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Chapter 2: Problem 34

Consider 1-bromopropane, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}\). (a) Draw a Newman projection for the conformation in which \(-\mathrm{CH}_{3}\) and \(-\mathrm{Br}\) are anti (dihedral angle \(180^{\circ}\) ). (b) Draw Newman projections for the conformations in which \(-\mathrm{CH}_{3}\) and \(-\mathrm{Br}\) are gauche (dihedral angles \(60^{\circ}\) and \(300^{\circ}\) ). (c) Which of these is the lowest energy conformation? (d) Which of these conformations, if any, are related by reflection?

Short Answer

Expert verified
Answer: The lowest energy conformation of 1-bromopropane is the anti conformation (dihedral angle of 180°). The two gauche conformations (60° and 300°) are related by reflection.

Step by step solution

01

Understand different conformations

In the Newman projection, we can show different conformations of the molecule by changing the dihedral angles between groups in the molecule. The anti conformation indicates that the two groups are opposite to each other (dihedral angle of 180°), while the gauche conformation indicates that the two groups are at an angle of 60° or 300° to each other.
02

Draw the Newman projection for the anti conformation

In the anti conformation, the -CH3 and -Br groups will be at a dihedral angle of 180°. To draw the Newman projection, place the -CH3 group on the front carbon atom (down) and the -Br group on the rear carbon atom (up). The other two -CH2 groups will be on either side of the two central carbon atoms, one in the front and one in the back.
03

Draw the Newman projections for the gauche conformations

In the gauche conformation, the -CH3 and -Br groups will be at dihedral angles of 60° and 300°. For the 60° gauche conformation, place the -CH3 group on the front carbon atom (down) and rotate the -Br group counter-clockwise for a 60° angle. The 300° gauche conformation can be obtained by rotating the -Br group clock-wise for a 300° angle. In both conformations, the two -CH2 groups will be on either side of the two central carbon atoms.
04

Determine the lowest energy conformation

Generally, the anti conformation has the lowest energy, because the two groups -CH3 and -Br are farthest from each other, reducing steric strain. In this case, the anti conformation (dihedral angle 180°) would be the lowest energy conformation.
05

Determine conformations related by reflection

Conformations related by reflection will have a mirror image of each other in the Newman projection. Here, the two gauche conformations (60° and 300°) are related by reflection, as they appear as a mirror image of each other in their respective Newman projections.

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Most popular questions from this chapter

Using a planar pentagon representation for the cyclopentane ring, draw structural formulas for the cis and trans isomers of the following. (a) 1,2-Dimethylcyclopentane (b) 1,3-Dimethylcyclopentane

\(1,2,3,4,5,6-\) Hexachlorocyclohexane shows cis,trans isomerism. At one time a crude mixture of these isomers was sold as an insecticide. The insecticidal properties of the mixture arise from one isomer, known as lindane, which is cis- \(1,2,4,5\)-trans-3,6hexachlorocyclohexane. (a) Draw a structural formula for \(1,2,3,4,5,6\)-hexachlorocyclohexane disregarding, for the moment, the existence of cis,trans isomerism. What is the molecular formula of this compound? (b) Using a planar hexagon representation for the cyclohexane ring, draw a structural Formula for lindane. (c) Draw a chair conformation for lindane, and label which chlorine atoms are axial and which are equatorial. (d) Draw the alternative chair conformation of lindane, and again label which chlorine atoms are axial and which are equatorial. (e) Which of the alternative chair conformations of lindane is more stable? Explain.

Following is a chair conformation of cyclohexane with the carbon atoms numbered 1 through \(6 .\) (a) Draw hydrogen atoms that are above the plane of the ring on carbons 1 and 2 and below the plane of the ring on carbon 4 . (b) Which of these hydrogens are equatorial? Which are axial? (c) Draw the alternative chair conformation. Which hydrogens are equatorial? Which are axial? Which are above the plane of the ring? Which are below it?

Provide an even more abbreviated formula for each structural formula, using parentheses and subscripts. (a) \(\left.\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2}\right|^{\mathrm{CHCH}_{3}}\) (b) (c)

Explain why each is an incorrect IUPAC name, and write the correct IUPAC name for the intended compound. (a) 1,3-Dimethylbutane (b) 4 -Methylpentane (c) 2,2-Diethylbutane (d) 2-Ethyl-3-methylpentane (e) 2-Propylpentane (f) 2,2-Diethylheptane (g) 2,2-Dimethylcyclopropane (h) 1-Ethyl-5-methylcyclohexane
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