Most people are familiar with the idea that oil and water do not mix. Crude oil is a complex mixture of mostly very long chain alkanes. During the refining process, the long alkane chains of crude oil are broken into shorter chains using heat and catalysts, then purified by distillation at refineries on the basis of boiling point into fractions such as gasoline (boiling point fraction between 100°C and 400°C) or diesel fuel (boiling point fraction between 180°C and 360°C). During the Deepwater Horizon oil rig disaster a little over two years ago, between 17 and 39 million gallons of crude oil spilled into the Gulf of Mexico. Thousands of birds, turtles and sea mammals died because they were covered by oil. Well over 125 miles of coastline were affected. The major cause of the destruction is that the oil was not simply diluted by sea water, but rather, it floated on the surface in large slicks that were blown by prevailing winds until they reached a shore. What is worse, animals that need to breath air such as turtles and sea mammals (dolphins, whales, manatees) became trapped under the oil slick and had to surface into it to breathe, sealing their doom. Unsuspecting birds landed on the surface of the oil slick, becoming covered in sticky oil that prevented them from flying away.
As mentioned in the passage, when crude oil is refined, heat and catalysts break the long carbon chains into much shorter ones. The resulting mixture is distilled and fractions with specific boiling point ranges are collected together and sold according to their intended use such as jet fuel, gasoline, heating oil, etc. Based upon what you know about the boiling points of branched vs. straight chain alkanes, which statement do you think will NOT be true as you analyze what actual molecules are in each fraction isolated based on similar boiling points.
A. The fractions with higher boiling points will generally have larger alkane molecules (higher molecular weights).
B. In a given fraction, the branched molecules will on average have a higher molecular weight compared to the straight chain molecules.
C. In a given fraction, the branched molecules will on average have a lower molecular weight compared to the straight chain molecules.
D. The fractions with lower boiling points will generally have smaller alkane molecules (lower molecular weights).
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