Ch. 6: Aldehydes and Ketones I

Question 1

basic components: ketone vs. aldehyde

Answer 1

ketone: two alkyl groups bonded to the carbonyl

aldehyde: one alkyl group and one hydrogen

Question 2

what do the components of ketone vs. aldehyde imply about where the group lies in a compound?

Answer 2

the carbonyl in a ketone is never a terminal group, whereas it always is in an aldehyde

Question 3

aldehydes and ketones are often strong-smelling compounds, give 5 examples of compounds containing volatile carbonyls

Answer 3

1. cinnamon
2. vanilla
3. cumin
4. dill
5. ginger

Question 4

how are aldehydes named and what are the common names of the first 5 aldehydes?

how are aldehydes named when they are substituents?

Answer 4

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-

Question 5

how is an aldehyde named if it is attached to a ring?

Answer 5

the suffix -carbaldehyde is used instead

Question 6

how are ketones named? when about when they are substituents?

Answer 6

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-

Question 7

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)

Answer 7

1. 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
2. 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
3. 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

Question 8

why do aldehydes and ketones both act as electrophiles in reactions?

Answer 8

due to the electron-withdrawing properties of the carbonyl oxygen, which leaves a partial positive charge on the carbon

Question 9

why are aldehydes generally more reactive toward nucleophiles than ketones are?

Answer 9

because they have less steric hindrance and fewer electron-donating alkyl groups

Question 10

why does the carbonyl carbon have a dipole moment?

Answer 10

oxygen is more electronegative and pulls electrons away from the carbon, making the carbon electrophilic and a good target for nucleophiles

Question 11

how can an aldehyde form?

Answer 11

the partial oxidation of a primary alcohol, only by PCC

Question 12

how can a ketone form?

Answer 12

the oxidation of a secondary alcohol, which can be done using reagents ranging from sodium or potassium dichromate salts to chromium trioxide to PCC

Question 13

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?

Answer 13

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

Question 14

why is the carbonyl carbon an electrophile, ripe for nucleophilic attack?

Answer 14

the C=O bond is polarized, with a partial positive charge on the carbonyl carbon and a partial negative charge on the oxygen

Question 15

explain the basics of nucleophilic attack on a carbonyl carbon (4)

Answer 15

1. when the nucleophile attacks, it forms a covalent bond to the carbon, breaking the pi bond in the carbonyl
2. the electrons from the pi bond are pushed onto the oxygen atom
3. oxygen happily accepts extra electrons due to its electronegativity
4. breaking the pi bond forms a tetrahedral intermediate

Question 16

what should one ask and decide any time a carbonyl is opened?

Answer 16

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

Question 17

what are the 4 groups of nucleophilic addition reactions involving aldehydes and ketones?

Answer 17

1. hydration
2. acetals and hemiacetals
3. imines and enamines
4. cyanohydrins

Question 18

explain the hydration reaction (steps (2), char (1), summary (1))

Answer 18

1. in the presence of water, aldehydes and ketones react to form geminal diols
2. 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

Question 19

how are hemiacetals or hemiketals formed?

how can these compounds be recognized?

Answer 19

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

Question 20

is the step of hemiacetals and hemiketals the final endpoint?

Answer 20

yes, in basic conditions

Question 21

how are acetals and ketals formed?

process (3)

summary

Answer 21

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

1. this proceeds by a nucleophilic substitution reaction (Sn1) and is catalyzed by anhydrous acid
2. the hydroxyl group of a hemiacetal or hemiketal is protonated under adicic conditions and is lost as a molecule of water
3. 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

Question 22

what are acetals and ketals frequently used as? why?

Answer 22

they are frequently used as protecting groups for carbonyl functionalities because they are comparatively inert

Question 23

how can molecules with protecting groups be easily converted back to carbonyls?

Answer 23

with aqueous acid and heat

Question 24

what are the electrophile and nucleophile in the formation of hemiacetals/hemiketals? acetals/ketals?

Answer 24

hemiacetals/hemiketals: alcohol = nucleophile, carbonyl carbon = electrophile

acetals/ketals: alcohol = nucleophile, carbocation carbon = electrophile

Question 25

why do nitrogen and nitrogen-based functional groups act as good nucleophiles?

what do they react readily with?

Answer 25

due to the lone pair of electrons on nitrogen

they readily react with the electrophilic carbonyls of aldehydes and ketone

Question 26

defn + how is it produced: imine

Answer 26

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

Question 27

defn: condensation reaction

Answer 27

a small molecule is lost during the formation of a bond between two molecules

Question 28

why is the formation of an imine a nucleophilic substitution reaction?

Answer 28

because nitrogen replaces the carbonyl oxygen

Question 29

what are 3 common ammonia derivatives that react with aldehydes and ketones?

Answer 29

1. hydroxylamine (H2N-OH)
2. hydrazine (H2N-NH2)
3. semicarbazide (H2N-NH-C(O)NH2)

Question 30

what do these common ammonia derivatives form when they react with aldehydes and ketones?

Answer 30

1. oximes
2. hydrazones
3. semicarbazones

Question 31

defn + how are they made: enamines

Answer 31

contain both a double bond and a nitrogen-containing group

are formed by the tautomerization of imines and related compounds

Question 32

what group codes hydrogen cyanide (HCN) fit into on the MCAT?

Answer 32

it is a classic nucleophile

Question 33

char (3): HCN

Answer 33

1. triple bonds
2. an electronegative nitrogen atom
3. relatively acidic with a pKa of 9.2

Question 34

process: cyanohydrin formation from aldehydes and ketones

Answer 34

after the hydrogen dissociates, the nucleophilic cyanide anion can attack the carbonyl carbon atom, forming a cyanohydrin once the oxygen has been reprotonated

Question 35

where does cyanohydrin gain its stability from?

Answer 35

the newly formed C-C bond

Question 36

Any oxidizing agent stronger than PCC can oxidize aldehydes further to carboxylic acids. What are 4 common examples of oxidizing agents?

Answer 36

1. potassium permanganate KMnO4
2. chromium trioxide CrO3
3. silver I oxide Ag2O
4. hydrogen peroxide H2O2

Question 37

func: hydride reagents

Answer 37

reduce aldehydes and ketones to form alcohols

Question 38

what are the two most common hydride reagents on the MCAT? which one is used when milder conditions are needed?

Answer 38

lithium aluminum hydride (LiAlH4) and sodium borohydride (NaBH4)

the second is used when milder conditions are needed