Alkanes: Conformational Isomerism and Preparation

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Questions and Answers

Explain why rotation around a C-C single bond is not completely free, and what term is used to describe the repulsive interaction that hinders this rotation?

Rotation around a C-C single bond is hindered by a small energy barrier (1-20 kJ/mol) due to weak repulsive interactions between electron clouds of different σ-bonds. This repulsive interaction is called torsional strain.

Define conformational energy. What does it represent and how is it determined?

Conformational energy is the difference in potential energy between the most stable conformation and the conformation under consideration.

How many types of H-atoms are present in 2-methylpentane? Explain your reasoning.

There are five types of H-atoms in 2-methylpentane. The methyl groups on the 2nd carbon, the two H atoms on the 1st carbon, the one H atoms on the 2nd carbon itself, the two H atoms each on the 3rd and 4th carbons, and the three H atoms on the terminal methyl group of the pentane chain are all chemically distinct.

Why is the concept of 'free rotation' around a C-C single bond in alkanes not entirely accurate?

The concept of 'free rotation' is not entirely accurate due to torsional strain. Although rotation is possible, it's hindered by energy barriers (1-20 kJ/mol) arising from repulsive interactions of electron clouds. Signup and view all the answers

If you were to draw Newman projections of ethane, what would be the key difference visually between the staggered and eclipsed conformations?

In the staggered conformation, the hydrogen atoms on the front carbon are as far away as possible from the hydrogen atoms on the back carbon. In the eclipsed conformation, the hydrogen atoms on the front carbon are directly aligned with, or eclipsing, the hydrogen atoms on the back carbon. Signup and view all the answers

Describe the relationship between conformations, conformational isomers, and the rotation around a single bond.

Conformations are different spatial arrangements of atoms in a molecule that arise from rotation around a single bond. Conformational isomers are the different forms that result from this rotation. Signup and view all the answers

How many chain isomers would be obtained on replacement of different H-atoms of n-pentane? Write their structures and IUPAC names.

Three chain isomers would be obtained. The structures and names are: 2-methylbutane, 2,2-dimethylpropane and n-pentane. Signup and view all the answers

Explain why Newman projections are useful for studying conformational isomerism.

Newman projections provide a direct view down a specific C-C bond, allowing for easy visualization and comparison of the relative positions of substituents and estimation of torsional strain in different conformations. Signup and view all the answers

Explain why the staggered conformation is more stable than the eclipsed conformation.

The staggered conformation minimizes torsional strain because the hydrogen atoms are as far apart as possible, whereas the eclipsed conformation has maximum torsional strain due to the hydrogen atoms being as close as possible. Signup and view all the answers

Describe the key difference in representing bonds using the solid wedge versus the hashed wedge in the flying wedge formula.

A solid wedge represents a bond that is above the plane of the paper, while a hashed wedge represents a bond that is below the plane of the paper. Signup and view all the answers

In the sawhorse projection formula, how are the front and rear carbon atoms indicated, and how are the bonds around each carbon arranged?

In the sawhorse projection, the front carbon is at the lower end of the elongated C-C bond line, while the rear carbon is at the upper end. Each carbon has three lines attached, representing bonds to other atoms, and the bonds are inclined at 120° to each other. Signup and view all the answers

What is unique about the conformation of methane compared to other saturated hydrocarbons, and why?

Methane does not have different conformations because it contains only one carbon atom, so it cannot exhibit the staggered, eclipsed, or skew conformations seen in molecules with multiple carbon atoms. Signup and view all the answers

Explain how the Newman projection formula represents the front and rear carbon atoms when viewing a molecule along the C-C bond.

In the Newman projection, the front carbon is represented by a point, and the rear carbon is represented by a circle. The three substituents attached to each carbon are shown as lines radiating from the point (front carbon) and the circle (rear carbon). Signup and view all the answers

Describe the dihedral angle in both the staggered and eclipsed conformations.

In the staggered conformation, the dihedral angle between two C-H bonds is 60°. In the eclipsed conformation, the dihedral angle between two C-H bonds is 0°. Signup and view all the answers

Define the 'skew' conformation and its relative stability compared to staggered and eclipsed conformations.

The skew conformation is any intermediate conformation where the dihedral angle is between 0° and 60°. Its stability is in between the two extreme conformations: Staggered > Skew > Eclipsed Signup and view all the answers

Why is it important to maintain consistent bond angles and bond lengths when considering different conformations of a molecule?

Maintaining consistent bond angles and bond lengths ensures that the conformational differences arise primarily from rotations around single bonds, affecting torsional strain rather than structural integrity. Signup and view all the answers

Explain the concept of torsional strain and how it relates to the stability of different conformations.

Torsional strain is the resistance to twisting about a bond. Eclipsed conformations suffer from maximum torsional strain due to repulsion between electron clouds of bonds. Staggered conformations minimize this strain, making them more stable. Signup and view all the answers

How does the flying wedge formula depict the three-dimensional arrangement of atoms around a carbon atom?

Two bonds attached to a carbon atom are shown in the plane of the paper using normal lines. A solid wedge represents a bond above the plane, and a hashed wedge represents a bond below the plane, indicating spatial orientation. Signup and view all the answers

A chemist attempts to synthesize pentane using the Wurtz reaction, but obtains a significant amount of butane as a byproduct. Explain the most likely reason for the formation of butane in this reaction.

The likely reason for the butane formation is the presence of ethyl halide as an impurity in the reaction mixture that reacts with sodium. Signup and view all the answers

Explain why the Wurtz reaction is generally not preferred for synthesizing alkanes with an odd number of carbon atoms.

The Wurtz reaction involves the coupling of two alkyl halides to form an alkane with twice the number of carbons, leading to symmetrical products. If you mix two different alkyl halides, you get a mixture of products that are hard to separate. Signup and view all the answers

Predict the major organic product formed when sodium propanoate undergoes decarboxylation with sodalime, and briefly explain the reaction mechanism.

The major organic product is ethane (CH₃CH₃). The sodalime (NaOH + CaO) facilitates the removal of carbon dioxide (CO₂) from the propanoate, leaving behind a two-carbon alkane. Signup and view all the answers

A researcher wants to convert butanoic acid to butane. Describe a two-step chemical process to accomplish this, including the necessary reagents for each step.

Step 1: React butanoic acid with NaOH to form sodium butanoate. Step 2: Heat the sodium butanoate with sodalime (NaOH and CaO) to induce decarboxylation and produce butane. Signup and view all the answers

You have been tasked with synthesizing 2,7-dimethyloctane using the Corey-House synthesis. Propose the alkyl halide and organocuprate reagents that would efficiently yield the desired product.

React 1-iodo-2-methylheptane with lithium to form the corresponding alkyllithium reagent. Then, treat this with copper iodide to generate the lithium dialkylcuprate reagent. Finally, react this cuprate with 1-iodopropane to form 2,7-dimethyloctane. Signup and view all the answers

Explain how a Newman projection represents the spatial arrangement of atoms in a molecule.

The front carbon is a point, with bonds radiating from it. The rear carbon is a circle, with bonds radiating from the edge. This shows the arrangement of substituents around a specific C-C bond. Signup and view all the answers

Describe the difference in potential energy between the eclipsed and staggered conformations of ethane. Why does this energy difference occur?

Staggered conformations have lower potential energy than eclipsed conformations. This is due to reduced steric strain (less bulky groups near each other) and minimal torsional strain. Signup and view all the answers

In propane, which C-C bond is considered for analyzing conformations, and why?

Either the C1-C2 or C2-C3 bond can be considered because they are equivalent due to the symmetry of the molecule. Signup and view all the answers

What is the primary reason for the difference in stability between staggered and eclipsed conformations in propane?

The staggered conformation in propane avoids the eclipsing interactions present in the eclipsed form, reducing torsional strain and minimizing steric hindrance between substituents. Signup and view all the answers

Explain why rotation about the C2-C3 bond in n-butane is more complex than rotation about the C1-C2 bond.

Rotation about the C2-C3 bond involves different substituents on each carbon (methyl and hydrogen), leading to distinct staggered (anti and gauche) and eclipsed conformations with varying steric interactions and energies, unlike the C1-C2 bond. Signup and view all the answers

Define the anti-staggered conformation of n-butane, and explain why it is the most stable.

In the anti-staggered conformation, the two methyl groups are 180° apart (dihedral angle). It's the most stable because it minimizes steric hindrance between the methyl groups. Signup and view all the answers

Describe the gauche conformation of n-butane, and explain why it is less stable than the anti-staggered conformation.

In the gauche conformation, the two methyl groups are 60° apart (dihedral angle). It's less stable because it introduces steric strain due to the proximity of the methyl groups. Signup and view all the answers

Explain the difference between the eclipsed and fully eclipsed conformations of n-butane and their relative stabilities.

The eclipsed conformation has one pair of methyl and hydrogen atoms in opposition. The fully eclipsed conformation has two methyl groups in direct opposition, causing greater steric strain and lower stability compared to the eclipsed conformation. Signup and view all the answers

Define dihedral angle and explain its significance in the context of conformational analysis.

Dihedral angle is the angle between two intersecting planes, specifically X-C-C and C-C-Y planes. It quantifies the relative positions of substituents and is used to describe and analyze different conformations. Signup and view all the answers

How does the dihedral angle change as a molecule transitions from a staggered to an eclipsed conformation?

As a molecule goes from staggered to eclipsed, the dihedral angle changes to align substituents on adjacent carbons, increasing torsional strain. For example, in ethane, angles change from approximately 60°, 180°, and 300° in staggered form to 0°, 120°, and 240° in eclipsed form. Signup and view all the answers

Describe how catalytic hydrogenation is used to prepare alkanes from unsaturated hydrocarbons, and what conditions are typically required for this process.

Catalytic hydrogenation involves reacting alkenes or alkynes with hydrogen gas in the presence of a metal catalyst (Ni, Pt, or Pd). This saturation process requires temperatures between 200-300°C (depending on the catalyst) to produce alkanes. Signup and view all the answers

Outline two methods for reducing alkyl halides to alkanes, mentioning specific reagents used in each.

Alkyl halides can be reduced to alkanes using either zinc with hydrochloric acid (Zn/HCl) or sodium borohydride (NaBH4). Signup and view all the answers

Describe how the Clemmensen reduction is performed and what type of carbonyl compounds can be converted to alkanes through this method.

The Clemmensen reduction uses amalgamated zinc and concentrated hydrochloric acid to reduce aldehydes and ketones to alkanes. Signup and view all the answers

If you wanted to synthesize ethane from ethene, which method described in the text would be most appropriate? Why?

Catalytic hydrogenation. Ethene (an alkene) can be directly converted to ethane by reacting it with hydrogen gas ($H_2$) in the presence of a metal catalyst. Signup and view all the answers

Explain why reduction reactions are used in alkane synthesis. What is being reduced and what is the general outcome in terms of hydrogen content?

Reduction reactions in alkane synthesis increase the hydrogen content of a molecule. Unsaturated hydrocarbons or alkyl halides are reduced, resulting in the formation of new C-H bonds and, consequently, an alkane. Signup and view all the answers

Consider the reduction of 2-chloropropane to propane. Which of the methods outlined could be used to achieve this transformation efficiently?

Reduction of 2-chloropropane to propane can be achieved using zinc/hydrochloric acid (Zn/HCl) or sodium borohydride ($NaBH_4$). Signup and view all the answers

What key structural feature must a compound possess for it to be a suitable substrate for catalytic hydrogenation, and why is this feature necessary?

The compound must contain a carbon-carbon multiple bond (alkene or alkyne). This unsaturation allows hydrogen atoms to add across the multiple bond, forming a saturated alkane. Signup and view all the answers

How does the Clemmensen reduction differ fundamentally from the direct hydrogenation of alkenes or alkynes in terms of the functional groups it targets?

The Clemmensen reduction converts carbonyl groups (aldehydes and ketones) into methylene groups ($CH_2$), while hydrogenation directly reduces carbon-carbon double or triple bonds to single bonds. Signup and view all the answers

If you wanted to convert propanal (an aldehyde) to propane, which method would you use? Explain the reaction in brief.

The Clemmensen reduction would be appropriate. Propanal would react with amalgamated zinc and concentrated hydrochloric acid, converting the carbonyl group to a methylene group, thus forming propane. Signup and view all the answers

In the reduction of alkyl halides with sodium borohydride, what is the role of the borohydride reagent, and what type of reaction does it facilitate?

Sodium borohydride ($NaBH_4$) acts as a reducing agent. It provides hydride ions ($H^−$) that replace the halide ion on the alkyl halide, leading to the formation of an alkane. Signup and view all the answers

Flashcards

Reduction of alcohols

Reducing alcohols, alkyl iodides, aldehydes, ketones, and carboxylic acids using red phosphorus and hydroiodic acid yields alkanes.

Grignard reagents

Chemical compounds formed from alkyl halides and magnesium, used to generate alkanes upon hydrolysis.

Decarboxylation

Elimination of carbon dioxide from carboxylic acids to form alkanes, using sodalime and heat.

Wurtz reaction

A reaction where two alkyl halides react with sodium to form an alkane with double the carbon count.