L6: Alkanes and Their Properties

Alkanes are called saturated hydrocarbons. Why?

• They only have single bonds.

• They contain double bonds.

• They are aromatic compounds.

• They have alternating single and double bonds.

Answer explanation

• (Answer: A – Alkanes only have single bonds, meaning they are "saturated" with hydrogen atoms.)

Fun Fact: The term "saturated" means that alkanes can't accommodate more hydrogen atoms without breaking their structure, making them chemically stable.

Methane is the simplest alkane. What is its molecular formula?

• C2H6

• CH4

• C3H8

• C4H10

Answer explanation

• (Answer: B – Methane’s molecular formula is CH4​.)

Fun Fact: Methane is not only a fuel but also a greenhouse gas, with 25 times the warming potential of carbon dioxide over 100 years

Alkanes form the basis of petroleum fractions. Which fraction is most valuable?

• Alkanes with 1-4 carbons.

• Alkanes with 5-12 carbons.

• Alkanes with 13-20 carbons.

• Alkanes with 21+ carbons.

Answer explanation

• (Answer: B – Alkanes with 5–12 carbons are highly valued as fuels like gasoline and kerosene.)

Fun Fact: Gasoline is primarily composed of octane, a key alkane that determines the fuel's quality.

Boiling points of alkanes depend on their structure. How does branching affect boiling point?

• Increases it.

• Decreases it.

• No effect.

• Depends on the molecule's size.

Answer explanation

• (Answer: B – Branching decreases boiling points due to reduced surface area and weaker intermolecular forces.)

Fun Fact: Isobutane, a branched alkane, boils at -11.7°C, whereas normal butane boils at -0.5°C.

Alkanes can form rings. What is the formula for cycloalkanes?

• CnH2n+2

• CnH2n

• CnH2n−2

• CnHn

Answer explanation

• (Answer: B – Cycloalkanes have the formula CnH2n.)

Fun Fact: Cyclohexane, a common cycloalkane, adopts a chair conformation to minimize strain, which is also found in glucose rings.

"Cracking" helps refine petroleum. What does cracking do?

• Combines smaller alkanes.

• Splits larger alkanes into smaller ones.

• Converts alkanes into alcohols.

• Purifies alkanes by filtration.

Answer explanation

• (Answer: B – Cracking breaks large alkanes into smaller, more useful molecules.)

Fun Fact: Catalytic cracking is widely used to produce gasoline and jet fuel, reducing waste and increasing efficiency.

Newman projections help visualize alkanes. What do they show?

• Stability of aromatic rings.

• Electron density of alkanes.

• Conformations around single bonds.

• Molecular resonance structures.

Answer explanation

• (Answer: C – Newman projections depict conformations of alkanes around single bonds.)

Fun Fact: The Newman projections, a tool used to visualize the conformation of molecules, play a key role in understanding how scents interact with our sense of smell? Perfume molecules often have flexible carbon chains that rotate around single bonds, creating different conformations. These conformations influence how the molecule binds to olfactory receptors in your nose, determining the specific scent you perceive. The "staggered" and "eclipsed" arrangements of atoms, visualized through Newman projections, help chemists design perfumes with desired aromas and long-lasting effects.

Cyclohexane has multiple conformations? Which is the most stable?

• Boat

• Chair

• Half-chair

• Twist-boat

Answer explanation

• (Answer: B – The chair conformation is the most stable due to minimized strain.)

Fun Fact: Did you know the chair conformation of cyclohexane is the reason why glucose is so stable in your body? Glucose, the primary energy source for living organisms, adopts a chair-like structure in its cyclic form. This conformation minimizes strain and maximizes stability, making it ideal for biological processes. In polysaccharides like cellulose (found in plants) and glycogen (stored in your body), the chair conformation ensures these molecules remain strong and stable under various conditions. It's why cellulose can form tough plant cell walls, and glycogen can efficiently store energy in your muscles and liver.

Butane can take up several conformations. Which is of the following is the highest energy conformation for butane molecule?

• Methyl-methyl gauche

• Methyl-methyl eclipsed

• Hydrogen-hydrogen eclipsed

• Methyl-methyl anti

Answer explanation

• (Answer: B – Methyl-methyl eclipsed contributed the highest energy to butane conformation.)

Fun Fact: Did you know the methyl-methyl eclipsed conformation, which creates significant torsional strain, is a key concept in optimizing fuel performance? In hydrocarbons like octane (used in gasoline), the arrangement of methyl groups impacts the molecule's stability and how efficiently it burns.

When methyl groups are in the eclipsed conformation, their close proximity increases repulsion, raising the molecule's energy. This instability is less favorable for efficient combustion. Understanding and minimizing such strain during refining processes allows engineers to create high-octane fuels that burn more cleanly and efficiently in your car engine.

Substituents on cyclohexane prefer to stay at equatorial rather than axial position. What is the disfavor energy in axial position?

• Angle strain

• 1,3-Diaxial interaction

• Torsional strain

• Gauche interaction

Answer explanation

• (Answer: B – 1,3-DIaxial interaction. Even the smallest atom such as fluorine would like to be at equatorial position.)

Fun Fact: Did you know the 1,3-diaxial interaction explains why some fats feel smooth and others feel waxy? In fats and oils, long carbon chains can form different arrangements. When bulky groups (like methyl or longer alkyl chains) are positioned axially in cyclic structures, they experience 1,3-diaxial interactions, increasing strain and rigidity.

Coconut oil, rich in short and flexible carbon chains, minimizes such interactions, making it feel smooth and easy to spread. In contrast, butter, with more complex and rigid fatty acids, has higher 1,3-diaxial strain in its structures, giving it a firmer and waxy texture.