Introduction to Organic Chemistry: Hydrocarbons and Isomerism

1. Chapter 9

2. We earlier defined a class of compounds called hydrocarbons (containing C and H and nothing else). Hydrocarbons form the backbone of an important area of chemistry called Organic Chemistry. More than 95% of all known substances are organic substances.

3. Original Definition – Any substance formed from living organisms or once living organisms. It was believed that there were organic and non-organic (inorganic) substances. Each were completely separate. No crossover. • Then in 1828, a chemist, named Friedrich Wohler showed that an organic substance could be made from inorganic substances and since then we know that this is common. A different definition was needed. • Modern Definition – Any substance containing C ( except CO, CO2, substances containing CO3-2 or CN-1). Often called Carbon Chemistry.

4. The uniqueness of C chemistry, is that C has 4 valence electrons and always forms exactly 4 covalent bonds, (not necessarily to 4 atoms; there can be double and/or triple bonds) but unlike N, O and F, C cannot form C2 molecule (a quadruple bond would be required and this does not happen.

5. The result is that when C bonds to itself in compounds there is always at least one electron left over to bond to other atoms, most commonly H but also frequently O, N and S. This results in C chains, sometimes extremely long containing thousands of C atoms. • Basic Structure and Nomenclature – There are so many organic compounds, that it is convenient to break down the many compounds into smaller groups with similar properties. The largest breakdown is into 2 major classes: • hydrocarbons and substituted hydrocarbons.

6. Hydrocarbons are further broken down into several types: 1. Alkanes – Every C atom has 4 single covalent bonds connected to it, either to other C atoms or to H atoms. Every bond is a single bond. Alkanes are said to be saturated, because there is no room for any more atoms to bond to the C atoms. We will explain this in more detail later. 2. Alkenes – Contains one or more C=C double bonds 3. Alkynes – Contains one or more CC triple bonds 4. Aromatics – We will come back to these later

7. It is also important to know more than the molecular formula for organic compounds. We also need to know the structure, or what atom is bonded to what atom and where in space. Let’s illustrate this with a couple of examples:

8. Before continuing our discussion of organic compounds, we need to understand another concept that we haven’t discussed so far; isomerism. • Isomers are 2 or more compounds with the same molecular formula but different structures or shapes. The most common are what are called structural isomers. There are 2 structural isomers of C4H10: • These actually have different names: butane and 2-methyl propane.

9. Also note, that the structures above look different than the first structures we saw. This is a condensed structural formula, just showing where the carbons are, since any H atoms must be bonded to the C atom using single covalent bonds. • These can be further condensed by eliminating all bond lines, as seen in Table 9.2 on page 226. • Also listed there are the names of the first 10 (containing 1 through 10 C atoms) alkane names, assuming all C atoms in one long continuous chain. • Also the number of possible isomers is listed for each. Note how that number increases dramatically as the number of C atoms increases.

11. Alkanes are the simplest hydrocarbons and the simplest of these have all the C atoms bonded to each other in a straight chain. We need to learn how to name hydrocarbons and the first step is to learn the names of these straight chained hydrocarbons. • You will need to know the names of these 10 basic alkanes.

12. The following set of rules for naming alkanes is not in your book.

13. Naming Alkanes In order to name alkanes correctly we need to know how to name a branch, such as the CH3 in the second structure above. These are called alkyl groups:

15. We only need to learn two of these for this course: • methyl • Ethyl

16. Nomenclature (naming) of Alkane Rules: 1) Find the longest continuous chain of C atoms. As long as you can pass from one C atom to the next by travelling along C to C bonds, then these C atoms are part of one chain. You should be able to trace along the longest continuous C chain without retracing any part of the line. Note that the longest continuous C chain does not have to be in a straight line. 2) Count the # of C atoms in the chain and give the compound the appropriate alkane name.

17. 3) Locate any alkyl branches. Number the C atoms in the chain, starting from the end that will result in having the branches on the lowest possible C atoms. 4) Number the position of the alkyl branches, name them in alphabetical order and use prefixes "di", "tri", "tetra", "penta", etc to denote more than one of a particular alkyl group. NOTE: Do not use these prefixes for alphabetizing purposes.

18. Some Examples:

19. 5) Note that hyphens separate letters from numbers and that commas separate numbers and there are no spaces between letters. Also, all branches are named first, followed by the primary chain. REVIEW NOTE: There are several ways to draw structures. The above examples illustrate 2 of them. The first 3 give the most detail but take the most time. The last example does not show the individual C-H bonds. Sometimes we will write structures with no H's written in, since it is assumed everyone can fill in the H's so that every C atom has 4 bonds. This would be the easiest and fastest.

20. Alkenes are very similar to alkanes except that they are unsaturated while alkanes are saturated. • C always has 4 covalent bonds attached to it(remember the octet rule). If there are 4 atoms bonded also, then each bond must be single and it is impossible to fit any more atoms bonding to it. Thus if all the C atoms have 4 atoms bonded to them, no more atoms can be added to the molecule and it is said to be saturated. • If there are any multiple bonds, such as in alkenes, then the C’s with the multiple bond will have less than 4 atoms bonded to them, and it is possible to break the second and/or third bond of these multiple bonds, and form all single bonds. • This would require adding other atoms to the molecule. Thus alkenes can fit more atoms in and are said to be unsaturated.

21. This is the cause for the other difference between alkenes and alkanes. Alkenes are more chemically reactive than alkanes.

22. Sometimes, instead of chains, C atoms will bond to each other to form a geometric shape (triangle, square, etc). These are called cyclic compounds. There are cycloalkanes, cycloalkenes and even cycloalkynes (although rare).

23. Aromatic Hydrocarbons: - Cyclic hydrocarbon that has the equivalent of several C=C double bonds. However, instead of the normal arrangement for the second bond between 2 atoms, in aromatic compounds all the C atoms forming the ring share all of the electrons in the second bonds of all the double bonds. This is called delocalization, which gives significant extra stabilization energy for this structure, called delocalization energy.

24. The important net effect is on the chemical behavior of aromatic hydrocarbons. The ring structure almost always stays intact, and it behaves like a saturated compound rather than the very unsaturated compound, that it is.

25. To indicate that all of the extra electrons are shared equally by all the C atoms in the ring, the multiple bonds are represented most commonly by a circle inside the ring, such as benzene, which is the simplest aromatic hydrocarbon, C6H6: Each corner represents a C atom and 1 H atom is bonded to each C atom:

26. The second large class of organic compounds, the Non-hydrocarbons are also called substituted hydrocarbons. • They are called this because in these compounds an element or group of elements replaces one or more of the hydrogens to form different classes of compounds. • These new classes are identified and their properties are controlled by these replacements, which are called Functional Groups.

27. A list of many of these functional groups and their class of compounds is found in Table 9.4 on page 236.

28. Functional Group Name Hydroxyl (OH) ( ) } carbonyl carboxyl (COOH) (COOR)