CHEM 102 Lecture Notes - Lecture 6: Elimination Reaction, Enol Ether, Regioselectivity

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10 Jul 2016

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Many aldehydes and ketones undergo substitution reactions at an alpha carbon, as shown in the following diagram (alpha-carbon atoms are colored blue). These reactions are acid or base catalyzed, but in the case of halogenation the reaction generates an acid as one of the products, and is therefore autocatalytic. If the alpha-carbon is a chiral center, as in the second example, the products of halogenation and isotopic exchange are racemic. Indeed, treatment of this ketone reactant with acid or base alone serves to racemize it. Not all carbonyl compounds exhibit these characteristics, the third ketone being an example. Two important conclusions may be drawn from these examples. First, these substitutions are limited to carbon atoms alpha to the carbonyl group. Cyclohexanone (the first ketone) has two alpha-carbons and four potential substitutions (the alpha-hydrogens). Depending on the reaction conditions, one or all four of these hydrogens may be substituted, but none of the remaining six hydrogens on the ring react.

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Detailed Reaction Procedure:

Step 1: Cinnamaldehyde + Cyclopentanone (Mix for 1 min).

Step 2: Add 10% NaOH + Water to the mixture above (Mix for 3 min).

Step 3: Add Ethanol to the mixture above (Mix for 3 min). Precipitate will form.

Step 4: Collect precipitate by suction filtration and wash it with Glacial Acetic Acid + Water (Continue suction for 5 to 10 min).

Step 5: Transfer the crude product known as BPC (or (2E,5E)-2,5-bis((E)-3-phenylallylidene)cyclopentan-1-one)) into beaker with Ethanol and heat it up until it boils (or also known as "recrystallization").

Step 6: Collect the pure BPC (crystals) by suction filtration and wash it with Ice-Cold Ethanol (Continue suction for 5 min).

Overall Reaction:

a) Provide a full mechanism for the synthesis of BPC and a full mechanism for a potential side reaction. Include all nonzero formal charges and intermediates. Please use arrows to clearly show bonds forming or breaking between atoms. *The mechanism is already shown above, however. I would just like to know if it's correct, and please still show what the side reaction would be.*

b) Please explain the difference between a "self" aldol condensation and a "mixed" aldol condensation.

c) In order to have a successful mixed aldol condensation, describe two requirements of the reaction conditions that should be met?

d) Please explain why the Î±-hydrogen atoms of aldehydes and ketones are weakly acidic (enolizable). Also show the general reaction mechanism of the deprotonation of an aldehyde or ketone using NaOH.

For each of the following, fill in the blank with the correct response. All of these fill-in-the-blank problems pertain to material covered in the sections on alkanes, alkenes and alkynes, aromatic hydrocarbons, and hydrocarbon derivatives.

a. The first "organic" compound to be synthesized in the laboratory, rather than being isolated from nature, was _ _ _ _ _, which was prepared from _ _ _ _ _.

b. An organic compound whose carbon-carbon bonds are all single bonds is said to be _ _ _ _ _.

c. The general orientation of the four pairs of electrons around the carbon atoms in alkanes is _ _ _ _ _.

d. Alkanes in which the carbon atoms form a single unbranched chain are said to be _ _ _ _ _ alkanes.

e. Structural isomerism occurs when two molecules have the same number of each type of atom but exhibit a different arrangement of the _ _ _ _ _ between those atoms.

f. The systematic names of all saturated hydrocarbons have the ending _ _ _ _ _ added to a root name that indicates the number of carbon atoms in the molecule.

g. For branched hydrocarbon, the root name for the hydrocarbon comes from the number of carbon atoms in the _ _ _ _ _ continuous chain in the molecule.

h. The positions of substituents along the hydrocarbon framework of a molecule are indicated by the _ _ _ _ _ of the carbon atom to which the substituents are attached.

i. The major use of alkanes has been in _ _ _ _ _ reactions, as a source of heat and light.

j. With very reactive agents, such as the halogen elements, alkanes undergo _ _ _ _ _ reactions, whereby a new atom replaces one or more hydrogen atoms of the alkane.

k. Alkenes and alkynes are characterized by their ability to undergo rapid, complete _ _ _ _ _ reactions, by which other atoms attach themselves to the carbon atoms of the double or triple bond.

l. Unsaturated fats may be converted to saturated fats by the process of _ _ _ _ _.

m. Benzene is the parent member of the group of hydrocarbons called _ _ _ _ _ hydrocarbons.

n. An atom or group of atoms that imparts new and characteristic properties to an organic molecule is called a _ _ _ _ _ group.

o. A _ _ _ _ _ alcohol is one in which there is only one hydrocarbon group attached to the carbon atom holding the hydroxyl group.

p. The simplest alcohol, methanol, prepared industrially by the hydrogenation of _ _ _ _ _.

q. Ethanol is commonly prepared by the _ _ _ _ _ of certain sugars by yeast.

r. Both aldehydes and ketones contain the _ _ _ _ _ group, but they differ in where this group occurs along the hydrocarbon chain.

s. Aldehydes and ketones can be prepared by _ _ _ _ _ of the corresponding alcohol.

t. Organic acids, which contain the _ _ _ _ _ group, are typically weak acids.

u. The typical sweet-smelling compounds called _ _ _ _ _ result from the condensation reaction of organic acid with an _ _ _ _ _.

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Detailed Reaction Procedure:

Step 1: Cinnamaldehyde + Cyclopentanone (Mix for 1 min).

Step 2: Add 10% NaOH + Water to the mixture above (Mix for 3 min).

Step 3: Add Ethanol to the mixture above (Mix for 3 min). Precipitate will form.

Step 4: Collect precipitate by suction filtration and wash it with Glacial Acetic Acid + Water (Continue suction for 5 to 10 min).

Step 6: Collect the pure BPC (crystals) by suction filtration and wash it with Ice-Cold Ethanol (Continue suction for 5 min).

Overall Reaction:

a) There are 9 geometric isomers (i.e. âcisâ and âtransâ or âEâ and âZâ isomers) of BPC; however in this reaction, the product mixture is composed of one major geometric isomer of BPC. Clearly draw the major geometric isomer of BPC; then draw any two of the other 8 geometric isomers. Briefly describe why the formation of one geometric isomer predominates over the formation of the other 8 geometric isomers.

b) Ketones with Î±-hydrogens can undergo a process called, "keto-enol tautomerism". Please answer the following sub-parts regarding this process.

I) Draw the keto-enol tautomers of cyclopentanone.

II) Which of the two tautomers is more stable? Why is it more stable?

III) Please show the mechanism for the formation of keto-enol tautomers of cyclopentanone in the presence of an acid.

c) The last step of the formation of BPC involves dehydration of the aldol by elimination of a hydroxide ion; which is generally considered a poor leaving group. Please justify why the elimination of OH^â occurs in this reaction.

CHEM 102 Lecture Notes - Lecture 6: Elimination Reaction, Enol Ether, Regioselectivity

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