Organic compounds are traditionally defined as compounds found in nature that contain the element carbon (C). The definition has been expanded to include man made compounds. The purpose of this section is to introduce organic compounds because they are found in so many areas; biology, medicine, foods, construction, and clothing. They also provide an opportunity to reinforce bonding and hybridization concepts introduced earlier and to relate these to the structure and physical properties of compounds.
Organic compounds can have a wide array of structures including linear, branched, and ring (cyclic) molecules.
Organic compounds are grouped according to bonding and structure. We begin with a group of organic compounds called alkanes. These compounds contain only carbon (C) and hydrogen (H). Every carbon in an alkane has sp3 molecular orbitals. Hopefully by now, you will associate sp3 hybridization with a tetrahedral shape. Recall the bonding in methane that is the smallest alkane since it contains only one (1) carbon. Other members of this group contain more carbon atoms. Organic compounds that are composed of only the elements carbon (C) and hydrogen (H) are also called hydrocarbons.
See Category: Alkanes for a list alkanes. Notice the formula indicates the number of carbon atoms and the number of hydrogen atoms in each alkane compound. Also take time to look at the names of each of the alkanes. With the exception of the first four alkanes, the names indicate the number of carbon atoms in that alkane. If you continue in chemistry, you will encounter the first 10 alkanes many times. In fact you might commit to memory these first 10 formulas and names. It will be useful in the future.
We begin with 3D structures of some representative normal alkanes. The adjective normal refers to the carbon atoms being bonded in a chain-like fashion. See Organic Molecular models (alkanes) for models of ethane (C2H6), butane (C4H10), decane (C10H22)
Click on the link Structure and Nomenclature of Hydrocarbons and scroll down to the Rotation section. Read this section regarding the motion of molecules.
Alkane molecules are constantly in motion. The molecules are tumbling in space, atoms within the molecule are vibrating back and forth, and atoms in the molecule are rotating about single bonds. Even though we are discussing organic compounds, this is true for all compounds. When you are presented with a formula, a structure, or a 3D model of a compound, realize that molecule has motion. It is not static.
What effect does the more linear shape of normal alkanes have on the physical properties of different alkanes?
Open a second window in your browser, copy, and paste http://en.wikipedia.org/wiki/Alkane in the address bar. Read the introduction and then scroll down to the Physical property section that has a table of alkanes and some of their physical properties.
First look at the name of the alkanes and notice the correlation between the name and the number of carbons in the formula in the second column. Although it is not shown, the gram formula weight of the alkanes increases with an increase in number of carbons in the formula. Also the length of the linear chain increases with an increase in the number of carbons in the formula. So this data displays the effect of mass and size of the alkane molecules on the physical properties of boiling point, melting point, and density of the compounds.
As was discussed in Covalent Compounds (Basics) any compound whose molecules have symmetry is considered to be a non-polar covalent compound with very small interaction between neighboring molecules and hence would be a gas at room temperatures. This is reflected in the first four alkanes: methane, ethane, propane, and butane. Even though they are gases, there is a large difference in the boiling points of the four compounds. The increase in boiling point is due to the increase in mass and the size of the molecules from methane to butane. Larger molecules provide more space in which bonding electrons could move and as result instanteous dipoles will form. The result is a greater attraction between molecules of larger mass and size. See Amination of intermolecular bonding.
The linear shape of alkanes facilitates van der Waals (London force) interaction by allowing neighboring molecules to get closer together. As can be seen from the data, this phenomenon continues as the mass and size of the alkane increases. When an alkane molecule has 20 or more carbons, the compound has enough van der Waals (London force) between molecules to be a solid. Paraffin is an example.
In the Covalent Compounds: (Hybridization) section, data was presented regarding solubility of compounds as an indicator of whether a compound has polar or nonpolar bonding. None of the normal (linear) alkanes are soluble in water and are classified as nonpolar compounds.
Alkanes can be branched. You can think of these compounds as smaller alkane entities attached to longer alkane entities. See examples at Organic Molecular models alkane 2 ( http://chemclass-ol.org/organic-molecular-models-alkanes-2/
Every carbon in each of the three branched alkanes shown above has sp3 hybridization. Hence each of the C atoms in the molecules of the compounds has a tetrahedral symmetry.
Look at BP of branched and their properties
The compound pentane is more linear. The branching in the other alkanes prevents the molecules from getting close to each other. Hence the lower boiling points. You can see this in the compounds shown at Organic Molecular models alkane 2
The remaining sections are presented to provide a glimpse at the wide range of bonding and structures found in organic compounds.
These are alkanes that form cyclic structures. See Organic Molecular models alkane 2 and scroll down to the cycle to the last model, cyclopentane.
Alkenes are organic compounds that contain at least one carbon carbon double bond. An example would be 2-pentene (C5H10).
The two carbon atoms making up the double bond have sp2 hybridization and are trigonal planar. See Orbital hybridisation and 3 Groups/Pairs Around the Central Atom. The remaining carbon atoms have sp3 hybridization. The double bond also prevents rotation from occurring about the bond.
Alkynes are organic compounds that contain at least one carbon carbon triple bond. An example would be 2-pentyne (C5H8).
The two carbon atoms making up the triple bond have sp hybridization and are linear. See Orbital hybridization. The remaining carbon atoms have sp3 hybridization. The triple bond also prevents rotation from occurring about the bond.
See examples of alkenes and alkynes at Organic Molecular models (alkene alkyne).
Benzene (C6H6) is an interesting compound. The six carbons form a ring structure. All the carbon atoms have sp2 (planar) hybridization. Hence the benzene molecule is planar. This compound is different than cyclohexane that has sp3 hybridization. You can see the difference at Organic Molecular models (cyclic).
Cyclohexane is not planar. It has the tetrahedral shape at each C that makes each staggered from its neighbor C
You probably did not notice, but the compounds butane and 2-methylpropane have the same formula (C4H10). Even though they have the same formula they are definitely different compounds with different chemical and physical properties. When two or more compounds have the same formula but different structures, they are called structural isomers of each other. See Isomers ( en.wikipedia.org/wiki/Isomer )
The compounds pentane 2-methylbutane, and 2,2-dimethylpropane mentioned above all have the same formula (C5H12) and would be structural isomers of each other.
The last section of this organic section is a brief look at functional groups. A functional group is a set of atoms (elements) that give a characteristic chemical behavior to a molecule. A list of common functional groups can be found at common functional groups. A more complete list of functional groups can be found at functional groups.
The following table has data for three alkanes: ethane, pentane, and decane and three functional groups: alcohol, amine, and carboxylic acid. These functional groups were selected because they are found in many biological compounds. You can see 3D models of ethanoic acid (Carboxylic acid function group), ethanol (alcohol functional group) and ethylamine (amine functional group) at Display/Organic (some functional groups).
As noted in the beginning of this web page, this is an introduction. There is so much more you will learn in a college level Organic chemistry course. Hopefully this will be beneficial to the student.