Organic Chemistry 51C. Lecture 05. Aldehydes and Ketones: Reactions. (Nowick)

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Chapter 21 covers carbonyl chemistry, highlighting reactions with nucleophiles, naming conventions, properties of aldehydes and ketones, spectroscopy data, reactions of acid chlorides, and the Wittig reaction's mechanism and synthetic applications. The chapter emphasizes electrophilic nature, conjugation effects on carbonyl stretching frequencies, and the importance of understanding nucleophile strength for various reactions in organic chemistry.

Insights

  • Carbonyl compounds can react with nucleophiles to form alkoxide anions, which can then be protonated to yield alcohols, showcasing the fundamental reactivity of carbonyl chemistry.
  • The Wittig reaction, a key synthetic method, involves the transformation of ketones or aldehydes into alkanes through the use of phosphorus ylids, highlighting a crucial tool for carbon-carbon bond formation with stereo selectivity.

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Recent questions

  • What are the main classes of carbonyl reactions?

    Reaction at carbonyl and alpha carbon as nucleophile.

  • How are aldehydes and ketones named?

    Follow IUPAC guidelines, common names for familiar compounds.

  • What properties vary between aldehydes and ketones?

    Volatility, boiling points based on molecular interactions.

  • How do carbonyl stretching frequencies vary?

    Decrease from esters to amides, impacted by conjugation.

  • What is the mechanism of the Wittig reaction?

    Involves phosphorus ylid reacting with ketone/aldehyde to form alkane.

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Summary

00:00

Carbonyl Chemistry: Nucleophilic Reactions and Properties

  • Chapter 21 focuses on carbonyl chemistry, specifically reactions of carbonyl compounds with nucleophiles.
  • Two main classes of reactions are highlighted: reaction at the carbonyl compound with nucleophiles and reaction of the alpha carbon as a nucleophile.
  • The electrophilic nature of carbonyl compounds allows nucleophiles to attack the carbonyl, leading to the formation of alkoxide anions.
  • Protonation of alkoxide anions with a source of H+ results in the formation of alcohols.
  • Weakly basic nucleophiles can react with carbonyl compounds when an acid catalyst is present, leading to protonation of the carbonyl and subsequent nucleophilic attack.
  • Naming aldehydes and ketones follows IUPAC guidelines, with common names often used for familiar compounds like acetone.
  • Properties of aldehydes and ketones, such as volatility and boiling points, vary based on their molecular interactions like dipole-dipole and hydrogen bonding.
  • Ketones are generally more reactive than aldehydes due to electron donation from alkyl groups and steric hindrance.
  • In IR spectroscopy, carbonyl stretching frequencies decrease as we move from esters to amides, with specific ranges for each compound type.
  • Conjugation in compounds like benzaldehyde can significantly impact carbonyl stretching frequencies.

22:09

Identifying Functional Groups in Spectroscopy

  • Carboxylic acids have a distinct OH stretch from 3500 to 2500, making them easily identifiable.
  • Ketones and acids have similar ranges in IR spectra, but carboxylic acids have a pronounced OH stretch.
  • Ester CO bond single stretch can sometimes be seen around 1300 to 1100 wave numbers.
  • NMR spectroscopy can provide hints like CH2 next to an oxygen in an ester at about four parts per million.
  • Aldehydes have characteristic bands around 2820 and 2720 in the CH stretching region.
  • Conjugation and ring strain can affect carbonyl stretching frequencies, shifting them accordingly.
  • Cyclobutanone has a higher stretching frequency of 1780 wave numbers due to ring strain.
  • Cyclopentanone shows a stronger CO bond with a stretching frequency of about 1745 wave numbers.
  • NMR spectra show alkyl CH's from 0.9 to 2 ppm, with aldehyde CH typically at 9 to 10 ppm.
  • C13 NMR spectra reveal carbonyl carbons of aldehydes and ketones around 190 to 215 ppm.

41:28

Reactions and Applications of Acid Chlorides

  • Acid chlorides react directly with water and alcohols due to the electron-withdrawing chlorine making them very electrophilic.
  • Reduction of acid chlorides to aldehydes can be achieved selectively using lithium aluminum hydride, with the addition of tert butanol to form lithium tritertbutoxycyaluminum hydride.
  • Reactions altering oxidation state without changing the molecule's structure are discussed, such as ozonolysis of alkenes to yield ketones or aldehydes.
  • The text revisits reactions of alkynes, including Markovnikov addition of water to yield ketones.
  • Hydroboration, adding in an anti-Markovnikov sense, is highlighted as a chemo-selective reaction for alkynes.
  • The concept of PKA is emphasized for understanding nucleophile strength, with examples ranging from very basic to weakly basic nucleophiles.
  • Strong nucleophiles like sodium acetylide can react with aldehydes and ketones to form alcohols.
  • Hydrogen cyanide can add to carbonyl compounds to form cyanohydrins, with precautions due to its extreme toxicity.
  • Sodium cyanide and sulfuric acid can be used to generate hydrogen cyanide in situ for safer handling.
  • The mechanism of cyanide poisoning is briefly discussed, with a focus on its binding to hemoglobin, contrasting with its nucleophilic reactivity in organic chemistry.

01:01:15

"Powerful Wittig Reaction: Carbon-Carbon Bond Formation"

  • Protonating from hydronium ion could protonate from another molecule of HCN as well.
  • Electrons flow from the oxyanion to the hydrogen, pushing electrons out of the OH bond back onto the oxygen to obtain cyanohydrin.
  • Chemistry analogy between alkyl metal addition and moderately basic nucleophile addition.
  • Moderately basic nucleophile, existing as an anion, can add to the carbonyl to form alkoxide anion, which can be protonated.
  • Strongly basic nucleophile cannot coexist with a proton source to prevent quenching.
  • Textbook introduces the Wittig reaction, involving a ketone or aldehyde reacting with a phosphorus ylid to form an alkane.
  • Wittig reaction replaces oxygen fully with a nucleophile, setting the stage for more complex reactions in the chapter.
  • Phosphorus ylid's conjugate acid is a triphenyl phosphonium salt, stabilized by phosphorus's di orbitals and charge separation.
  • Wittig reaction mechanism involves the formation of an oxaphosphetane intermediate, leading to the alkene and triphenylphosphene oxide.
  • Wittig reaction is a powerful synthetic tool, forming carbon-carbon bonds and exhibiting stereo selectivity.
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