Alkenes

Four Classes of Hydrocarbons

General formula: CnH2n+2 CnH2n CnH2n-2 none

• How many degrees of unsaturation are there in the molecule of prizmane?

1. 12. 23. 34. 45. 5

An Introduction to Alkenes • Draw the skeletal structures of all hydrocarbons with the formula C4H8.

What can we note about these structures?

• Some of the above structures contain a C=C double bond. What orbitals are involved in a C=C double bond?

• What do we call different molecules that have the same molecular formula?

Naming Alkenes • Each of the following names describes an alkene. However, these names are not the

correct names for these molecules. Draw the skeletal structure of the molecule and provide a correct systematic name.

• 4-ethyl-4-pentene

• 2-vinylpropane

• allylethane

Naming Cycloalkenes

• Carbon atoms of the ring double bond are numbered 1 and 2 in the direction that gives the substituent encountered first the smaller number

• Cis,trans isomerism• In small-ring cycloalkenes, the configuration of the double bond is cis

• Rings are not large enough to accommodate a trans double bond

• Trans-cyclooctene is the smallest trans cycloalkene that has been prepared in pure form and is stable at room temperature

• Is chiral even though it has no chiral center• The cis isomer is more stable than its trans isomer by 38 kJ (9.1 kcal)/mol

Naming Cycloalkenes

CyclohepteneCyclohexeneCyclopentene

H

H

H

H

H

H

H

HCyclobutene

Dienes, Trienes, and Polyenes

• Vitamin A is a biologically important compound for which a number of cis,trans isomers are possible

• There are four carbon-carbon double bonds in which cis,trans isomerism is possible

• 24 = 16 stereoisomers are possible

The Problem with Cis and Trans

• How would you name the following alkene?

• We need better rules: The E/Z rules for naming alkanes require that we determine the priority of the four groups attached to the alkene, as follows:

Step 1. Greater atomic number = higher priority.If atomic number is the same, heavier isotope = higher priority. If there is a tie, go on to Step 2.

Step 2. If the atoms are the same, consider the atoms that are attached. (For carbon, there will be 3 other atoms). List those atoms in descending order of priority, and make a pairwise comparison between the two sets. One “wins” at the first point of difference. If there is a tie, go on to Step 3.

Step 3. If the two sets of atoms are the same, follow the path of highest priority and move one atom away along each path. Then go back to Step 2, but with the new atoms. Continue in this fashion until one of the branches ultimately “wins” over the other.

Step 4. If the high-priority groups are on the same side of the double bond, then the configuration is Z (German “zusammen” = together). Otherwise, the configuration is E (German “entgegen” = opposite).

Systems for Designating Configuration in Alkenes - Cis,Trans System

• Configuration is determined by the orientation of atoms of the main chain for complex alkenes

• Determine whether the double bonds in the following alkenes has the E or Z configuration:

• What is the IUPAC name for the following compound?

1. (E)-2,5-dimethylhex-2-ene2. (Z)-2,5-dimethylhex-2-ene 3. 1-isobutyl-2-methylprop-1-ene4. 1-isopropyl-2-methylbut-2-ene5. 2,5-dimethylhex-2-ene

• What is(are) the double bond configuration(s) in the following compound?

1. E2. Z3. E, Z4. E, E5. Z, Z

NC

CN

COOH

H2N

OH

Reaction Mechanism

Developing a Reaction Mechanism

• Chemists, by combining experience and intuition, propose several sets of steps or mechanisms, each of which might account for the overall chemical transformation

• Next, they test each proposed mechanism by designing and carrying out experiments that provide experimental observations that will allow them to exclude those mechanisms that are not consistent with the facts

• A mechanism becomes generally accepted by showing that it is consistent with every test that can be devised

• Does not mean that a generally accepted mechanism is a completely accurate description of the chemical events, but it is the best that chemists have been able to devise

• It is important to keep in mind that as new experimental evidence is obtained, it may be necessary to modify a generally accepted mechanism or even discard it in favor of an alternative

Reaction Mechanisms

• Describe:• How a reaction occurs• Which bonds are broken and which new ones are formed• The order and relative rates of the various bond-breaking and bond-

forming steps• The role of the solvent if the reaction takes place in a solution• The role of a catalyst, if any• The position of all atoms and energy of the entire system during the

reaction

• Why do chemists go to the trouble to establish them, and why must students spend time learning about them?

• Mechanisms provide a framework within which to organize a great deal of descriptive chemistry

• For example, with information on how reagents add to a particular alkene, it is possible to generalize how they might add to other alkenes

• Intellectual satisfaction is derived from constructing models that accurately reflect the behavior of chemical systems

• To a creative chemist, a mechanism is a tool to be used in the search for new information and new understanding

• Mechanism that is consistent with all that is known about a reaction can be used to make predictions about chemical interactions as yet unexplored, and experiments can be designed to test these predictions

Additional Terminologies Used in Discussing Reaction Mechanisms

• Nucleophile• From the Greek meaning nucleus loving, seeking a region of low

electron density• An electron source that can donate a pair of electrons to form a new

covalent bond; alternatively, a Lewis base • Electrophile

• From the Greek meaning electron loving, seeking a region of high electron density

• An electron sink that can accept a pair of electrons to form a new covalent bond; alternatively, a Lewis base

Writing Reaction Mechanisms

• There are a surprisingly small number of different mechanistic patterns of individual steps, even for complex reaction mechanisms

• Following are the most common patterns:• Pattern 1: Make a new bond between a nucleophile (source for the electrons) and an

electrophile (sink for the electrons)

Writing Reaction Mechanisms

• Pattern 2: Break a bond to create relatively stable molecules or ions

Writing Reaction Mechanisms• Pattern 3: Add a proton

• Pattern 4: Take a proton away