Aldehydes & Ketones I: Electrophilicity and redox

Question 1

Carbonyl

Answer 1

C=O

Question 2

Ketone

Answer 2

internal functional group with two alkyl groups bonded to the carbonyl

Question 3

aldehyde

Answer 3

terminal functional group with one alkyl group and one hydrogen

Question 4

aldehyde nomenclature

Answer 4

-replace -e with the suffix -al
-when named as substituents use the prefix oxo-
-in rings indicated by suffix -carbaldehyde

Question 5

ketone nomenclature

Answer 5

-replace -e with the suffix -one
-when naming ketones by common name, the two alkyl groups are named alphabetically followed by -ketone
-when named as substituents use either the prefix oxo- or keto-

Question 6

the physical properties of aldehydes and ketones are governed by the presence of _______

Answer 6

the carbonyl group.
-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

Question 7

Carbonyl reactivity

Answer 7

-dictated by the polarity of the double bond
-carbon has partial positive charge and is therefore electrophilic
-most common electrophile

Question 8

boiling points of carbonyl-containing compounds

Answer 8

-higher than equivalent alkanes because of dipole interactions
-lower than alcohols because alcohols have hydrogen bonding

Question 9

generally, ________ are more reactive towards nucleophiles since they have less steric hindrance and fewer electron donating groups than ketones

Answer 9

aldehydes

Question 10

PCC

Answer 10

used for synthesizing aldehydes

Question 11

Ketone synthesis

Answer 11

oxidizing agents
Na2Cr2O7, K2Cr2O7, CrO3, PCC

Question 12

Given an alkane, an aldehyde, and an alcohol with equal-length carbon chains, which will have the highest boiling point? Why?

Answer 12

The alkane will have the lowest boiling point, followed by the aldehyde and then the alcohol. The boiling point of the aldehyde is elevated by its dipole, but the boiling point of an alcohol is further elevated by hydrogen bonding.

Question 13

Is the carbon of a carbonyl electrophilic or nucleophilic? Why?

Answer 13

The carbon in a carbonyl is electrophilic; it is partially positively charged because oxygen is highly electron-withdrawing

Question 14

what is one method for forming an aldehyde? a ketone?

Answer 14

Aldehydes can be formed by the oxidation of primary alcohols, but can only be produced using weaker (and anhydrous) oxidizing agents like PCC—otherwise, they will oxidize fully to carboxylic acids. Ketones can be formed by the oxidation of secondary alcohols. Other methods can be used as well (ozonolysis, Friedel-Crafts acylation), but these are outside the scope of the MCAT.

Question 15

Nucleophilic addition reaction mechanism

Answer 15

the nucleophile attacks the carbonyl carbon, opening the carbonyl. the carbonyl cannot reform because there is no good leaving group; thus, the O- is protonated to generate an alcohol

Question 16

hydration reaction

Answer 16

in presence of water aldehydes/ketones react to form geminal diols
-nucleophilic oxygen in water attacks the electrophilic carbonyl carbon

Question 17

hemiacetal

Answer 17

one equivalent of alcohol (acts as a nucleophile) reacts with an aldehyde

Question 18

hemiketal

Answer 18

one equivalent of alcohol reacts with a ketone

Question 19

Hemi is known as the _____________ and is the endpoint in basic conditions

Answer 19

halfway point

Question 20

when ___ are added, the reaction proceeds to completion which results in the formation of an acetal and a ketal

Answer 20

two equivalents of alcohol

-reaction proceeds by SN1 and is catalyzed by anhydrous acid
-hydroxyl group of hemiacetal or hemiketal is protonated under acidic conditions and lost as a molecule of water. which leads to the formation of a carbocation
-another equivalent of alcohol attacks the carbocation which results in the formation of an acetal or ketal
-can be converted back to carbonyls with aqueous acid and heat

Question 21

nitrogen and nitrogen-based functional groups act as good nucleophiles due to since nitrogen has _________

Answer 21

a lone pair of electrons

Question 22

imine

Answer 22

simplest case. a compound with nitrogen atom double-bonded to a carbon atom
-a condensation reaction (small molecule is lost in the formation of the bond. in this case H2O)
-also an example of a nucleophilic substitution

Question 23

common ammonia derivatives

Answer 23

hydroxylamine (H2N-OH)
hydrazine (H2N-NH2)
semicarbazide (H2N-NH-C(O)NH2)

Question 24

imine formation

Answer 24

Question 25

enamines

Answer 25

contain both a double bond and a nitrogen-containing group
-imines can undergo tautomerization to form these

Question 26

Hydrogen Cyanide (HCN)

Answer 26

-classic nucleophile on the MCAT
-has both triple bonds and an electronegative nitrogen atom which makes it relatively acidic (pKa of 9.2)

Question 27

Cyanohydrin Formation

Answer 27

-after the hydrogen dissociates, the nucleophilic CN- can attack the carbonyl carbon atom
-reaction with aldehydes and ketones produce them
-gains stability from newly formed C-C bond

Question 28

When an aldehyde or ketone is reacted with one equivalent of an alcohol, what occurs? What would be different if it were reacted with two equivalents in acidic conditions?

Answer 28

With one equivalent of alcohol, aldehydes and ketones will form hemiacetals and hemiketals, respectively. With two equivalents of alcohol, the reaction will run to completion, forming acetals and ketals, respectively.

Question 29

When nitrogen or nitrogen-containing derivatives react with aldehydes and ketones, what type of reaction happens, and what functional group is formed?

Answer 29

The reaction that occurs is a condensation reaction because a small molecule is lost, and also a nucleophilic substitution reaction. This reaction results in the formation of an imine (or, for nitrogen-containing derivatives: oximes, hydrazones, or semicarbazones).

Question 30

When HCN reacts with an aldehyde or ketone, what functional group is produced? Is the product stable?

Answer 30

When HCN reacts with an aldehyde or ketone, a cyanohydrin is produced, which is a stable product

Question 31

oxidation-reduction reaction (aldehydes and ketones)

Answer 31

-aldehydes are in the middle of the redox spectrum since they are more oxidized than alcohols but less oxidized than carboxylic acid
-ketones are as oxidized as secondary carbons can get

Question 32

oxidizing agents for aldehydes

Answer 32

-KMnO4, CrO3, Ag2O, or H2O2
-oxidized to carboxylic acids

Question 33

Reduction by Hydride Reagents

Answer 33

Aldehydes and ketones can also undergo reduction to form alcohols.

This is performed with hydride reagents.
-Lithium Aluminum Hydride (LiAlH4)
-Sodium Borohydride (NaBH4)