Chapter 19 Aldehyde and Ketones, Nucleophilic Addition Flashcards
How are aldehydes prepared in chapter 19?
Hint: Oxidation and Reduction
1) Oxidation of primary Alcohols with Dess Martin Periodinane, in dichloromethane solvent at room temperature.
2) partial reduction of carboxylic acid derivatives, such as ester, by DIBAH (diisobutylaluminum hydride) this is usually carried out at Dry Ice Temperature -78* in Toluene.
What are the four different ways in which ketones are prepared in chapter 19?
Hint: O, O, FCA, CAD
1) The oxidation of secondary alcohols using Dess Martin Periodinane or Cr03 is one of the more common choices.
2) Ozonolysis of an alkene with at least one disubstituted unsaturated carbon yields the (C=O) Ketone
3) Freidel Crafts Acylation of aromatic compounds in the presence of AlCl3
4) Preparation from Carboxylic acid derivatives
Aldehydes can be oxidized by which reagents to yield Carboxylic Acids?
1) CrO3 (Chromium Trioxide) is used to oxidize normal aldehydes
2) KMnO4
3) Hot HNO3
Ketones are generally Inert under conditions that would oxidize aldehydes, what reagent readily causes ketones to Oxidize?
Using Strong Oxidizing Agents, such as LiAlH4, Ketones undergo oxidation.
Also, Hot alkaline KMnO4 will slowly cleave the adjacent C-C bonds to the carbonyl group. This reaction is hardly used.
Nucleophilic Addition Reactions, specifically Hydration, describe how water interacts with Aldehydes and Ketones..
1) Water adds to aldehydes and Ketones to give 1,1-diols (known as gem-diols or hydrates)
What is one structural difference between aldehydes and ketones that enables aldehydes to undergo oxidation and yet make ketones inert?
“Aldehydes are easily oxidized to yield carboxylic acids, but ketones are generally
inert toward oxidation.”
The difference is a consequence of structure: aldehydes
have a ] CHO proton that can be abstracted during oxidation, but ketones do not.
Describe the intermediate step of an aldehyde undergoing oxidation into a carboxylic acid..
intermediate 1,1-diols are formed, also known as hydrates, which is formed by a reversible nucleophilic addition of water to the carbonyl group. One of the alcohols is then oxidized back to a carbonyl compound.
the most general reaction of aldehydes and ketones is the nucleophilic addition reaction, describe this stepwise reaction in detail..
1) A nucleophile approaches the carbonyl group from the 105* opposite side of the oxygen and forms an electrophilic bond to the carbon on the C=O bond.
2) Simultaneously a sp2 to sp3 rehybridization occurs and a pair of electrons are pushed to the electronegative oxygen creating a sp3 hybridized carbon with an alkoxide.
3) protonation of the alkoxide by an acid then yields an alcohol.
Nucleophilic additions of aldehydes and ketones have two variations, describe these two variations
1) the first variation results in the tetrahedral intermediate being protonated by water or acid and yielding an alcohol.
2) in the second variation, the carbonyl oxygen atom is protonated, into -OH or H2O (stable leaving groups), to then give a product with a C=Nu double bond.
Aldehydes undergo nucleophilic addition reactions more readily than ketones, for reasons of electronics and sterics. Explain.
Sterically,
Aldehydes have only one large substituent bonded to the carbonyl group while ketones have two large groups. Aldehydes provide more room for incoming Nucleophiles and thus more readily undergo nucleophilic addition reactions.
Electronically,
There is greater polarization within the aldehyde function group than the ketone group. A primary Carbocation is higher in energy and less stable than a secondary carbocation and thus the primary is much more reactive because there is only one inductive group rather than two.
Nucleophilic addition of H2O: Hydration
Both aldehydes and ketones react with water to form 1,1-diols. These are also known as?
Geminal Diols.
Equilibrium favors the carbonyl compound for steric reasons, but the gem-diol is favored for a few simple aldehydes.
the nucleophilic addition of water to aldehydes and ketones is slow under normal conditions, what increases the rate of reaction?
acids and bases both catalyze this reaction.
1) Under acidic conditions, the Carbonyl Oxygen is protonated by the acid which then makes the carbonyl group more electrophilic. (Carbonyl oygen now has a positive charge)
2) Then a neutral nucleophile such as H2O, uses a pair of electrons to bond to the electrophilic carbon atom, and a pair of pi electrons are then pushed from the C=O bond to the oxygen atom neutralizing the positive oxygen.
3) the nucleophile (H2O) then gains a positive charge and is then deprotonated by H2O which yields a neutral addition product and regenerates the H3O+ catalyst.
Describe the nucleophilic addition of HCN, the Cyanohydro Formation…
Tip: The cyanohydrin is formed similar to that of every other nucleophilic addition that has been covered thus far.
Aldehydes and unhindered ketones are required for the -CN nucleophilic addition. The reaction occurs just as other tetrahedral intermediates are formed by the nucleophilic addition.
The alkoxide is once again protonated by the HCN, yielding an cyanohydrin (R-C-OH-CN-H) and regenerating -CN.
[Helpful Note: Nitrile R-CN can be reduced by LiAlH4 to yield a primary Amine RCH2NH2 and can then be hydrolyzed by hot aqueous acid to yield a carboxylic acid]
Nucleophilic addition of hydride and Grignard reagents yields alcohol formation.
The formation of the Alcohol follows the formation of the tetrahedral intermediate form of a standard nucleophilic addition reaction.
NaBH4 and LiAlH4 are merely a means of transportation for the Hydride Ion. After the alkoxide is formed addition aqueous acid protonates the alkoxide and water is formed as a by product
The addition of hydride reagents leads to the reduction of a compound, by what means?
By providing a reducing agent, such as -H (Hydride), that then enables a nucleophilic addition to occur.
