Question

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chemistry 313 exam ii march 26, 2013
\example is not only the best way of propagating an opinion, but it is the only way worth taking into account.\ -- samuel butler
instructions: choose the best answer to each of the questions. put the letter corresponding to your answer on your scantron. also circle your answers on these pages. the scantron will not be returned.

1. your test form letter is circled below. enter the circled letter on the scantron as your answer to this question.

a) a b) b c) c d) d

1. in the newman projection shown below, what is the dihedral angle between the methyl and ethyl groups that are gauche to each other?

a) \\(0^\circ\\) b) \\(30^\circ\\) c) \\(60^\circ\\) d) \\(120^\circ\\) e) \\(180^\circ\\)

1. the molecular model shown is a staggered conformation of \\(\text{ch}_3\text{ch}_2\text{ch}_2\text{br}\\). what term best describes the atom that is anti to bromine?

a) a primary carbon
b) a primary hydrogen
c) a secondary carbon
d) a secondary hydrogen
e) a tertiary hydrogen

Explanation:

🆕 New Concept Discovered: Newman Projections and Conformations
Visualizing 3D spatial arrangements along a carbon-carbon bond.

Step 1: Identify the groups of interest

We need to find the dihedral angle between a methyl group (\(-\text{CH}_3\)) and an ethyl group (\(-\text{CH}_2\text{CH}_3\)) that are gauche to each other in the provided Newman projection.

• Front carbon (Y-shape in the center): Has three attached groups:
• Top: Ethyl (\(-\text{CH}_2\text{CH}_3\))
• Bottom-right: Methyl (\(-\text{CH}_3\))
• Bottom-left: Hydrogen (\(-\text{H}\))

• Back carbon (circle): Has three attached groups:
• Bottom: Ethyl (\(-\text{CH}_2\text{CH}_3\))
• Top-left: Ethyl (\(-\text{CH}_2\text{CH}_3\))
• Top-right: Hydrogen (\(-\text{H}\))

Step 2: Understand the term "gauche"

In a Newman projection, two non-hydrogen groups are described as gauche when they are adjacent to each other in a staggered conformation. This spatial relationship corresponds to a dihedral angle of \(60^\circ\).

Step 3: Locate the gauche methyl and ethyl groups

Looking at the front carbon's methyl group (\(-\text{CH}_3\)) at the bottom-right:

• The adjacent group on the back carbon to its right is a hydrogen (\(-\text{H}\)).
• The adjacent group on the back carbon to its left (pointing straight down) is an ethyl group (\(-\text{CH}_2\text{CH}_3\)).

Because these two groups are adjacent in a staggered conformation, they are gauche to each other. The dihedral angle between any two adjacent staggered positions on a Newman projection is \(60^\circ\).

Answer:

c) 60°