Unsaturated Hydrocarbons

Hydrocarbons that have double or triple bonds between carbon atoms are called unsaturated hydrocarbons; they are unsaturated in the sense that more hydrogen atoms can be added when H₂reacts across the double or triple bonds.

Virtually free rotation exists about a carbon-carbon single bond A methy group (CH3-) can spin like a top about the single bond joining it to another atom.

In contrast, a molecule such as ethylene cannot be twisted about one of its double bonds without breaking the second bond of the double bond. Double bonds are impor­tant in defining the geometry of many biologically important molecules, and in helping to make them rigid.

As was mentioned previously, saturated hydrocarbons are called alkanes, and iden­tified by the suffix "ane" in the series methane, ethane, propane, butane, pentane, and hexane, which have one through six carbon atoms, respectively.

Unsaturated hydrocarbons, which have double bonds, are called alkenes and have similar names ending with the suffix "-ene", as in ethene (СД), propene (СД), butene, pentene, and hexene. Ethene, propene, and butene are commonly known as ethylene, propylene, and butylene.

Many oxidations and other useful chemical reactions are spontaneous (accompanied by a decrease in free energy) and exothermic (accompanied by a decrease in enthalpy) and hence are useful as energy sources; yet the reactions often are extremely slow.

We must distinguish between spontaneity and rapidity in chemical reactions. Spon­taneous reactions eventually will take place without outside help, but they may take from a microsecond to a billion years to occur.

At room temperature hydrocarbons are spontaneously oxidizable with 0^, but are inert. Heat is required to trigger a reaction.

If an initial heat supply is provided to start the process, then the heat given off by oxidation is enough to keep the reaction going.

Once ignited, combustion is self-sustaining thereafter. A high temperature is needed to overcome the high activation energy (E_a) of the reaction.

Alkanes are relatively unreactive; the term "paraffins" often applied to them means "little affinity."

Saturated hydrocarbons undergo few other reactions. The halogen derivatives are not important for their own sake, but because they are a bridge to other, more useful

compounds.

Once formed, halogen compounds can react to form alcohols, acids, amine bases, and other types of molecules.

Unsaturated hydrocarbons are considerably more reactive than alkanes; their reac-tions take place at moderate temperatures with the help of catalysts.

The Achilles’ heel of alkanes is the double bond, and the main alkene reaction is the addition across this bond of a variety of reagents.

Notice that only one product is formed when an asymmetric reagent such as HCor H2O is used. 2-Chloropropane is formed to the exclusion of 1-chloropropane.

Some plants synthesize the all-trans isomer of polyisoprene, known as guttapercha. Guttapercha is hard and horny rather than rubbery, because the orderly trans-polyisoprene chains can pack next to one another easily in crystalline regions within the polymer.

Simple polymerization of isoprene in the laboratory yields a mixture of cis and trans bonds. More subtle methods of polymerization had to be perfected before a de­pendable method of making a pure cis polymer was developed in 1955.

As we saw first, 1,3-butadiene is a planar molecule because its double bonds are delocalized along the entire four-carbon chain.

Although the conventional representation shows double bonds between the end pairs of carbon atoms, resonance structures can be drawn in which the central two car­bons are double-bonded, and the electron pair of the other double bond is either given to one of the two outside carbon atoms or divided between them.

Twisting about the central carbon-carbon bond is as restricted as twisting about either of the outer carbon-carbon bonds, so all ten atoms of the 1, 3-butadiene molecule are constrained to he in one plane.

Just as three electron pairs are delocalized around six carbon atoms in a benzene ring, so two electron pairs are delocalized along the four-carbon chain of butadiene.

Delocalization can occur whenever single and double bonds alternate along a chain of atoms, so that after all single bonds are formed, each atom along the chain has an unused p orbital and one unused electron. Such molecules are termed conjugated.