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Magazine Chapter 2: Problem 42
Using Newman projections, draw each of the following molecules in its most stable conformation with respect to the bond indicated: (a) 2 -methylbutane, \(\mathrm{C} 2-\mathrm{C} 3\) bond; (b) 2,2 -dimethylbutane, C2-C3 bond; (c) 2,2 -dimethylpentane, C3-C4 bond; (d) \(2,2,4\) -trimethylpentane, C3-C4 bond.
Short Answer
Expert verified
a) CH3-CH2 anti b) Both CH3 staggered c) Methyl groups anti d) CH3 groups anti
Step by step solution
01
- Understand the Newman Projection
A Newman projection is a way to visualize the spatial arrangement of atoms around a particular bond. We look straight down the bond axis and draw the atoms attached to the front carbon as a dot, and the atoms attached to the back carbon as a circle.
02
- Drawing the 2-Methylbutane (C2-C3 bond)
First, identify the groups around the C2-C3 bond in 2-methylbutane. C2 has a methyl group (CH3), a hydrogen (H), and a CH3-CH group bonded to it. C3 has a hydrogen (H), another hydrogen (H), and a CH3 group. In the most stable (staggered) conformation, the large groups (methyl groups) should be as far apart as possible. Hence, one CH3 on C2 will be anti to the CH3 on C3.
03
- Drawing 2,2-Dimethylbutane (C2-C3 bond)
For 2,2-dimethylbutane, each carbon (C2 and C3) has a similar attachment due to two identical groups. On C2 there are two methyl groups (CH3) and a CH group. On C3 there is a single hydrogen and two methyl groups (CH3). In the most stable conformation, the large groups (methyl groups) on adjacent carbons should be staggered as far apart as possible.
04
- Drawing 2,2-Dimethylpentane (C3-C4 bond)
At C3, attachments include two methyl groups and a CH group. On C4, the attachments are a hydrogen and two alkyl groups (CH2 and CH3). Arrange these groups to ensure the bulky groups are farthest apart in staggered conformation in the Newman projection.
05
- Drawing 2,2,4-Trimethylpentane (C3-C4 bond)
For 2,2,4-trimethylpentane, focus on the bonds between C3 and C4. C3 has two methyl groups (CH3) and another CH2 group. C4 has similar alkyl groups - a CH3, CH, and H. Align these groups to reduce steric hindrance with large groups anti to each other.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Staggered Conformation
In the study of Newman projections, staggered conformation is a key concept that refers to the arrangement where the atoms or groups attached to two adjacent carbons are positioned as far apart as possible. This conformation minimizes repulsions between electron clouds of bonded atoms and is generally more stable than other conformations. In the staggered conformation, the dihedral angle between atoms or groups is usually 60 degrees, leading to decreased steric hindrance and lower potential energy. When drawing Newman projections, always aim to place bulky groups (such as methyl groups) at 180 degrees (anti) to each other to achieve maximum stability.
Steric Hindrance
Steric hindrance occurs when atoms or groups of atoms within a molecule are too closely packed, leading to repulsion between their electron clouds. This repulsion can cause strain and an increase in the molecule's potential energy. Steric hindrance is a critical factor to consider when determining the most stable conformation of a molecule. In Newman projections, minimizing steric hindrance is achieved by placing large groups as far apart as possible. For instance, placing bulky groups like methyl groups at an anti-position (180 degrees apart) reduces their interactions and helps achieve a more stable conformation.
2-Methylbutane
2-Methylbutane is an organic molecule with the formula C5H12. When considering the Newman projection of the C2-C3 bond in 2-methylbutane, it's important to identify the groups attached to these carbons. C2 is connected to a methyl group (CH3), a hydrogen (H), and a methylene group (CH3-CH2). C3 has two hydrogens (H) and another methyl group (CH3). In the most stable staggered conformation, the methyl groups on C2 and C3 should be anti to each other to minimize steric hindrance, which means placing them 180 degrees apart. This arrangement results in a lower energy and more stable conformation.
2,2-Dimethylbutane
2,2-Dimethylbutane, also known as neohexane, has the formula C6H14. When drawing the Newman projection through the C2-C3 bond, it's important to note that both C2 and C3 have two methyl groups attached, along with other groups. On C2, the groups are two methyl groups (CH3) and a methylene group (CH3-CH). On C3, the groups are one hydrogen and two methyl groups (CH3). The staggered conformation can be obtained by positioning these large groups (methyl groups) as far apart as possible. This results in a more stable structure due to minimized steric hindrance and lower potential energy.
2,2-Dimethylpentane
2,2-Dimethylpentane is a branched alkane with the formula C7H16. For capturing the staggered conformation in a Newman projection along the C3-C4 bond, we need to examine the groups at these positions. C3 is bonded to two methyl groups (CH3) and a methylene group (CH). At C4, you'll find a hydrogen (H) and two larger alkyl groups (CH2 and CH3). To achieve the lowest energy conformation, place the bulky groups (methyl groups on C3 and alkyl groups on C4) anti to each other (180 degrees apart). This arrangement minimizes steric hindrance, producing a more stable molecular structure.
2,2,4-Trimethylpentane
2,2,4-Trimethylpentane, commonly known as isooctane, has the formula C8H18. When considering the Newman projection along the C3-C4 bond, specific group placements around these carbons matter. C3 has two methyl groups (CH3) and a methylene group (CH2). C4 is bonded to a smaller hydrogen (H), a methyl group (CH3), and another alkyl group (CH). The most stable (staggered) conformation is obtained by arranging these groups such that the larger groups (methyl groups and alkyl groups) are anti to each other. This configuration reduces steric hindrance, leading to a lower-energy and more stable molecule.
