Question

1. Draw two molecules that are examples of constitutional isomers. Are your examples skeletal or positional isomers?

2. Draw the cyclohexane ring in the chair and boat conformations.

3.What is meant by the phrase "plane of symmetry"? Do enantiomers have a paln of symmetry? How do you justify your answer about enantiomers?

4. What is meant by the term "optical activity"? Why is it that only certain kinds of molecules are optically active. How do chemists measure the optical activity of molecules?

5. How is priority order determined for atoms attached to a chiral carfbon center?

Answer 1

1.Constitutional isomers are compounds that have the same molecular formula and different connectivity. To determine whether two molecules are constitutional isomers, just count the number of each atom in both molecules and see how the atoms are arranged. If both molecules have the same count for all of the different atoms, and the atoms are arranged in different ways (their connectivity is different), the molecules will be constitutional isomers. here 2 types are here

Positional isomers

this isomers that have the same carbon skeleton and the same functional groups but differ from each other in the location of the functional groups on or in the carbon chain.

Skeletal isomers

This isomers that have the same functional groups but differ from each other in the connectivity of the carbon skeleton.

2. chair conformations are in a state of constant flux. Because all the C-C bonds are interconnected, they cannot rotate independently but have to move together. For example, one end of the chair could "flip up" to put the cyclohexane ring in a boat conformation.

3. Plane of Symmetry

A plane of symmetry is an imaginary plane that bisects a molecule into halves that are mirror images of each other.

the vertical plane that passes through the red broken line perpendicular to the plane of the cyclopropane ring bisects the molecule into halves that are mirror images of each other. Therefore, it is a plane of symmetry.

Lack of a symmetry plane or center causes enantiomerism which means No symmetry should be present in enantiomer. but enantiomer pair is mirror images of each other

4. optical activity

Optical activity describes the phenomenon by which chiral molecules are observed to rotate polarized light in either a clockwise or counterclockwise direction. This rotation is a result of the properties inherent in the interaction between light and the individual molecules through which it passes. Material that is either achiral or equal mixtures of each chiral configuration (called a racemic mixture) do not rotate polarized light, but when a majority of a substance has a certain chiral configuration the plane can be rotated in either direction. Hence optical rotation is a chiral phenomenon.

Measuring Optical Activity

When rotation is quantified using a polarimeter it is known as an observed rotation, because rotation is affected by path length (l, the time the light travels through a sample) and concentration (c, how much of the sample is present that will rotate the light). When these effects are eliminated a standard for comparison of all molecules is obtained, the specific rotation, [a].

[a] = 100a / cl    when concentration is expressed as g sample /100ml solution

5. The four different groups attached to the chiral center atom are ranked from highest priority (1) to lowest priority (4).

1) Start with the first atom of each group that is directly attached to the chiral tetrahedral center. The atoms with higher atomic number will have highest priority. For example, the ranking for the four groups around the chiral center of the molecule CHBrClF would be:

If two groups have the same first atom, then compare the second atom from the chiral center. If there are multiple second atoms, then compare them in order of atomic number. Stop, when there is a difference. For example:

Although -CBr3 has more atoms that are higher in atomic number, it is the lower priority group. The first atom in -CBr3 is a carbon, which has a lower atomic number (6) than -F (9).

The first atom of the two groups are the same (carbons). So, we compare the "second" atoms. However, in each case there are three "second" atoms so we have to proceed in order of atomic number (high to low). The first pair of "second" atoms are the same (Br and Br) so we proceed to the next highest pair (Cl and F), which are different. Chlorine has a higher atomic number and so the group on the left will have the higher priority.

3) If all the first and second atoms from the chiral center are the same, then proceed to the next furthest atom (in order of atomic number) until you find a difference.

4) Treat double and triple bonds as if they are a series of single bonds to the same atom. This will involve the creation of singly bonded dummy atoms (highlighted in blue) of the same type as involved in the double or triple bonds. For example:

Note: The first carbon is singly bonded to hydrogen and doubly bonded to carbon 2. For the purposes of ranking priorities, we would consider the first carbon as being singly bonded to hydrogen, singly bonded to carbon 2, and singly bonded to another 'dummy' carbon.

The same rules can be applied to carbon two, it is singly bonded to a H and Br and doubly bonded to carbon one. Therefore, we would consider carbon two as being singly bonded to H, Br, and carbon one. In addition, it also forms a single bond to a another 'dummy' carbon.