Ch. 6: Aldehydes and Ketones I Flashcards
basic components: ketone vs. aldehyde
ketone: two alkyl groups bonded to the carbonyl
aldehyde: one alkyl group and one hydrogen
what do the components of ketone vs. aldehyde imply about where the group lies in a compound?
the carbonyl in a ketone is never a terminal group, whereas it always is in an aldehyde
aldehydes and ketones are often strong-smelling compounds, give 5 examples of compounds containing volatile carbonyls
- cinnamon
- vanilla
- cumin
- dill
- ginger
how are aldehydes named and what are the common names of the first 5 aldehydes?
how are aldehydes named when they are substituents?
named: replacing the -e at the end of the alkane name with the suffix -al
first 5 aldehydes: common (real)
formaldehyde (methanal)
acetaldehyde (ethanal)
propionaldehyde (propanal)
butryaldehyde (butanal)
valeraldehyde (pentanal)
when substituent: named with the prefix oxo-
how is an aldehyde named if it is attached to a ring?
the suffix -carbaldehyde is used instead
how are ketones named? when about when they are substituents?
replacing the -e with -one
when naming by their common names: the two alkyl groups are named alphabetically, followed by -ketone
when they are named as substituents: use the prefix oxo- or keto-
how are the physical properties of aldehydes and ketones governed by the presence of the carbonyl group in comparison to the forces and physical properties of alcohols? (3)
- the dipole of the carbonyl is stronger than the dipole of an alcohol because the double-bonded oxygen is more electron-withdrawing than the single bond to oxygen in the hydroxyl group
- in solution, the dipole moments associated with these polar carbonyl groups increase intermolecular attractions, causing an elevation in boiling point relative to their parent alkanes
- however, even though aldehydes and ketones have dipoles more polar than those of alcohols, the elevation in boiling point is less than that in alcohols because no hydrogen bonding is present
why do aldehydes and ketones both act as electrophiles in reactions?
due to the electron-withdrawing properties of the carbonyl oxygen, which leaves a partial positive charge on the carbon
why are aldehydes generally more reactive toward nucleophiles than ketones are?
because they have less steric hindrance and fewer electron-donating alkyl groups
why does the carbonyl carbon have a dipole moment?
oxygen is more electronegative and pulls electrons away from the carbon, making the carbon electrophilic and a good target for nucleophiles
how can an aldehyde form?
the partial oxidation of a primary alcohol, only by PCC
how can a ketone form?
the oxidation of a secondary alcohol, which can be done using reagents ranging from sodium or potassium dichromate salts to chromium trioxide to PCC
why is there concern about oxidizing too far when oxidizing a primary alcohol, but not a secondary alcohol? what impact does this have on what oxidants can be used?
when oxidizing a secondary alcohol, there is no concern because the reaction will stop at the ketone stage
whereas, when oxidizing a primary alcohol, if the oxidant is stronger than PCC, primary alcohols would continue to be oxidized PAST aldehydes, all the way to carboxylic acids
why is the carbonyl carbon an electrophile, ripe for nucleophilic attack?
the C=O bond is polarized, with a partial positive charge on the carbonyl carbon and a partial negative charge on the oxygen
explain the basics of nucleophilic attack on a carbonyl carbon (4)
- when the nucleophile attacks, it forms a covalent bond to the carbon, breaking the pi bond in the carbonyl
- the electrons from the pi bond are pushed onto the oxygen atom
- oxygen happily accepts extra electrons due to its electronegativity
- breaking the pi bond forms a tetrahedral intermediate
what should one ask and decide any time a carbonyl is opened?
Can I reform the carbonyl?
If no good leaving group is present (as is true with aldehydes and ketones), the carbonyl will not reform. Generally O- will accept a proton from the solvent to form a hydroxyl group, resulting in an alcohol
If a good leaving group is present (as is true with carboxylic acids and derivatives), the carbonyl double bond can reform, pushing off the leaving group
what are the 4 groups of nucleophilic addition reactions involving aldehydes and ketones?
- hydration
- acetals and hemiacetals
- imines and enamines
- cyanohydrins
explain the hydration reaction (steps (2), char (1), summary (1))
- in the presence of water, aldehydes and ketones react to form geminal diols
- the nucleophilic oxygen in water attacks the electrophilic carbonyl carbon
char: this reaction normally proceeds slowly, but we can increase the rate by adding a small amount of catalytic acid or base
summary: the carbonyl is hydrated by water, then protonated, resulting in a geminal diol
how are hemiacetals or hemiketals formed?
how can these compounds be recognized?
when one equivalent of alcohol (the nucleophile) is added to an aldehyde or ketone, the product is a hemiacetal or hemiketal
they can be recognized by the retention of the hydroxyl group
is the step of hemiacetals and hemiketals the final endpoint?
yes, in basic conditions
how are acetals and ketals formed?
process (3)
summary
when two equivalents of alcohol are added to an aldehyde or ketone, the reaction proceeds to completion, resulting in the formation of an acetal or ketal
- this proceeds by a nucleophilic substitution reaction (Sn1) and is catalyzed by anhydrous acid
- the hydroxyl group of a hemiacetal or hemiketal is protonated under adicic conditions and is lost as a molecule of water
- a carbocation is thus formed, and another equivalent of alcohol attacks this carbocation, forming an acetal or ketal
summary: once a hemiacetal or hemiketal is formed, the hydroxyl group is protonated and released as a molecule of water, alcohol then attacks, forming the acetal or ketal
what are acetals and ketals frequently used as? why?
they are frequently used as protecting groups for carbonyl functionalities because they are comparatively inert
how can molecules with protecting groups be easily converted back to carbonyls?
with aqueous acid and heat
what are the electrophile and nucleophile in the formation of hemiacetals/hemiketals? acetals/ketals?
hemiacetals/hemiketals: alcohol = nucleophile, carbonyl carbon = electrophile
acetals/ketals: alcohol = nucleophile, carbocation carbon = electrophile
why do nitrogen and nitrogen-based functional groups act as good nucleophiles?
what do they react readily with?
due to the lone pair of electrons on nitrogen
they readily react with the electrophilic carbonyls of aldehydes and ketone
defn + how is it produced: imine
imine = a compound with a nitrogen atom double-bonded to a carbon atom
produced = ammonia adds to the carbon atom of the carbonyl and water is lost
defn: condensation reaction
a small molecule is lost during the formation of a bond between two molecules
why is the formation of an imine a nucleophilic substitution reaction?
because nitrogen replaces the carbonyl oxygen
what are 3 common ammonia derivatives that react with aldehydes and ketones?
- hydroxylamine (H2N-OH)
- hydrazine (H2N-NH2)
- semicarbazide (H2N-NH-C(O)NH2)
what do these common ammonia derivatives form when they react with aldehydes and ketones?
- oximes
- hydrazones
- semicarbazones
defn + how are they made: enamines
contain both a double bond and a nitrogen-containing group
are formed by the tautomerization of imines and related compounds
what group codes hydrogen cyanide (HCN) fit into on the MCAT?
it is a classic nucleophile
char (3): HCN
- triple bonds
- an electronegative nitrogen atom
- relatively acidic with a pKa of 9.2
process: cyanohydrin formation from aldehydes and ketones
after the hydrogen dissociates, the nucleophilic cyanide anion can attack the carbonyl carbon atom, forming a cyanohydrin once the oxygen has been reprotonated
where does cyanohydrin gain its stability from?
the newly formed C-C bond
Any oxidizing agent stronger than PCC can oxidize aldehydes further to carboxylic acids. What are 4 common examples of oxidizing agents?
- potassium permanganate KMnO4
- chromium trioxide CrO3
- silver I oxide Ag2O
- hydrogen peroxide H2O2
func: hydride reagents
reduce aldehydes and ketones to form alcohols
what are the two most common hydride reagents on the MCAT? which one is used when milder conditions are needed?
lithium aluminum hydride (LiAlH4) and sodium borohydride (NaBH4)
the second is used when milder conditions are needed
