Hydrocarbons

What are hydrocarbons?

• The simplest aliphatic and aromatic compounds
• Only contain hydrogen and carbon
• Can contain single, double, or triple bonds
• Classified by the types of bonds.

Alkanes

• Alkanes are aliphatic hydrocarbons that only contain single bonds.
• The general formula for alkanes is C_nH_2n+2
• Example: CH₄, C₂H₆, C₃H₈
• Considered saturated because carbon is surrounded by the max number of hydrogen.
• Methane is the simples alkane (CH4)
• Each consecutive alkane adds a carbon and its respective hydrogens.
• Methane: 1 carbon
• Ethane: 2 carbons
• Propane: 3 carbon
• Butane: 4 carbons
• Properties of Alkanes
• Very low melting and boiling points. They rise as carbons are added.
• Ex. methane
• Melting point (C):  -183
• Boiling point (C): -164
• Non-polar

Alkenes

• Hydrocarbons that contain double bonds between carbon atoms
• Contain the prefix -ene.
• The smallest is ethene. Why not methene?
• Considered unsaturated, because the double bond prevents the max number of hydrogen from bonding.
• Naming of alkenes requires numbering the carbons to identify the place where the double bond is.
• You start at the carbon that will give you the smallest number.
• This allows us to know where the double bond is. 
• Properties of Alkenes:
• Slightly higher melting/boiling points
• The first couple are gases at room temperature
• Relatively non polar

Alkynes

• Hydrocarbons that contain triple bonds between the carbon atoms
• Uses the prefix -yne
• The simplest is the most common, ethyne (acetylene).

Cyclic Aliphatic Compounds

• Not all hydrocarbons are open chains of carbon atoms.
• Some form a ring.
• 5 and 6 alkane rings are most abundant. 
• Some can have more than one double bond.

Aromatic Structures

• All contain a form of a molecule benzene
• They are called aromatic because they often smell good
• C₆H₆ is the simplest aromatic compound known
• It was hard to figure out the structure:
• Behaves like an alkane, but they knew from the molecular weight that it had several double and triple bonds.
• When they measured the bond length, the found that it should contain 1.5 bond lengths.
• Showed that carbon was in a ring and all had identical bonds
• In 1865, August Kekule proposed the structure. 
• He said that it was a dynamic equilibrium of the two.
• The double bonds were not “tied dow”, but are more or less shared.