Ch.3 - Structure and Stereochemistry of Alkanes

Wade9th EditionOrganic ChemistryISBN: 9780135213728당신이 사용하는 게 아니라요?교과서 변경

Wade 9th Edition

Ch.3 - Structure and Stereochemistry of Alkanes

문제 25a

3장, 문제 25a

Draw the most stable conformation of
a. ethylcyclohexane.

검증된 단계별 안내

Start by understanding the structure of cyclohexane. Cyclohexane is a six-membered ring that can adopt different conformations, with the chair conformation being the most stable due to minimized steric strain and torsional strain.

Identify the substituent group in ethylcyclohexane, which is the ethyl group (C₂H₅). This group will be attached to one of the carbon atoms in the cyclohexane ring.

Consider the chair conformation of cyclohexane. In this conformation, each carbon atom has two positions for substituents: axial (pointing up or down, perpendicular to the ring) and equatorial (pointing outwards, parallel to the ring).

Place the ethyl group in the equatorial position on the cyclohexane ring. This position is preferred for larger groups like ethyl because it minimizes steric hindrance and is more stable than the axial position.

Draw the complete structure of ethylcyclohexane in the chair conformation with the ethyl group in the equatorial position. Ensure that the hydrogen atoms are appropriately placed in axial and equatorial positions to complete the conformation.

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주요 개념

질문에 올바르게 답하기 위해 반드시 이해해야 하는 핵심 개념들은 다음과 같습니다.

Cyclohexane Conformations

Cyclohexane can adopt several conformations, with the chair conformation being the most stable due to minimized steric strain and torsional strain. In the chair form, carbon atoms are staggered, reducing repulsion between hydrogen atoms. Understanding these conformations is crucial for predicting the stability of substituted cyclohexanes.

Axial and Equatorial Positions

In the chair conformation of cyclohexane, substituents can occupy axial or equatorial positions. Axial positions are perpendicular to the ring plane, while equatorial positions are parallel. Substituents prefer equatorial positions to minimize steric hindrance, which is key to determining the most stable conformation of substituted cyclohexanes like ethylcyclohexane.

Steric Hindrance

Steric hindrance occurs when atoms or groups are too close, causing repulsion and destabilizing the molecule. In cyclohexane derivatives, larger groups in axial positions experience more steric hindrance due to 1,3-diaxial interactions. Placing bulky groups like ethyl in equatorial positions reduces steric strain, leading to a more stable conformation.

관련 실천

교과서 질문

Draw 1,2,3,4,5,6-hexamethylcyclohexane with all the methyl groups

b. in equatorial positions.

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교과서 질문

Draw the most stable conformation of

c. cis-1-tert-butyl-4-isopropylcyclohexane.

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교과서 질문

Draw the most stable conformation of

b. 3-isopropyl-1,1-dimethylcyclohexane.

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교과서 질문

Table 3-6 shows that the axial–equatorial energy difference for methyl, ethyl, and isopropyl groups increases gradually: 7.6, 7.9, and 8.8 kJ/mol (1.8, 1.9, and 2.1 kcal/mol). The tert-butyl group jumps to an energy difference of 23 kJ/mol (5.4 kcal/mol), over twice the value for the isopropyl group. Draw pictures of the axial conformations of isopropylcyclohexane and tert-butylcyclohexane, and explain why the tert-butyl substituent experiences such a large increase in axial energy over the isopropyl group.

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교과서 질문

Draw a Newman projection, similar to Figure 3-25 down the C1—C6 bond in the equatorial conformation of methylcyclohexane. Show that the equatorial methyl group is also anti to C5. (Using your models will help.)

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Name the following compounds. Remember that two up bonds are cis; two down bonds are cis; one up bond and one down bond are trans.

(c)

(d)

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