Introduction to Electrochemistry and Tricks to Solve Problems in Electrochemistry

 Introduction to Electrochemistry

Tricks to Solve Problems in Electrochemistry

After watching the videos, attempt for the following MCQ test in Electrochemistry

https://forms.gle/wqLWStot2Y8wDsr59

Conformations of Ethane and Butane

 

Conformational Analysis of Ethane and Butane

•      Different spatial arrangement of molecules that are generated by rotation about single bonds.

•      Conformational analysis is the study of the effect of rotation on the properties of a molecule.

Torsional strain

It exists when neighboring carbon posses hydrogens that overlap in space (eclipse). The torsional strain arises when a molecule is rotated around a bond.

Steric strain

It is the repulsion between two atoms or groups of atoms when the distance between them is decreased. Steric strain is formed when two or more bulky groups get close to each other.

Conformation of Ethane

•   Although there are SEVEN sigma bonds in the ethane molecule, rotation about the six carbon-hydrogen bonds does not result in any change in the shape of the molecule.

•  Rotation about the carbon-carbon bond results in many different possible molecular conformations.

•      Types of conformations are

§  Eclipsed conformation

§  Staggered conformation

§  Skew conformation (infinite intermediate conformation)

The least stable (high energy) conformation is the one in which the six carbon–hydrogen bonds are as close as possible (eclipsed in a Newman projection). More torsional strain due to eclipsing hydrogen atoms and the energy of the molecule is high- hence less stable. 

The most stable (low energy) conformation is the one in which all six C–H bonds are as far away from each other as possible (staggered when viewed end-on in a Newman projection). less torsional strain due to less interaction between hydrogen atoms and the energy of the molecule is less- hence more stable.

Potential Energy Diagram of conformations of ethane are as follows. The staggered conformation is more stable by 12.6 kJ/mol than the eclipsed conformation. The order of stability of conformation of ethane is as follows. Staggered > skew > Eclipsed

Conformations of butane

There are THREE C-C sigma bonds in the butane molecule, rotation about the C2-C3 carbon results in many different possible molecular conformations.

Types of conformations are

• Fully Eclipsed conformation (Syn)
• Eclipsed conformation
• Gauche conformation
• Fully staggered conformation (anti)
• Skew conformation (infinite intermediate conformation)

Fully Eclipsed Conformation (Syn Conformation)

This conformation arises when the dihedral angle between two CH3 group is 0 degree leading to maximum torsional and steric strain. Hence this conformation is unstable compared to all other conformations.

Gauche Conformation

As rotation around the C(2)–C(3) bond occurs by 60 degree from fully eclipsed conformations, staggered conformation is reached in which dihedral angle between Me–Me groups are 60 degree. Due to less torsional and steric strain, it is relatively stable than fully eclipsed conformation.

Eclipsed Conformation

As rotation around the C(2)–C(3) bond occurs from gauche conformation by 60 degree, another eclipsed conformation is reached in which there are two Me–H interactions and one H–H interaction. Due to torsional and steric strain, it is relatively unstable than gauche conformation.

Anti Staggered conformation

The lowest-energy arrangement, called the antiperiplanar (or anti) conformation, is the one in which the two large methyl groups are as far apart as possible. Hence It experiences minimum torsional and steric strain

The potential energy diagram of butane conformers is as follows. The stability of conformations of butane follows the order ;

anti-staggered > gauche > Eclipsed > Fully eclipsed

Baeyer Strain Theory - Stability of Cycloalkanes

 

Baeyer Strain Theory

Cyclopropane reacts with chlorine and bromine in dark to form addition products (ring opening reaction). Cyclobutane and higher cycloalkanes do not give this ring opening reaction at ordinary conditions. This can be explained by the basis of angle strain theory or Baeyer Strain Theory.

Assumptions of Baeyer Strain Theory

All ring systems are planar.

Deviation from normal tetrahedral angle results in angle strain that makes the cycloalkanes to become unstable. 

The large ring systems involve negative strain hence do not exist.

Besides torsional strain and steric strain, the conformations of cycloalkanes are also affected by angle strain.

Angle Strain in Cyclopropane

•  In cyclopropane, C-C-C bond angle is 60°.
• This revealed that normal tetrahedral angle of 109.5° between any two bonds is compressed to 60°. Hence it has more angle strain.     

Calculation of Angle strain

θ + 60 + θ = 109.5

2θ = 109.5 – 60

θ = 49.5 / 2

θ = 24.5^o

Hence, cyclopropane has an angle strain of 24.5 ^o.

Angle Strain in cyclobutane

• In cyclobutane, C-C-C bond angle is 90°.
• This revealed that normal tetrahedral angle of 109.5° between any two bonds is compressed to 90°. 
  
  
  

Calculation of Angle strain

            θ + 90 + θ = 109.5 

2θ  = 109.5 – 90

θ = 19.5 / 2

θ = 9.75^o

Hence, cyclobutane has an angle strain of 9.75 ^o. 

Angle Strain in cyclopentane

• In cyclopentane, C-C-C bond angle is 108°.
• This revealed that normal tetrahedral angle of 109.5° between any two bonds is compressed to 108°. 

Calculation of Angle strain

θ + 108 + θ = 109.5

2θ  = 109.5 – 108

θ = 1.5 / 2

θ = 0.75^o

Hence, cyclopentane has an angle strain of 0.75 ^o. 

Angle Strain in Cyclohexane

• In cyclohexane, C-C-C bond angle is 120°.
• This revealed that normal tetrahedral angle of 109.5° between any two bonds is expanded to 120°. 
  
  
  

Calculation of Angle strain

θ + 120 + θ = 109.5

2θ  = 109.5 – 120

θ = -10.5 / 2

θ = - 5.25^o (Expansion)

Hence, cyclohexane has an angle strain of -5.25^o. 

According to Baeyer’s Strain theory, cyclopentane is the most stable cycloalkane than cyclopropane, cyclobutane and cyclohexane due to less angle strain. The order of stability follows

Cyclopropane < cyclobutene < cyclohexane < cyclopentane 

Cycloalkane	Actual C-C-C bond angle	Angle Strain
Cyclopropane	60	24.5
Cyclobutane	90	9.75
Cyclopentane	108	0.75
Cyclohexane	120	-5.25

Limitations of Baeyer Strain Theory

• According to enthalpy of combustion, cyclohexane is more stable than cyclopentane. Baeyer Theory does not explain this behavior. The correct order of stability of cycloalkanes are as follows.  Cyclopropane < cyclobutene < cyclopentane < cyclohexane
• Baeyer assumed all the cycloalkanes as planar molecules.
• Actually, except cyclopropane, all other cycloalkanes are not planar.