Question

5.) (12 points) For 2,3-dimethylpentane, (CH3)2CH-CH(CH)2 (a) sighting along the indicated bond, draw all possible staggered conformers using Newman projections, identify the strains, ergy of each conformer, and rank the conformers according to stability (b) draw the least stable eclipsed conformer, show the strains, and give the total strain energy. (See Reference Section at end of test for strain-energy values.) (4 points) Cyclopropane has a total strain energy of 115 kJ/mol. How much of this is due to angle strain? Show and explain your calculations. (See Reference Section at end of test for strain-energy values.) (14 points) For trans-1,4-dichlorocyclohexane, (a) draw the two chair conformers, and compare their stabilities, pointing out what types of strain they each exhibit; which one is more abundant in the equilibrium? (b) draw a twist-boat form of this molecule and indicate what forms of strain it has. (c) explain the relationship between the chair, boat, and twist-boat forms. Reference Section: CH-CH torsional strain-4 kJ/mol CH-CCH3 torsional strain = 6 kJ/mol CH,C-CCH, eclipsed steric strain = 11 kJ/mol

Please help to answer this, so I can understand this, Thanks !!

Answer 1

There are multiple questions with multiple sub-parts , student have not specifically mentioned which question to be answered . so, as per guideline only first question with its sub-parts is being answered,

1. Shown compound is   (CH₃)₂CH--CH(CH₃)₂

(Structure shown is different from 2,3-dimethyl pentane , it is 2,3-dimethyl butane ).

It is solved for  (CH₃)₂CH--CH(CH₃)₂ .

Newman projection is way of viewing a molecule along a carbon-carbon single bond to help evaluate the relative orientations of attached groups.

In Staggered conformation about a carbon-carbon single bond, atoms or groups on one carbon are as far apart as possible from atoms or groups on an adjacent carbon.

In dealing with the relative stabilities of the various conformations , we encounter torsional strain, Angle strain and steric strain.

Torsional strain arises when non-bonded atoms separated by three bonds are forced from a staggered conformation to an eclipsed conformation.It is also called eclipsed interaction strain.

Steric strain occurs when non-bonded atoms separated by four or more bonds are
forced closer to each other than permissible by their atomic radii. It is also called non-bonded
interaction strain or van der Waals strain.

(a) All possible staggered structures of (  (CH₃)₂CH--CH(CH₃)₂ ) 2,3-dimethyl butane have been shown by viewing along C2-C3 bond :

CMa CH3 (a) 1H И? (7D CHa .CHa Potsible Stagzioed Cermformaton to=6011e0/30) CHa CH3

This difference in energy is of these possible conformers is almost entirely by the steric strain (non-bonded interaction strain) between the two methyl groups; we have not been provided with data , but these can be arranged in order of energy , baesd on No. of  CH₃--CH₃ non-bonded interaction strain (gauche interaction).

Strain energy : E ( Conformer II ) = E ( Conformer III ) > E ( Conformer I )

(b) In Eclipsed conformation about a carbon-carbon single bond, atoms or groups on one carbon are as close as possible to the atoms or groups on an adjacent carbon.

Least stable eclipsed structure is one in which in which all Methyl groups are in eclipsed conformation.

(b) H Lust stable Eclibed HD

It has Torsional strain, which arises when non-bonded atoms separated by three bonds are forced from a staggered conformation to an eclipsed conformation.It is also called eclipsed interaction strain.

From given data energy of it : 2*( (CH₃)C-C(CH₃)₂ eclipsed steric strain) + 1*(CH-CH torsional strain )

Total strain Energy   = 2*11 kJ/mol + 4 kJ/mol = 26 kJ/mol