Test: Stereochemistry Level - 1

Diastereomers have different physical properties. They can be separated by ordinary separation techniques such as fractional crystallization.

Gauche and anti-conformers are stereoisomers not related by mirror-image relationship, hence diastereomers.

• Fully eclipsed conformation: the rotation of the gauge conformation to 60 degrees results in the gauge conformation and it is the least stable conformation of all.
• Hence, the most stable conformation of ethylene glycol is gauche conformation.

• Eclipsed atoms are not valid in boat configurations.

• Flagpole interaction generally raises the strain and then energy in cyclohexane (boat conformational structure).
• You can imagine this as two hydrogen atoms at pole position (1, 4) and the interaction between them.
• The boat conformations have higher energy than the chair conformations. The interaction between the two flagpole hydrogen atoms, in particular, generates steric strain.
• Torsional strain also exists between the C2–C3 and C5–C6 bonds, which are eclipsed. Because of this strain, the boat configuration is unstable (i.e. is not a local energy minimum).

Hence D is correct.

• The reaction in which stereochemically different molecules react differently is called stereospecific reaction.
• Stereospecificity is the property of a reaction mechanism that lead to different stereoisomeric reaction products from different stereoisomeric reactants, or which operates on only one of the stereoisomers.

• There is a possibility of Hydrogen Bonding between "Hydrogen(H)" of OH group and "Oxygen(O)" of OMe group. 
• So if they are oriented in such a way that these two groups are close to each other , the strength of hydrogen bond would increase making it more stronger.
• Hence, Option A, because of Hydrogen Bonding becomes the most stable isomer. 

Louis Pasteur was the first documented person to separate stereoisomers.

• The above reaction is an example of E2 elimination.
• For E2 elimination both the leaving group must be anti w.r.t each other in the adjacent carbon atoms.
• In option C, we get this requirement of anti geometry of the Br atoms, so it undergoes reaction. 
   

The two substituents in the cis 1,4-disubstituted chair cyclohexane shown are necessarily axial and equatorial, as in (a), (b) and (d). In (c), however, the 1,4-substituents are both equatorial so this is the trans isomer, i.e. a different compound.

In identifying stereogenic centers, any quaternary or tertiary carbons would be a good place to look. There are no quaternary carbons, and all the tertiary carbons are part of a benzene ring structure.