MS 920 - Aldehydes and Ketones

Submitted by Matt on August 7, 2011.

Textbook and Chapter: Carey and Giuliano 8th Ed. (2010), Chapter 17

Keywords: aldehydes, ketones

Description: Problems related to aldehydes and ketones: Wittig, imines, acetals, and protecting groups.

Total Problems: 6

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Problem # 710

Rank the carbonyls A-D below in order of decreasing electrophilicity (reactivity with nucleophiles).

(1 = Most reactive). Explain your reasoning.
Solution
The carbonyl carbon is electrophilic because it has a partial positive charge.

Are there any groups that make a carbon more positive? Yes, electron withdrawing groups (EWG). They pull away electron density, which increases electrophility. So C is the most reactive.

Conversely, electron donating groups (EDG) add electron density, and so make the carbonyl carbon less positive, and less electrophilic. Alkyl groups (carbon chains) are mildly EDG so ketone B will be less reactive than C.

Hydrogen is neither a EDG or EWG, so the aldehyde A will be in between the B and C. Also, the hydrogen is very small, so the carbonyl carbon is easily to get to (less steric bulk to block an attack).

Ester D is the least reactive because it has a resonance (the lone pair on the oxygen gets involved).

So overall, the order form most reactive to least reactive is C > A > B > D.

Problem Details
MendelSet practice problem # 710 submitted by Matt on July 22, 2011.
Subject: Organic Chemistry
Subject Topic(s): Nucleophilic Acyl Addition (Aldehydes & Ketones, Imines/Enamines, Acetals/Ketals)
Question Type: Concept (explain why this is so..)
Keyword(s): EDG and EWG, carbonyl trends
Problem # 711

Show two ways of preparing the alkene below via the Wittig reaction starting from triphenyl phosphine (PPh₃).

Is one route better than the other? Why?
Solution
Alkenes can be prepared from a combination of ylide and aldehyde or ketone. This is the Wittig reaction.

One carbon on the alkene comes from the carbon bonded to the PPh₃ on the ylide, and the other carbon comes from the carbonyl.

There are usually two different ways to make an alkene via the Wittig reaction. So is one way better than the other?

Yes. Ylides come from the S_N2 reaction of PPh₃ with an alkyl halide. Because it's an S_N2 reaction, you want to use the least substituted alkyl halide! (1º is better than 2º). So of the two reactions below that would result in the desired alkene, the top method is better because it involves the less bulky alkyl halide.

This principle also holds true for the Williamson ether synthesis (problem 703). Less substituted alkyl halides are better for S_N2 reactions.

Problem Details
MendelSet practice problem # 711 submitted by Matt on July 22, 2011.
Subject: Organic Chemistry
Subject Topic(s): Nucleophilic Acyl Addition (Aldehydes & Ketones, Imines/Enamines, Acetals/Ketals)
Question Type: Synthesis (show how to prepare X from Y..)
Keyword(s): Wittig reaction
Problem # 712

Show what combination of amine and carbonyl would result in each imine or enamine.
Solution
For both imines and enamines, the sp² carbon comes from the carbonyl, and the nitrogen comes from the amine.

Problem Details
MendelSet practice problem # 712 submitted by Matt on July 22, 2011.
Subject: Organic Chemistry
Subject Topic(s): Nucleophilic Acyl Addition (Aldehydes & Ketones, Imines/Enamines, Acetals/Ketals)
Question Type: Synthesis (show how to prepare X from Y..)
Keyword(s): imines, enamines
Problem # 713

When a carbonyl is treated with semicarbazide under acidic conditions an "imine" is produced called a semicarbazone.

Which of the two products below is the correct structure for a semicarbazone? Explain.
Solution
What this problem is really asking is "Which nitrogen on semicarbazide is more nucleophilic?"

The nitrogens bonded to the carbonyl on semicarbazide all have resonance forms, so their lone pairs aren't as available for nucleophilic attack.

The other nitrogen (on the left) is the most nucleophilic because it isn't involved in any resonance forms, so its lone pair isn't shared with another atoms.

Problem Details
MendelSet practice problem # 713 submitted by Matt on July 22, 2011.
Subject: Organic Chemistry
Subject Topic(s): Nucleophilic Acyl Addition (Aldehydes & Ketones, Imines/Enamines, Acetals/Ketals)
Question Type: Concept (explain why this is so..)
Keyword(s): nucleophilicity, imines
Problem # 714

Complete each synthesis below. All carbon sources must come from alkenes.

Each synthesis will involve protecting groups.
Solution
The protecting group most commonly used for aldehydes and ketones (in undergraduate orgo) is ethylene glycol.

It is put on using HOCH₂CH₂OH/H⁺ and removed with H₂O/H⁺.

For a), the reaction calls for the use of acetylide with an alkyl halide. But acetylides also react with carbonyls.

So before the alkyne is deprotonated using a strong base (such as NaNH₂), the carbonyl must be protected with ethylene glycol.

For b), the reaction calls for the addition of two equivalents of phenyl Grignard to the ester. The problem is that esters aren't as reactive as ketones (or aldehydes), so the Grignard would react with the ketone before it ever touched the ester! To prevent this, the ketone must be protected before Grignard is added.

Problem Details
MendelSet practice problem # 714 submitted by Matt on July 22, 2011.
Subject: Organic Chemistry
Subject Topic(s): Nucleophilic Acyl Addition (Aldehydes & Ketones, Imines/Enamines, Acetals/Ketals)
Question Type: Synthesis (show how to prepare X from Y..)
Keyword(s): protecting groups, acetals and ketals
Problem # 715

α-D-glucose is shown below.

a) Is α-D-glucose an acetal, hemiacetal, ketal, or hemiketal?

b) Draw the carbonyl form of α-D-glucose.
Solution
a) You can spot the carbonyl carbon because it's the one with two oxygens bonded to it (oxidation state II).

The carbonyl carbon has an -OH (alcohol) bonded to it, and not an -OR (ether), so it's a hemiacetal or a kemiketal.

The carbonyl carbon also has a hydrogen bonded to it, so it must have come form an aldehyde (instead of ketone). So it's a hemiacetal. (remember: aldehyde -> acetal, ketone -> ketal).

b) The hemiacetal carbon's -OH (or -OR in the case of a ketal or acetal) is the oxygen that used to be the carbonyl C=O.

Problem Details
MendelSet practice problem # 715 submitted by Matt on July 23, 2011.
Subject: Organic Chemistry
Subject Topic(s): Nucleophilic Acyl Addition (Aldehydes & Ketones, Imines/Enamines, Acetals/Ketals), Carbohydrates
Question Type: Concept (explain why this is so..)
Keyword(s): acetals and ketals

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MS 920 - Aldehydes and Ketones

MS 920 - Aldehydes and Ketones

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