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16. ALDEHYDES AND KETONES. (approx. lecture time: 3 lectures) Topics covered:16.1-16.8A/B/C,16.9-16.10A,16.11. Most students find the next four (4) chapters (all dealing with chemistry of the carbonyl (C=O) group) to be the most challenging in CHEM 232. Here you will learn many new reactions and you will need to have a system for learning how to recall them. SEE NEXT SLIDE…….
How to Learn Organic Reactions (Organic reactions interconvert organic compounds. They are the basis of organic chemistry and of living systems) What happened? (Learn the reaction. Give the reaction a title or name. Use “reaction lists” and other techniques. Know your reactions “forwards” and “backwards” Recognize variations. This is hard work. How did it happen? (Learn the mechanism, if required. You are not required to know all them) Why did it happen? (Not always answerable. Wait for advanced courses…maybe)
Nomenclature of Aldehydes and Ketones Some well-known aldehydes (preservative) (your hangover) (cinnamon) (almonds) (vanilla flavour)
Diacetyl (The 1,2-diketone that gives microwave popcorn its “butter” flavour) Sorry, it’s not real butter
CHANEL No 5 (A perfume for “older” people with deeper pockets) The key ingredients for the most expensive and famous perfume in the world are simple aldehydes! Coco Chanel (circa 1920) Created a fashion empire from humble beginnings This aromatic aldehyde used in fragrances and perfumes (Lily of the Valley scent) is known to attract human sperm, by causing them to swim twice as fast when they sense this molecule in vitro.
Useful method: Ketones can be synthesized from a nitrile and a Grignard reagent
General Reaction of Aldehydes and KetonesNucleophilic Addition to the Carbon of the C=O Mechanism for Addition of STRONG Nucleophiles
WEAKnucleophiles such as water and alcohols (ROH) need acid catalysis as shown below: Aldehydes are more reactive than ketones: The aldehyde carbonyl carbon is less hindered for attack. The extra R group in ketones stabilize the carbonyl carbon by electron donation.
Hydration: Addition of Water (hydrates are generally unstable) Chloral hydrate is a stable aldehyde hydrate used historically as a sedative before minor medical. It is still used in veterinary medicine. Addition of Alcohols: Formation of Acetals In general, hemiacetalsare not isolated; acetalsare isolable and are often made.
Mechanism for Formation of Acetals hemiacetal hemiacetal Longest mechanism in this course acetal
Glucose (a stable hemiacetal) (acetals are unstable in acid; they regenerate the aldehyde or ketone) (acetals are stable in base) Acetals are often used in “functional group protection”
The Sex Life of the Olive Fruit Fly(BacroceraOleae) Where is the ketone hiding?
Ecstasy(The illicit Drug)(MDMA; 3,4-methylenedioxy-N-methylamphetamine) Very stable acetal! BC is well-known for being a major exporter of marijuana and Ecstasy.
TATP (“triacetonetriperoxide”) One of the most unstable of explosives Readily made from acetone (ADDITION of hydrogen peroxide) Commonly used by terrorists; difficult to detect Explosive used in the London subway bombings (July 7, 2005) acetone White crystalline powder with bleach odour
Cyanohydrins (Cyanide as the nucleophile, giving cyanohydrins)
Amines as Nucleophiles Think C=N !
C=N does not form here! (but you don’t need to know this one) (Tertiary amines do not react with aldehydes and ketones)
Methamphetamine (“crystal meth”) The utility of reduction in the synthesis methamphetamine Possible Methods: Hydrogenolysis or Birch Reduction Sudafed and Claritin contain ephedrine or pseudoephedrine Ephedrine, isolated from Ephedra sinica, is an old medicine for colds and asthma.
Wittig Reaction Think C=C ! (A new way to make alkenes)
The Nobel Prize in Chemistry 1979 "for their development of the use of boron and phosphorus reagents in organic synthesis" Herbert C. Brown For boron reagents such as BH3 Georg Wittig For phosphorus reagents such as Ph3P=CH2