 hybrid orbitals and

3. sp hybrid orbitals

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sp3 Hybridization

This is when four orbitals are mixed-or hybridized-and four new hybrid orbitals are obtained.

The hybrid orbitals are called sp3 orbital to indicate that they have one part the character of an s orbital and three part the character of a p orbital.

Let us now look at sp3 hybridization in a selected compound, methane, as an example.

According to quantum mechanics, the electronic configuration of a carbon atom in its lowest energy state-called the ground state-is given here:

.

sp3 Hybridization

Ground State of A Carbon Atom

The valence electrons of a carbon atom (those used in bonding) are those of outer level, i.e., the 2s and 2p electrons. Note that hybridization procedure applies only to the orbitals not to the electrons.

The 1s2electrons are too deep inside the atom to be involved in bonding. The only electrons directly available for sharing are the 2p electrons.

When bonds are formed, energy is released and the system becomes more stable. If carbon forms 4 bonds rather than 2, twice as much energy is released and so the resulting molecule becomes even more stable.

There is only a small energy gap between the 2s and 2p orbitals, and so it pays the carbon to provide a small amount of energy to promote an electron from the 2s to the empty 2p to give 4 unpaired electrons.

The extra energy released when the bonds form more than compensates for the initial input.

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sp3 Hybridization

1 2s orbital + 2 2p (px, py) orbitals

3 sp2 orbitals

Energy-level diagram of sp2 hybridization

Un-hybridized pz orbital

Carbon uses the sp2 hybridized orbitals for forming sigmal (σ) bonds within the plane

The remaining 2pz orbital is used for forming the pi (π) bond.

Note that the double bond consists of one σ and one π bond.

Energy hybridization

C atom orbitals in ethylene

2p

2s

sp2

Orbitals in isolated C atom

E 2p

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Copyright © Houghton Mifflin Company. All rights reserved.

Figure 14.10: When one s and two p oribitals are mixed to form a set of three sp2 orbitals, one p orbital remains unchanged and

is perpendicular to the plane of the hybrid orbitals.


Figure 14.11: The sigma bonds in ethylene.

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Figure 14.12: A carbon-carbon double bond consists of a sigma bond and a pi bond.


Figure 14.13: (a) The orbitals used to form the bonds in ethylene.

(b) The Lewis structure for ethylene.

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An atom surrounded by 3 effective electron pairs uses sp2 hybridized orbitals for bonding.

Example H2CO formaldehyde

Lewis Structure

— 12 valence electrons — 3 effective pairs around C

Sp2 hybridized orbitals are used to form the C-H bonds and the C-O σ bond, the un-hybridized 2pz orbital is used to form the C=O π bond.

Other sp2 hybridized carbon atoms

C O

H

H

.. ..

sp Hybridization Carbon in carbon dioxide, CO2 uses another type of hybridization (rather than sp2 or sp3)

2 hybrid orbitals required to meet the 180° (linear) geometry requirement are sp orbitals.

2 effective pairs around C atom sp hybrid orbitals

3 effective pairs around O atom sp2 orbitals

O=C=O

En

erg

y

2p

1s

2p

sp Hybridization

Orbitals in a free C atom

Orbitals in sp hybridized orbitals in CO2

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Figure 14.14: When one s orbital and one p orbital are hybridized, a set of two sp orbitals oriented at 180 degrees results.


Figure 14.15: The hybrid orbitals in the CO2 molecule

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Figure 14.19: (a) Orbitals predicted by the LE model to describe (b) The Lewis structure for carbon dioxide

Electron Delocalization and Resonance

• Localized electrons =

restricted to a particular region

• Delocalized electrons do not belong

to a single atom or exclusively to a

bond between 2 atoms

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• Benzene C6H6

– Rapid Equilibrium between 2 structures

– Proposed by Fredrich Kekule (1865 German chemist)

Kekule Structure

Rapid Equilibrium

• Kekule Structures of Benzene were

accepted in the 1930’s when X-ray

studies showed ALL SIX C-H bonds

equal and ALL SIX C-C bonds equal!

Benzene Structure

H

H

H

H

H

H

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• Each C is sp2 hybridized

• Each C has an unhybridized p orbital

perpendicular to the plane of the ring

• The 6 p orbitals overlap to form a cloud

Bonding in Benzene

• A compound with delocalized e- is said

to have resonance – resonance contributor

– resonance structure

– contributing resonance structure

Resonance Hybrid

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• Benzene

– contributing resonance structures

Resonance Hybrid

Drawing resonance hybrids

• 1) Only e- move (not atoms)

• 2) Only and non-bonding e- move

• 3) Total # e- stays same (as does unpaired e-)

Resonance Hybrids

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e- can be moved only by…

e- move toward + or toward bond

Resonance Hybrids


Nonbonding pair e- toward a bond

Resonance Hybrids

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Nonbonding single e- toward a bond

Resonance Hybrids

• Drawing resonance hybrids

Resonance Hybrids

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Resonance Hybrids

• What makes a Resonance Structure

Have Decreased Stability?

– 1) an atom with an incomplete octet

– 2) a negative charge that is not on the most

electronegative atom

– 3) a positive charge not on the most

electropositive atom

– 4) charge separation

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Examples To Examine

B is less stable than A Equal Stability

• 1) The greater the predicted stability of a resonance contributor, the more it contributes to the resonance hybrid.

• 2) The greater the number of relatively stable resonance contributors, the greater the resonance energy.

• 3) The more nearly equivalent the resonance contributors, the greater the resonance energy.

Resonance Energy

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Resonance Energy

The more nearly equivalent the resonance contributors,

the greater the resonance energy

Inductive Effect

The polarization of a σ bond due to electron withdrawing or electron donating effect of adjacent groups or atoms is called inductive effect.

Salient features of inductive effect It arises due to electronegativity difference between two atoms forming a

sigma bond.

It is transmitted through the sigma bonds.

The magnitude of inductive effect decreases while moving away from the groups causing it.

It is a permanent effect.

It influences the chemical and physical properties of compounds.

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Illustration of Inductive Effect

The C-Cl bond in the butyl chloride, CH3-CH2-CH2-CH2-Cl is polarized due to electronegativity difference.

The electrons are withdrawn by the chlorine atom. Thus the first carbon atom gets partial positive charge.

In turn, this carbon atom drags electron density partially from the next carbon, which also gets partial positive charge.

Thus the inductive effect is transmitted through the carbon chain.

Types of Inductive Effect

The inductive effect is divided into two types depending on their strength of electron withdrawing or electron releasing nature with respect to hydrogen.

Negative inductive effect (-I): The electron withdrawing nature of groups or atoms is called as negative inductive effect. It is indicated by -I. Following are the examples of groups in the decreasing order of their -I effect: NH3

+ > NO2 > CN > SO3H > CHO > CO > COOH > COCl > CONH2 > F > Cl > Br > I > OH > OR > NH2 > C6H5 > H

Positive inductive effect (+I): It refers to the electron releasing nature of the groups or atoms and is denoted by +I. Following are the examples of groups in the decreasing order of their +I effect.

C(CH3)3 > CH(CH3)2 > CH2CH3 > CH3 > H

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Why alkyl groups are showing positive inductive effect?

Though the C-H bond is practically considered as non-polar, there is partial positive charge on hydrogen atom and partial negative charge on carbon atom.

Therefore each hydrogen atom acts as electron donating group. This in turn makes an alkyl group, an electron donating group.

Applications of Inductive Effect

Stability of carbonium ions: The stability of carbonium ions increases with increase in number of alkyl groups due to their +I effect. The alkyl groups release electrons to carbon, bearing positive charge and thus stabilizes the ion. The order of stability of carbonium ions is :

.

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Stability of free radicals:

In the same way the stability of free radicals increases with increase in the number of alkyl groups.

Thus the stability of different free radicals is:


Stability of carbanions:

However the stability of carbanions decreases with increase in the number of alkyl groups since the electron donating alkyl groups destabilize the carbanions by increasing the electron density.

Thus the order of stability of carbanions is:

structure & formulae

Documents

carbon atoms

carbon forms

p orbitals

sp3 hybrid orbitals

occurrence of carbon

p electrons

carbon displays tetravalency

new hybrid orbitals