Of the 10 million or so compounds that are known today, about 9 million of those are carbon containing compounds.  Nearly everything you touch contains something organic.  Gasoline, oil, most of the medicines you take, the plastics you encounter are all organic based compounds.  Needless to say, organic chemistry is a very large area of chemistry.  Considering the very large area of chemistry that organic chemistry encompasses, it might be surprising to know that organic chemistry is of of the youngest areas of chemistry, less than 200 years old!  Prior to 1828, a theory was in place called the Vital Force theory which held that organic compounds must originate from living substances, i.e., plants or animals.  During that year, a German chemist by the name of Friedrich Wohler did an experiment which totally dismantled the theory.  By heating an aqueous solution of ammonium cyanate, an inorganic compound,  he produced urea, an organic compound.  Wohler must have realized he had hit on something big, in 1835 he wrote a letter to a collegue, Jons Jakob Berzelius, where he stated ÒOrganic chemistry is like a primeval tropical forest, full of the most remarkable thingsÓ.  The field of organic chemistry continues to grow today and scientist are still discovering those Òremarkable thingsÓ Wohler refered to some 170 years ago.

 

Saturated Hydrocarbons (Alkanes)

 

A saturated hydrocarbon is a compound composed of only carbon and hydrogen and all carbon atoms have the maximum number of hydrogen atoms attached to it, i.e., 4.  Carbon is tetravalent, which means it must have 4 bonds and all of the angles between the atoms in an alkane are 109.5¡.    Picture yourself inside a three-sided pyramid and draw a line to each apex, that will give you an idea of the angles.  These four bonds can be made up of single, double, or triple bonds as long as the total is four.  With the saturated hydrocarbons, all of the bonds are single bonds.  The simplest saturated hydrocarbon is methane, CH4.  The generic formula of a hydrocarbon is CnH2n+2 where the n represents the number of carbon atoms present.

 

 

Methane (CH4)

 

 

The saturated hydrocarbons can be either straight chains or branched chain.  Some examples of straight chains are:

 

Ethane (CH3CH3)

 

A note about how we draw the formulas.  The formula is and the expanded formula of CH3CH3 which is the condensed formula.  In the expanded formula, all of the bonds are explicitly shown.  Another way to show the compound is to draw a stick formula.  The stick formula only shows the bonds between the carbon atoms and the hydrogen atoms are not drawn.  Each point on the stick formula is a carbon atom.  To determine the number of hydrogens attached to that atom, just subtract the number of bonds shown from Ò4Ó since you know the total must be four.  For ethane, the stick formula would be:

 

 

 

 

 

Propane (CH3CH2CH3)

The stick formula of propane would be:

 

 

   

Butane (CH3CH2 CH2CH3)                Stick Formula

 

 

    

Pentane (CH3CH2 CH2 CH2CH3)                   Stick Formula

 

 

      

Hexane (CH3CH2 CH2 CH2CH2CH3)                        Stick Formula

 

 

        

Heptane (CH3CH2 CH2 CH2CH2CH2CH3)                            Stick Formula

 

 

 

Octane (CH3CH2 CH2 CH2CH2CH2CH2CH3)                                   Stick Formula

 

 

 

Nonane (CH3CH2 CH2 CH2CH2 CH2CH2CH2CH3)                          Stick Formula

 

 

 

Decane (CH3CH2 CH2 CH2CH2 CH2CH2CH2CH3)                           Stick Formula

 

 

The chains can be much longer.  As the chain becomes longer the solubility of the hydrocarbon in water decreases and the boiling point increases.  The first four are gases at room temperature, the rest shown above are liquids.  The first hydrocarbon that is a solid at room temperature is icosane (C20H42).  For a list of some of the longer chain hydrocarbons, click here.

 

icosane

 

 

Branched Chain Alkanes

 

Life is not made up of straight chain alkanes.  Many alkanes have branches coming off the main chain, for example,

 

 

 

Do not confuse a branch with part of the main chain.  Because there is free bond rotation about the single bonds, groups may rotate from one position to another to give a new conformation of the same molecule.

 

and  are the same molecule, pentane.  Note that each has the formula CH3CH2 CH2 CH2CH3

 

 

Isomers

Some compounds have the same formula but different structures.  These types of compounds are referred to as constitutional isomers.  These isomers are different compounds with different properties.  Take C4H10 for example, there are two compounds than are different but have this formula.

 

 

 and

Butane                                                 Isobutane

MW = 58.123 g/mol                           MW = 58.123 g/mol

Liquid Density = 604.4 kg/m3             Liquid Density = 593.4 kg/m3

Boiling Point = -0.5¡C                        Boiling Point = -11.7¡C

Critical temperature = 152¡C   Critical temperature = 134.9¡C

Critical pressure = 37.96 bar               Critical pressure = 36.48 bar

 

As you can see, the compounds have different properties, similar but different.

 

C5H12 has three constitutional isomers.  They are

 

                                                   

 

 

 

Of course as the number of carbon atoms increase, the number of isomers increase as well.  C6H14 has 5, C7H16 has 9, C10H22 has 75, and C40H82 has 6.2 x 1013!!!

 

 

Isomer Practice:  Draw the 9 isomers of C7H16.  Click here for the answers.

 

 

Naming Organic Compounds

 

Although there are many common names for organic compounds that have been around for years, the official names of the compounds are governed by the International Union of Pure and Applied Chemistry (IUPAC system).  Naming of the compounds is very systematic, every organic compound ever made will have itÕs very own name and no two different compounds will have the same name.  When naming the straight chain alkanes, you simply use the prefix that denotes the number of successively bonded carbon atoms.  1 C = butane, 2 C = ethane, etc. (see above).  When naming branched chain alkanes, you must show what and where the branch is located.  So how do you know what the branch is?  You must learn the names of several, what we call alkyl groups.  An alkyl group has the generic formula CnH2n+1.  An alkyl group is basically any alkane with one hydrogen removed.  When naming the alkyl group, determine the alkane name, drop the –ane, and add –yl.  Here are a few of the alkyl groups you must learn to be successful at some basic nomenclature.

 

 

 

 

Methane                                                                      Methyl

 

Ethane                                                                         Ethyl

 

With methane and ethane, it would not have made any difference which hydrogen you removed.  Removing any one of the four on methane would have given you methyl while removing any one of the six in ethane would have given you ethyl.  We say that all of the hydrogens on these two compounds are ÒequivalentÓ.  Once you reach a three-carbon compound and higher, this is no longer the case.  Propane has two different types of hydrogens.

 

 

Notice that all the red hydrogens are equivilant and the blue hydrogens are equivalent but the red ones are different than the blue ones.  What this means is that we will get a different alkyl group depending on which of the two types of atoms we remove.

 

Propane                                                                                   Propyl or (n-propyl)

 

Propane                                                                                   Isopropyl

 

When we get two a four carbon alkakne (C4H10), we have two isomers, butane and isobutene.

 

 and

Butane                                                 Isobutane

 

Like propane, each of these have different types of hydrogens.  LetÕs begin with butane.

 

Butane                                                                                     Butyl or (n-butyl)

 

 

Butane                                                                                     sec-Butyl

 

 

Now for isobutane.

 

Isobutane                                                                                             Isobutyl

 

Isobutane                                                                                             tert-butyl (or t-buyl)

 

The prefixes sec- and tert- are short for secondary and tertiary.  A secondary carbon atom is bonded to two other carbon atoms while  a tertiary carbon is bonded to three.  A primary carbon atom is bonded to only one.  Primary, secondary and tertiary are often symbolized by 1¡, 2¡, and 3¡ respectively.

 

There are many more alkyl groups but if you know the eight above, youÕll be able to name a lot of compounds.  Now that we know how to identify our branches, we need to know how to show where the branch is on the chain.  Number the chain so that the branch gets the lowest possible number and that you have the longest possible chain of successively bonded carbon atoms.  The longest chain is the base name 1 is methane, 2 is ethane, 3 is propane, etc. (see above)

 

 

 

 

 

 

                                                

Correct                                                                                    Incorrect

 

 

Now you name the compound showing what the alkyl group is and where it is i.e., to which carbon atom is it bonded.  Numbers are separated by comas; letters and numbers are separated by dashes.

 

 

3-methyloctane

 

If there are more than one branch, number so the branches get the lowest possible number and name them in alphabetical order.  If there are more than one branch of the same type, use di-, tri-, tetra-, penta- to show 2, 3, 4, 5 respectively.

 

                        

4-ethyl-2-methyloctane                                    2,4,6-trimethyloctane

 

 

Click here for some practice naming alkanes.  For more practice, check out the following links.

 

Nomenclature Tutorial

Alkane Nomenclature

Rules for Naming Alkanes

 

 

Saturated Hydrocarbons (Cycloalkanes)

Not only do carbon and hydrogen atoms come together to form the open chained alkanes, they also can come together in a way to form closed chain alkanes called cycloalkanes.  Cycloalkanes have the generic formula CnH2n where n must be at least three.  When naming the cycloalkanes, simply use the prefix cyclo- before the parent name.

 

                                                                                                                   

cyclopropane               cyclobutane                 cyclopentane                cyclohexane                 cycloheptane                cyclooctane

 

 

 

If there are alkyl groups attached to the cycloalkane, name the alkyl group first followed by the parent name.

 

Methylcyclopentane

 

 

 

If there is more than one alkyl group, number so that you have the lowest possible number.

 

 

                                            

1,3-dimethycyclohexane          NOT 1,5-dimethylcyclohexane

 

 

 

If the alkyl groups are different, number the ring as above but name the alkyl groups alphabetically.

 

2-ethyl-4-methyl-1-propylcyclohexane

 

 

Cycloalkane Practice.

 

 

Unsaturated Hydrocarbons

 

Unsaturated hydrocarbons, which contain one or more double bonds, are referred to as alkenes and have the general formula CnH2n.  Those unsaturated hydrocarbons, which contain one or more triple bonds, are referred to as alkynes and have the general formula CnH2n-2.  The unsaturated alkenes and alkynes will react with H2 to form the corresponding alkane with the same number of carbon atoms.  The simplest alkene is ethane (CH2=CH2) and the simplest alkane is ethyne ().

 

 

When naming the alkenes, number the chain so that the carbon atoms of the double bond have the smallest possible number.  The longest parent chain must contain the two atoms of the double bond.  When you name the alkene, show where the double bond is by giving the first carbon atom of the double bondÕs number.  The parent name is determined just like the alkanes, but the –ane suffix is exchanged with a –ene (alkenes) or –yne (alkynes).

 

   1-Butene             NOT`           3-Butene

 

If there are branches present on the alkene, the carbon atoms of the double bond still get precedence, that is, we still number the ring so that the carbon atoms of the double bond get the lowest possible numbers.

 

 

5-Methyl-1-hexene

 

If itÕs a cyclic alkene, the carbon atoms of the double bond will always be number 1 and 2 and therefore, there is no reason to show where the double bond if.

 

 

 

                   

cyclobutene     cyclopentene    cyclohexene

 

If branches are present, number around the ring so as to give the lowest possible number.

 

 

Alkene Nomenclature

More Alkene Nomenclature