Ch 6 - Aldehydes and Ketones: Electrophilicity and Oxidation-Reduction

Question 1

What are aldehydes and what is their nomenclature?

Answer 1

• terminal functional groups containing a carbonyl bonded to at least one hydrogen
• in nomenclature, they use the suffix -al and the prefix oxo-
• in rings, they are indicated by the suffix -carbaldehyde

Question 2

What are ketones and what is their nomenclature?

Answer 2

• internal functional groups containing a carbonyl bonded to 2 alkyl chains
• in nomenclature, they use the suffix -one and the prefix oxo- or keto-

Question 3

What dictates the reactivity of a carbonyl (C=O)?

Answer 3

• the polarity of the double bond

- the carbon has a partial positive charge and is therefore electrophilic

Question 4

How do carbonyl-containing compounds compare to alkanes and alcohols?

Answer 4

• carbonyl containing compounds have higher boiling points than equivalent alkanes because of dipole interactions
• alcohols have higher boiling points than carbonyls because of H bonding

Question 5

What produce aldehydes and ketones?

Answer 5

oxidation of primary and secondary alcohols

Question 6

What must be used for synthesizing aldehyes?

Answer 6

weaker, anhydrous agents like PCC must be used or the reaction will continue oxidizing to the level of the carboxylic acid

Question 7

What must be used for synthesizing ketones?

Answer 7

various oxidizing agents can be used dusch as dichromate, chromium trioxide, or PCC because ketones are the most oxidized functional group for secondary carbons

Question 8

What happens when a nucleophile attacks and forms a bond with a carbonyl carbon?

Answer 8

electrons in the pi bond are pushed to the oxygen atom

• if there is no good leaving group (aldehyde or ketone), the carbonyl will remain open and is pronated to form an alcohol
• if there is a good leaving group (carboxylic acids and derivative), the carbonyl will reform and kick off the leaving group

Question 9

What happens in hydration reactions?

Answer 9

water adds to a carbonyl, forming a geminal diol

Question 10

What happens with an equivalent alcohol reacts with an aldehyde or ketone? What happens if reacted with 2 equivalents?

Answer 10

• 1 equivalent: hemiacetal/hemiketal via nucleophilic substitution (alcohol is the nucleophile and the carbocation carbon is the electrophile)
• 2 equivalents: acetal and ketal

Question 11

What forms when nitrogen and nitrogen derivatives react with carbonyls via what reaction?

Answer 11

• condensation because a small molecule is lost and also a nucleophilic substitution
• results in imines (N=C), oximines, hydrazones, and semicarbazones
• imines can tautomerize to form enamines

Question 12

What is formed with hydrogen cyanide reacts with carbonyls?

Answer 12

cyanohydrins (stable product)

Question 13

What are aldehydes oxidized and reduced to?

Answer 13

• oxidized to carboxylic acids using an oxidizing agent like KMnO4, CrO3, Ag2O, or H2O2
• reduced to primary alcohols via hydride reagents (LiAlH4, NaBH4)

Question 14

What are ketones oxidized and reduced to?

Answer 14

• can be further oxidized thought they are the most oxidized functional group
• reduced to secondary alcohols using the same hydride reagents

Question 15

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

Answer 15

electrophilic, it is partially positively charged because oxygen is highly electron withdrawing

Question 16

What is one method of forming an aldehyde and ketone?

Answer 16

• 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

Question 17

Why is it difficult to isolate hemiacetals and hemiketals?

Answer 17

• the molecules are unstable
• the hydroxyl group is rapidly protonated and lost as water under acidic conditions, leaving behind a reactive carbocation