More on Hydrocarbons

Isomers: Simple definition: Different compounds with the same molecular formula Isomers Constitutional isomers (connectivity differences) Stereoisomers (configuration differences) EnantiomersDiastereomers GeometricOptically activeMeso

Constitutional or Structural Isomers So far we have drawn all carbon atoms connected like beads on a string, but… What if we rearrange how the carbon atoms are connected or what if we move double and triple bonds around? We get constitutional isomers:  same molecular formula but different structures  different chemical and physical properties (We can draw them in several ways: structural, skeleton, line.)

Comparison of physical properties of constitutional isomers: melting point (◦C)boiling point (◦C) Alkanes: C 4 H 10 :n-butane-1380 isobutane C 5 H 12 : n-pentane isopentane neopentane Alkenes: C 4 H 8 : 1-butene cis-2-butene trans-2-butene C 5 H 10 : 1-pentene methyl-1-butene-13831

Consider free rotation of bonds versus restricted rotation of bonds: Single bonds: free or restricted? Double bonds: free or restricted? Cyclic compounds: free or restricted?

Geometric isomers - alkenes Two systems to specify geometric isomers: Trans = opposite ; or cis = together : each double bond C has a hydrogen and another group attached E (entgegen) = opposite ; or Z (zusammen) = together : a more general system

Geometric isomers – cyclic compounds Cyclic compounds also have restricted rotation between carbons in the ring.

Naming organic compounds We can draw a lot of different compounds now. But what to call them? Every organic compound has a unique name. IUPAC = International Union of Pure and Applied Chemistry The IUPAC name for each compound is unique, descriptive and specific.

IUPAC Rules of Organic Nomenclature Organic compounds are named very systematically.  From the name, you should be able to draw the compound!  From the compound’s structure, you should be able to write the name! Rules for naming hydrocarbons are listed in your text.  p. 309alkanes  p. 311cycloalkanes  p. 326 alkyl halides  p. 340 alkenes  p. 356 alkynes  p. 359aromatics

The Rules for Alkanes, as listed in your text: 1. The name of the longest chain becomes the BASE, or parent name, of the compound. The suffix (ending) of the family name is added to the end of this base name. Note that, even if the subsequent rules are correctly applied, a failure to correctly identify the longest chain will result in an incorrect name for the compound.

2. The base name accounts only for the carbons in the longest chain. The carbons that are not part of the longest chain – those attached as branches to the longest chain and called substituents or groups - must also be included in the name. Substituents are included as follows: A. The name(s) of any alkyl group(s) in the compound is placed in front of the base name. B. Use the prefixes di, tri, tetra, penta, and hexa, before the name of the alkyl group when there are two, three, four, five, or six, respectively, of the same group. C. alphabetize the names of alkyl groups when there are two or more different types of groups. Ignore all prefixes (both the branching prefixes s- and t- and the multiplying prefixes such as di-, tri-, and tetra-) in alphabetizing, with one exception: iso- is not ignored in alphabetizing.

3. Number the carbons in the longest chain, starting from whichever end will result in the lowest number (or set of lowest numbers) for the alkyl group(s). An alternate rule is useful for most compounds: number from the end nearest a branch. 4. In front of the name of each alkyl group, place the number of the carbon to which the group is attached. 5. Use hyphens to separate numbers from words; use commas to separate numbers.

More on bonding: Hybridization  bonds– interactions between two s-orbitals of two atoms  Geometry of s-orbitals?  bonds – interactions between two p-orbitals of two atoms  Geometry of p-orbitals? Orbital hybridization  sp 3  sp 2  sp

Or electron density looks like this:

Benzene and Aromatic compounds How can you form a molecule with 6 carbons and 6 hydrogens fulfilling all bonding requirements for each atom?

Some proposed structures: (They seemed reasonable enough at the time)

August Kekule and his dream “I was sitting writing at my text book, but the work did not progress; my thoughts were elsewhere. I turned my chair to the fire and dozed. Again the atoms were gamboling before my eyes. …But look! What was that? One of the snakes had seized hold of its own tail, and the form whirled mockingly before my eyes….Let us learn to dream, gentlemen, and then perhaps we shall learn the truth.

We draw benzene like this and say the electrons in p-orbitals are delocalized: p-orbitals are perpendicular to the plane of the molecule. Does it matter where you draw the double bonds in this case?

How do we prove this is correct for benzene’s unique structure? Fancy words for this: Electron delocalization And aromaticity Benzene C-C bond lengths are all: 1.39 A From X-ray crystallography, we can determine the following for carbon- carbon bond lengths: Single bond length: 1.47 A Double bond length: 1.33 A Hmmm.

Some aromatic structures to recognize (associate name with them) 1. Benzene 2. Phenol 3. Toluene 4. Aniline 5. Benzoic acid

Other aromatic structures: Polycylic aromatics and heteroatomic aromatics

Note on Definitions AliphaticAromatic

What do you think about the electrons in a situation like this? Consider overlap of p-orbitals: Conjugated? Delocalized? Aromatic?

What about the electrons in this molecule? Overlap of p-orbitals: Conjugated? Delocalized? Aromatic?

What about these electrons: Overlap of p-orbitals: Conjugated? Delocalized? Aromatic?