Boiling points and hydrocarbon structures
The boiling points of organic compounds provide important clues to other physical properties.

A liquid boils when its vapor pressure equals atmospheric pressure. The vapor pressure is determined by the kinetic energy of molecules. The kinetic energy is related to temperature and the mass and velocity of the molecules. When the temperature reaches the boiling point, the average kinetic energy of fluid particles is sufficient to overcome the attractive forces that hold the molecules in the liquid state. Subsequently, these molecules are separated from the liquid forming the gas state.

The vapor pressure is caused by a balance between molecules in a gaseous state and molecules in a liquid state.

Beginning of boiling
The boiling point is a rough measure of the amount of energy required to separate a liquid molecule of their closest neighbors.

The molecular weight and trends of the hydrocarbon chain in the boiling points
A series of alkanes shows the general principle that boiling points increase with increasing molecular weight or length of the string.

Focus on oil and fossil fuels
Fractional distillation of petroleum (oil refining) is the process of separation of the compounds present in many crude oil. The principle used is that the longer the carbon chain, the higher the temperature at which boiling these compounds. Crude oil is heated and becomes a gas. The gases pass through a distillation column cools as altitude increases. When a compound in the gaseous state cools below its boiling point, it condenses into a liquid. The liquid can be extracted from the distillation column at different heights.

Although all fractions of petroleum, the greatest demand is for gasoline. A barrel of crude oil contains only 30-40% gasoline. Transportation demands require that over 50% of crude oil becomes gasoline. To meet this demand some petroleum fractions must be converted to gasoline. That can be achieved by “cracking.”

The “cracking” is decomposition of large molecules of heavy heating oil, “reform” – changing the molecular structures of molecules of low quality gasoline, or “cure” – and the formation of smaller molecules.

For example, if pentane is heated to about 500 C, the carbon-carbon covalent bonds begin to break during the cracking process. Many types of compounds including alkenes are made during the cracking process. Alkenes are formed because there are not enough hydrogen atoms to saturate all membership positions after the carbon-carbon bonds are broken.

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