More on Hydrocarbons

Isomers: Simple definition: Different compounds with the same molecular formula

Isomers

Constitutional isomers(connectivity differences)

Stereoisomers(configuration differences)

Enantiomers Diastereomers

Geometric Optically active Meso

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:C4H10: n-butane -138 0

isobutane -159 -12

C5H12 : n-pentane -130 36isopentane -160 28neopentane -17 9.5

Alkenes:C4H8 : 1-butene -185 6.3

cis-2-butene -139 3.7trans-2-butene -106 0.9

C5H10 : 1-pentene -165 302-methyl-1-butene -138 31

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 - alkenesH

Cl

H

Cl

H Cl

HCl

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

H OH

OH H

H H

OH OH

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. 309 alkanes p. 311 cycloalkanes p. 326 alkyl halides p. 340 alkenes p. 356 alkynes p. 359 aromatics

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 sp3

sp2

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 matterwhere 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 delocalizationAnd 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.

C

C

C

C

C

C

Some aromatic structures to recognize (associate name with them)

1. Benzene2. Phenol3. Toluene4. Aniline5. Benzoic acid

OH CH3 NH2 C

O

OH

phenol toluene aniline benzoic acid

Other aromatic structures:Polycylic aromatics and heteroatomic aromatics

Naphthalene

Anthracene

Phenanthrene

N

NHN

N N

N

Purine Pyrimidine

Building blocks of DNA, RNA:

Note on Definitions

Aliphatic Aromatic

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

H3C CH3

CH3

OH

CH3 CH3

Vitamin A (trans-retinol)

O

CH3

H3C

HO

CH3

CH3 CH3 CH3 CH3

CH3

Vitamin E (-tocopherol)

What about the electrons in this molecule?

Overlap of p-orbitals:Conjugated?Delocalized?

Aromatic?

O

OCH3

Vitamin K2

What about these electrons:

Overlap of p-orbitals:Conjugated?Delocalized?

Aromatic?